water hammer noises from stove

[Matt_matt]

Hello, I have a wood buring stove that has an integrated heat exchanger and is linked to the hot water tank and the radiator in the bathroom. I understand that the way it works is that it heats the water in the tank until a particular temperature is reached and then a three way valve swaps the flow to heat the radiator. The system is un pumped (so that it will work even in a power cut) and has a small header tank in the loft.

The system works well but the issue is that once it is up to temperature, and every so often after that, there is a lot of banging from the pipework and the pressure relief valve that is next to the fire actually blows and a small amount of water splashes out (only about a tea spoon full).

What could be causing the water hammer type noises and what can I do to fix it? Any advice would be great.

Thanks,
Matt

 

[Robtheplumb]

Sounds to me that you are producing to much heat for your system to handle, Do you have a radiator as a heat dump in the system? how big is your pipe work? Where is the cylinder in relation to the wood burner? Are your other radiators pumped?

 

[Matt_matt]

There is a hot water tank and one radiator as a heat dump. The pipework from the stove and into the water tank is 22mm and then 15mm into the radiator. The tank and radiator are both above the stove (upstairs) and the stove is downstairs. The other radiators are on a conventional heating system and so the water is pumped around with a conventional pump. The stove is a 7kW stove and should be the right size for a water tank and 1 radiator.

 

[Renewable M]

Energy in (from stove) is exceeding losses or ability to circulate thermodynamically. 22mm pipe from the stove would be better at 28mm. The 'heat' would find it 'easier' to get away.

We have a 3kw back boiler direct to 300 litre thermal store, and the temperature starts racing up when lit. We have to dump heat.

7kw it would seem is overwhelming the systems ability to lose it.

 

[Matt_matt]

Your suggestion sounds about right. I have just looked at some stoves on the net and it appears that a 7kW stove can heat a water tank and 2.6 600mm x 1000mm radiators. In our case it's fitted to the water tank and then one 600mm x 400mm radiator in the en suite. I've attchached a sceen shot of the web site I found. The stove I have was fitted by the house builder, the house was only built 1 year ago, because I asked that they fit one and plumb it in as part of the house buying negotiations. It has meant that we didn't need to turn the heating on until much later than otherwise and even now we are only using the boiler for central heating as the fire does all of the hot water.

The question now it what to do next. The options are; replace the stove for one with a lower rated heat exchanger - this seams like a shame to waste the energy that is clearly being produced; fit a second radiator to the system besides the one in the en suite - this would mean putting in another radiator in another room and would be a mess; link the stove into the central heating system to dump the rest of the heat and assist the boiler to warm the other radiators - this would be the best solution but I don't know if it would be possible.

 

[Matt_matt]

I've just been looking at the system and realise that the central heating circuit is a sealed system, with no header tank, where I just have to turn on a valve to maintain pressure in the system. This means that connecting in an un-pumped circuit would no be possible... unless...

The boiler is an electric boiler in the airing cupboard along with all of the pipework from the stove and so is it possible to fit a cross flow heat exchanger immediately before the boiler inlet? This would pre-heat the return water before it enters the boiler and reduce the need for the boiler to heat the water so much, if at all.

 

[Renewable M]

dont throw away all that cheap energy from the stove, in this age of high fuel prices.

Simply change the indirect hot water cylinder for a thermal store. Lots of hot water, and the stove contributes to your heating too. They are being fitted like they are going out of fashion.

heres an example, just scroll down the page

Obviously yours wont have the solar. any questions, just ask. these have been standard on the continent since the 70's but we've always been gas and oil rich, so never cared for energy efficiency until about last thursday afternoon. All of a sudden we're having to play catch up.

Your sealed circuit for the heating would come off a correctly sized coil inside the cylinder. viola.

 

[Matt_matt]

So, correct me if I'm wrong, the thermal store is like a very well insulated hot water cylinder with say 3 input coils and a couple of output coils. The store is maintained at, for example, 70degC by heat from the input coils which could be fed from stove, solar, and conventional boiler, and the output coil supplys the radiators. The water in the thermal store, as well as being the medium for transfering energy from one circuit to another, doubles up as the hot water to the taps in the house. The control circuit simply maintains the setpoint temperature (in my example 70degC) by switching on the conventional boiler only if required when the energy from solar or stove are not enough. What a perfect solution for obtaining energy from many sources. My hot water tank is a standard 210litre tank with 2 input coils. One fed from the boiler and the second labled 'solar' and is fed from the stove.

Thanks for your help, I wish I'd spoken to an expert before I had the system installed and then I could have had the correct thing fitted.

 

[Renewable M]

in a nutshell, you're about right. any other questions, just ask.

 

[stevetheplumber]

dont throw away all that cheap energy from the stove, in this age of high fuel prices.

Simply change the indirect hot water cylinder for a thermal store. Lots of hot water, and the stove contributes to your heating too. They are being fitted like they are going out of fashion.

heres an example, just scroll down the page

Obviously yours wont have the solar. any questions, just ask. these have been standard on the continent since the 70's but we've always been gas and oil rich, so never cared for energy efficiency until about last thursday afternoon. All of a sudden we're having to play catch up.

Your sealed circuit for the heating would come off a correctly sized coil inside the cylinder. viola.

im a great supporter of thermal stores ive got a old one i salvaged from a job in my loft coupled to a open flue vaillant which is 20 years old and running great instant heating lashings of hot water at high preasure and no mains fed regs to get involved with