# A standalone dual-pass solar air heater using downspouts



## precaud

As part of my ongoing project to reduce wood consumption, I've just installed another solar air heater, designed and built incorporating some of the concepts that I learned from woodstoves over the years. It's a simple design that uses aluminum downspout tubes as the absorber and heat exchanger. That in itself is nothing new; downspout tubes have been used in solar heaters for a few years and have proven to be a good choice for horizontally-displaced air heaters. But the way this one works addresses the major shortcomings of earlier designs.

Previous tube designs were basically insulated boxes with tubes attached to the back and glazing on the front. Air flows through half of the tubes in one direction, then reverses through the other half. And therein lies the two major problems. Attaching the tubes directly to the back, separated only by an R6 insulating board, creates a heatsink directly from the heat exchangers to the outdoors, cooling them off. And the long path length that the air travels through is not an efficient use of the exchanger surface area, requiring high air velocity through the system to achieve reasonable efficiency.

Both problems can be solved with minimum added complexity by suspending the tubes away from the back, using that channel to rout the incoming air to the other end, and then flow it through all of the tubes at once in parallel. The heat that was previously conducted through the back now preheats the incoming air, and air velocity is cut in half or better.

All of the materials needed to build this can be found at your local big box store, total cost around $200.

This is what the finished heater looks like, vented into a basement window to heat my lab (yes, the duct tubes will be insulated at some point...). The dimensions are 12' L x 2' H x 8" D. The heat exchanger area is 10' x 2', which will heat 200-400 sq ft. depending on the insulation, height, etc. I'm guessing it will reduce my wood use downstairs by a third or more.

A snap switch is built in for fan control, turning on at 110F and off at 90F, making it completely automatic.

Today is the first day I'll be able to operate and test it - should be fun!


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## semipro

Very cool.  I'd like to understand your airflow better.  Have any more pictures that would help illustrate?


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## precaud

Sure. The first pic shows the unit face down, the tubes are placed on the front panels to position them.

The second one shows the baffle, which isolates the input and output air.

The third one shows one of two 3/16" steel rods that support the downspouts. And everything is siliconed in place. It is important to have no gaps anywhere, including between the tubes.


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## precaud

The first pic shows the first section of the back in place, showing the 2-1/2" gap between the back and the tubes, where intake air is preheated.

The second pic shows the section of the back that mates with the input/output baffles.

The third pic shows the baffles with the front cover removed so they can be sealed up. The snap switch is wired in at this point.


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## precaud

The first pic shows the front of the unit with tubes installed.

The second pic shows the back with duct connectors and painting completed (yes, I built it in my living room... there are advantages to not being married!)

The third pic shows the front with snap switch and glazing supports.


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## precaud

The first pic here shows the glazing is in place.

Second pic shows the ducting into the basement window, which will be insulated soon.

The third pic shows the 7" fan roughed in. The fan is on a variac so the optimum air flow throughput can be found. The intake duct will extend down to near the floor and mate up with an input filter and backdraft damper.


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## precaud

And lastly we have the output temp in degrees F measured by a Fluke temp probe at the end of the output duct. 65F in, 120F out at this flow rate. I like it.


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## begreen

Neat. I love your projects. Is the ducting just PVC pipe? Have you considered insulating the pipe to reduce heat loss?


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## precaud

BG, yes, the ducting is PVC sewer pipe. It will be insulated shortly, I have the material, just waiting for a little warmer day to do it.


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## Dune

This is awesome Precaud. Great work, and great presentation as well.


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## precaud

Thanks Dune. It's working well, and will work even better when I get the ducts insulated over the next few days. I just got my old Anemotherm anemometer working, so soon I'll be able to present credible output btu/hr data. And once I get the solar radiation input variables figured out, I can calculate overall efficiency. Fun stuff.

Added: The output duct is now insulated, it's just R6 foil-faced fiberglass and not a permanent solution, but it made a nice difference.


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## precaud

I ran measurements on the collector today. It was clear all day except the last hour or so, when some high clouds moved in. See the attached graph of output temps, from 8:40am to 4pm.

With solar radiation data calculated for this latitude and the collector's orientation, a collector area of 18.5 ft^2, airflow of 65.4cfm, and an air density correction factor of 0.83 for the 7,000 ft. altitude, we get this set of numbers for btu/hr output and efficiency over the burn cycle I mean collection cycle:

Local time_____Tdelta______btu/hr_______Efficiency
8:50am________35.92______1950________45.4%
9:50am________44.45______2412________45.3%
10:50am_______51.32______2785________48.6%
11:50am_______50.32______2731________48.6%
12:50pm_______47.62______2585________50.1%
1:50pm________41.42______2248________54.2%
2:50pm________29.96______1626________55.2%
3:50pm________15.71_______852________53.5% 

Without the correction for altitude, the efficiency would be a good 10% higher across the board. Sigh. But even in its present form, it is giving me 17,200 btus per day for the cost of running a 20 watt fan for 7 hours. A pretty good deal.

I'm told that the best commercial collectors are in the 50% efficiency range, so this is comparable to slightly better. I could probably eek a bit more out of it if I insulated the intake duct. That will have to wait until next year - it's supposed to start getting cold here in the next few days, so its time to get the woodstoves ready.


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## begreen

Not too shabby precaud. Free heat is free heat. I will be curious to see if the numbers drop mid-winter when intake heat loss becomes more of an issue, or not.


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## karl

How did you calculate your flow rate?


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## precaud

Yes BG, I'm pretty pleased with it. Would have liked to have made it twice as big, but there wasn't room for anything bigger. I too am interested to see the numbers when it is truly cold out. My bigger heaters aren't affected by it, but they don't have exposed input ducts, either. Plus, if there's snow on the ground, efficiency actually improves from the reflected light.

karl, flow rate is air speed (measured at the intake) times duct area.


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## fishingpol

Nice work.  I have never seen downspouts used. I love the fact that you are harnessing the free heat and putting 120 air into your basement.  Very cool... er warm.  I am doubling the size of my basement window heater and will just need glazing to finish.  I'll have more pics up on that thread soon.  I love free heat.  I just need sunny days.


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## fishingpol

I have another question.  If you used a smaller or lower rpm fan, could you realize a higher output temp as the air would be in the collector longer, or is overheating the materials a concern?


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## karl

fishingpol said:
			
		

> I have another question.  If you used a smaller or lower rpm fan, could you realize a higher output temp as the air would be in the collector longer, or is overheating the materials a concern?



He's getting on average a 40 degree delta.  That's pretty good.  Plus 120 is hot.  Without going to evacuated tubes,  I don't see how you could get it much hotter in the winter.  

I bet a bigger fan would result in more total btus extracted.


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## btuser

That's about $.40 worth of heat/day.


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## precaud

fishingpol, I look forward to seeing your collector details.

Someone in another forum suggested I try using Reflectix Reflective Foil Insulation on the intake duct. So I went and got a roll of it this morning. I must admit I've been skeptical about this stuff in the past, but as we'll see, it appears to work pretty well in this application.

The roll I got is 16" wide. The duct pipe is 4-1/8" OD, putting the circumference approx. 13.5". I have some 1/4" heavy-duty  paper-backed bubble wrap laying around, so I put a layer of it on the pipe first to build it out. That worked out very well, adding another insulation layer and making the diameter perfect for the 16" Reflectix.

After sealing it up with Gorilla Tape and giving the system a half hour to adjust, I logged temps every hour from 11:50am to 3:50pm to compare with yesterday's data. The results were better than I expected; temps up 3F to 5F across the board, with approx. 200 btus more output per hour:

Local time_____Tdelta______btu/hr_______Efficiency
11:50am_______53.77______2919________51.8%
12:50pm_______51.12______2774________54.7%
1:50pm________44.38______2409________58.1%
2:50pm________34.10______1851________62.9%
3:50pm________20.05______1088________68.3%

So BG, your intuition was correct: a 6' long bare intake duct was costing about 1400 btus per day, roughly 9% of total output.

Lesson learned: Make ducts as short as possible, and be absolutely FANATICAL about insulating and sealing everything in the air path! (Sound familiar? I've been saying the same thing about air/combustion systems in wood stoves...)

I won't be doing any more measurements on this thing until it turns much colder out. The hunt for quieter fans is all that remains to be done for now.


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## precaud

karl, you're right. The sweet spot for output temps appears to be in the 110-120F range for best efficiency. With the fan on a variac, I've run tests at 45 cfm, temps peaking at about 135F, and efficiency was about 5% lower. At 65.4 cfm, it tops out around 123F. Something around 70-75 would be ideal. Pulling that with 0.5" H2O static pressure with low noise levels in a 4" deep space is the next puzzle to solve.



> btuser: Thatâ€™s about $.40 worth of heat/day.



Measured in what? And what's your point?


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## fishingpol

Couple of more questions:

I am curious why the snapstat is in the collector housing?  It just seems like a bit of work to get at it if it fails.  Is this where the first usable temps are realized to start the fan?

Could this design work if say air flowed horizontally down 2 spouts and then did a 180 degree turn and came back on the next 2 above it and so on?  If the air flowed longer through the collector back and forth, wouldn't it pick up more temp along the way?

_____

Sorry Karl, I am not really a formula and numbers guy.  I just run ideas through my head and hope for a good number in the end.  Building these are half the fun without getting to complicated.  I just think of any way to extract more juice of the the giant lemon called the sun.


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## precaud

fishingpol said:
			
		

> I am curious why the snapstat is in the collector housing?  It just seems like a bit of work to get at it if it fails.  Is this where the first usable temps are realized to start the fan?



Where else would you put it? Yes, it will be a pain to get to if it fails. A temp sensor with leads back to a controller in the house would have been more elegant.



> Could this design work if say air flowed horizontally down 2 spouts and then did a 180 degree turn and came back on the next 2 above it and so on?  If the air flowed longer through the collector back and forth, wouldn't it pick up more temp along the way?



Nothing of heating value happens in a 180 degree turn. It only adds back pressure to the system, requiring a more powerful fan to maintain cfm throughput.

IMO, these downspout heaters are only a good choice for heaters with horizontal form factors. It's better if you can go vertical, take advantage of natural thermosiphoning, and use black screen for the collector/heat exchanger. Cheaper and easier to build, too.


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## fishingpol

Ok, thanks for the answers.  I am used to looking at thermosiphons, so I need to wrap my head around the horizontal set ups to understand them a little more.


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## karl

Glad to see insulating your output helped.  I would suggest insulating the input.  There won't be as big of an improvement as the input, but it's still warmer than ambient temperature.

I'm impressed hat you're getting over 900 btus a day per square foot.  I wonder if the soda can collectors would do as well as the downspouts.  I suppose they would, and be much cheaper.


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## precaud

karl, the output was insulated last week, the input yesterday.

I don't share your enthusiasm for the soda can collectors. Cheaper, yes. But a pain to work with. And higher flow resistance.


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## Todd

Very cool, thanks for sharing. I've thought of doing something similar, but I don't think we have enough sun to make it worth it.


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## precaud

Todd, NREL keeps pretty good records of available solar energy in many places across the country:
http://rredc.nrel.gov/solar/old_data/nsrdb/bluebook/state.html
For example, a south-facing wall in Eau Claire gets 1,000 btus/ft^2/day in January. While that's less than the 1600 that we get here, you just have to scale your collector up in size to make it work. Two of the biggest solar devotees I know of live in places you wouldn't think solar was viable - Maryland and Pennsylvania.


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## karl

precaud said:
			
		

> karl, the output was insulated last week, the input yesterday.
> 
> I don't share your enthusiasm for the soda can collectors. Cheaper, yes. But a pain to work with. And higher flow resistance.



Point taken, but $70.00 cheaper makes a huge difference, especially when you're wanting to make 5 or 6 of them.


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## btuser

precaud said:
			
		

> btuser: Thatâ€™s about $.40 worth of heat/day.
> 
> 
> 
> 
> Measured in what? And what's your point?
Click to expand...


I'm just running numbers through my head.  Believe me, I'm jealous when I see stuff like this (you've already given me ideas for a greenhouse booster next Spring) but I've got a big giant pig of a house that is hungry hungry hungry.  I need serious btus.  

17.5k btu per day would mean about $.40/day for me.  I heat with oil.  139k btu/17.5k btu is about .125 gallons of oil (I'm counting my oil system at 100% efficiency)  which at todays market price of $3.5/gallon is 43.75 cents.  If I count my wood stove its about 15 cents/day if I have to buy wood, natural gas about 20 cents, electric about 50 cents.   A pair of 100 watt lightbulbs would put off the same amount of heat at a constant 24hr rate rain/shine night/day for a cost of $.86/day with an electric rate of $.18 per Kwh. 
A heating space would have to have a heat load of 750 btu/hr, so your basement at 200sqft would need a heat load requirement of about 3-5 btu/hr per square foot.  That's not enough for even 1 air change per hour.  

The main problem I see is its only going to heat during the day, but if its heat gain it counts.   What happens when its 20 degrees outside?


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## precaud

Sounds to me like you're not a candidate for this.



> I need serious btus.


Sounds like you need serious insulation.



> ...so your basement at 200sqft...


Who said I had a 200 sq ft basement?

As to the rest of it, why must everything be measured in $$ ? On average over the season, I expect this little heater to replace one load of wood in the stove per day. That's a lot of wood I don't have to gather and process. Very worthwhile to me.



> The main problem I see is its only going to heat during the day, but if its heat gain it counts.   What happens when its 20 degrees outside?


You have a way of casting everything in a negative light. And what's the point of your 20 degrees question? Several things can happen. If it's sunny, I have enough passive solar to heat the house to 75F even on sub-zero days. If it's not sunny, I have woodstoves.


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## karl

As to the rest of it, why must everything be measured in $$ ? On average over the season, I expect this little heater to replace one load of wood in the stove per day. That's a lot of wood I don't have to gather and process. Very worthwhile to me.



> The main problem I see is its only going to heat during the day, but if its heat gain it counts.   What happens when its 20 degrees outside?




Ok. I'm definitely not a naysayer as my previous posts show.  I'm truly interested and would like to build a larger system for my house.  I hope you keep posting results.  There are a bunch of us who would love to know so we would have the extra push to go build one ourselves.

Anyway, I got to ask.  What sort of Barbie doll playhouse of stove do you have if it one load of wood is only 17,000 btus or so?


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## precaud

karl said:
			
		

> Anyway, I got to ask.  What sort of Barbie doll playhouse of stove do you have if it one load of wood is only 17,000 btus or so?



You're making erroneous assumptions. It's not all about quantity, but location and timing too.

Location: This particular heater is vented directly into my lab in the basement where I work. All of its heat output has to pass by me in order to get to another room down there. My barbie-doll X33 is located more centrally in the 1250 sq ft basement so a good chunk of its output doesn't make it into the lab.

Timing: When I'm not down there working, I don't make fires. But the solar heater will produce whether I'm there or not, raising the baseline temperature (except in periods of prolonged cloudiness, which aren't often), making the space easier to heat to comfortable temps. I can see this already. Last year I started making fires down there by the second week of October and this year I haven't had to yet. Shorter wood heating season = less wood used.


