GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Thermal solar panels to pre-heat hot water tank

user-5659458 | Posted in GBA Pro Help on

My 88 y/o mother who was a pioneer in the solar energy movement has 8 liquid-based thermal solar panels from the 1990’s sitting in storage. She never installed them, and now would like to donate them to us. Each panel is 8’x3′ in size. She installed 2 in her mountain home near Fairplay, CO (9000′ elevation and seriously COLD in the winter!) and they supply enough heat for her hot water needs. They passively circulate water, and have worked well for many years.
My question is, how do we estimate the number of panels we would need? We have a natural gas boiler and radiators for our home heating system, and the hot water  heater is piggy-backed onto the boiler. The whole boiler system has to blast on every time we even use a cup of hot water in the sink – even in the summer. It seems very inefficient. Can we simply run glycol through thermal panels and then to a coil in the hot water tank to pre-heat the water? I’m thinking that then at least the boiler won’t need to fire up in the summer.
I know that photovoltaic would be the other way to pre-heat our water, but these panels would be free (except the installation). What would you recommend?
Thank you!

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. tommay | | #1

    Just a few details needed. When you say radiators, are they cast iron or baseboards? You say there is a coil in your hot water tank, so is it hooked up to a zone or is it hooked up to a tankless in your boiler?
    The number of panels you have would more than take care of your hot water needs in the summer, spring and fall and would probably be enough to maintain in the winter if you have some good exposure. You can install them so that you can isolate some of them in the summer when not needed or add an additional storage tank. Two or three should more than enough take care of an 80 gal tank. Not exactly sure of what you meant by passively circulate (thermo-syphon?) so you may just have to purchase a differential controller and pump.
    You could also pipe and/or valve off (bypass) some of your radiators to be used with the panels to keep your boiler off as much as possible. You would have to use glycol in the entire system if you decide to interconnect them, and it is subject to freezing, which isn't such a bad idea anyway. If you have room for a ground installation, it may make it easier to decide how many panels you may need once you decide on how you want to incorporate them. It will also allow you to "cover up" or tilt away any panels you don't need at the time to prevent overheating.
    If you have room for ground installation, you could set up something this summer, just lean them against something, use pieces of hose and hose clamps to couple the panels together, along with hoses connected to your tank drain and cold water input, using just water, and no pump to get started and thinking.......your mom knew what she was doing......i luckily still have customers nearly that old who know what she does.

  2. Expert Member
    NICK KEENAN | | #2

    I think you should price installation, and then price grid-tied PV and a heat pump water heater. You may be surprised.

    Thermal solar has largely been eclipsed. The problem is that it delivers the most heat when you need the least, and vice versa.

  3. Expert Member
    BILL WICHERS | | #3

    I think with free panels the economics will probably shift in your favor, even though usually people are going for photovoltaics these days (PV). You might want to add some thermal storage (big water tanks) to help store the heat from the day if you find the panels can produce more than you need. A lot of people convert old propane tanks for this purpose (big tanks for houses, not the little BBQ size tanks).

    There is a lot of good hydronic info on the forums that you might want to check about regarding system design. Just swap your solar panels in place of their outdoor wood boilers, the rest of the system is going to be pretty similar.


  4. Nola_Sweats | | #4

    I had a solar-thermal system in my old house, a 2200 square foot house in zone 2A, with lots of sun. We had two Schuco collector panels, roughly 4x7, with a glycol loop running to a small electric motor and through a special solar-thermal tank. It worked great for us, supplying nearly 100% of our hot water.

    That solar tank had an electric heating coil backup, but we never used it. Instead, we fed the solar tank into our existing gas-fired water heater, so we had extra water when we needed it (big tub!). The gas water heater was set to 120 degrees and would receive a feed of hotter water (about 130 degrees) and did not need to run often, unless we had several consecutive overcast rainy days, which would happen a few times a year in midwinter. We'd shut the gas off entirely in the summer.

    We did not have any kind of radiators, so you might need more collector surface area, and we're in a hot Gulf Coast climate, which probably helped us. I understand PV and a heat pump are good options, but I loved my old system. It ran for 10 years without any problem and is still working fine AFAIK. Sounds like it might be a nice tribute to your mom, and it's always nice to use existing stuff instead of buying new stuff.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |