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I have a solid wood burner that contains a heat exchanger. There is no guarantee that the combustion can be regulated. This heat exchanger feeds a loop that acts as the source heat exchanger in a hot water cylinder. The loop is pumped (pump not shown in diagram) when the furnace outlet exceeds a certain minimum temperature.

As we cannot control the combustion, it is possible that the loop and hot water cylinder would overheat. I am looking to ensure that we do not exceed a specified maximum temperature.

It is very possible that there is a component that already exists to do this, so please let me know if this is the case (the more analogue the better). In lieu of such a component, I came up with the following diagram:

enter image description here

When the temperature in the loop exceeds the maximum desired, the thermostatic control valve injects cold water into the loop. This creates an excess of water in the loop which will then exceed the pressure managed by the pressure relief valve, which will open and then 'drain' some of the coolant from the loop.

I would appreciate some feedback on this idea. The existing implementation (not my doing) basically has no control whatsoever, and isn't immediately replaceable.

I am totally aware that this is a waste of energy, but its just a safety control - I wouldn't expect to ever trip it.

EDIT: I can see that the more sensible option here, as hinted a few times by others, would be a thermostaticaly actuated valve to release hot water from the hot water cylinder when it exceeds a certain temp which would then handle its own "mixing". I think thats what im going to do for now. I also like the idea of a separate loop that gets activated to discharge excess heat through an alternate sink. Could be handy for keeping a hot-tub, greenhouse, etc, warm - should I ever decide to take on that task :D

Thanks for help everyone. Wish I could accept a combination of answers, as they are all great!

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  • A pressure and temperature valve would be better. Check out woodstoves as they usually state the minimum for that stove. Also there are usually specific local regulations you have to meet.
    – Solar Mike
    Mar 9 '20 at 13:40
  • Thanks @SolarMike - is that a combined valve (as opposed to the individual ones I mentioned)? Im not sure who "woodstoves" are, unfortunately. When i type it into google i get lots of results.
    – Matt
    Mar 9 '20 at 13:47
  • How much circulation (system flow) are we talking about here? Is this woodstove the sole source of BTUs to the cylinder? Also, how many circulators are you willing to throw at this problem? Mar 9 '20 at 23:37
  • @ThreePhaseEel its not high flow by any stretch. Theres probably about 10 litres in the heat exchange loop and the tank is about 100 litres. Wood stove is sole source of BTUs. Im not sure what you mean by circulators, im afraid.
    – Matt
    Mar 10 '20 at 7:17
  • @Matt -- a hydronic system will have something that motivates the flow around the loop (some people call them pumps, but they're more properly called circulators, unless your system lets gravity and convective currents do the work instead) Mar 10 '20 at 11:46
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A woodstove would never be connected to a cylinder in such a fashion.

The only use for the thermostatic valve is to control the output of the hot water tank to the user and a common setting is 46 degrees C.

The hot out pipe of the woodstove should have the combined P&T valve fitted with an expansion pressure vessel to allow for the normal water expansion due to the rise in temperature.

The woodstove manufacturer will normally specify the minimum volume of tank to be used and, most likely, so will the local regulations - perhaps also set by the local fire department.

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  • This isnt a conventional woodstove and it is probably older than both of us combined. It is already connected in such a way to a hot water cylinder, but without a way to handle thermal runaway. This isn't a regulatory issue, its a practical one - and as mentioned it is not immediately replaceable. I am looking for a way to handle this by adding some hardware to the existing solution - otherwise i would just swap in with a thermostatically controlled furnace, obviously.
    – Matt
    Mar 9 '20 at 15:43
  • Thanks - I have found a temperature actuated valve that i will add to the hot water cylinder. This will provide the desired solution without the need for managing the pressure. I looked at PT valves, but couldn't find one that I could adjust the temperature - but it was your idea that lead me on that path, so thanks again! I will likely add a PT valve as well just in case something goes drastically wrong. I will also add expansion vessel to the loop.
    – Matt
    Mar 10 '20 at 7:26
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Look up the installation manual for the HS Tarm OT-70.

In the 70s, these were hooked up to pressurized hydronic systems. To prevent boiler explosions, they were required to be installed with at least one zone having an air-conditioner like thermostat -- above a certain temp, the zone opens, and the entire set of radiators works to radiate heat. It isn't perfect, but it has worked pretty well for the last 40 years.

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  • Thanks - so it would use an exterior radiator as a sink for excess heat? Thats a nice and simple idea. I can bury a coil of pipe under the ground and divert excess heat there.
    – Matt
    Mar 10 '20 at 5:56
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    Actually, it had a zone valve on a regular hydronic heating zone that both opened when it called for heat, and was forced open when the temperature got too hot, thus using the existing house radiators as the heat sink. Worked pretty well.
    – gbronner
    Mar 10 '20 at 20:57
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Is there any other place where you could use this high temperature water? Is this for a residential or commercial building? What is the maximum allowable water temp in the cylinder? An alternate to dumping the hot water through a pressure and/or temperature device would be to install a fan coil to reduce the excess water temperature and not introduce any amount of fresh cold water. Most good wood stoves can reduce the speed of the burn and thus reduce the amount of hot water being heated.

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  • Its for a remote cabin. There is no real use for excess hot water. I was originally thinking to just have a larger thermal store, but its not practical given the situation. A few hours of neglect resulting in overheating the coolant recently, so i am looking for a simple way to safeguard that. Unfortunately, its not possible to regulate the intake, as it is a combined stove, oven and water heater. i.e. when cooking, it is possible to overheat the heat exchanger if the boiler is already at a high temperature. I am being careful with this now, but I would rather not have to think about it.
    – Matt
    Mar 9 '20 at 15:48
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In the 70’s we did install heat loops in wood stoves , they were open tank pumped units.

Why open tank pumped? The system had to be open to prevent a pressurized system because pressurized systems are boilers and boilers are regulated, require steam certified pipe fitters. The systems we used had check valves that allowed water in the heated tank as it was heated water expands and pushes out to a 2nd check valve then cold water entered (the double check valves were much like your coffee pot but instead of pumping up the 2nd valve stopped the back flow. when there was a power failure the system still worked but did not have any real control other than the heat in the stove). Normal operation was temp controlled pump based on 2 temp sensors, one was on the stack and I think the other was on the discharge side outside the stove. The water had to go to a heat exchanger or open tank when used for things like hot tubs and swimming pools (this was the leading use in the San Francisco Bay Area that we installed). My mom had one for over 30 years but when the hot tub was removed so was the heating loop. I do remember the early units were sealed and had temp pressure valves but because of scalding the open tank heat exchanger was later required I don’t know if law or liability issues were the reason but know we did not require a steam fitter as the system was open. The hot water had to enter the bottom of the tank and the recirculating pump was above for the open tank style, on the tube heat exchanger(s) I don’t remember as much we only did a couple of these I think they were sealed but required a pipe / steam license so they were much more expensive.

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