Adding another solar air heater

[precaud]

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:
https://www.hearth.com/talk/threads/24643/

I estimate that, on sunny days in winter, I get 50k-60k btus per day from this heater. It's working so well, I thought I'd add another one right next to it. So I'll share my process of building the second heater, which I started on July 5th.

The first photo shows the space where it will go. The walls of this house are thick (16 in.) masonry, so I won't be putting any new holes in. The idea for this heater is to bring cold air in through the basement window (bottom right), use a sub-flooring with partitions to distribute it evenly across the front glass, take the heated air into the house through the top 1/4 of the upper window with fan assist, and install a floor-level vent in a hallway 25 feet into the house to complete the circulation loop. When operating, it will heat upper and lower rooms at once.

As I discovered while building the first one, construction is painfully slow when building triangular spaces, probably 5 to 10 times slower. So this indeed is a labor of love.

The second photo shows the concrete slab poured, giving a better idea of what will be happening.

 

[precaud]

The stuccoed walls are built to last, but are very uneven. 2x4 frames with shims are Tapcon'ed into the walls to make the outlines parallel and vertical. What in the world did folks do before Tapcons were invented?

The triangular vertical beams are milled on a table saw from 4x4 doug fir. Big fun. The bottom one is a redwood 4x4 just in case moisture finds its way in. The beam across the arch is a 2x6. Thanks to careful frame layout, everything is vertical and square at the top. {Phew.}

This heater will be about a foot taller than the first one, or 8' x 10'. The old porch wall blocks part of the early morning direct sunlight, so ultimate heat output will probably be just a bit more the first one, I'm guessing around 60k-65k btus/day. But since the walls will now be insulated, consider that we are also moving about 120 sq ft of massive masonry wall inside the heated envelope. So the ultimate impact on overall house heating is much greater than just the btu numbers would suggest.

 

[precaud]

And now for my least favorite part - the roof. It will only have a 6º slope, but that will be plenty for such a small roof.

A 6º roof slope on the approx. 65º / 35º structure makes for some ridiculous angles in the places where they converge...

And that's where it stands as of today. Next step is to cover and seal the roof. I plan on using modbit roll roofing, it's easy to work with and I think I have enough of it left over from the last heater for this one.

 

[billjustbill]

And now for my least favorite part - the roof. It will only have a 6º slope, but that will be plenty for such a small roof.

A 6º roof slope on the approx. 65º / 35º structure makes for some ridiculous angles in the places where they converge...

And that's where it stands as of today. Next step is to cover and seal the roof. I plan on using modbit roll roofing, it's easy to work with and I think I have enough of it left over from the last one for this job.

Ask your local roofing supply about Ice and water shield going down first, instead of normal roofing felt. With the two side walls being your house, keeping water out on a low slope roof is the the main long term issue to keep your solar air heat going and keeping out black fungus. You might consider adding the slope of the roof to match your Latitude, so you could put an 80 watt PV panel or two to power the storage battery and 12v-24v blower so there's no inverter needed. If you'll do this, then the $1500 max tax credit you only get once can be transfered to the unlimited 30% tax credit of a solar electric system. When there is a low sun or no sun day for making electricity to power the blower, your air heater wouldn't be adding much heat anyway.

What type and cfm size of blower are you going to use? What kind of collector plate and glazing? There are many DIY versions on the web, but here is a commerical air heater that uses soda cans. With their thin walls, not only is the heat transfer better, but the heated air never comes in contact with the glass....

(broken link removed)

I built a 40 gallon batch heater back in the mid-80's using wood as the main framing. I always worried about the amount of heat it had. After four years and a major hailstorm, I removed it when the new roof was put on. If I were to do it again, for water or air solar heat, I would use metal studs, fiberglass insulation, and the correct foil-faced foam insulation inside the heater itself.

Hope you will keep taking pictures of your progress and posting them. Great project!!

Bill

 

[precaud]

Ask your local roofing supply about Ice and water shield going down first, instead of normal roofing felt. With the two side walls being your house, keeping water out on a low slope roof is the the main long term issue to keep your solar air heat going and keeping out black fungus.

Thanks for your suggestion, Bill, but I have never seen fungus of any kind here in New Mexico. The roof I did two years ago has been leak-free so I'm comfortable doing this one the same way. The roof area is small (half of 84" x 64") and south-facing, so water/snow clears very quickly. No need to go crazy on the construction.

You might consider adding the slope of the roof to match your Latitude, so you could put an 80 watt PV panel or two to power the storage battery and 12v-24v blower so there's no inverter needed. If you'll do this, then the $1500 max tax credit you only get once can be transfered to the unlimited 30% tax credit of a solar electric system. When there is a low sun or no sun day for making electricity to power the blower, your air heater wouldn't be adding much heat anyway.

The roof is already built, not about to change it... The electricity needed to run a small fan for six hours a day is so miniscule, it's not worth putting money into PV panels to power them. Intellectually satisfying, but economically hard to justify.

What type and cfm size of blower are you going to use?

Very little is needed from the fans to assist the natural convection - only 50-75cfm. Haven't decided exactly which fan(s) to use yet.

What kind of collector plate and glazing?

Suntuf polycarb glazing, two layers of black aluminum screen for the collector. Black vented aluminum soffit is an interesting material but noone in this area stocks it. I guess there are not many soffits in New Mexico...

There are many DIY versions on the web, but here is a commerical air heater that uses soda cans. With their thin walls, not only is the heat transfer better, but the heated air never comes in contact with the glass....

(broken link removed)

Yes, I looked at that a couple years ago and it looks like a pita to make and not any more efficient than the screen.

Hope you will keep taking pictures of your progress and posting them. Great project!!

I certainly will!

 

[precaud]

Be glad you weren't born a roof. The most under-appreciated part of any structure.

I put the roofing on this weekend. SBS modified bitumen roll roofing. Dirty, difficult work, and every detail has to be exactly right or it will leak. We had a thunderstorm last night and it passed the test with no leaks. And I haven't even sealed along the termination bar with PU caulk yet.

In 30 days, after the roofing cement cures, I'll paint the SBS with 2 coats of white elastomeric roof coating. That stuff lowers daytime rooftop temperatures bigtime and extends the life of the roof.

 

[billjustbill]

A tight roof on and after a rainy spell is a reward all its own!!

One of the things I learned about the using the white elastomeric roof coating is that the one I used on my 16'x 44' patio roof, required a clear primer to be rolled on first. Be sure to read the brand's requirements.

The other thing is that the one place that cracked and peeled, despite the expensive primer and putting on the second coat at 90 degrees to putting the first one on. I rolled the second coat on within the manufacturer's recoat time, but it was where I left a place that thicker than the norm two-coat thickness. The drying and weathering of the extra thickness caused it to shrink more and caused cracking in that area. I believe that extra thickness in the coat was either at the beginning of the job when I had plenty of coating, so I rolled it on thick, or at the end of the job when I had some left over and rolled the it on "For good measure".... ;>)

Hope this helps,
Bill

 

[precaud]

Bill, I currently have about 2,000 sq ft of modbit roofing that I have coated, and the only problem areas I see are those where the substrate has issues. I've used two different Henry products and one from Lowes, and after 3 years I don't see any significant difference between them. I pressure-sprayed the surface before applying, and coated in the early morning before temps had risen. So far, so good.

 

[Jags]

Great project Precaud. Keep those pics coming.

 

[precaud]

I removed the top half of the window. It was very interesting tearing it apart and seeing how they built things back in the 1930's - iron weights on ropes nailed to the window frame for slide counterweights, and still functional after 80 years of use. Given the tools of the day, their work was amazingly precise, except for the squareness issues again... but they knew how to fudge them inconspicuously.

The top 12" is now an oak plywood hatch door that opens inward to let the heat in, the remainder was fixed in place. It will get covered with polyiso so finishing on the outside is not needed. The bottom sliding window is still operational, as it will be the only possible "service entrance" once the front is closed off.

Then all the frame gaps were caulked with expanding PU foam.

Next we start dealing with the interior.

 

[precaud]

Made good progress this week. It is primed, caulked, the floor manifold (not shown in the pic) is cut to rough shape, and most of the insulation is installed. I'm hoping the good weather will hold so I can hang the rest of the fiberglass and get it covered!

It turns out I made one significant error. The concrete lip extends 5.5" in front of the glazing mounting plane, and the roof drip edge is 3" in front. So it drips onto the lip, which, of itself, is OK. Problem is, I forgot to slope the lip forward so the water runs off. So... it puddles there when it rains. Not good. The best idea I've come up with is to put a sloped flashing there to keep the water off the concrete completely. If you have a better idea, please let me know.

On the first one I built, I was able to get the glazing up and then finish the interior at my leisure. Not this time. With no easy access from inside the house, the interior of the heater must be finished first and the glazing goes on last. Everything has to be right the first time. I liked it the other way better.

 

[semipro]

Great project Precaud. I love seeing this sort of work. The work I'm doing on my house now is much less fun...fixing water damage. It amazes me the difference in building techniques between dry areas like NM vs. wetter areas like where I live.

Is there a surface that goes over the fiberglass batt? If so do you plan to paint that surface black for better heat absorption? Maybe your glazing is not clear so it won't matter as much.

 

[precaud]

Thanks Semi. Yes, not having to deal with consistently high moisture levels makes things a bit easier. Good luck with your repairs.

The batts get covered with black polyethylene sheeting. If the weather cooperates, I'll be getting to that today.

 

[precaud]

Insulation is complete and the covering installed. The intake manifold is partitioned to distribute the air is evenly across the front, and is braced well enough to withstand my weight on it. If it weren't for that, foil-faced polyiso would have been a better material to use. But the interior will need to be cleaned every few years.

