Sorry, another query. We are planning on using a heat store with a gas system boiler to top it up. The boiler manufacturer has said that we should use a coil to heat the heat store, but the reasons were not very clear, i.e.:
- it stops the boiler water mixing with the domestic hot water (but the domestic hot water is heated indirectly using a coil).
- that's the way they usually do it.
By contrast, the heat store company shows a boiler connected directly (see attached). Can somebody explain why one might need to use a coil?

Yes, an air source heat pump. We're not going to have one (at this stage), but it seemed that the principle in the diagram is similar to ours. We'll have the following inputs: stove (direct), PV, gas boiler (direct/indirect?) and the following outputs: domestic hot water (indirect), underfloor heating (direct), radiators (direct).

Ok, I think one benefit is that it helps with the stratification to have an indirect boiler supply. Any thoughts?

Posted By: barneyA further point is that it's very difficult to get a sensible return temperature…

Isn't that why they have a mixer valve on the return to the boiler? That's actually easier to arrange with a direct rather than indirect system, I'd have thought, as there's the option to have three or more tappings from the store if necessary.

Maybe it's to do with the size of the pressure vessel in a sealed system.

If the boiler is indirectly connected there will be very little water in the boiler/heating coil system so you can use a small PV. If you include all the stored water plus the radiator/UFH circuits you will need a much bigger, more expensive, harder to source PV.

I don’t like that setup; you have UFH that would normally result in the gas boiler condensing 100% while running the heating. By adding the thermal store, you require the output of the gas boiler to be 60c (or over), so that your DHW coil works, hence the gas boiler will condense very little.

I just don't believe wood burning stoves are worth the effort if mains gas is on site, and can't see most people lighting them enough.

But if it was a modem wood boiler and the owner had a joiner company creating waist clean wood on site, then my views would be VERY different.

For the OP system I would want two thermal stores, or a thermal store with two clearly defined section, both with a boiler coil. Then the boiler can be set to heat the bottom section to 40c for UFH and DHW preheat, and the top section for 60c, hence letting the gas boiler condense a lot of the time.

(The other option is not to heat the thermal store with the boiler, just consider the thermal store to be a 2nd heat source, connected to the boiler and the rest of the system with a low lose header.)

But if it was me, I would JUST have a simple gas boiler and a normal DHW tank (for the PV), spending the money on low temperature radiators, so the radiators can run at the same temperature as the UFH.

@Peter, it looks like we'll have a reasonably reliable supply of wood for a few years yet. We're also planning on having gas installed as a backup. In terms of future options, we'd like to leave our options open to use more renewable sources when we can afford it, hoping that prices will come down and we'll have more money in the future.

ComeOnPilgrim If it is mains gas and even if you don't have to pay for the wood, just work for it, then

Posted By: ringiI just don't believe wood burning stoves are worth the effort if mains gas is on site,

And I agree with

Posted By: ringiBut if it was me, I would JUST have a simple gas boiler and a normal DHW tank (for the PV), spending the money on low temperature radiators, so the radiators can run at the same temperature as the UFH.

But I would I would keep the stove as a dry stove to supplement space heating and for aesthetics.

@ComeOnPilgrim If its mains gas it would be a big cost saving just to have a normal DHW tank and a dry wood stove. The price difference would put in lots of extra insulation! Over heating is not an issue with a dry stove - you just stop putting wood in to it

Posted By: ComeOnPilgrimWe'll have the following inputs: stove (direct), PV, gas boiler (direct/indirect?)

For both the stove (assuming one wants one feeding to water) and the PV a heat store is part of the solution.

We install all manner of heating systems gas, oil, LPG, biomass, solid fuel and in our experience if you have gas, just keep it simple and use it either via a combi or a cylinder, weather comp the circuits and oversize the rads if you want to, job done.

We do link-ups but to do them properly the costs are high and usually won't be recovered, a dry stove and an immersion gives you back-up to gas. A thermal store unless it is designed for a heat pump at the outset is unlikley to work well for a variety of reasons and heat pumps are notoriously difficult to combine with solid fuel for various reasons. Happy to elaborate if required.

Don't underestimate the labour needed for solid fuel and the volume of wood required if it is to be a primary source albeit backed up by gas.

Is suspect the coil for the boiler referred to in the OP is as others have said becuase usually a boiler is sealed and a stove is vented, there are sealed stoves and vented boilers and different ways of separating them. If you do coil the boiler be sure that it is rated to the output of the boiler and for the correct delta T at max output (20 degrees for condensing boilers).

Gas is so simple and the extra cash as said elsewhere could go into insulation and controls to reduce lifecycle expenditure.

CWatters gas boilers can take very low returns without issue 25 and lower, stoves, oiler and biomass boilers need back end protection in so,e format to prevent premature failure of the jacket.