Pre-heating water using biomass for use in combi CH system

[old duffer]

Renewal price on energy starting Aug has risen from £130 to £736/mth so now is the time to look for savings in gas (the biggest cost is heating @ 73% )
My idea is to add heat to the water in the CH circuit before it returns to the boiler so that the boiler can modulate way down and so reduce gas usage.
My idea is to have a direct heated tank (water flow into the wood burner and back to the tank) with the heating return being fed into the bottom of the tank and coming out of the top of the tank.
What things need to be considered for this to be a success?

 

[ianmcd]

Not all boilers can be used this way if you contact the boilers tech help line thwy will be able to tell you if your boiler is suitable

 

[oldbutnotdead]

So a poor man's thermal store or low loss header?
Your tank will have to be metal, well insulated and open vented. This means your heating system effectively becomes open vented (not pressurised), some modern boilers won't operate if pressure is below 1 bar. You could get round this by using a proper thermal store (with multiple coils) or even a standard dhw cylinder (woodburner heats coil via thermal Syphon, small metal header tank, open vented etc required. Heating system return connected where you'd usually have cold in/hot out, that part of the cylinder can be pressurised subject to MIs and may not be DIYable (could be seen as G3 territory).
Pipework also should be all metal- this includes pipework to all rads, placcie is not suitable for use with uncontrolled heat sources

 

[old duffer]

So a poor man's thermal store or low loss header?
Your tank will have to be metal, well insulated and open vented. This means your heating system effectively becomes open vented (not pressurised), some modern boilers won't operate if pressure is below 1 bar. You could get round this by using a proper thermal store (with multiple coils) or even a standard dhw cylinder (woodburner heats coil via thermal Syphon, small metal header tank, open vented etc required. Heating system return connected where you'd usually have cold in/hot out, that part of the cylinder can be pressurised subject to MIs and may not be DIYable (could be seen as G3 territory).
Pipework also should be all metal- this includes pipework to all rads, placcie is not suitable for use with uncontrolled heat sources

Never seen a "poor man's thermal store" so no idea what that looks like. Do you have a link or image of it or better still, a video of it working?
I've also been looking at how low loss headers work but can't see how they attach it to a secondary heat source for use with a combi. Can you send a link for that one as well please

 

[oldbutnotdead]

Low loss header is essentially a neutral point allowing you to combine heat sources. They're used a lot in commercial premises (where there will often be 6 small boilers linked rather than one large boiler). The flows and returns from all heat sources and heat loads will all meet there. Google is your friend here.
Thermal stores are sometimes just a big low loss header, more often they'll have multiple internal coils allowing you to combine heat from solar, heat pump, wood burner, gas or oil burner and keep the heat sources hydraulically separated where it is advantageous (heat pumps tend to use glycol or other refrigerant coolant, you don't want 500 litres of that stuff running round your radiators). Usually the flow and return for the heating will connect to the store as well. Volume wise- depends how hard you're going to drive the woodburner and what the thermal load of your house is. 300 litres as a minimum, 500 litres is good.
You need a large volume of water to store the heat from your woodburner- mine delivers 6 or 7kwh per hour to water at full burn. A small volume of water will boil quite quickly which would annoy your boiler and waste energy.
Your plan is a sort of halfway house. First thing is check your boiler manual, see if it'll run properly unpressurized and whether the control system allows you to preheat the return (you may find that if the return is at 40 degrees or so the boiler won't even fire rather than modulating down, so your room comfort may be compromised). Also check what maximum temperature the boiler will tolerate on the return.
A thermal store plumbed in a conventional fashion would definitely work whatever boiler you have- if you are paying for design and installation payback time will be 5 years minimum. If you are buying firewood or smokeless you will never recover your investment.