I took a different route to deal with the front lip drainage issue. The thought of living with the sound of water dripping on metal flashing was not appealing at all. So I cut grooves into the cement every five inches and dished out channels from the areas where water was standing. Not elegant, but it solved the problem.

Only three more steps before the polycarb goes in and we close her up.

 

[precaud]

Everything is prepped and ready to go. The double screen is in, intake manifold installed, outtake manifold is in (hard to see it in the pic, it's above the window and insures that air is drawn from the heater at the very top, where it's hottest.) The intake manifold supports my weight easily, and crawling into and out of the heater through the window is not as difficult as I thought it might be. When I'm in my 70's I might feel differently about it...

The forecast today is for 86 and calm - couldn't ask for better weather for installing the polycarb.

 

[Seasoned Oak]

If that works well imagine a sun room along the south facing part of your house, mine is about 24Ft long and 6 Ft wide. It warms a 3000 SF home on a sunny day and at night when it gets colder in the sun room than the indoor temp (about 77) i simply close the door to keep from giving back the heat at night. Worls like a charm ,and no moving parts (except the front door).

 

[precaud]

Well the polycarb is installed and she's all sealed up. Functionally it's ready to rock, just a few exterior cosmetic details remain to take care of - coating the roof, removing masking tape, some gravel in front, little stuff. High chance of heavy rain is forecast for the next 24 hours, so I get to see if it's watertight. Since the roof already passed that test, it should be no problem.

Though I haven't cut the hallway floor vent yet, I opened the heater vents today to carry off the chemical offgassing (very nasty smells). Even with the longest possible loop length - house front to house rear, down the stairs, basement rear to basement front, and in - it thermosyphons very well. So I'm going to hold off on the floor vent and see how this arrangement works once it gets colder out. This would pull more heat into the kitchen at the north end.

I think I'll step away from this for a couple days, this thing has dominated my life since early July and it's time to take the blinders off and reacquaint myself with the rest of the world...

 

[precaud]

These pics are interesting, taken from the same place and cropped as equally as possible, showing the south side of the house before the project started in July 2008, and as it is now, with 154 sq. ft. of solar air heating in place.

 

[begreen]

Nice job precaud. Thanks for the visual documentation. Looks like you won't be heating with wood until December with the new setup.

 

[PapaDave]

Yep, I love seeing these "how I did it" threads.
Nice job, and let us know how it works for you when it starts getting cold.

 

[4acrefarm]

Looks great, if it works half as good as it looks i'd call it a success.

 

[billjustbill]

These pics are interesting, taken from the same place and cropped as closely as possible, showing the south side of the house before the project started in July 2008, and as it is now, with 154 sq. ft. of solar air heating in place.

Very nice and professional looking!! How much do you have in materials? Are the pictures showing the shadows in the before and no shadows in the after shots taken in the same month and time of day?

I'd bet that in the next 12-24 months, you'll have a couple of 100 watt solar PV panels on each solar heater to power the blowers.... ;-)

Bill

 

[precaud]

Thanks guys. I've been basking in that "post-project contentment", which will last until I finally start cleaning up the messes I made...

The color coat on the stucco above the first heater will get done next year, I have some other stucco repairs to do and we'll get them all at once.

Total bill of materials for the second heater, about $550. I didn't have to buy any tools for this one so that helped keep the cost down.

Looks like you won’t be heating with wood until December with the new setup.

I hope so BG, but methinks that may be optimistic, we'll see. No doubt the burning season for the Quad upstairs will be shorter on both ends, and maybe slightly fewer fires once winter arrives in force. But not the X33 in the basement. I'll probably start having fires down there sometime next month. Once ground temps start dropping the basement is a giant btu sponge, even with the walls insulated.

 

[precaud]

Are the pictures showing the shadows in the before and no shadows in the after shots taken in the same month and time of day?

No, the first one was taken early July 2008 in the morning, when the project started. The other was taken a couple days ago in the afternoon, hence the shadows are different.

EDIT: I've replaced that photo with one taken about mid-day.

I'd bet that in the next 12-24 months, you'll have a couple of 100 watt solar PV panels on each solar heater to power the blowers.... ;-)

Could very well be!

 

[billjustbill]

Are the pictures showing the shadows in the before and no shadows in the after shots taken in the same month and time of day?

No, the first one was taken early July 2008 in the morning, when the project started. The other was taken a couple days ago in the afternoon, hence the shadows are different.

I'd bet that in the next 12-24 months, you'll have a couple of 100 watt solar PV panels on each solar heater to power the blowers.... ;-)

Could very well be!

Thanks for the answers. Great job!! And remember, keep those reciepts and estimated labor costs to put toward this year's 30% tax credit... Heck, with the tax breaks, Federal and Local, I'd bet you'd get enough to pay for those PV panels.... ;>) Or at least that's the excuse you can use to tell your wife...

Here's a thought that might really help you with how much heat you'll bet getting. Look for a thermometer that shows today's highest temp and lowest temp. Might help if you log your readings and keep record of what it does other than the amount of wood and energy used you would normally know ....

Well done,

Bill