Solid fuel heating

17 Oct 2022
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United Kingdom
Afternoon all,

I'm planning on installing a solid fuel heating system at home. This will replace an old inefficient wood burner. It will run completely independently from DHW and central heating which is run from a gas boiler to an unvented cylinder.

I think the independent system would be a cheap and simple solution to get heat into selected rooms (bedrooms) when the fire is lit. Also save complications with integrating a solid fuel boiler into an unvented system.

I have a local plummer who will help me install, but I want to do pretty much all the legwork, hence the post.

A couple of questions...

Regulation wise, if it is replacing a less efficient wood burner, and reducing reliance on fossil fuels, can it still be done within regs?

Looking at the diagram below, can anyone see any glaring issues?

Priorities are getting heat into rads as quickly as possible. Most likely not all the rads in the pumped circuit will be in use.

The existing central heating feeds 30 rads and has a large cylinder. Hense why I feel feeding into that system would be ****ing in the wind.

Thanks for your time


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Just an opinion :-

If your boiler is going full chat and there's a power cut - what happens - a 2kw dump radiator won't handle the thermal load

Apart from the above you need a pipe-stat on the flow out of the boiler to turn on the pump when the boiler gets hot. You also need some form of 'ineligence control' to prevent excessive pump cycling since the boiler outflow will dip as you draw hot water away from it.
Thanks Jackrae,

My thoughts were exactly the same on the heat leak. However the only advice I could find was for the heat leak to be 10% of the maximum output of the stove (16kw, therefore 1.6kw). If that's the case 2kw would be well in limits. However as I say, it doesn't seem like enough to me either.

Absolutely agree with the stat. I plan for the pump to be variable so that it doesn't pull too much from the rest of the system
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No reason why you shouldn't make the heat sink bigger. You can also isolate it with a zone valve, normally open energise closed and controlled with a pipe stat, so that you can concentrate the heat where you need it when operating normally.
Bit of a bore having to light the fire every day to get heat into the bedrooms.
Unless you leave the fire banked all day you're looking at 30 mins absolute minimum to deliver any useful heat to the rads- you'll then have a pile of wasted energy from the coalburner once the rooms are up to temperature. Running direct like that with small system volume will make it very hard to control the burn so you get adequate heat without kettling or boiling, especially with such a monster coalburner.
Much better off to stick a thermal store in the system- yes you can integrate your pressurised gas boiler (coalburner heats store direct, rads run direct from store, gas boiler heats store via internal coil. DHW from store either plate hex or additional high surface area coil in the store).
Main running advantage of thermal store- you can have a good solid burn in the afternoon/evening, store can then run heating for evening and (if you've sized it right) morning as well.
You'll also see some useful gains from zoning the system- 30 rads will get a bit pricey for WiFi TRVs but it does make targeting the heat much easier. You can link control systems as well (might need a relay or 2)- my gas boiler is set to not respond to call for heat from the store if the woodburner flue temp is above 150 degrees. I can override this if I want to...
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Couple of edits in that first post. Your design with the heatloss rad means you've programmed in that 2kw energy loss forever since no other part of the system is running on the thermal siphon set up by that rad. Usually you'd have hot water cylinder (or thermal store) below the heat loss rad so in normal running most of the energy goes into the store/cylinder by siphon, the heat leak maintains thermal circulation even when the cylinder is at the same temp as the boiler.

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