Sunday, January 24, 2010

Tom's Larger Solar Heating System

Tom sent in the final update on his new solar space and water heating system.  This update covers the storage tank, controls, heat exchanger and integration with the boiler.


This is a really interesting project.  It includes all of the following:
  1. Design and construction of a 336 sqft solar collector -- including some new wrinkles on fin fabrication and installation.
  2. Building the 410 gallon heat storage tank that doubles as a workbench.
  3. Details on the solar  domestic water heating system that utilizes a heat exchanger made from about 200 ft of 3/4 inch rigid copper pipe. 
  4. Details on implementing the radiant floor heating system that pumps hot water directly from the storage tank and requires no heat exchanger.
  5. Details on adding heat input to the tank from a nearby wood boiler in a very simple way.
  6. There is even a way to make use of solar heat on seasonal refills of a hot tub.
Tom's system integrates all of these functions in a simple, straight-forward and cost effective design.

Wood boiler heat source as alternative to solar.

Full details on Tom's space and water heating system...

Many other solar space heating projects...

Tom's other projects on Build-It-Solar....

Thanks again to Tom for taking the time to document this project!



  1. Tom,
    what an awesome project. I am glad you finished and it turned out so great. Congratulations!! :-)
    I hope, once the weather is better, you can collect some performance data and share it with us?!! That would be very informative.
    My own project is going a little slower than hoped for, but that is just the way it is.
    Let's stay in touch - and once again: fantastic job - well done!

  2. Hi Tom
    Looks great wish I had that much room!
    Couple of questions
    How much better do you think the copper performs compared to PEX?
    Twin wall poly glazing compared to single poly glazing?
    Was there a flow rate you were trying to achieve through the collector?
    Also do you know of any DIY solar groups or clubs in MI? I have been looking but no luck

  3. I forgot Copper for the heat exchanger instead of PEX.

  4. Axel,
    Thanks for the nice remarks. I knew we were going to have a nice sunny day yesterday, so before going to work I shut off everything that pulls heat from the solar tank. In 3 1/2 hours of sun (9:30 AM to 1:00 PM) the tank tempurature came up 28 degrees (94,000 BTU's). That computes to about 27,000 BTU's per hour for the array, 3,400 BTU's per 4' panel. Since the solar gain the first hour is probably lower than the middle of the day, I suspect I'm getting closer to 4,000 BTU's an hour, per 4' panel, during the peak solar exposure hours. That is certainly within my target performance goals.

  5. Knick,
    The copper and PEX are comparable for stagnated water, water that's been sitting in the heat exchanger for a while. I believe the copper is appreciably better once I've depleted the stagnated water, as evidenced with the minimal temp difference from the inlet to the outlet temps. With times of significant hot water usage in my household, the added efficiency of the copper was important in my design.
    The twin wall was used because it should be more effective in cold climates, insulating the very cold ambient air from the collector. Axel is the one that moved me in this direction. In fact, he bought triple wall for his "test collectors", which I hope he's working on soon.
    Flow rate was determined by the formula Gary provided me with; 1 1/2 to 1 3/4 gallons a minute for a standard 4' x 8' collector. Since mine are 10 1/2' tall, I provided 2 gallons a minute per panel (16 gallons a minute for the panel). Needless to say, the flow rate out the return line is pretty sustantial.
    I'm not aware of any solar DIY clubs in MI.

  6. Tom
    Did you factor in the cost of going with copper for the heat exchanger compared to Pex when making your decision? Do I understand that you built your system to not depend so much on stagnated water but on being able to pull heat from the tank once that stagnated was gone?
    Gary please jump in if you had done any testing as to how the pex compared to copper in heat exchange performance?
    Tom did I read that your exchanger outlet temp was only 10 degrees less then the tank temp?
    I am not sure if I have this right I see under the pump info on Garys $1,000 system he had a total flow rate of 1.7 gpm for 50 sf (6-4x8 collectors) and you are getting 2 gal a min for each collector?

