Connecting a wood burner to a thermal store

28 Oct 2008
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United Kingdom
I am currently planning the detailed layout of our new house and I would be very grateful for some advice.

The main heating (UFH) for the house will be an air source heat pump connected to a thermal store (either DPS or Akvaterm). We want to have a wood burning stove in the living room, mainly for the real fire effect rather than the heat so I was planning to get this connected to the thermal store to use most of the stove output to heat the DHW.

As I understand it, I need to have some sort of thermodynamically controlled heat dump in case of a power cut. I can fairly easily position a radiator upstairs almost directly above the wood burner so that should get round that problem. The thermal store, however, will be on the same floor as the wood burner and the other side of the house. The pipes to and from the thermal store will have to go up to the ceiling, across and then down to the thermal store.

Am I right in assuming that the loop to the thermal store will be driven by a thermostatically switched pump? Is it possible to have this loop open while the loop to the radiator upstairs is off? (I only want the radiator for emergency use if the loop to the thermal store is not working - I want the output from the wood burner to go the top of the store to provide the DHW). What I would like is that if there is a power cut then the loop to the radiator will open. Similarly, if the return from the thermal store gets too hot, then the radiator circuit should kick in.

I am not intending to install any of this myself but I would like to know if this is possible / standard / horribly complicated (i.e. expensive). If it sounds like a bit of a nightmare then I will keep it simple and just keep the wood burner for the occasional room warming fire.

Thanks very much.
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I am not an expert in these matters, but the logic is that there should be no form of valve, pump or any other type of potential blockage between the back boiler and the heat sink (radiator) so that you are not relying on any device, thermo-mechanical, electric or electronic, to keep the system safe from the dangers of overheating.

I suppose you could have a pump and pipe layout that when operating will divert the flow to the thermal store, but in the event of pump or power failure, the bypass to the heat sink will allow gravity circulation to resume. The gravity pipework is always connected, but when the pump is running it persuades the flow to go to thermal store rather than the heat sink.

If you get what I mean.
What Axel says is good advice.

Usually in these circumstances you would have the thermal store above the wood burner and allow it to be heated by gravity. When the sore stat says store is warm enough then water is pumped to the rads (from the store itself or from the flow and return pipes). In this case you would ensure that the store is of a sufficient size so as never to boil (or it would take somebody fueling the fire like a madman for many, many hours for this to happen).

In your case you are swapping the position of the store and the radiator/s. You are heating the rad and when it gets warm you will activate a pump and send flow and return to store. Problem here is that you will need one or more radiators of a sufficient size so as to be able to provide enough of a heat sink so that the water in the primary circuit cannot boil. What size this heat sink rad/s needs to be I am not sure and maybe somebody can assist here (e.g. 20% of stove kw output so if stove is 12kw boiler then rad needs to be 2.4kw).

Is it not possible though to use the first method of gravity to store and then pump to rads. All you need is a gradual rise in the floiw pipe from top of fire to top of store. I know store is on first floor but if you have a tall store (say six or eight feet) and a fire at two feet then you can achieve a gravity flow straight to the store maybe? I suppose it depends just how far away the store is and how the pipework route is.
Personally I wouldn`t bother. You will need a large loop of 28mm copper for the gravity circuit - incorporating the heat store . forget about rads pumps etc. keep it simple and if you can afford meters of 28mm pipe and get someone to put it in so it will work - a bit like getting a farrier in Cental London- . then go for it.
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If your having a dps store you'll fin it comes with an anti boil to drain operated valve.

As for tappings your better off letting them design where to puthem.

Normally I would pump the stoves ouput and have it controlled through a high and low limit stat ensuring pump holds off till gravity temp is near store temp but also to ensure pump runs if the stor gets to hot if controlled through a timer.
I have a very similar situation at my house: thermal store on same level as log burner. I was going to put it upstairs til I realised it would weigh 450KG when full! I decided that was asking too much for my 17th century floors to hold.

My thoughts are to pump the feed to the thermal store and have a solenoid valve to prevent flow to the heat sink rad up stairs. When energised this would shut off the rad. In the event of a power cut or the thermal store getting too hot the valve would de-energise and the excess heat would be dumped.

Does this sound like a viable option? My back boiler is 3Kw and the proposed rad is 1Kw.
It would be easier to use one of these and more reliable......
A Danfoss AVTA cooling valve!
You could use a normally open motorised valve for both the store and radiator. When the store is calling for heat the store valve is open and radiator is energised closed. Once store is satisifed store valve is energised closed and vise versa for heat leak radiator. In event of power failure both are open to dissipate as much as possible.

Normally open motorised valves are the only type of valve recommended by HETAS for a gravity circuit. Although maybe possible, very unlikely to happen being sring operated returns especially on the same day of power cut if ever :LOL:

Also both would have to fail for you to be in real trouble. The heat leak radiator size is not a particular stated size/ratio but you may be able to get infomation off the appliance manufacturers.

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