I am looking to heat my home as currently I have no system to heat the house.

I have a 300L (65 gallon) thermosiphon water solar heater that heats sanitary water. My house is built on a platform so it is easy to add plumbings underneath as everything is accessible. I need to heat 2*45m2 (480 sf)rooms.

I am thinking of installing 2 hydronic fan coil heaters and somehow use the excess hot water from the solar heater tank to heat them.

Is this a good approach? How do you use the water from the solar heater tank to heat the closed loop system of the hydronic fan coil heaters ?


  • how hot does your solar heater get?
    – JACK
    Commented Oct 11, 2022 at 21:19
  • @JACK it gets quite hot, up to 90 degrees C (190 F) on sunny days. But I use a mixer on the exit to mix it with cold water as the sanitary water doesn't need to be that hot. It's sunny where I live, winter has quite a lot of sunny days as well, however it does gets cold in night. I haven't experienced it yet during the winter yet so I can't tell for sure how it will be
    – Michael
    Commented Oct 11, 2022 at 21:27
  • 1
    The first thing I would do is get a few estimates from the local shops. This will give you several options and the most important thing the BTU needed to heat your home. Then find out how many days you will not have heat, this will help you size your reserve heating tank. Consider using an auxiliary boiler of some time to keep you warm on the cloudy days. You can also expand your solar collection system to get more heat. At this point you will have a good idea of what you need to accomplish what you want.
    – Gil
    Commented Oct 11, 2022 at 21:58
  • Does it get below 0°C when it gets "cold at night?" - if so, plumbing under the platform may freeze.
    – Ecnerwal
    Commented Oct 12, 2022 at 2:16
  • @Ecnerwal it might, not often, but I do put the PEX tubes in insulator conduit
    – Michael
    Commented Oct 12, 2022 at 10:24

3 Answers 3


Those fan coils are supposed to be used with a heat pump for heat and AC. What you're proposing to do will only give you heat so you might be better off with just hydronic baseboards. You'd need two circulating pumps, one for each room and attach to the in and out from the solar tank.

  • I thought they work as a hydronic baseboards only that they also have a fan to push the heat out more efficiently. So basically they have a hot water entry, cold water exit and it's a closed loop system, simply put
    – Michael
    Commented Oct 12, 2022 at 10:28
  • @Michael You're right. What I was getting at is that since they are part of a complete designed system, you'll pay a whole lot more for the fan coils than you would basic hydronic baseboards
    – JACK
    Commented Oct 12, 2022 at 12:09

Normally a heat exchanger is used to separate the closed-loop recirculation and the potable hot water. IF all the parts contacting the liquid in the heating path are potable-water safe, you might not need to do that, but it's the usual way.

Whether it will work is going to depend on a lot of unknowns - winter water heating effectiveness, temperature, insulation (or utter lack thereof, if the climate is one where it doesn't freeze and "nobody bothers to insulate" or insulation is hard to even buy) - it's rather easy to use a LOT more heat for heating a building than a hot water tank stores, so you may be disappointed unless the climate is mild and the sun still shines a good deal in winter.

  • So, a heat exchanger will involve having a water tank with a heat exchanger in it, right ? In that case, I will have to pump water from the solar heater tank ?
    – Michael
    Commented Oct 12, 2022 at 10:30
  • There are different types of heat exchangers - plate style (external to any tanks) are common these days for domestic size heat/hot water interactions. One or two pumps are pretty much required for any type.
    – Ecnerwal
    Commented Oct 12, 2022 at 12:40

Try this link it will give you a lot of information especially from the links contained within. https://www.energy.gov/energysaver/solar-water-heaters You need to determine how much heat you need in your area for your home. The home uses the same BTUs regardless of how it is heated so use the tables for a furnace or maybe a friendly contractor can look it up for you. Big hint insulate well especially the windows. My hydronic system is not solar but it is potable water. I put pex on the floor and covered it with cement and ceramic tile. I then sized it for 180F hot water then doubled the amount of tubing and zoned it. I put a small pump on the outer circuit and it runs when the main loops are not when it is below 55 outside. It works very well and the room temperature is stable My boiler is a hot water heater rated for heating. Surprise when I purchased it the higher temperature ones were no longer available. I am very happy I added tubing. It works great in a 4 seasons room and basement in Michigan.

The basement has fin tubing in the ceiling and the loss goes up through the 'roof' and into the main floor. That had a big impact on the furnace it now runs a lot less and cycles in sub zero days. If you plan on putting in a fireplace you can get hot water exchangers for it as a heat source when you have a spell of non sun days. I found a ~75 gal 180F hot water heater rated for heating, it is in the crate in my garage as a replacement.

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