# solar air heating collectors



## GaryGary (Jul 24, 2013)

Hi,
I know its a long ways from winter, but I just did some testing on two DIY solar air heating collectors and thought I would pass it on.

http://www.builditsolar.com/Experimental/PopCanVsScreen/PopCanVsScreen.htm

One is the pop can collector in which the absorber is made from columns of alum pop cans with the ends cut out, and the other is the screen collector in which the absorber is made from 3 layers of black insect screen. 

The screen collector is kind of amazing -- very quick and cheap and easy to build, but performs very well.  Can be a couple year payback in a descent sun location.



GAry


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## Ehouse (Jul 24, 2013)

What would happen with a finer mesh screen, like landscaping fabric?  More than three layers?


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## Seasoned Oak (Jul 24, 2013)

WHat about just a black painted sheet of metal or a piece of sheet metal roofing,the wavy kind for the heat sink?


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## jebatty (Jul 25, 2013)

In the early '80's I built a 4' x 8' hot air collector mounted on the roof, 3/12 pitch, 45 degree latitude, and used steel corrugated panel as the collector, painted black, fiberglass greenhouse panel for the glazing, foam insulation panel on bottom and sides. Air flow was under the collector and then back over the top of the collector, 4" ducting and a 50 cfm bathroom exhaust fan for the blower. I used a snap thermostat, on at 100F in the collector. Air was blown in from our basement at about 60F and returned to the basement as high as 140F during winter on a calm, -20F outside temp day. Very satisfactory, but the problem was condensation inside the collector and rotting of the wood. The collector lasted about 6 years before I removed it due to deterioration.


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## Seasoned Oak (Jul 25, 2013)

jebatty said:


> but the problem was condensation inside the collector and rotting of the wood. The collector lasted about 6 years before I removed it due to deterioration.


SO treated wood could solve this problem,RIGHT?


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## Ehouse (Jul 25, 2013)

What about combining functions using a collection material porous enough to allow fan induced air flow but acting as insulation when the fan is off?


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## GaryGary (Jul 25, 2013)

Ehouse said:


> What would happen with a finer mesh screen, like landscaping fabric? More than three layers?


 

Hi,
Some of the people who have tried landscape fabric have reported a short life, but that may depend on the brand and material.

2 or 3 or 4 layers of screen is an open issue, and I may have a go at testing some combinations.  One thing that does happen is that the solar radiation that gets through how ever many layers of screen hits the black painted back of the collector and gets absorbed there, and the air flowing up the box picks of that heat as well.  But, it may be more efficient to have it absorbed in the screen as the back probably has to run quite hot in order to get the heat transferred into the air.  

All good things to try.

Gary


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## GaryGary (Jul 25, 2013)

Seasoned Oak said:


> WHat about just a black painted sheet of metal or a piece of sheet metal roofing,the wavy kind for the heat sink?


 

Hi,
This works, and people use it, but I think that the flow through absorbers are better.

Scott and I did some testing a couple years back comparing several types of absorbers, and the screen was the efficiency winner -- I think this has to do with the large surface area and lots of heat transfer area that thousands of strands of the screen wire provide.

This is the stuff on the earlier test: http://www.builditsolar.com/Experimental/AirColTesting/Index.htm  there is nothing exactly like what you propose, but some are fairly close.  I do think there is a lot left to learn about these collectors and more experimenting would be a good thing.

Gary


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## GaryGary (Jul 25, 2013)

jebatty said:


> In the early '80's I built a 4' x 8' hot air collector mounted on the roof, 3/12 pitch, 45 degree latitude, and used steel corrugated panel as the collector, painted black, fiberglass greenhouse panel for the glazing, foam insulation panel on bottom and sides. Air flow was under the collector and then back over the top of the collector, 4" ducting and a 50 cfm bathroom exhaust fan for the blower. I used a snap thermostat, on at 100F in the collector. Air was blown in from our basement at about 60F and returned to the basement as high as 140F during winter on a calm, -20F outside temp day. Very satisfactory, but the problem was condensation inside the collector and rotting of the wood. The collector lasted about 6 years before I removed it due to deterioration.


 

Hi,
Nice.

The 60F to 140F is a bit high on the temperature rise and it probably would have produced a bit more heat with more airflow and a smaller temperature rise, but clearly it worked well.

I think that if you live in a climate where moisture problems can occur, its better to use a metal box for the collector.  There is a design that uses galvanized metal sill track for the box.  Its not too expensive and pretty easy to build -- should last a long time.


One example from Kennith: http://www.builditsolar.com/Projects/SpaceHeating/KenSoffitCollector/Main.htm
This one has a flow through absorber that uses aluminum vented soffit panels -- it was a performance tie with the screen absorber in our testing.

Gary


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## jebatty (Jul 25, 2013)

As mentioned, winter performance was good, but the heating period was very short in Dec - January, about 2 hours maximum. The low sun angle does not provide heat for very long.


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## Augie (Jul 25, 2013)

I have been thinking of Building Two 4x8 Panels for Shoulder Season Heating. 10-15k BTU's per hour would keep my house nice and warm when temps are in the 40's or 50's. This is my hardest time to heat as well because it is easy to over heat the house with the stove. I have an area on my house that is shaded in the summer but not after the leaves fall, this is when I would be interested in the type of heat that I could get form this kind of setup. Im thinking that If I placed The Units on the Roof I could run them right into the house, with a temp controlled fan I think this could be a great solution. I actually have a bunch of leftover materials and might be able to get away with almost no cost on this one...


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## GaryGary (Jul 25, 2013)

jebatty said:


> As mentioned, winter performance was good, but the heating period was very short in Dec - January, about 2 hours maximum. The low sun angle does not provide heat for very long.


 

Hi,
This is not as bad as it first seems when you look at the day length in summer vs day length in winter -- the summer day may be 16 hours long and the winter day 8 hours, but in the summer the sun is behind the panel for much of that 16 hours.  The real useful solar heating time is 9 am to 3pm -- if you can get that, then you are good.  As long as the collector is not shaded during those times it will do well, winter or summer.

Clear winter days here in MT at 46 deg N lat produce a ton of heat -- the combination of the low sun shining nearly directly on a vertical collector and the strong reflection off snow in front of the collector provides much more radiation on the collector than that same collector in the summer.

Its more the amount of cloud cover in winter than the length of the day that limits solar heating in the winter, and that depends on your local climate.

-----------

People often rate the economic viability of solar PV on a peak watts per dollar (eg PV SYSTEMS are now selling for about $5 per peak watt).

So, what's the cost per peak watt of one of the DIY solar air heating collectors?

You can build a 4 by 8 ft air heating collector for about $200 in materials (this can include collector, ducting, blower, and thermal control switch).

This collector (if its a good design) under sunny conditions (1000 w/sm) and with a 35F ambient temperature and 120F outlet temperature operates at 53% efficiency with 168 BTU/sf-hr output.  Its total output 5380 BTU/hr or 1575 watts.
http://www.builditsolar.com/References/Calculators/Collector/ColEfic.htm

Since the collector costs $200, the cost in dollars per peak watt is:  ($200/1575 watts) = $0.13 per peak watt.  
About 38 times less than a PV system at $5 per peak watt.

Of course, this is not fair because the PV system operates all year and solar heating system only during the heating season, but the difference is still pretty staggering (at least to me 

I'm not trying knock PV ( I have a PV system and love it), but I think people don't appreciate what a good deal these air heating collectors are.


Gary


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## Seasoned Oak (Jul 25, 2013)

jebatty said:


> As mentioned, winter performance was good, but the heating period was very short in Dec - January, about 2 hours maximum. The low sun angle does not provide heat for very long.


Wow 2 hours, My passive solar room gets about 7+ in the dead of winter from 9 AM to 4PM and also heats my entire house on sunny days in winter.


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## woodgeek (Jul 26, 2013)

In my case, the PA resource is pretty good, say >1200 hours of peak output per year (for PV). During the 5 mos where I need real heating, I might have 400 hours of peak output resource. For PV, those partly cloudy and hazy days still contribute to those 400 hours. For thermal, a 50% cloud cover will hurt the eff a lot, so I will get less than 50% rated heat out. On the other hand, during a lot of that time, my outdoor temps are higher than Gary's, then my eff might be a tad higher.

So, if I assume I still get 400 hours of peak output per heating season, then that is 1.575 kW * 400 hours * 3414 BTU/kWh = 2 Million BTU/season.

My wind-powered BTUs from my heat pump cost me ~$15 /MMBTU (an estimate, they will be more expensive on colder cloudy days and at night), so a 4'x8' would save me ~$30/yr, for a 7 year payback at a $200 upfront cost (costing my 'hobby' labor at $0). The panel would offset 3% of my ~65 MMBTU annual heating load, or save me 1/10th cord of wood equivalent.

In contrast, a few weekends with $300 of weatherstripping, caulk and foamboard netted me a reduction of ~30 MMBTU/season, 15x what I would get from building a panel, and with a 150% annual ROI (on material cost).


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## jebatty (Jul 26, 2013)

Low hanging fruit often is not glamorous. Glitz is attractive. I am as guilty as many others, but I do a pretty good job of both picking the low hanging fruit as well as picking the shiny apple at the top of tree. In this regard perhaps the lowest hanging "nutritional" fruit I picked was the insulation and plumbing modifications of my electric hot water heater at a one time cost of less than $50 and with a payback of $60+ per year in perpetuity, increasing as electric rates go up. That fruit was so low on the tree that no one even knows I plucked it unless I tell them, and when I do most look on with disinterest because it's not exactly pretty.


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## GaryGary (Jul 28, 2013)

woodgeek said:


> In my case, the PA resource is pretty good, say >1200 hours of peak output per year (for PV). During the 5 mos where I need real heating, I might have 400 hours of peak output resource. For PV, those partly cloudy and hazy days still contribute to those 400 hours. For thermal, a 50% cloud cover will hurt the eff a lot, so I will get less than 50% rated heat out. On the other hand, during a lot of that time, my outdoor temps are higher than Gary's, then my eff might be a tad higher.
> 
> So, if I assume I still get 400 hours of peak output per heating season, then that is 1.575 kW * 400 hours * 3414 BTU/kWh = 2 Million BTU/season.
> 
> ...


 
Hi,

There is the common advice to work on the basics like insulation and sealing and you will get the greatest return, and I tell people the same thing all the time.
But, I've kept track of all the projects we have done around the house to save energy and estimated cost, energy saving, carbon saving, and return on investment (ROI)  on each one -- they are all listed here:

http://www.builditsolar.com/References/Half/ProjectCharts.htm
The last chart is the ROI one.

Each project and the calculations are described here:
http://www.builditsolar.com/References/Half/Projects.htm
I'll be the first to admit that some of these energy and cost calculations are very hard to do accurately, but I decided I would attempt a calculation for every project.

If you look at the ROIs for the different projects, the insulation ones do generally do well, but some of the DIY solar thermal projects do as well or better.  And, some of the oddball ones like the electric mattress pad heaters do even better.

The message for me is that nothing is cast in concrete, you have to try to estimate the cost and saving for each project and let stand on its own merits -- you might be surprised.

Some insulation projects can be quite expensive depending on what the current insulation level is and how hard it is to add more.  We are having our roof redone, and part of the house has a cathedral ceiling that is hard to add insulation to except on top of the roof deck before the new shingles go on.  The current ceiling is insulted to R38 with FG, and I was just not able to make adding insulation on top the roof deck pay at all.

---------
I'll see if I can find a more accurate way of estimating the seasonal heat output of the air collector -- I think you may be on the low side. 
One rule of thumb that is rough, but is used a lot is 1 gallon of oil per sqft of collector.  That would give about 4.5 million BTU per year for a 32 sqft collector.  
For $2 propane (which is what we use) in an 80% efficient furnace, this would be $130 a year for a 1.5 year payback -- not counting any state rebates.  The MT $1000 tax credit would actually pay for 5 collectors.

Agree that one 4  by 8 collector is not going to revolutionize anyone's heating bill in a cold climate.  More area is definitely better, and its easy to build bigger -- south walls are made for space heating collectors.

Not sure how low the sun gets under cloudy conditions (probably varies a lot), but at half sun (500 w/sm) with 40F ambient and 100F collector outlet temperature, a well designed collector will still have about 40% efficiency -- still pretty good.  But, I suspect that cloudy days are typically well under the 500 w/sm. 
http://www.builditsolar.com/References/Calculators/Collector/ColEfic.htm

When I look at the power output on my Enpase PV system, the output on cloudy days is pretty bad.  It does still produce a little power, but my impression just looking at the graphs is that the bulk of our PV power comes from sunny days.  I wish I had saved some of the winter graphs, but did not.  The next time we get an honest cloudy day, I'll keep track of it.

-------
My reservation about heat pumps is that for most people with typical fossil fuel generated electricity the carbon emissions including generation are about the same as if you heat with natural gas in an efficient furnace.  To me the carbon emissions are as important as reducing costs.
In MT, each KWH comes with a 1.6 lbs of CO2 emissions even with some hydro and wind in our power mix.

There is also the eventual compressor replacement cost.

It looks like you have (or buy?) wind power, so the carbon is not an issue for you.  
Also possible to put in enough PV to run the heat pump, but, to me, that seems like a lot of cost and complication compared to a simple solar thermal system.

Lots of trades to consider.

Gary


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## Where2 (Jul 28, 2013)

Seasoned Oak said:


> SO treated wood could solve this problem,RIGHT?


ACQ (the wood treatment) generally is intended to reduce fungi growth and decay damage. If you want to avoid water problems, use several coats of a quality paint to keep the moisture out of the wood. Not sure I'd want to have long term exposure to whatever off-gasses from ACQ as it ages. I'd seal it with paint...


Seasoned Oak said:


> What about just a black painted sheet of metal or a piece of sheet metal roofing,the wavy kind for the heat sink?


What I would propose, is to use a corrugated black metal surface elevated with stand offs from the rear insulated back. That way, air can flow over the top and the bottom of the absorber.

The OP's screen collector is a curious thought. Were you using fiberglass, or metal screen mesh? I wonder how much additional surface area is available using multiple layers of screen vs using a corrugated metal plate? The whole conversion efficiency of a thermal system is driven on surface area for thermal transfer.


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## woodgeek (Jul 28, 2013)

Gary,

I love the projects, and especially the ROI calcs you do which are right on. But every case is different. I still think the rockies and west are much better for solar space heating than New England, due to the very different winter resource. I would like to make DIY solar work on my site, but I can never get it to pencil out (even if I weren't shaded by neighbors trees/hills).

Lots of folks seem to be going net zero with super-insulation+grid-tiePV+minisplits. We can say that that is dependent on grid, govt spending, complicated tech, but it is being done. Active solar thermal sometimes gets incorporated into those projects, sometimes not, depending on how it pencils out.


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## 4acrefarm (Jul 28, 2013)

I have built 2 solar collectors using black polyester felt and I am very happy with the results.

One is a small 61/2'x 21/2' in my master bathroom . This one is powered by a pv fan. Air enters near floor at 65*  and returns at 130*. It runs 4 or more hours per day.

The other one is 3 times larger and suck air out of the basement and into our tv room, this one runs a 120v snap disc setup.This one stars around 10:00 am and runs till at least 3:00.

The bathroom temp is increasing  about 15* and the attached bedroom is also heated by it. The large panel helps heat 400sf. Iam very happy with the results.
https://www.hearth.com/talk/posts/53589/


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## Where2 (Jul 28, 2013)

4acrefarm said:


> I have built 2 solar collectors using black polyester felt and I am very happy with the results.
> 
> One is a small 61/2'x 21/2' in my master bathroom . This one is powered by a pv fan.


 
^^^ Makes plenty of sense to me. I've found some 130CFM server fans that only take 6W@12V to run. Throw a snap switch in the system, and a dryer vent style self actuating flap and it would be automatic.


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## GaryGary (Jul 28, 2013)

Where2 said:


> ACQ (the wood treatment) generally is intended to reduce fungi growth and decay damage. If you want to avoid water problems, use several coats of a quality paint to keep the moisture out of the wood. Not sure I'd want to have long term exposure to whatever off-gasses from ACQ as it ages. I'd seal it with paint...
> 
> What I would propose, is to use a corrugated black metal surface elevated with stand offs from the rear insulated back. That way, air can flow over the top and the bottom of the absorber.
> 
> The OP's screen collector is a curious thought. Were you using fiberglass, or metal screen mesh? I wonder how much additional surface area is available using multiple layers of screen vs using a corrugated metal plate? The whole conversion efficiency of a thermal system is driven on surface area for thermal transfer.


 

Hi,
Scott and I did some side by side testing of various solar air heating collectors a couple years ago  Results here:
http://www.builditsolar.com/Experimental/AirColTesting/Index.htm
Also just got done comparing the pop can collector to screen collector -- basically what started this thread.

None of them are exactly what you propose, but it might give some useful info on how the various designs do.

We were both surprised that the screen collector was tied for best performer of the bunch -- happily, its also the easiest one to build.

I've used both fiberglass and alum screen.  I don't believe that there is any significant difference in performance between the two.
I did attempt a calculation of absorber area for a three layers of screen collector, and came out with 2.5 times the flat absorber area.  In addition to that, any light that gets through the screen hits the back of the collector, so, to some degree this also counts as absorber area.

I agree that absorber area is a big factor in air collector design, but I think there are some issues I'd call airflow quality.  
Like:
- is the flow mixed well enough that all of the flow actually contacts the absorber, or is the airflow layered enough that some air goes through the collector without enough contact with the absorber to pick up heat?
- Getting the airflow velocity over the absorber high enough for good heat transfer is important.
- Is there moving air contacting the glazing, and how warm is that air -- both increase heat loss out the glazing.  

Collectors like the pop can collector and the downspout collector have the advantage of no airflow against the glazing and a lot of absorber area (about Pi times the flat plate area).  Yet they don't test out at the top of the heap.

The picture that people like to think about for the screen collector is that the air resistance of the screen makes the air spread out uniformly over the screen and then flow through the screen (the air path is arranged so that the air must flow through the screen to get out).  There is airflow against the glazing, but its the lowest temperature air in the collector (direct from the inlet).  Whether this picture is strictly accurate or not I don't know, but the testing does indicate its an efficient collector.  The thermal images of the screen collector also seem to indicate pretty uniform flow without much in the way of hot spots.

Gary


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## Augie (Jul 28, 2013)

I think what gives the Screen Collector the advantage is greater surface area of the absorber. You want the absorber to both adsorb as much solar radiation as possible and give it us as quickly as possible. To do this you need surface area. And the screens have a huge surface area, I don't know how much more exactly but Im guessing on the order of 1.5x2 times as much as pop cans. So even if transmission from screen to air is not as good as the pop can to air, you have so much more surface area that is doesnt matter


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## woodgeek (Jul 28, 2013)

GaryGary said:


> ---------
> I'll see if I can find a more accurate way of estimating the seasonal heat output of the air collector -- I think you may be on the low side.
> One rule of thumb that is rough, but is used a lot is 1 gallon of oil per sqft of collector. That would give about 4.5 million BTU per year for a 32 sqft collector.
> For $2 propane (which is what we use) in an 80% efficient furnace, this would be $130 a year for a 1.5 year payback -- not counting any state rebates. The MT $1000 tax credit would actually pay for 5 collectors.
> ...


 
Part 1:

Here's a more careful take. I still think the major difference is the Montana vs East Coast resource....

Using PVWatts with a vertical '1 kW collector with 100% inverter efficiency', I get ~500 kWh of total projected output from Oct 1st to May 1st (I gave you a seventh month). I interpret that to mean that my total projected harvest is equivalent to 500 hours of operation of a system that would give 1 kW perpendicular to the sun under clear skies, spread out over about 2000 hours of daylight, i.e. averaging only 25% sun.

It appears that your test panel scaled to 4x8' would give 6400 BTU/h. TImes my 500 hours, it would give 3.2 MBTU/year in my location, mounted vertically, assuming the perpendicular efficiency you got at all insolation angles and cloud cover (i.e. PV assumes similar eff across insolation levels). That is a rather favorable assumption for your case...your test is in the summer, with perpendicular incidence under clear skies. At half sun (or 30° incidence), your stated eff drops by a third. What about 40°F ambient? Let's derate you 25% to 2.4 MBTU/year. I trust your figure at 4.5 MBTU, which is almost twice as high....I figure it is the resource difference.

Part 2:

If I was still burning oil, 2.4 MBTU would require ~22 gallons of oil, maybe $80 and ~550 lbs CO2. My payback on a $200 panel is 2.5 years.

You are a little queasy on my heat pump, but at HSPF = 8.5 (verified), I can make 2.4 MBTU using 2400/8.5 = 282 kWh = $40. My simple payback versus the solar air is now 5 years.

If I used my conventional grid power, which is 1.06 lbs CO2/ kWh, running the heat pump releases 300 lbs CO2. This is 300 lbs more than the solar case, but only 55% that of the oil case. IOW, the HP/conventional grid is ~45% less CO2 intensive than my legacy oil system. This is indeed more than natgas, but like you I have no access to that.

Solar air thermal is effectively carbon free, but IMO it would be hard to get to more than 50% space heating fraction. My HP/conventional power and 50% solar air + oil backup would have **similar** CO2 emissions (while requiring 10 4x8 panels, some good thermal mass and some complex controls to avoid overheating).

But I can and do buy 100% wind power for the same price as conventional kWh, and get effectively carbon free BTUs for 100% of my heating needs, which I think would be effectively impossible with solar air.

And you're right, if I get only 20,000 run hours out of a compressor, and replacement is $3-5k, it increases my cost per BTU 25-40%, still comfortably less than oil, and still very low carbon. I'm at ~13000 hours now, and hoping to get 15-20,000 more. Time will tell.

Still, I think my HP would choke on your climate, the same as I think your collector would disappoint in mine.  Different strokes.


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## georgepds (Jul 29, 2013)

woodgeek said:


> Part 1:
> 
> ....You are a little queasy on my heat pump, but at HSPF = 8.5 (verified), I can make 2.4 MBTU using 2400/8.5 = 282 kWh = $40.......


 

FWIIW, I just installed a Fujitsu 12RLS2 heat pump, it has a HSPF of 12 (bigger is better here) . As to verified,you can find a good lab tests comparing similar Fujitsu/Mistsubishi 12 k BTU/h units here

www.nrel.gov/docs/fy11osti/52175.pdf


This Fujitsu units can go down to - 5 F .. at this temperature the unit can put out 12 kBTU/H with an COP of ~2 (according toboth Fujitsu and NREL( figs 4  and 5))


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## isipwater (Feb 4, 2014)

Seasoned Oak said:


> Wow 2 hours, My passive solar room gets about 7+ in the dead of winter from 9 AM to 4PM and also heats my entire house on sunny days in winter.


Tell us more about your passive solar room. Do you build it? Will you share some photos?  I am interested in doing a passive solar project on my house. Thanks!


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## isipwater (Feb 4, 2014)

jebatty said:


> Low hanging fruit often is not glamorous. Glitz is attractive. I am as guilty as many others, but I do a pretty good job of both picking the low hanging fruit as well as picking the shiny apple at the top of tree. In this regard perhaps the lowest hanging "nutritional" fruit I picked was the insulation and plumbing modifications of my electric hot water heater at a one time cost of less than $50 and with a payback of $60+ per year in perpetuity, increasing as electric rates go up. That fruit was so low on the tree that no one even knows I plucked it unless I tell them, and when I do most look on with disinterest because it's not exactly pretty.


What modifications did you do to your electric hot water heater?


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## Seasoned Oak (Feb 4, 2014)

isipwater said:


> Tell us more about your passive solar room. Do you build it? Will you share some photos?  I am interested in doing a passive solar project on my house. Thanks!


All i did was close in a south facing porch 6x24 Ft with some surplus windows 6@ 3Wx8T Ft Gave me 144Sq Ft of window area. Cost about $400 for the windows and some lumber. Can get up to 100 in there if i dont open the door to the inside soon enough,(by 11AM)even in winter. At the time i was heating with oil and it reduced the heating load a whole 250 gal tank of oil per season. In my next house ill just build a greenhouse style solarium on the south facing side


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## isipwater (Feb 4, 2014)

Seasoned Oak said:


> All i did was close in a south facing porch 6x24 Ft with some surplus windows 6@ 3Wx8T Ft Gave me 144Sq Ft of window area. Cost about $400 for the windows and some lumber. Can get up to 100 in there if i dont open the door to the inside soon enough,(by 11AM)even in winter. At the time i was heating with oil and it reduced the heating load a whole 250 gal tank of oil per season. In my next house ill just build a greenhouse style solarium on the south facing side


This sounds wonderful!!  I would like to build a passive solar addition on the southwest part of my house in Massachusetts.  

One of my fears was that it would not produce enough heat in the winter, living in a cold climate state.  Sounds like you are getting plenty of heat. 

My other fear is that it would produce too much heat in the summer.  Has that been a problem for you?  I have read you can use roof overhangs or awnings to block the summer sun.  

Please post a photo if you have time. I would love to see what your passive room looks like.


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## Seasoned Oak (Feb 4, 2014)

No problem in summer .sun is directly overhead so, you dont want one of those curved glass transparent roofs. I also have windows on the sides that open so in summer they remain open,and can regulate the heat in spring and fall. You will want to keep this room separate from the main house so you can close it off at night as the temps plummet with all that glass and will siphon off all your gains in short order. WHen the sun goes down i simply close the steel insulated door between the sun porch and the main house to retain all the heat gained during the day.


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## jebatty (Feb 5, 2014)

> What modifications did you do to your electric hot water heater?


For electric hot water heaters only: 
1) if on a basement concrete floor, probably any floor, raise off the floor with 2 layers of 2" foam underneath (R20)
2) wrap the entire tank and top with at least 6" fiberglass, then cover with foil wrap insulation to seal all air leaks and for appearance (R19+); you can wrap the entire tank, no gaps
3) make U-shaped heat traps to prevent thermo-siphoning of hot water into both the cold and hot water lines
4) put pipe insulation on all the hot water pipes you can get at (don't insulate the heat traps)

I did mine about 10 years ago, cost for insulating materials may have gone up since that time, but these things reduced electricity for hot water by 50% (hot water heater on a separate meter and easy to see usage change)


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## Seasoned Oak (Feb 5, 2014)

The new hot water heaters are very well insulated from the factory. The biggest factor IMO is not turning the temp up too high.
If you have to add cold water for a shower the temp is set too high. Big savings and less standby loss can be had keeping the temps down.


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## bmblank (Feb 5, 2014)

This is a little off topic, but it's driving me nuts so I'm gonna say something...
There is no point in heating the water if it's already hot. It's just a "water heater," not a "hot water heater."
I know this is the internet and by saying something like that I'm just opening myself up to getting a bunch of replies from a bunch of trolls, but I had to say something.


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## Ehouse (Feb 5, 2014)

Seasoned Oak said:


> The new hot water heaters are very well insulated from the factory. The biggest factor IMO is not turning the temp up too high.
> If you have to add cold water for a shower the temp is set too high. Big savings and less standby loss can be had keeping the temps down.




My wife complained  because I set the temp to be shower ready with just a hair of CW.  She likes to have it at scald and then mix in the CW.  She's responded somewhat to my explanation but still grumbles a bit.


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## Seasoned Oak (Feb 5, 2014)

Ehouse said:


> My wife complained  because I set the temp to be shower ready with just a hair of CW.  She likes to have it at scald and then mix in the CW.  She's responded somewhat to my explanation but still grumbles a bit.


That would be a luxury that you pay xtra for. Luxuries cost extra.


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## Seasoned Oak (Feb 5, 2014)

GaryGary said:


> Hi,
> 
> The screen collector is kind of amazing -- very quick and cheap and easy to build, but performs very well.  Can be a couple year payback in a descent sun location.
> GAry


The enclosed sun room is a collector that you dont really see. I like solar solutions that are sort of invisible. I have a cement foundation wall under my sun porch and i d like to cover it with glass so it would look like more windows but in reality it would be a solar hot air collector.


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## GaryGary (Feb 5, 2014)

Seasoned Oak said:


> The enclosed sun room is a collector that you dont really see. I like solar solutions that are sort of invisible. I have a cement foundation wall under my sun porch and i d like to cover it with glass so it would look like more windows but in reality it would be a solar hot air collector.




I agree that sunspaces can be very effective solar collectors, not to mention giving the whole family some extra daytime living/playing space.

I did some testing on how a sunspace collector compares to a good quality commercial collector, and depending on how the sunspace is designed, the sunspace can provide just as much space heating per sqft of glazing as a commercial solar thermal collector.  

You get the highest space heating output from the sunspace by minimizing thermal mass in the sunspace and by insulating all the surfaces that are not glazed.  Couple this with a good fan and duct system to move the heated air from the sunspace to the living space, and you have a really effective solar space heater.

This is a bunch of collected info on low thermal mass sunspaces: http://www.builditsolar.com/Projects/Sunspace/sunspaces.htm#LowMassSS
and, this is the testing that gives the performance: http://www.builditsolar.com/Projects/Sunspace/LowMassSunspace/TestsLowMassSunspace.htm

They are amazing -- you can be sitting inside your collector having a cup of coffee and checking the Hearth Forum while the same collector you are in is providing 90+F air to your house for space heating when its 10F outside!  

Gary


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## Circus (Feb 5, 2014)

isipwater said:


> Please post a photo if you have time. I would love to see what your passive room looks like


 
Dec/ Jan Home Power Issue 158 has samples of low thermal mass sunspaces. The author seems familiar


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## semipro (Feb 6, 2014)

jebatty said:


> For electric hot water heaters only:
> 1) if on a basement concrete floor, probably any floor, raise off the floor with 2 layers of 2" foam underneath (R20)
> 2) wrap the entire tank and top with at least 6" fiberglass, then cover with foil wrap insulation to seal all air leaks and for appearance (R19+); you can wrap the entire tank, no gaps
> 3) make U-shaped heat traps to prevent thermo-siphoning of hot water into both the cold and hot water lines
> ...


Jim, why no insulation on the heat traps? 
BTW, I believe loop heat traps work equally well and and don't take up as much lateral space as u-shaped ones. 
I'm not sure how effective they are but these heat traps are available also.


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## Ehouse (Feb 6, 2014)

semipro said:


> Jim, why no insulation on the heat traps?
> BTW, I believe loop heat traps work equally well and and don't take up as much lateral space as u-shaped ones.
> I'm not sure how effective they are but these heat traps are available also.
> View attachment 126683




I just bought a cheapo at Lowes and it came with those installed.  Check Your WH so you don't double up (don't know if it would hurt anything though).


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## Seasoned Oak (Feb 6, 2014)

GaryGary said:


> You get the highest space heating output from the sunspace by minimizing thermal mass in the sunspace and by insulating all the surfaces that are not glazed.  Couple this with a good fan and duct system to move the heated air from the sunspace to the living space, and you have a really effective solar space heater.
> They are amazing -- you can be sitting inside your collector having a cup of coffee and checking the Hearth Forum while the same collector you are in is providing 90+F air to your house for space heating when its 10F outside!
> Gary


Mine is as simple as it gets,no moving parts. I simply open the door between the sunroom and the main house when the sunroom temp exceeds that of the house. And close it when it goes below. About 10 hours on a sunny winter day. The house stays above average temps for another 3-4 hours after that. Effectively 12 hours of free heat. Main house(3000Sqft) heat not needed during those hours.


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## Circus (Feb 6, 2014)

Seasoned Oak said:


> Mine is as simple as it gets,no moving parts. I simply open the door between the sunroom and the main house


 
You won't even need to open a door if vented. Thermal air collectors work fine without fans but losses go up from operating at higher temperature.


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## Seasoned Oak (Feb 6, 2014)

Circus said:


> You won't even need to open a door if vented. Thermal air collectors work fine without fans but losses go up from operating at higher temperature.


Door opening of 17.78Sq ft  keeps a lot of  air moving. Plus vents leak heat at night when the sun porch is 25 degrees.


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## jebatty (Feb 7, 2014)

> Jim, why no insulation on the heat traps? BTW, I believe loop heat traps work equally well and and don't take up as much lateral space as u-shaped ones.I'm not sure how effective they are but these heat traps [picture] are available also.


I didn't use insulation because I wanted to make sure the hot water rising out of the hwh cooled by the time it rose to the top of the traps to insure no siphoning. Maybe it would work fine with insulation. No reason why loop heat traps would not work, so long as siphoning is stopped. The commercial [picture] devices may work, but my hot water heater had those and I still had siphoning, thus put in the heat traps.


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## woodgeek (Feb 7, 2014)

I think your right Jim, it not clear if they would work if insulated.  As for the other kind, they have a ball that can get stuck in the wrong position....I have loops on mine, no moving parts to fail.

A stuck check valve and thermosiphoning prob cost me >$1000 on my old hydronic heating system before I figured it out.  HVAC guys wanted $600 to change the valve out, I decided to just close a zone cutoff valve during the warm months.


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## semipro (Feb 7, 2014)

Hmm.  It seems that leaving the heat traps uninsulated would actually allow the temp to equalize across the loop and weaken the effect that makes them effective.  The first reference I could find recommended insulating the whole loop.  http://www.energycodes.gov/moodle/mod/resource/view.php?id=583




Mine are easily accessible and insulated.  I think I'll take my IR gun and do some tests.  Could be they work whether insulated or uninsulated.

The off the shelf heat traps I referenced earlier have plastic flap valves in them sort of like an artificial heart valve.  This type doesn't have the check balls.


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## GaryGary (Feb 7, 2014)

Seasoned Oak said:


> Mine is as simple as it gets,no moving parts. I simply open the door between the sunroom and the main house when the sunroom temp exceeds that of the house. And close it when it goes below. About 10 hours on a sunny winter day. The house stays above average temps for another 3-4 hours after that. Effectively 12 hours of free heat. Main house(3000Sqft) heat not needed during those hours.



Sounds like that works really well for you.

What I found in the testing was that if you want to optimize the sunspace for producing space heating for the house you can get a lot more house space heating by 1) minimizing thermal mass in the sunspace, 2) insulating all the sunspace surfaces that are not part of the south glazing to reduce heat loss, 3) having as much south facing glazing at a good winter angle as possible, 4) double glazing is also helpful.  

