Solar garage heater - using that ball of fire in the sky to my advantage

I built this a few years ago, but just filled it and turned it on for the season.  Thought I'd share.  It's a pretty basic water heating set-up.  Here's the panel on the garage.  It has 3/4" PEX tubing with aluminum flashing formed over it.  Everything inside is painted flat black, and then I covered the face with corrugated polycarbonate.   The panel drains to the inside of the garage when flow is turned off.  A "drain back" system.  The gable is on a nice south-facing exposure, though my spruce tree blocks it for a little while in the mornings.  A reasonable compromise.

Here's the heater box.  Again, basic stuff here.  Box fan for moving air, car radiator (new from Rock Auto - only about $40) and some aluminum flashing for a plenum.  The sump pump moves the water thru the solar panel.

Here's a view of the radiator.

Control is with an Arduino board and a 110 v relay outlet from Spark Fun.  My cousin is a programming whiz so he did all the heavy lifting here.  There are two temp sensors - one in the panel and one in front of the radiator.  When the panel temp is higher than the interior temp, then fan and pump turn on.  Turns back off as the evening cools down.

That's pretty well it! 

It doesn't get super hot or anything, just pumps in a little heat at a time.  So it's about 40 out right now and there's probably 60 or 65 deg air coming out of the heater.  I could have gotten better heat transfer with copper tube, but the pex is cheap and works ok.  I didn't go to heroic measures to seal off the panel either.  I'm sure I get some leakage.

I never did the math on it, but I think I spend maybe $300 on the whole system, and I easily paid that back in heating bills last winter.  I used to spend about $150 a month keeping the garage warm with an electric heater.  Big difference!

Nice!

Love it! Good idea with the drain back- no worries about freezing.

I lived in a friend's house in my 20s that had a whole house heating system that worked this way. His grandfather had built the house and he later owned it. The house had a large set of panels on the south facing side of the house. The house had a very large peaked roof without any dormers so the whole side of the roof was covered in these panels. The panels had water circulated through them during the day to heat up and then that water would run back into a rock storage that was built into the basement of the house. The house had ducts like you would with any forced air heating system, but part of that heat came from the rock storage when it was at a certain temperature. The difference was made up by propane heat. The house was also very well insulated. The rooms all had thick shutters on rollers that were recessed into the walls and you would pull these out and they would completely cover up the windows to the point that it would block off any cold coming in and any light as well. It was like you pulled the wall closed.

Cool system.  Now you need to make reservoir to store larger volumes of water so when you aren't in the garage for a few days you can save all that energy and use it at once.

That tilt looks pretty extreme for winter in Denver, no?  Or Is it just the angle of the photo.

ProDarwin said:

Cool system.  Now you need to make reservoir to store larger volumes of water so when you aren't in the garage for a few days you can save all that energy and use it at once.

That tilt looks pretty extreme for winter in Denver, no?  Or Is it just the angle of the photo.

The whole idea with this was to make it a very simple system.  I didn't want to go crazy.  So there are some definite compromises.  The panel is vertical rather than being tilted, so I give up a bit of efficiency there.  But it's much easier to mount a panel flat to a wall than off the wall at an angle.

The controller runs the panel automatically, so no need for me to be there!  When I filled it all up for the first time last fall I would just plug it in in the morning and unplug at night.  That was fine to start with.  But now the Arduino compares temps and runs on auto.

I have a daydream/plan of converting the house to water-heated solar.  For that I'd use high-efficiency panels, and run them into a larger water storage tank.  Then this tank could heat hot water for showers and such, and have another circuit for radiant floor heating.  Some of the systems I've read about will heat the storage tank way up to 160 deg or so!  That's a monster thermal battery to have available.

I already have solar panels that spin my electric meter backwards for most of the summer.

Is there some way to store the heat thru the night to keep the garage above freezing ?

Asking from noon freezing Los Angeles 

californiamilleghia said:

Is there some way to store the heat thru the night to keep the garage above freezing ?

Asking from noon freezing Los Angeles 

Well, short answer is yes,  but maybe not in the way you think.  My garage is pretty well insulated, and by keeping the internal temp at around 60 the slab heats up and holds in the heat.  So it usually stays pretty constant regardless of outside temp.

The 5000 lb lump of cast-iron that is my milling machine helps regulate the temp too!

If I spend a lot of time with the door open then some of my heat will go away, so I try to keep things closed up as much as possible.

I have a few areas where I could improve my insulation and air-intrusion.  I'll be doing some of that to keep the heat in a bit better.

this is very cool. you use straight water or some sort of anti-freeze?

Stuff like this is the core of Net-Zero energy homes- and funny enough, even with those gains you've made you're not even CLOSE to what "proper" systems can do.

Nice work Scott! Can you post more pictures of it? I also want to know if you have antifreeze mixed in or not.

Thanks for all the kudos, everyone!

At the moment I'm running just straight water.  I plan to switch to antifreeze water mix, mostly for anti-bacterial properties.  I didn't before because my reservoir was at ground level and didn't want to give my dog access to that.

I may look into the RV antifreeze stuff, since I understand it's both non-toxic and keeps the buggies at bay in sufficient strength.

This is definitely not an optimized system at all.  With better panel construction and orientation, I would guess the performance would be much better.  But this gives you an idea of what simple materials and design can do.

Very cool! I am researching converting my Denver house to water/radiant heat as well, but my roof doesn't face south, so I'd be losing some efficiency there. Once I figure out how to "idle" my furnace to keep air filtration active, that is...

In reply to sobe_death :

Fist-bump for fellow Denver resident!

With solar, like anything, there are trade-offs from the ideal.  My solar PV panels are on the East-facing roof.  I have a small N/S roof section but not a big enough one for the PV array.

Solution?  Add another panel!  Generally deviating from the ideal south-facing roof give you about 85% of the solar gain of the ideal.  So we have 16 panels instead of maybe 14.  Used to be people would add brackets to angle their panels the right direction, or even have tracking mechanisms that would rotate the array to follow the sun all day.  Last few years anyway, PV panels have gotten so cheap that you just add a panel or two to make up the difference.

So same idea with solar water heat.  Just add some square-footage to your collector to make up for the lost efficiency.

Water heating for space-heating is an interesting case too.  You can successfully do a vertical panel on a South-facing wall.  The low winter sun hits the wall really nicely, while the panel won't overheat as bad in the summer.  And you really need maximum input during the winter.

Okay this is pretty interesting. What sort of temps do you see in the winter where you are?

I was debating building one of the air to air version of this for the attached garage (my house is south facing) but this would be way better.

In reply to The0retical :

We get some crazy swings in temp thru the winter.  Some days we'll have a high of 10 deg, and the next we'll have a 60 deg out of nowhere.  Generally speaking, Nov. til March we'll be sub-freezing overnight and somewhere between 20 and 40 during the day.  We have very intense solar access, even on cold days.  And it's frequently sunny.  Even on cloudy days, this system usually kicks on.

I've seen some really neat air-heating panels, and am planning a variation for my next solar project.

Also, I just thought of another advantage of vertical South-facing panels for winter heat: no snow accumulation!

Quick update:  My heater panel is currently pumping cheap heat into the garage.  A+

Longer update: I've actually had quite a lot of trouble with this.  I converted the pex tubing to copper last fall, and didn't get my slope quite right.  Froze the panel again, burst the copper in multiple places, and burned out the cheap sump pump.

This fall I repaired the burst copper, corrected the slope, and swapped in a different fountain pump.  Everything is now working fine.

Overall this project has been pretty high on the pain in the butt scale, but I really like it now that it's operating correctly.  The garage drops to about 50 deg at night, with outside temps in the low 20's.  The heater boosts it back up to between 60 and 70 each day.  Very comfortable and my old milling machine is happy.

Would I recommend everyone run out and make one too?  Only if you're ready and willing to mess with it.

In reply to TVR Scott :

How many square ft are you heating?  Do you see a consistent outside:inside temp differential?  Sq ft of panels?

Very cool project! I love the idea of solar to generate power. For us, here in FL, it would be to stop using so much electricity to power the A/C 9 months of the year. :(

Garage is 400 square feet.

The panel itself is roughly 36 sq ft "inside the box".  It's got roughly 66 ft of 3/4" copper tubing, and aluminum flashing covers the tubes and the whole inside surface.  Matte black over the whole inside, since everyone loves a rat-rod solar panel.

Not sure what you mean by running a consistent outside:inside temp differential.  The way I've got it controlled now is when the panel air temp is warmer than the garage time the pump and fan turn on.

In reply to TVR Scott :

I guess I was asking if you notice it's cooler inside on colder days or is the temp a pretty consistent 60ish degrees.  Meaning are the heat sinks effective to hold you over on colder days.

Stampie said:

In reply to TVR Scott :

I guess I was asking if you notice it's cooler inside on colder days or is the temp a pretty consistent 60ish degrees.  Meaning are the heat sinks effective to hold you over on colder days.

Gotcha.  Inside temps are more consistent than you'd expect.  Low of 50, high of 70, average right around 60.

The system often runs on cloudy days.  We can have very cold sunny days here, and I still get lots of heat on those days.  Seems like it's indifferent to outside air temp.

Great idea! Thanks for the full details and honest stories. I'm up in Longmont and this sounds like a great idea.

Do you have an HOA? Do they have any control/input on something like this?

Any thoughts or info on how it would compare to do similar size solar electric panels and an electric heater instead of the water system?

In reply to BoulderG :

No HOA here!  My house is one of the original 1949 houses and I'm surrounded by gentrified mega-mansions.

This water system was mostly for fun with the added benefit of energy savings.

For a commercially installed system, the no-brainer nowadays is PV panels and a heat-pump heating/cooling system.  I have a 5 kW system mounted on the house that gently spins our electric meter backwards.  I would like to eventually add some more panels and switch to heat pump systems and electric appliances.  And my wife has her eye on an Electric MINI.

My house in Monument CO faces south.  There's a huge difference between working in the garage on a cold morning and a sunny but cold afternoon.  The non-insulated steel doors really shed some heat.  It would be even better if we had a system like this, but for now I just wait for the afternoon to hit the garage.

Nicely done.

Tried a low budget heated air system some years ago when we were building the exocet in a unusually cold Wisconsin winter.  Helped, but not enough to make the garage comfortable, even with two space heaters going.

The glass is a door from habitat...

Have a heat pump and that works well here.