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## maple1

precaud said:
			
		

> IMO, these downspout heaters are only a good choice for heaters with horizontal form factors. It's better if you can go vertical, take advantage of natural thermosiphoning, and use *black screen for the collector/heat exchanger*. Cheaper and easier to build, too.



I'm curious about the bolded - would you or someone have a link to more info about that? Does that mean replacing downspouts or pop cans with screening?


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## precaud

Many ways to do a screen collector - here's a simple one:
http://www.builditsolar.com/Experimental/AirColTesting/ScreenCollector/Building.htm

and test results:
http://www.builditsolar.com/Experim...llector/110111TestScreenCol/ScreenColTest.htm


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## btuser

I've got a pretty tight house with r50 in the attic and r19 in the walls with modern double glazed windows and pretty good Southern exposure.   My negative outlook comes from a cold hard winter.  

I"m in NH, which is a lot different than New Mexico.  My degreed days are 7000 and my design temp is -10.   17,000 btus is a little more than 3 pounds of wood in an average stove, which is like a whole piece.   How many cord do you burn?  

Yeah, its about money.


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## precaud

Perhaps you're thinking of southern New Mexico. I am at 7,080 feet, degree days are 6487, of which heating are 6092. Forecast low tonight is 29. We had about 4 hours of sun today so I won't need to light a fire.

Each year for the last three, I have been steadily lowering my wood consumption by adding solar air heaters. Last year I cut upstairs wood use by 60%, documented in this thread:
https://www.hearth.com/econtent/index.php/forums/viewthread/57663/

I still don't understand why you're on this thread. You're clearly not interested in the subject matter and you seem intent on minimizing it.

This is the only forum I participate in regularly where people who aren't interested in a subject feel the need to weigh in on it anyway. I don't get it.


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## madrone

precaud said:
			
		

> But even in its present form, it is giving me 17,200 btus per day for the cost of running a 20 watt fan for 7 hours. A pretty good deal.



How big of a PV would it take to run 20 watts for 7 hours?


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## karl

In your other link. You mentioned using window screen as a collector.  Why did you switch to gutter material on this one?


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## precaud

karl said:
			
		

> In your other link. You mentioned using window screen as a collector.  Why did you switch to gutter material on this one?



Different form factor. I think I explained this in the first post;  this one had to be horizontally-displaced (low and long) to fit the space, with input and output on the same end. The other two were vertical, built into the house structure, and could take advantage of natural convection. This one is small; less than 1/4 the size of the others.

Madrone, I've never worked with PV panels so I don't know. Worth looking into some day, though.


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## btuser

precaud said:
			
		

> I still don't understand why you're on this thread. You're clearly not interested in the subject matter and you seem intent on minimizing it.
> 
> This is the only forum I participate in regularly where people who aren't interested in a subject feel the need to weigh in on it anyway. I don't get it.




I'm very interested in the subject, and if you fancy yourself a scientist you shouldn't have a problem defending your project.  I was impressed with the results of your other project so of course I'd be interested in this.  But I'm not a cheerleader, I'm looking for BTUs.  60k/day for one of your other collectors is what you claim which seems to be a pretty good return.  17k on a good day is about the same as a window with an insulated shade.  My original invasion of this thread was simply putting the thermal gain in perspective.  That's a lot of work for 40 cents on a good day, but you heat with gas so you're solar air heater is more valuable to me than it would be to you!

Sounds like its cold where you live.  What's your heat load when its -20?  Mine is about 100k/hr (typical construction for our area).  I heat with oil/wood.  In the last 4 years I've reduce my usage from 1600 gallons to 823 last year. You say you've dropped a substantial amount of wood use since the solar air projects.  Do you have hard numbers because that pile of wood I estimate between 1-2 cord.  I don't know many people in a heating enviroment that can heat their house on only a cord of wood/year.


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## precaud

btuser said:
			
		

> I'm very interested in the subject, and if you fancy yourself a scientist you shouldn't have a problem defending your project.


Thanks, but I have better things to do with my time. This project is what it is. If you're not interested, move along, please.



> My original invasion of this thread was simply putting the thermal gain in perspective.  That's a lot of work for 40 cents on a good day, but you heat with gas so you're solar air heater is more valuable to me than it would be to you!



I don't heat with gas. Another bad assumption on your part. And we clearly measure the value of things differently. Hence my disinterest in engaging with you.


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## begreen

karl said:
			
		

> As to the rest of it, why must everything be measured in $$ ? On average over the season, I expect this little heater to replace one load of wood in the stove per day. That's a lot of wood I don't have to gather and process. Very worthwhile to me.
> 
> 
> 
> 
> The main problem I see is its only going to heat during the day, but if its heat gain it counts.   What happens when its 20 degrees outside?
> 
> 
> 
> 
> 
> Ok. I'm definitely not a naysayer as my previous posts show.  I'm truly interested and would like to build a larger system for my house.  I hope you keep posting results.  There are a bunch of us who would love to know so we would have the extra push to go build one ourselves.
> 
> Anyway, I got to ask.  What sort of Barbie doll playhouse of stove do you have if it one load of wood is only 17,000 btus or so?
Click to expand...


Please take a few minutes to read a few of precaud's past postings. If you did you would realize how incredibly off the mark and insensitive this question is. We are lucky to have people like precaud posting. He takes combustion, burning and efficiency very seriously and has contributed a lot here with previous posts on stove efficiency and improving performance. I've lost track of how many stoves he has owned.


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## begreen

btuser said:
			
		

> precaud said:
> 
> 
> 
> 
> I still don't understand why you're on this thread. You're clearly not interested in the subject matter and you seem intent on minimizing it.
> 
> This is the only forum I participate in regularly where people who aren't interested in a subject feel the need to weigh in on it anyway. I don't get it.
> 
> 
> 
> 
> 
> I'm very interested in the subject, and if you fancy yourself a scientist you shouldn't have a problem defending your project.  I was impressed with the results of your other project so of course I'd be interested in this.  But I'm not a cheerleader, I'm looking for BTUs.  60k/day for one of your other collectors is what you claim which seems to be a pretty good return.  17k on a good day is about the same as a window with an insulated shade.  My original invasion of this thread was simply putting the thermal gain in perspective.  That's a lot of work for 40 cents on a good day, but you heat with gas so you're solar air heater is more valuable to me than it would be to you!
> 
> Sounds like its cold where you live.  What's your heat load when its -20?  Mine is about 100k/hr (typical construction for our area).  I heat with oil/wood.  In the last 4 years I've reduce my usage from 1600 gallons to 823 last year. You say you've dropped a substantial amount of wood use since the solar air projects.  Do you have hard numbers because that pile of wood I estimate between 1-2 cord.  I don't know many people in a heating enviroment that can heat their house on only a cord of wood/year.
Click to expand...


Perhaps you should try setting up your own solar heater project? Armchair critiques aren't too helpful. Not everyone lives in your situation. This thread is exceptionally helpful to many that live in milder climates for example. At least precaud it actually trying out systems. Maybe they won't all succeed or have the best results. But I see a steady approach that improves on designs with each successive season. That is in the spirit of this forum. His sharing info with others is very helpful. 

FWIW, in your region, my brother-in-law built his own, 2300 sq ft house in 1980. It used about 2 cords of wood/yr to heat. And this was so for 3 decades. With the addition of an attached solar greenhouse a few years ago, they are now down to about 1.5 cords/yr. The house was designed for this eventual addition of the greenhouse and the plan has worked out beautifully.


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## btuser

Point taken.  Sorry I came off like a grouch, I was only trying to quantifiy it for myself and was thinking out loud.  My own solar project involves cutting and burning the trees shading my house.  I'll post pics when I'm done.