The difference in space heating output for a sunspace that follows these rules and one that does not is a lot.  On my test sunspace I measured its heat output with a dirt floor (lots of mass), no wall insulation, lighter colored (bare wood) walls, and before a complete job of sealing was done, and compared this to the heat output after insulating the walls, ceiling, and floors, better sealing, dark surfaces.  The heat output of the heat output of the optimized space was nearly 3 times the non-optimized  -- this may seem like a lot and it certainly surprised me, but if you do some rough calculations on what the lack of insulation and warming the mass in the sunspace costs you, it comes pretty close to the same number.  The measured efficiency of the optimized sunspace was 55% and the efficiency of the non-optimized was 20%.  
This page goes into excruciating detail on the actual test: http://www.builditsolar.com/Projects/Sunspace/LowMassSunspace/TestsLowMassSunspace.htm

If you take the steps to optimize the sunspace for house heat collection, then you will almost certainly need some form of forced ventilation to move the heat to the house. On my 200 sqft test space, it took fans operating at an actual 650 cfm to remove the heat from the sunspace in good sun winter conditions -- if you don't have the enough flow rate, the sunspace gets uncomfortably warm and you lose some of the space heating potential.  The heat output per hour in good sun conditions for the 200 sf sunspace was up to 41,000 BTU/hr -- kind of like a 60K BTU/hr house furnace operating at 70% efficiency!

The downside of optimizing for space heating is that the sunspace will go cold very quickly once the sun goes off of it because it has no mass to store heat, and it will not be a good place for plants in cold climates.  

I'm not saying by any stretch that everyone with a sunspace will want to take the steps to optimize it for house heating, but just saying that if you do, the gain may be a lot more than you might think (certainly a lot more than I thought it would be).  

Gary


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## Seasoned Oak (Feb 7, 2014)

I do have the ceiling well insulated as well as the side walls. I dont have a lot of thermal mass,just flat black ceramic tile on the floor with cementboard under it. The room only overheats (80-100) if i keep the door to the main house closed. when i open it the sunroom rarely goes over 75-78 in winter.The air moves constantly warm air flowing into the main house and cool air flowing along the floor out into the sun room.  This gets the main house up to about 78 and it takes me well into the evening hours for the house to fall  into the low 70s and require any kind of additional heat. The fact that the sunroom does NOT overheat in winter tells me i dont need any additional artificial forced air devices.


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## Cynnergy (Feb 7, 2014)

Thought this might be of interest to this thread:

http://www.cbc.ca/news/canada/new-b...olar-furnace-decreases-heating-bill-1.2527065

Sort of like a solar hot water panel but using air.  Not sure how he keeps it from bringing in cold air at night?  A sunroom seems preferable to me if you have the space - no fans, can close it off at night, presumably more solar gain.


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## Seasoned Oak (Feb 7, 2014)

Interesting yes ,but not everyone will be parking an old window outside their house to collect a few BTUS on a sunny day. 
Not sure this thing looks too good in front or even on the side of his house. The best designs are invisible


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## GaryGary (Feb 7, 2014)

Seasoned Oak said:


> Interesting yes ,but not everyone will be parking an old window outside their house to collect a few BTUS on a sunny day.
> Not sure this thing looks too good in front or even on the side of his house. The best designs are invisible



They can be made to look pretty nice, or at least not to ugly...

http://www.builditsolar.com/Projects/SpaceHeating/DHWplusSpace/Main.htm

http://feherhome.dyndns.org/solar/

http://www.builditsolar.com/Projects/SpaceHeating/SolAirColWithDHW/AlAirCol.htm

http://www.builditsolar.com/Projects/SpaceHeating/solar_barn_project.htm

Some of these are putting out 30K+ BTU per hour.

It seems to me that one of the promising areas that has not gotten much attention is integrating solar air heating collectors into the south walls of new construction homes.  When you account for the material saving to the wall (eg no siding) the cost could be near zero and it would not take any kind of specialist to build it -- it could be just another part of the carpenters job and add a new profit center for the local builder.  

But I agree that if your house is well set up for it, there is probably no better solution than a low mass sunspace on the south side of the house -- it can provide a lot of heat, make the house look better, and adds some value for everyone in the family and when you go to sell the house.

Gary


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## Seasoned Oak (Feb 7, 2014)

Some of those designs look OK .The foundation where its exposed on the south side would be a good place for solar. Venting could be easy with natural rising of heated air.  
Same line of thinking with solar panels. Integrating panels with roof coverings will be more accepted ,like "solar shingles".
I do love solar. It was like spring at my house today and every sunny winter day, despite never rising above freezing just on the other side of those windows. Ill always have a sun room. Mine is only 144SF of floor space,but 150+sq ft of glazing, next one will be 2x as big.


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## denjohn (Feb 7, 2014)

Could a solar air heat collector be used to preheat the inflow of a mini split?


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## jebatty (Feb 8, 2014)

I looked at the window screen and downspout designs for a solar air heater. My available space is about 2.5' x 24', and I think a screen design would be difficult due to the length, but a downspout design would work very well: 2 x 10' sections per run, several runs, cost about $50 for the downspouts. This may be worth a try to see how much heat my install location could produce, and what a dent that might make in my electric consumption for heat.


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## Circus (Feb 8, 2014)

denjohn said:


> Could a solar air heat collector be used to preheat the inflow of a mini split?


 
No.
Solar air works when it's collector is warm, isolated when it's cold. Sucking air through when it's cold is counterproductive.


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## GaryGary (Feb 8, 2014)

jebatty said:


> I looked at the window screen and downspout designs for a solar air heater. My available space is about 2.5' x 24', and I think a screen design would be difficult due to the length, but a downspout design would work very well: 2 x 10' sections per run, several runs, cost about $50 for the downspouts. This may be worth a try to see how much heat my install location could produce, and what a dent that might make in my electric consumption for heat.



The downspout might be a good choice.

There are some comparative tests of downspout and screen collectors here: http://www.builditsolar.com/Experimental/AirColTesting/Index.htm
The results were a bit dissapointing for the downspout, but that may have been in part due to the test -- I could never get the air distribution in the downspouts as even as I would have liked.

There is a test of a horizontal screen collector here: http://www.builditsolar.com/Experimental/HorizontalScreenCol/HorizontalScreenCol.htm
A pretty crude test, just laid collector on its side, but it did do pretty well.
So, maybe a screen collector would work OK in your case?

