I am looking to combine a pellet stove water heater with a solar water heater. First of all I wold like to apologise for the ugly drawing, though I think it represents my idea well enough. The whole thing will be a closed loop cycle or two cycles rather. What we are looking to do is use whatever energy we can collect from the solar panels in the winter time and use it to aid the heating of the boiler/buffer vessel. The main source of energy will be the pellet stove. My question is what can we do in order to use the solar energy first so we don`t end up stealing energy from the boiler. P.S. The drawing is just the direction of my thoughts. Please feel free to butcher it :) enter image description here


There are a lot of things that can go wrong here, and for a small system, the expense is probably not worth the risk.

The primary coolant loops for the hydronic pellet stove and the solar collector are totally isolated and neither should go to the domestic water tank.

If you buy a hydronic pellet stove, get one with the domestic HWH exchanger installed in the buffer tank. Domestic water is routed from the domestic tank to the buffer tank and back to the domestic tank using a pump and control circuit.

Buy a hydronic solar collector kit with a remote heat exchanger. Domestic water runs from the domestic tank to the exchanger and back to the domestic tank using a pump and control circuit.

You need a pressure regulating temperature control mixing valve on the domestic tank outlet. This effectively means you need a three (or four) port tank.

You need domestic water at four points - makeup water on the pellet primary, makeup water on the solar primary (fitted with shut-offs and check valves), supply water to the mixing valve, supply water to the domestic tank.

You need two decent little circulating pumps that are rated for 250F. Many aren't rated for hot water, so be careful.

You probably need a excess heat dump outside for the solar collector. People don't realize how hot these things get. They can and do blow the pressure-temperture blowoffs on the domestic water tank.

You want to install pressure control valves on both primaries that ensure the pressure in the primary loops is lower than the domestic tank pressure.

All components need their own anodes and these need to be checked regularly. These systems can eat anodes faster than a typical electric HWH tank.

Doing this in my head, you need:

  • Five check valves

  • four drain valves or shut-offs with drains.

  • Two extra thermal isolators more than the port count of the tank.

  • About 11 valves on the domestic water piping

  • Two circulator pumps

  • Pressure controls for the primary loops

  • Temperature controls for the domestic loops to turn the pumps on (sometimes can be slaved to the controls of the stove and solar collector kit).

  • An air-gapped drain for each primary loop.

  • An additive system to get the glycol or whatever into the two primary loops.


So you need a tank with a solar coil at the bottom and a mid coil for the pellet stove to add heat.

Decent tank manufacturers can do this, but you have to tell the tank manufacturer about the recovery time etc

I am not going to do a range of diagrams on how solar panels can be used with a boiler - too many options single tank, two tanks, solar tank feeds on demand boiler directly etc etc etc

Do a google search on "solar panels and boiler" and you will see the range: but each has advantages and disadvantages, but they will all be better than what you showed.

  • $\begingroup$ We have a tank with two coils hence the two cycles. To further my thoughts, is there any kind of automated temperature controlled valve where it wold open at say 40+ degrees and close when below? Yes there is :D So do you think it wold work? I`m planning to put the temp valve between the boiler and solar panel. $\endgroup$ Mar 9 at 20:04
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    $\begingroup$ Don’t connect the boiler to the solar panel, boiler to tank, solar panel to tank and boiler only cuts in when tank not heated sufficiently by solar. $\endgroup$
    – Solar Mike
    Mar 9 at 20:18

I suggest this setup:

  • Pellet stove directly heats buffer tank, radiators are directly supplied from buffer tank
  • solar panel indirectly heats buffer tank (heating coils low in the tank) - you'll probably want a glycol-water mix for frost safety
  • water tank is indirectly heated from buffer tank.
  • radiator heated from buffer tank

To make the above a bit clearer, this means three media that are never mixed:

  • water-glycol in the panel and coil
  • potable water in the domestic water tank
  • water anywhere else

Be aware that a large, warm potable water tank is a legionaires disease risk, you'll want to heat it to 70°C for ~20 min or so every once in while.


Thanks a bunch for the input everybody. I`ll probably mix and match as from what you guys suggested as I see fit after a bit extra research. FYI the domestic tank I am using has two separate coils for heating the water in it and as an addition it has an electrical heating element so there is no threat of legionaires. The water in it will come from the public water supply for now.


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