No doubt I would not have built my home as it stands.  Unfortunately even with the harsh Winters here we don't build em right, or even like they used to with a center hearth.


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## begreen

It's hard to plan for solar when you are in a valley of shade. But there are lots of folks with good southern exposure that have the potential for a lot of free warmth. Wish our house wasn't on the north slope of the hill, but we do our best.


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## precaud

So true. I have had to do some serious limbing and felling to a row of large, leggy Siberian Elms on the south side of my property to clear the way for unobstructed solar exposure. I had zero remorse doing it - I hate the species. They have taken over the entire perimeter of my property, and every year I spend a day or two clearing them back. But they are winning the war, no question.

PS - we had very little sun today, so I'm going to have to make a take-the-chill-off fire upstairs. First of the season.


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## begreen

Ha, I have an English elm directly to the south of the house that has equal disdain from me. They spread like wildfire underground.


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## billjustbill

Precaud,

Thank you for taking the time to post your construction pictures.  Seeing it go together is great.

Years back, for $10 each, I picked up four of those vertical, chrome framed, double pane freezer doors you see in your local grocery store's frozen food isle and thought they would be great for such a project like yours.

Since then, I've read much about air heaters because I live in Texas, and my home's roof has a great Southern exposure.  I went through the articles and the commerical versions of the soda can heaters and do like the thin shell of the cans allowing heat transfer so well.  This type of thin material used in the thin wall tubing allows the makers of A/C units to raise their effiency values, too

Because of all the seams to be seal with all those cans, it appears that the next best thing would be using your working concept with three changes I'd like your comments on:  1. use a duct booster fan (found at Home Depot & on line) farther up into the intake to help with intake noise and some models can be used with a speed control that addresses the noise and sunlight conditions that might benefit a slower air speed. 2. use metal downspout material to improve heat transfer.  3. use a spiral piece of metal down the center of each downspout to swirl more air against the hot downspout surface area.

All the gas water heaters I've seen have some type of swirl or zig-zag baffle in the tube that allows the combustion fumes to go from the bottom burner to the double wall vent pipe.  The rising heat slows and transfers the heat of the combustion fumes to heat more of the water before it goes out the roof vent.  Maybe if you could insert some type of baffle through the use of a hole saw to cut a hole in line with each downspout, and pluged with expandable foam and fully sealed with an over lapping board, it would raise your efficency rate even higher.

Having built a cyclone dust collection system for my workshop, using 4", 6", and 8" ducting, the learning curve about air movement taught me that using standard 90* elbows or T's lessen effeciency.  Using wye's or long sweep elbows allow air to move much better.

Just some ideas for your thoughts.  Keep up your good work,
Bill


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## precaud

Hi Bill, thanks for your comments and questions, replies inline.



			
				billjustbill said:
			
		

> Since then, I've read much about air heaters because I live in Texas, and my home's roof has a great Southern exposure.  I went through the articles and the commerical versions of the soda can heaters and do like the thin shell of the cans allowing heat transfer so well.  This type of thin material used in the thin wall tubing allows the makers of A/C units to raise their effiency values, too.



Yes, I generally like the approach of using low-mass heat exchangers.



> ...three changes I'd like your comments on:  1. use a duct booster fan (found at Home Depot & on line) farther up into the intake to help with intake noise and some models can be used with a speed control that addresses the noise and sunlight conditions that might benefit a slower air speed.



I like the booster fan concept, but reports from users show they don't perform as described. Their cfm rating is under low-load conditions, which of course isn't how they will be used.  And I like the idea of varying the cfm to light conditions. There are many ways to go about that. Using a PV panel for at least one of the fans in the system is one way I've thought about it.



> 2. use metal downspout material to improve heat transfer.


That's why I used aluminum downspouts and not the steel; I wouldn't recommend the vinyl ones for this.