Scott's site has some nice stuff on downspout collectors and he has one that is similar to what you want to build.
http://www.n3fjp.com/solar/index.htm

Gary


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## jebatty (Feb 8, 2014)

I'll check out the links. Thanks.

The screen collector has the screen laid at a slope from end to end, works over a short length but what about a 20' length? I like the idea of a downspout collector because it keeps moisture out of the collector box, recirculates it, and any condensing moisture stays in the down spouts, to be evaporated out on the next solar run. The flat plate collector I made years ago would collect moisture condensation in the collector box which ultimately rotted out the box.

As to even air flow through the spouts, spout restrictors/dampers could be used with a ribbon (very high tech) one could visually observe the airflow, adjust the damper(s) as needed, and get relatively even flow.

I think I could build a 20' collector for about $200, have some parts lying around I can scavenge. What I really like about solar collectors is the minimal operating and maintenance cost. I have all the stove wood I need, but it all takes work and time to get the btu's. I also have a 30w solar panel that might be incorporated to power a 12v fan.


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## Where2 (Feb 8, 2014)

jebatty said:


> I also have a 30w solar panel that might be incorporated to power a 12v fan.



I've got a 20W 12V panel I keep lying around for experiments. I have noticed that running my 0.5A muffin fan (130cfm) from the 20W panel, I see 18-19V on the fan leads if I don't run a connection to a battery to load down the panel. You might want to consider this when choosing a fan, or designing your setup. The ultimate setup would have a fan controller with a user selectable delta-T and a couple of different resistance loads in series with the fan motor so you could operate the fan at multiple speeds to extract the available heat. Then again, I design complicated control systems for theme parks, and may be thinking too complicated for simple air heating.


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## jebatty (Feb 8, 2014)

Thinking off the cuff now. Most DC motors will operate over a pretty wide voltage range. Might a panel operate to start a12vdc motor when panel voltage rose to, say 8v, fan would speed up as panel voltage rises, max out at about 18-19V and likely not damage the motor?


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## GaryGary (Feb 9, 2014)

jebatty said:


> I'll check out the links. Thanks.
> 
> The screen collector has the screen laid at a slope from end to end, works over a short length but what about a 20' length? I like the idea of a downspout collector because it keeps moisture out of the collector box, recirculates it, and any condensing moisture stays in the down spouts, to be evaporated out on the next solar run. The flat plate collector I made years ago would collect moisture condensation in the collector box which ultimately rotted out the box.
> 
> ...



Hi, 
Sounds like the downspout collector might work best for you.

Just as a side point, I don't think that the sloping screen is a necessary feature for good performance.  The idea of the slope of the screen is that air enters on the glazing side of the screen, and works its way through the screen from front to back as it goes up, so the slope allows for more flow area on the inlet side at the bottom when there is more flow there, and then slopes to allow more flow area behind the screen at the top when most of the flow is on the back side of the screen.  Its a nice mental picture of the flow, but no one that I know of has actually verified that this is what really happens inside the collector.

I'm doing some more testing with 2 by 3 ft collectors of various air collector configurations, and I tried one with two layers of screen very quickly and kind of randomly installed -- it was stapled to the sides of the collector box and the only "rule" was to not let the screen touch the glazing or the back of the collector.  Itt took less than 5 minutes to cut out and install the absorber.  It ended up performing a little better than the carefully sloped screen.  So, I think that the important thing is to have the layers of screen that the sun heats up and good airflow around the screen to pick up the heat.  The difficult thing with air collectors is getting good heat transfer from the absorber to the air -- air is low density and low specific heat, so the transfer is hard -- lots of absorber area and good flow over the absorber help.

Where you are, the twinwall polycarbonate glazing would be a good choice for higher efficiency.



Gary


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## Circus (Feb 9, 2014)

Collectors with black sheet metal (metal furnace duct metal) hung within, cooled by rising air on both sides are usually only about 2 1/2" deep. As long as one wall duct is higher than the other, any shape works. Removing siding behind the collector and using the house's sheathing as the collector's backside insulation exposes the collector only 1 1/2" beyond the house's siding. Forget baffles, restrictions etc. Twin walled polycarbonate, good stuff


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## jebatty (Feb 9, 2014)

In the early '80's I made a 4 x 8 corrugated metal, painted black, hot air collector. It performed very well, cool basement air in and 110-140F air out, 50 cfm bathroom exhaust fan for the air movement, snap switch. But condensing moisture killed it after a couple of years. 

Twin walled polycarbonate looks very good.


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## arbutus (Feb 9, 2014)

I love solar ... where it makes sense.  Our previous house had a large air solar collector on it when we bought it.  16' x 16', 2x6 framed vertical wall added onto the south side of the original A Frame ... in Michigan's western lower peninsula, where the sun does not shine more than half a dozen days during the winter. 

Those few days that the sun did shine, as well as sunny spring days, it would heat the house all by itself, for about six or seven hours.  9 - 4 or so.  There was a simple thermostatic switch set to 90F, a blower ducted into the upper part of the A Frame living room that blew downwards, and two wall inlets near the floor.  The panels were single glazed, with an commercial dimpled aluminum collector, and about an inch of airspace behind the collector and plywood, with fiberglass insulation behind that.

It really did help in the spring, but wasn't needed in the summer when it was usually sunny, and there just wasn't enough sunlight to make it worth while for winter.


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## jebatty (Feb 9, 2014)

I really do love wood for the stove and the Tarm, and heating with wood has saved us a ton of money, but wood also is work, and after putting in the solar electric and watching it put out power (sun is shining, will be a little over 33kwh today) and not have to do anything, now that is sweet. Hence, add low cost solar hot air to supplant some of the electric used to heat the basement. Extra free, no work heat in Oct (Nov and Dec usually very cloudy), Jan and into spring will be very welcome.


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