Al's Solar Air Heating Collector with Hot Water Heating

This page describes Al's new solar air heating collector.

Some nice features of this collector:

- Its large enough to do some real heating -- people tend to make collectors too small to be effective for space heating.

- The design of the collector and airflow path look good and should make for an efficient collector.

- While the collector is mainly an air heater, Al is experimenting with including some water heating as well. 

- The collector looks good!

Thanks very much to Al for sending this material in!





Overview of Al's new air collector along with earlier experiment.
Has a nice, tidy appearance. 


Collector Description

I used the higher temperature rated (3/4") Polyisocyanurate insulation board for the backs. Silicone caulk was used to secure it to all the vertical sides and 'runners' using plenty of roofing nails.


The picture below shows how the collector bays are built, and the airflow path through the collector from the inlets in the lower left and right corners to the outlets in the center bays at the top.



For the right half of the collector, the air enters at the collector inlet just out of the picture
on the lower right. The air flow up the right most bay, then down the 2nd bay in, then up the
third bay in, and exits through the round exit vent visible in the center of the picture.
The left half of the collector works in the same way.

The collector frame is made from pine 2x6's and 2x4's for the middle vertical 'runners'.  The middle runners are actually 3" wide though instead of the full 3.5".  I had to rip a half inch off to fit right.  I jigsawed out the 'wiggle strips' out of the 2x6 top and bottom and used plenty of Silicon to seal. 



Collector showing the metal absorber partially installed.
The metal is spaced about 3 inches from the insulation board.


The collector glazing is 2 ft wide corrugated polycarbonate from Lowes.  Most Home Depots sell a similar product called SunTuf.  Just be sure to use the polycarbonate corrugated glazing, as it holds up to the high temperatures well.


Fans and Vents

The arrows show the flow direction of the hot air thru the collector, and as it slowly heats the water flowing inside the PEX tubing. There are two 4" diameter collector inlet ports in each lower corner made from 4" diameter thin-wall drain pipe. (Not shown in photo)
Therefore, there are essentially TWO separate collectors, the Left and the Right side.

The wires you see in the photo go to the thermostat above one of the exhaust/output ports (4" PVC drain pipe) , and to the two fans which are located on the bedroom wall inside.
Everything is working as planned and I am quite happy with it. Next time I will be able to make things even better."

Click on pictures for full size

Picture shows the two collector exit
vents with fans in place.

Board on which the fans are installed.


It may of been better to go with somewhat higher flow rate fans than these. But the reason I went with these weaker fans is, they were very affordable, quiet, and they use very little power so I can easily run the whole system with my small 12 volt RE-system. In fact, the whole thing could run on just ONE 12V battery and a 30Watt solar panel! Which is nice if the grid goes down for extended periods of time.



The picture below shows one of the two collector inlet vents.  The vents incorporate a good sized air filter to reduce dust, and also have a flip down lid to prevent reverse air circulation at night, which would cool the room.


Click on pictures for full size

Collector inlet vent with front
cover and filter removed.
The actual inlet vent duct is visible
at the lower right.

Top view of the collector inlet vent
showing the air filter.





The fans are automatically controlled by a typical baseboard heat type thermostat located inside the collector up near one of the 4" output ports. I can adjust the thermostat (if need be) by taking one of the fans off and reaching my arm thru the 4" pvc port (thin-wall drain pipe).

The thermostat goes to a very simple electronic FET-relay circuit (I made) located in my bedroom closet near the 55gal water drum. The circuit reverses the 'on/off' sequence of the thermostat so the fans can go on and off at the right times.
The thermostat is set at 70F to turn on the fans. It automatically turns off the fans after the sun goes down when the thermostat drops below 70F.
This basic control system is not as perfect as real electronic controllers, but it works fine.



Water Heating

There is about 70' feet of 1/2" PEX tubing that has two parallel runs which angle downward for good drainage to avoid freezing when the pump turns off. Water pumps upward from the bottom to the top, then drains back out the bottom.
 I used some Romex wire pieces to squeeze and hold the PEX "U" turns inward.

Click on pictures for full size


This shows the PEX tubing used to
pick up some heat from the air to
heat water in the drum below.



The 55 gallon drum used to store
some of the heat collected during
the day time for use at night.

Water heated up to 100F


I primarily made a Solar AIR heater which is what does by FAR the most heating of the total MASS inside my 740sf apartment/house. But I also thought I would try my hand at making a basic water heating system inside the Air collector in order to heat up water in a 55 gallon plastic drum in my bedroom closet so as to gain some extra BTU's in my bedroom during the night.
I was of course just guessing at many things, and even though it could have been better I'm sure, I am happy it worked out as planned. I only had room for the one 55 gallon drum in my tiny closet, so I knew the stored heat would be limited.

5" wide aluminum flashing was tightly wrapped around a piece of 5/8" steel rod using 'Channel locks' (Tool) to squeeze into the shape of the PEX. I then just riveted the aluminum pieces to the PEX with a big bead of Silicone down each one for better thermal connection as Gary showed. As you can see in the photo, there is only about 25' feet of the PEX with aluminum wrapped around it. I wish now I had used even more aluminum around more of the PEX and wider and thicker aluminum for this, which would have improved the thermal exchange. Probably copper piping would have been even more effective than PEX in this limited space. But oh well.

I simply open up the closet door at around 11pm to allow a small heat flow from the barrel into the bedroom during the night hours.
On decent sunny days the water in the tank gets up to 100 to 110F which helps reduce the normal heat loss by 5 or 6F by 9am in the morning. The The tank temp drops about 12F over night.
For half sun days, the tank temp only gets to around 82F, but I suspect it is still helping to reduce heat loss by a couple degrees thru the night hours, which I will gladly take.
However, I can see now, the water heating system would have been much better if I had used copper pipes and used more aluminum around it with bigger aluminum 'wings'.


I added a little chlorine bleach to the water in the barrel to keep anything from growing in it.


  Update August 29, 2012:

Al has taken the 55 gallon barrel out of the system because 1) the benefit of the barrel heat storage was not large, and 2) the closet space was needed.

I've left in the description because I think it is a technique some may want to use to get some storage in the system in a simple way, and the technique Al used seems to work OK. 

The 6F drop overnight that Al mentions is worth about 2800 BTU -- as Al says, this is a relatively small fraction of what the collector produces (perhaps 10%).  More stored heat might be recovered from the barrel by using a small fan to blow room air over it.  More barrels would of course also increase the stored heat.   The same technique might also be used to preheat domestic water.

Al also reports that he is quite pleased with the system performance, and about the only change he would make would be to go with fans with a higher flow rate, as collector temperatures run a bit high.  Al also sees the potential for domestic water preheat using the PEX tubing already in the collector, and may give this a try down the line.





I am very pleased with the Heat this Solar Collector produces when the sun is out here in the State of Ohio.  Over this Fall season (and I'm sure the Springtime too) there were many days I had to open a window slightly because it got TOO hot inside my rooms during the daytime.  But this is no big deal, since I like to heat up the total Mass inside the rooms as much as I can tolerate in order to help keep it warmer thru the cold nights. (But you have to remember to close the window back!)

How well it does this heating of the Mass really depends on how much sun there was, how cold it was outside, and how cold it gets at night.  It of course does best with TWO (or more) consecutive sunny days in a row I noticed.
On a good full sunny day, the heat coming out the fans is 120-145F.  This is also with the Water Heating system on.  The Water heating system seems to use up only a few degrees of this (Flowing Air) heat when it is on at the same time as the Air heater.   So if I know the sun will be good and consistent all day, I turn on both the Air AND Water heating together.  But if the sun is  forecast to be sporadic, I only turn on the Air heater since the Water heating does not work very well on these type days.
I was also surprisingly pleased that the Air heating collector does fairly well even on 'Bright gloomy days', though the temps coming out the fans are significantly less. (65-80F)
 It was also interesting that the collector temps react almost instantly with the sun light strength.  Many times the sun peaks out for several seconds and the fans almost immediately were blowing out 100F air within 5 seconds!!  That was surprising too as it was unexpected.  I was watching my hand held thermostat and the light outside my bedroom window at the same time.   It does take a few seconds more though if the temps outside are around freezing. (30's) 
Overall, I am very happy I made this Solar Collector and could not do without it now.  Thanks a million Gary and everyone else!"