  7. Knick,
    I did factor in the cost of copper vs. PEX when deciding to use copper, but that was not the only deciding factor. I was concerned about the issue the HVAC "experts" were telling me about; that I couldn't use water directly out of the tank for a hydronic system that was elevated 8' above the tank. When I made the heat exhanger, I was thinking it was possible this would end up being used for the hydronic system. In that application, I really think copper is much more efficient. As it turned out, using the water directly from the tank for the hydronic system has worked perfectly.
    The copper heat exchanger should also be more effective for the pre-heat feature I will use for my hot tub water. When heating 350 gallons of water, while filling the tub, I needed the most efficient heat transfer design.
    As far as the copper heat exchanger efficiency, after purging the "stagnated water"; yes, I see within 10 degrees of tank temp on the discharge water. Having that benefit is more luck than design. I didn't plan on seeing that kind of heat transfer while designing the system. Keep in mind, I'm not knocking the PEX designs. I would have probably gone that route if I'd known the hydronic system was going to work without a closed loop (pressurized) system.
    My pump flow for the collector was based on exchanges between Gary and me alomst two years ago, while I was building "test panels". If Gary is pumping 1 3/4 GPM on 50 sf, we're not far off from each other on flow.
    The pump on my system is rated for 14-16 GPM at the lift required for my array, giving each 4' x 10 1/2' panel (42 sf)just under 2 GPM. Either way, I think there's a range where the efficiency won't suffer much based on reasonable flow, and the output of my panels suggest I must be pretty close to maximum efficiency for a home built panel.

  8. Hi Knick,
    There is a test on the PEX coil heat exchanger here:
    I've not done any comparisons between PEX and copper, but Tom Gocze has done some work on this, and figures that it takes about 5 times as much PEX surface area as copper to have the same steady state transfer. The 300 ft of 1 inch PEX I use has about 88 sqft of surface area. Tom's 200 ft of 3/4 copper has about 42 sqft of surface area -- so, I'd guess that Tom's hx has significantly better steady state performance for large and long hot water draws.
    The thing I like about the large PEX coil is that it gives you a 100% efficient HX for about the first 12 gallons (which is already in the pipe coil), and does well after this for moderate flow rates. And, its pretty cheap :) For our use, the 12 gallons already preheated in the coil takes care of nearly all our demands.
    I feel that both of these approaches work well -- it just depends on what your hot water demands are like.

  9. Tom and Gary thanks for insight. Gary one thing
    I see during the HX test, the storage tank temp dropped what looked to be from 135F down to 120F.
    Do I have that correct? Would that temp maybe have came back up some, after testing once everything equalized? Gary and Tom do you feel there is a relationship between the sq ft of collectors you have and the capacity of your storage tank? At what point is the storage tank size not going to benefit you anymore?

  10. Knick,
    I think the accepted formula is 2 gallons of storage for every SF of panel. Gary was originally seeking 500 gallons for his system with a 240 SF collector (pretty sure he ended up with less). I was looking for 600 gallons for my 336 SF collector, but just didn't have the room. I'm at 420 gallons on 336 SF of collector, not really the ideal size, but it works for my limited space.
    As far as pros and cons to over sizing or undersizing from that formula, my thoughts are;
    oversizing - more storage of the heat energy for periods of no sun / takes longer to get temp up once heat energy is sapped.
    undersizing - heat is more quickly available during sunlight and tank temp goes up quicker / storage temp drops quicker upon home energy demand, and will not carry long during periods without good sunlight.
    I think a lot of builders size the tank on available space and/or tank availability as much as by collector size.
    Tom Sullivan

  11. Very ambitious project Tom.
    Heavy duty, lots of copper, should last a hundred years without an earth quake to bring it down.
    Good pictures.
    How far is the dwelling from the house?
    Where will you store the heat?
    Good luck with your project.

  12. John
    If you look just below the picture of the boiler you will see a link labeled - Full details on Tom's space and water heating it has more pics and info

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