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Add radiant loop to indirect solar DHW?

Orion_134 Member Posts: 35
I currently have a 3-panel indirect solar 120gal DHW setup in northern VA. I'm redoing to master bathroom and am considering adding hydronic floor heating. My thought is that there has to be some kind of thermostatic valve that I can set a temp for the floor from the bathroom, and it will cycle the valve to bleed off some of my hot water to the floor loop. My concern is that I think my DHW should trump my floor heating and there should be controller logic such that the radiant loop will only turn on if there is already sufficient DHW temps, and sufficient collector temps to satisfy the on state.

Does that already exist? Am I overthinking it? Am I missing something, like the flowrate needed for radiant flooring would overtax my collectors and it would end up never turning on? 


  • Jamie Hall
    Jamie Hall Member Posts: 21,514
    Not a bad idea in itself. You'd need either a pump or a zone valve (if there is already a circulating pump) to run the water through the floor, but that could be controlled by two things: first, a thermostat to decide when the floor needed heat, and second an aquastat to ensure that your domestic hot water in storage never dropped below 130 F (to control bacteria growth). You could use a simple tempering valve, if you had a pump for the floor, to control the water temperature in the floor if you needed to.

    Now. All that said. I've never been too keen on mixing heating water with domestic hot water. The floor water may be none too clean -- and is close to the ideal temperature in most installations for bacteria to grow. However, it is done.

    The other problem is heat demand vs. heat supply -- your question on the flow rate needed for radiant heating. Typically a radiant floor is expected to deliver about 20 BTUh per square foot to the space. More or less. But much over that and the floor is too hot for comfort, and much less than that it's chilly. Now in full sun on a clear day, with the sun pretty well overhead the collector, you can expect to get as much as 100 watts of heat per sqaure foot from the collector -- perhaps 300 or so BTUh. But that is only when the sun is shining pretty much overhead, and assumes a really good collector. Is that enough to be worth it? Quite possibly, if the collector area is large enough in relation to the floor, and if you have a way to store the excess hot water for when the sun isn't shining. It's not so much a question of flow rate as it is a question of how much heat is actually available. Then you can fiddle with the flow rate and tempering valves from there.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    Larry WeingartenBloemie
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 6,023
    edited May 2022
    This sounds like an idea in progress. After you try to use your existing water storage, you may find that you do not have enough to satisfy your needs, you may then decide to add a storage tank to hold some of the unused solar heat in the tank to use it when there is an overcast sky. Then add more collectors since you have more storage capacity, The project may never end.

    That said, design an ideal finished product that may handle both DHW and more space heating than just the master bedroom. This way you can work to that goal a little at a time. Design heat storage and collectors so you don't end up getting a better idea and tossing out parts of the project that will not work with your next step. I hate when I get that better idea and need to discard pieces that could have been used if I only thought ahead and used a bigger or better design "thingie" last year.

    Just some rambling thoughts of some old man in a wheelchair

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
    Larry Weingarten
  • hot_rod
    hot_rod Member Posts: 19,846
    Is the 120 solar tank your only source of DHW? If so it probably has a 4500w element in the bottom for no-sun and evening hw

    So the tank is always maintained by both solar and resistance element

    Essentially you have about 15,000 btu/hr to work with. How large is the floor area?

    Any issues with the solar over heating in the summer months? I assume it is a closed loop glycol system?

    several ways to shut off the floor when the tank drops below about 110F. A differential control, or a 3 way thermostatic valve as a diverting valve

    Ideally you would have a heat exchanger between the potable water in the tank and the radiant water.

    A dual coil solar tank is great for taking a small radiant zone off the upper coil, also
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream