# Solar air heat?



## stoveliker (Nov 29, 2020)

I guess this is a contentious subject, but for such subjects learning the (perceived) pros and cons is useful.

I've been playing with the thought of getting solar air heaters. I have a South facing wall and like the idea of having a 20W fan providing like a 1500 W space heater. Especially upstairs where my basement stove is not very effective.
But also in the LR; I'm not leaving the minisplit (running on now free solar electricity) on when I am out. But a 20 W fan I could leave on.

I liked the Cansolair, bit it appears this never got off the ground with volume and they seem to be out of business.

Any other contenders? Ideas about (not) doing it?

Or point me to an older thread that I could not find...


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## begreen (Nov 29, 2020)

Several years back Precaud made a homebrew solar collector, then added another. It took some fiddling and rejiggering, but he ended up getting decent output eventually. This is in the desert SW however, so lots of sun.






						Adding another solar air heater
					

In the fall of 2008 I converted my south-facing front porch into a 72 sq ft solar air heater, and the results have been excellent. Upstairs wood consumption dropped by about 1/3 and I no longer use ANY nat gas for heat. I posted some in-process photos in this thread...




					www.hearth.com


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## EatenByLimestone (Nov 29, 2020)

I worked on a house this year that used the sun to heat the air in his workshop.  I was originally confused as the collector was behind the building, about 4ft from the trees, in the shade.   When I asked him why it was in the shade (how could that work?)  He explained that all the leaves would be gone when he needed to heat his shop.   He said it worked pretty well.


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## peakbagger (Nov 30, 2020)

Take a look at the Build it Solar website. Lots of home brew info. Solar air heaters can work for space heating when the sun is out, once you start talking about storing the heat it has major limitations. You also need to figure out how to deal with the hot air if you dont need it. Shading and landscaping are important.


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## jebatty (Nov 30, 2020)

In the early 1980's I built a solar hot air collector and put it on the roof of our house, 3/12 pitch. It was just a box, 4' x 8', with the collector being a corrugated steel panel painted flat black and installed half way between the top and bottom, a clear fiberglass panel on the top, insulation on the bottom, with 4" duct from the basement into the box below the collector and 4" duct from above the collector back down to the basement. I used a bathroom exhaust fan to move air through the box controlled by a snap disc thermostat in the box with "on" being  100F. On a clear sunny day, as low as -20F outside, the collector would take air from the basement and return it to the basement with air temp into the basement as high as 140F. The collector lasted for a few years before it rotted out from moisture condensing in the collector. 

For a commercial "solar furnace" take a look at https://8thfiresolar.org/our-products -- or do a web search on "solar furnace", a lot more sophisticated and can work quite well. I'm aware of a facility with two of these installed for space heating two rooms, been in operation for several years, no issue of which I am aware.


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## andym (Nov 30, 2020)

I had forgotten about solar air heaters. I may need to think about building one to put some heat in my shop. I currently have no heat source other than electric space heater. Sounds like a fun thing to tinker with anyway.


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## stoveliker (Nov 30, 2020)

Thanks all. I had googled quite a bit; I don't have time to design and build one myself (ever, or yet?). Most of what's out there is people documenting their builts. Of course a self-designed unit will work (it's easy to get some heat in), but is it reasonably efficient (I don't want to spend many hours doing something that when designed better could be three times as productive), and what about its longevity..

If there are building plans out there that use off the shelf materials (that could be bought e.g. in a big box stores), and that include data regarding heat output (see e.g. the well documented CanSolair), I might be interested in tackling a project on my own.

Otherwise I prefer to buy. But the few companies that I found are either out of business or did not respond (in a professional way). That makes me wonder.

I'll check out the company mentioned by @jebatty .

Thanks all for useful info so far!


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## stoveliker (Nov 30, 2020)

peakbagger said:


> Take a look at the Build it Solar website. Lots of home brew info. Solar air heaters can work for space heating when the sun is out, once you start talking about storing the heat it has major limitations. You also need to figure out how to deal with the hot air if you dont need it. Shading and landscaping are important.


Wouldn't switching off the fan (and having an insulated block off) be sufficient for that?


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## EatenByLimestone (Nov 30, 2020)

I suppose it's possible that you could get it drafting hard enough to push through your block off plates.  I was reading that they were getting 140F in the box.   If you're pulling 50F air from the basement and plumbing it into an above floor, it could draft pretty well on its own!


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## peakbagger (Dec 1, 2020)

Of the ones I have seen over the years, generally the owners end up covering them over in the summer. Most just have a gravity flap on the inlet or outlet with fan somewhere in the mix. On a clear night in the winter a black collector will radiate a lot of heat and will try to pull a lot of heat out of the house. Sure someone could probably make some insulated plugs to block off the ducts that are well sealed but most commercial systems dont have them.


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## EbS-P (Dec 1, 2020)

I was seriously thinking of building my own. I did the math building it as cheap as I could two 4’x8’ panels. I figured to do it would be 300$. Break even time given I have a heatpump was something like 5 years. I installed a clothesline instead.  I gave some thought to 1kw PV to DC hot water.    Again break even point was longer than the life of the hot water tank.    Spending 100-500 $ on insulation /air sealing made more sense. Save money for both heating and cooling season. Takes less of my time and the I stall would still be functional long after I’m gone.

evan


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## Corey (Dec 1, 2020)

Seems like you'd have a lot to consider.

Solar hot air would be simple, but then you'd cover it up all summer long and don't use it.

Solar hot water could provide heat during the winter and hot water during the summer, if needed, so that would seem to double the usefulness of the system?

Solar electric, or 'more' solar electric (sounds like you already have some?)  Seems like that would essentially provide hot air, or hot water, or some surplus power selling back to the grid?  so useful all the time the sun shines.  

Not sure what the overall current efficiency of air, water or electric would be, though taking into account the 'usage' of each system plus running a heat pump on solar electric would also likely net some COP > 1 which would multiply the net 'efficiency', it might be that electric nets the best overall gains?


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## stoveliker (Dec 1, 2020)

Yes, a lot to consider. Indeed I have PV and a very efficient minisplit for AC and heat. Solar water is a bit tough for me; either very long lines, or the collector on the North side (and in the shade of a half-hour second story).

Having a solar air heater that I would just leave on (with a Tstat) when I'm out (a 20 W fan after all, possibly even independently powered with its own little PV panel, to be unaffected by power outages) helps a lot keeping the chill off - I'm still a bit hesitant to consistently run the stove all day when nobody is home, given the work needed for wood. Assuming I'd be able to go back to work at some happy day in the future...

I read @precaud thread of his build - and while I don't have the space to do it as he did, the functional results are precisely what I'm hoping for.

I'll contact that company in MN mentioned above. If someone knows of other places where one could buy things like this, pls post them here.

Thank you all!


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## woodgeek (Dec 5, 2020)

@stoveliker, I appear to be stalking you today.

I thought a LOT about solar air heaters 15 years ago when I was paying $5/gal for fuel oil, using 1300 gal/yr for heat and hot water and hadn't yet airsealed my house.    

You can do solar air heaters.  You can do solar hot water.  The equipment for both is hard to find commercially, and as the cos have gone under the equipment prices have gone up if anything.  Or you can DIY one yourself using designs from the 1970s.

Did those companies go under bc of cheap fossil fuels?  Nope. They went under bc of cheap PV solar.  Even when fossils were cheap before, there were enough enviros and early adopters around to keep those companies alive and making panels.  Now all those folks (like you) just have PV.  And while there was a period where people would put PV and solar HW on their roof, it didn't take long for people to realize that just putting more PV penciled out cheaper, lower maintenance and greener.  PV + HPWH = solar HW.

in 2012:








						Solar Thermal is Dead - GreenBuildingAdvisor
					

In the northern half of the U.S. — and even much of the South — installing a residential solar hot water system doesn’t make any sense. It’s time to rethink traditional advice about installing a solar hot water system, because it’s now cheaper to heat water with a photovoltaic (PV) array than...




					www.greenbuildingadvisor.com
				



in 2014:








						Solar Thermal Is Really, Really Dead - GreenBuildingAdvisor
					

Back in early 2012, in an article called “Solar Thermal Is Dead,” I announced that “it’s now cheaper to heat water with a photovoltaic array than solar thermal collectors.”  Now that almost three years have passed, it’s worth revisiting the topic. In the years since that article was written, the...




					www.greenbuildingadvisor.com
				




Note that the price of rooftop PV has dropped by more than 50% since those articles were written, and HPWH COP's have gone from 2.2 to 3.0.

Of course, you were asking about solar air heat.  Same story.  Use you net-metered PV to run a minisplit.  Want more heat? add more PV.  It will be cheaper than the air heater per BTU, unless you are pulling the materials out of the garbage and budgeting your labor as $0/hr.

I hate to be a (renewable energy) killjoy, but you are already living the dream.  The kicker for me, when I came to this conclusion many years ago is the seasonal solar resource for the East Coast in winter.  Look it up. Not only are the hours fewer, but the average cloud cover is waaay higher.  So that air heater is going to be soaking up BTUs all summer which are completely useless.  A PV in the same place will bank a ton of kWh all summer long, that will run your mini all winter (and at night).  

PV is also way better in partial sun.  A thermal system has parasitic losses (designed in to prevent overheating in fault cases), and when the sun is in and out of clouds, or filtered, those parasitic losses eat up most of the output.  But the PV will still bank 50% of the kWh. So when you are looking up your winter solar resources, you need to look up the thermal resource, not the PV one.  Its even lower.


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## stoveliker (Dec 5, 2020)

woodgeek said:


> @stoveliker, I appear to be stalking you today.
> 
> I thought a LOT about solar air heaters 15 years ago when I was paying $5/gal for fuel oil, using 1300 gal/yr for heat and hot water and hadn't yet airsealed my house.
> 
> ...



I can't add more PV on the roof - it's full.
I was hoping for these air heat panels on my South facing wall to free up some kWhs for a future electric vehicle.

Basically, unless I add tilted PV panels on my wall there is not much I can do anymore. Same for solar hot water (that I have disliked for some reason).

Essentially I am not willing to have the minisplit run when I am not at home, even if it runs for free... But having a 20 W fan going is ok with me. Cansolair claimed that with 15 mins of sun per hour it would (at least) heat my living room. Meaning that on half the days I would not have to heat from scratch (which is 60 F for me on a working day) when I get home.

I still have to contact 8th fire solar for a price as that is not listed on their website.

So, I do see your point and despite the wood heating that is also free...,I'm hoping to be even more efficient with the kWhs in view of my only 11 mile, eventually electric commute (that hopefully will start again in a few months).

I'm not yet ready to throw in the towel on this  even though you have good arguments.

Btw, it might very well take a year or two before I make a final decision on these things - I do tend to mull, calculate (physicist after all), think, play devil's advocate, think more, get more input etc. After all I am already living the dream (no heating cost between wood and PV minisplit, no other electric bill, free AC) - so I have time...


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## stoveliker (Dec 5, 2020)

stoveliker said:


> I can't add more PV on the roof - it's full.
> I was hoping for these air heat panels on my South facing wall to free up some kWhs for a future electric vehicle.
> 
> Basically, unless I add tilted PV panels on my wall there is not much I can do anymore. Same for solar hot water (that I have disliked for some reason).
> ...





woodgeek said:


> @stoveliker, I appear to be stalking you today.
> 
> I thought a LOT about solar air heaters 15 years ago when I was paying $5/gal for fuel oil, using 1300 gal/yr for heat and hot water and hadn't yet airsealed my house.
> 
> ...



And the stalking is friendly and informative, so I do appreciate it!


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## woodgeek (Dec 5, 2020)

No worries, just two physicists having a chat.    

I think the math says that tilted panels on the wall would give you more kWh/$$ into the mix than the solar air would produce.

Not clear what the issue is running a mini when you are not around?


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## stoveliker (Dec 5, 2020)

The issue with the minisplit is that it uses much more than 20 W. And while it may be more (financially) efficient to run the minisplit for heat rather than a solar air heater, that jumps over the issue that my kWhs are finite (if I don't install PV on the vertical wall, which I am unlikely to do, given that in summer they would not produce much).
Same argument as in your first link from 2012: electricity can be used for a few things (light, refrigeration, entertainment, heating), but when it's gone it can't be used for anything - so taking some usage off of the heating part (at still a low cost per BTU) allows me to use it for my (future electric) car instead.

I have the same hesitation with the heat pump water heater. (Apart from the fact that I still like to have the unlimited oil heat back up, and intermittently running the furnace to keep my water hot keeps the furnace in better shape than having it cool down to room temp. This is a trust issue; will I be able to always get enough wood...?)

So, the mind keeps circling. Your analysis does make sense though.


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## stoveliker (Dec 5, 2020)

Basically your kWh/$$ argument ignores the boundary condition that kWhs are finite.


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## woodgeek (Dec 5, 2020)

stoveliker said:


> Basically your kWh/$$ argument ignores the boundary condition that kWhs are finite.



Hmmm.  I am just saying that the BTU of a solar air heater would offset much fewer kWh (replenishing your finite supply) than a PV panel would.  And as a physicist, you can run the amount of expected kWh in your area for different tilts.  Its not zero for a vertical panel, and not too shabby with a 20° tilt.   Or you can think outside the box: if you have a yard, you can do ground mount PV like a lot of folks here have.

?


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## EatenByLimestone (Dec 5, 2020)

can't a minisplit be zoned so only a small area is heated?   Using it as a space heater shouldn't be bad on kwh usage.


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## EbS-P (Dec 6, 2020)

best looking design I saw utilized metal downspouts as the heat exchanger.  As I recall the pictures showed a horizontal configuration.  Probably the dimensions of a suntuff sheet.   I like the idea using it as a solar clothes drier in the summer.   I just looked up the numbers figuring you average 2.5 kW hours per day per m2 October through March, It’s not going to heat your house but it sure could keep a room warmer.     I figure 150$ a sq meter in materials.


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## SpaceBus (Dec 6, 2020)

My house came with a "solar air heater" installed on the outside. It consisted of a layer translucent fiberglass with an air gap, then corrugated and stippled aluminum painted black with an air gap, and then some 6"x8" ducts in both of the upstairs bedrooms. Those ducts ran down to the slab, then joined in a channel running under the floor to a blower under the stairs. We tried to use it and didn't really think much of it. The blower is loud and the exterior treatments are hideous. I'll try and find a picture taken before we removed it. I covered the ducts coming out of the slab and sealed them. The large 12" round duct coming out of the floor under my stairs is also closed in. This system probably worked better in the summer to cool the house since the intake for the slab duct was facing the front door, which also faces North and has loads of shade. We never got to find out since we started tearing the solar wall down before it got hot. Before destroying all of that work the builder put into the house I asked some folks on this site and around town, most everyone, besides our home inspector, said it is not very effective.

Edit: Our house also has a solar water heater on the roof, but the way it was integrated into the house (completed 1976) did not work well. This means we have a 1975 SWH on the roof that has had water run through it once by me as a test for leaks and probably once by the builders. Eventually I'd like to repurpose it to use a heat exchanger and glycol to assist our current DHW system, which is a 40 gallon tank heated by my cookstove with a on demand electric heater for backup.