The total cost for materials was about $500.









Some comments on Al's collector:

- One thing I really like about this collector is that it looks good.  I suppose looks should not be important to an engineer type, but its a lot more likely that more collectors will get built if they are not ugly.


- The flow path seems about right to me both in depth and in length.  For these backflow collectors, the flow path should not be too deep in order to keep the air velocity fairly high -- this makes for better heat transfer from the absorber to the air.  The flow path length needs to be long enough so that the air has is in the collector long enough to raise its temp about 50F, so that its useful for heating.  A A path length of 20 ft is about right, and that's about what Al has.  The flow rate should be around 2 cfm per sqft of collector in order to have enough airflow for good heat transfer and to get about the right temperature rise.  By constraining the air to flow up and down the collector bays, Al has also avoided the tendency of air flow to short cut directly between the inlet and outlet, which leaves larger dead air spaces that make for poor heat transfer from the absorber to the air, and lower efficiency.  I think that this likely to be a collector that works well.


- I think the idea of trying to incorporate some heat storage for use at night by adding the 70 ft of PEX tubing and the water barrel mass is very interesting.  I think this makes sense in the case of Al's collector because the collector at 100 sqft is more than 10% of his living space area, so there should be some excess heat to store on sunny days.  From the initial readings, it looks like the water storage system is working, and it will interesting to see more performance data over time.
Perhaps the same type of system might be used to preheat domestic water?

If you live in a place that gets freezing weather, you really needs to be careful to incorporate slope in the tubes so that the drain back to the storage when the pump stops, as Al has done on his.


- An alternative to the fan on/off control that Al used would be to use a Thermal Snap Switch.  These come in versions that come on at a set temperature, and turn off below that temperature, so no rewiring is needed.  A replacement thermostat for an attic vent fan can also be used -- some Home Depots sell these.


- I think that both the fans and the inlet and outlet ducts may be a little bit undersized.  The airflow through the collector is about right when the temperature rise from inlet to outlet is about 50F.  If its more than this, than the collector is running hotter than it needs to, and this will result in higher heat losses out the glazing and less efficiency.  If the temperature rise is much less than 50F, the moving air coming out of the outlet will feel cool -- it wants to be around 120 or 130F in order to "feel" like warm air. 

Normally, this will mean about 2 cfm per sqft of collector.  The fan has to produce this with the pressure drop through the collector.  Note that the advertised rating for the fan will normally be for essentially zero flow resistance -- it will not produce this airflow with the flow resistance of the collector and ducts.

But, its also nice that Al can run these two small fans on solar electricity with a relatively small PV panel.


- Collectors like this may thermosyphon at night and pull cold air into the room.  Al handles this with a flip open lid on the collector inlet vents, and this works fine.  It is also possible to rig automatic backdraft dampers if you want to automate the process.


-  Al experienced quite a bit of paint odor from the paint used to paint the back of the collector absorber and the collector housing.  This happens once in a while, and (I think) just has to do with the kind of paint used.  Its a bad deal with air collector because the odor gets circulated into the room being heated.


The steps you can take to reduce the chances of this:

- Use paint that will not produce and odor -- I wish I could give some good advice on what types and brands are safe, but I really don't know.  I've not heard of anyone having a problem with the Rust-Oleum high temperature spray can paint for barbeques, so that might be a good choice.  It seems like a high quality latex paint for the collector housing should be OK.
It may be that low VOC paints have an advantage.


- Let the paint fully dry and cure.  Set the painted absorber up in the sun so that it heats to a good high temperature and let it cure for a long time.  For an air collector, I don't think that two or three days is too long.  I think that its important to get the absorber up to the same sort of temperatures it will operate at.


- If you still get odor when you turn the collector on, try to rearrange the flow path so that the air circulates through the collector, but not into the room.  This may require some jury rigging of ducts or absorber and glazing.

If you do experience some odor, I've not heard of any cases where the odor does not go away over time, and the odor did go away on Al's collector after about a dozen sunny days.





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Gary December 5, 2009