> 3. use a spiral piece of metal down the center of each downspout to swirl more air against the hot downspout surface area.



Introducing turbulence (resistance) in the air path is great, in theory, but must be weighed against the effect it has on the system as a whole, especially the static pressure, which will require more power from your fan system to overcome. Other variables are in play, but that is basically what it comes down to with these systems. All of the resistances in the system are cumulative, and as you noted, every element in the system (diameter, length, elbows, other transitions) contribute to it. So you can add as much resistance as you want, as long as you're willing to deal with the pressure consequences.

Actual cfm needed to pull through these heaters is higher than one might think going in. When I reviewed horizontal collectors others have built and viewed measurements of their performance, the cfm requirements were always higher than they expected, and the fan performance under load was always worse. One guy built a 30ft-long downspout collector and even with 190+ cfm of throughput, it is in the low 40's for efficiency. So a major priority with my unit was to minimize system pressure while increasing air-to-exchanger contact area, allowing lower system air velocity. Instead of increasing resistance, I increased the amount of time the air spends going through the exchanger.


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## billjustbill

Thanks for your help and comments.  

Bill


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## precaud

This system has been in use for more than a month so I thought I'd give an update. Comparisons to last year are pretty valid, since La Nina conditions (generally warmer and drier than normal) prevailed then as now. Still winter-ish though; last night's low 23F, today's high 42F.

There are three main ways I evaluate this project: 1. how well does the heater work, 2. how well is it sized to the space it's heating, and 3. how much is it reducing my wood consumption in that space.

The numbers give #1, the effectiveness of the design, high marks and I do also. On a btus/sq ft basis, it works really well.

In #2 it falls short. But I knew this would be the case. I wanted to build it twice the height but decided there wasn't room for that. At 2'x12" it's undersized for the space, so it can only carry the heat load requirements in the shoulder seasons. On the other hand, it's putting btus into the basement on days where previously none would have been (weekends and holidays), and that is a definite plus.

All that being the case, it is still going to put a significant dent into the wood use downstairs. Here we are at the end of November, and I have only had 5 small fires in the X33 to take a morning chill off. Last year at this time, I was burning a couple loads a day on workdays.

So, overall I'm very pleased with it. There are still improvements to be made. The outdoor vent pipes could be better insulated. At some point, a quieter fan would be nice. And in the back of my mind, I'm thinking this unit could perhaps be put on the roof and piped into the bedroom on the north end, and I'll find a way to put a larger unit in its place...


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## karl

That's great.  How about some current numbers.  Or are you still measuring them.


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## PapaDave

Precaud, glad to see you're still at this.
Do you think there might be a benefit to boxing in/insulating the intake/exhaust runs to further reduce heat losses?
Would that create other problems for you? 
Are you using schedule 30 or 40, and I wonder if it would make a difference in heat retention?
This is VERY interesting stuff.


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## precaud

> How about some current numbers.  Or are you still measuring them.


karl, my data logger went on the fritz right after the last set I took, and I haven't found time to fix it yet.



			
				PapaDave said:
			
		

> Precaud, glad to see you're still at this.
> Do you think there might be a benefit to boxing in/insulating the intake/exhaust runs to further reduce heat losses? Would that create other problems for you?


Hi Dave, yes I think there would be real benefit to that. An yes, there is a problem with it, the vent pipes run right next to the only outdoor water outlet, which I can't block access to.



> Are you using schedule 30 or 40, and I wonder if it would make a difference in heat retention?


Sorry, I don't know what that is, pls explain.


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## PapaDave

Sorry, referring to the pvc pipe.....schedule 30 is thin wall and schedule 40 is thicker. 
I thought I saw an outdoor spigot.


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## precaud

It's the thinner wall pvc, used for sewer pipe. My thought was, pipe area was more important than thickness. In hindsight, it would have been better to go bigger than 4", but I didn't see any pvc larger than that.


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