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## DBoon (Dec 6, 2020)

I'll second everything woodgeek said. I did a bunch of calculations for solar thermal, and it would work great...at delivering lots of BTUs in the summer when I didn't need it and nearly none in the November through February period when I really needed it. Same would be true of hot air panels. 

So I punted on that for the new house, went with a PV array addition, geothermal radiant heat, and a newly installed HPWH. 

Having said that, Central NY is a pretty lake-effect cloud driven climate. Down closer to to NY City (east of the Catskills), there is a lot more sun during the winter. This week, our forecast is for cloud cover all week. Down closer to the city it looks like sun 50% or more of the time. If eastern long island is sunnier like that, I'd say home build one panel and see how it works and looks. Then go from there.


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## stoveliker (Dec 6, 2020)

EatenByLimestone said:


> can't a minisplit be zoned so only a small area is heated?   Using it as a space heater shouldn't be bad on kwh usage.



Correct. We have 4 zones.


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## peakbagger (Dec 6, 2020)

By the way I do have wall mounted tilted PV panels.  They work really well as they shade an angle bay window so I cut down on summer cooling. I adjust the tilt seasonally.


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## stoveliker (Dec 6, 2020)

SpaceBus said:


> My house came with a "solar air heater" installed on the outside. It consisted of a layer translucent fiberglass with an air gap, then corrugated and stippled aluminum painted black with an air gap, and then some 6"x8" ducts in both of the upstairs bedrooms. Those ducts ran down to the slab, then joined in a channel running under the floor to a blower under the stairs. We tried to use it and didn't really think much of it. The blower is loud and the exterior treatments are hideous. I'll try and find a picture taken before we removed it. I covered the ducts coming out of the slab and sealed them. The large 12" round duct coming out of the floor under my stairs is also closed in. This system probably worked better in the summer to cool the house since the intake for the slab duct was facing the front door, which also faces North and has loads of shade. We never got to find out since we started tearing the solar wall down before it got hot. Before destroying all of that work the builder put into the house I asked some folks on this site and around town, most everyone, besides our home inspector, said it is not very effective.
> 
> Edit: Our house also has a solar water heater on the roof, but the way it was integrated into the house (completed 1976) did not work well. This means we have a 1975 SWH on the roof that has had water run through it once by me as a test for leaks and probably once by the builders. Eventually I'd like to repurpose it to use a heat exchanger and glycol to assist our current DHW system, which is a 40 gallon tank heated by my cookstove with a on demand electric heater for backup.



Fair enough. In what I was thinking about, there would be no long ducts. Just directly thru the wall. So the fan power and noise would be much less, if convection wouldn't do enough.


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## SpaceBus (Dec 6, 2020)

stoveliker said:


> Fair enough. In what I was thinking about, there would be no long ducts. Just directly thru the wall. So the fan power and noise would be much less, if convection wouldn't do enough.


I did remove the registers from the bedroom ducts and put my hand against the panel during our first winter and it was almost too hot to touch on a clear day. How much of that heat made it into the bedroom is questionable. Whatever heat gain we had in the house was offset by how hideous the solar air heater was.
The attached photo is from right before we closed on the house. In full disclosure I did make the small hole you can see towards the bottom right corner.


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## begreen (Dec 7, 2020)

I'm not sure if it will pan out, but this branch of research is intriguing in its potential for solar heating. 









						Study shows promising material can store solar energy for months or years
					

As we move away from fossil fuels and shift to renewable energy to tackle climate change, the need for new ways to capture and store energy becomes increasingly important.




					www.lancaster.ac.uk


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## woodgeek (Dec 8, 2020)

begreen said:


> I'm not sure if it will pan out, but this branch of research is intriguing in its potential for solar heating.
> 
> 
> 
> ...



Meh.  Azobenzenes get used in all kinds of 'magic trick' experiments with light, but they are not very efficient or durable.  And the energy stored per mass of the material is likely rather low.


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