 

[old duffer]

Low loss header is essentially a neutral point allowing you to combine heat sources. They're used a lot in commercial premises (where there will often be 6 small boilers linked rather than one large boiler). The flows and returns from all heat sources and heat loads will all meet there. Google is your friend here.reservoir
Thermal stores are sometimes just a big low loss header, more often they'll have multiple internal coils allowing you to combine heat from solar, heat pump, wood burner, gas or oil burner and keep the heat sources hydraulically separated where it is advantageous (heat pumps tend to use glycol or other refrigerant coolant, you don't want 500 litres of that stuff running round your radiators). Usually the flow and return for the heating will connect to the store as well. Volume wise- depends how hard you're going to drive the woodburner and what the thermal load of your house is. 300 litres as a minimum, 500 litres is good.
You need a large volume of water to store the heat from your woodburner- mine delivers 6 or 7kwh per hour to water at full burn. A small volume of water will boil quite quickly which would annoy your boiler and waste energy.
Your plan is a sort of halfway house. First thing is check your boiler manual, see if it'll run properly unpressurized and whether the control system allows you to preheat the return (you may find that if the return is at 40 degrees or so the boiler won't even fire rather than modulating down, so your room comfort may be compromised). Also check what maximum temperature the boiler will tolerate on the return.
A thermal store plumbed in a conventional fashion would definitely work whatever boiler you have- if you are paying for design and installation payback time will be 5 years minimum. If you are buying firewood or smokeless you will never recover your investment.

The whole point is to reduce gas consumption, not replace it. I have a perfectly balanced heating system now so not going to change that.
Coming winters gas bill is going to be £5000 so even If I halved that it still gives a decent ROI.

You keep mentioning pressure as an issue so the simple solution in my mind is to extend the CH circuit with a coil and heat the coil...no pressure loss..
As a plumbing expert, can you explain why that will not work? (as you have not mentioned it I assume it won't work)
So how do I heat the coil as the heat exchanger?
Pot bellied stove @ £95 6kw output. Wrap the coil around it. Its going to be outside the property so what does it matter.
70% of gas consumption is on CH so this would make a big difference.

DHW is used mostly in evenings so needs to have a preheated store.
Copper water water tank with coil inside connected to coil around pot bellied stove.
Main water into bottom, warmed water out of top and into heater for final boost.

Is there a reason why this very low tech simple approach would not work?

 

[oldbutnotdead]

So you plan to wrap a coil on the return of your (we assume pressurised) system ( you haven't enlightened us and it does make a difference) round a pot bellied stove outside and light a fire in it?
It'll work. For about an hour. The PRV on your system will operate once the stove boils the system water (might take more than an hour, your coil wrapped round the stove might capture 20% of the energy, might be as low as 5%). You can then spend £5000 replacing your boiler and any plastic pipework in your heating system (modern boilers aren't designed to boil water).
Heating dhw from your lash-up is far more doable- though again you're describing a pressurised cylinder (mains in bottom, hot out top, coil to heat it)- pressurised cylinders are not DIY. Again the heat transfer from stove to coil wrapped round it will be cr*p. Plus you can't use the same coil as you are planning to destroy your heating system with (hydraulic separation).

Other things to think about. Your potbellied stove will go through a lot of wood to emit 6kwh- from my experience you'll be outside every 30 minutes stoking the thing. If you're buying the firewood it costs more per kWh than mains gas. If you're cutting it yourself it takes time, effort and space (mine goes through about a cubic metre of timber per week in winter- mind, that's heating the whole house with no gas input so you'll be on half that. Except my woodburner is indoors so any energy not going up the flue heats the house whether through the rads or direct).

Annoyingly, blending heat sources isn't that cheap (it is low tech though).

 

[old duffer]

So you plan to wrap a coil on the return of your (we assume pressurised) system ( you haven't enlightened us and it does make a difference) round a pot bellied stove outside and light a fire in it?
It'll work. For about an hour. The PRV on your system will operate once the stove boils the system water (might take more than an hour, your coil wrapped round the stove might capture 20% of the energy, might be as low as 5%). You can then spend £5000 replacing your boiler and any plastic pipework in your heating system (modern boilers aren't designed to boil water).
Heating dhw from your lash-up is far more doable- though again you're describing a pressurised cylinder (mains in bottom, hot out top, coil to heat it)- pressurised cylinders are not DIY. Again the heat transfer from stove to coil wrapped round it will be cr*p. Plus you can't use the same coil as you are planning to destroy your heating system with (hydraulic separation).

Other things to think about. Your potbellied stove will go through a lot of wood to emit 6kwh- from my experience you'll be outside every 30 minutes stoking the thing. If you're buying the firewood it costs more per kWh than mains gas. If you're cutting it yourself it takes time, effort and space (mine goes through about a cubic metre of timber per week in winter- mind, that's heating the whole house with no gas input so you'll be on half that. Except my woodburner is indoors so any energy not going up the flue heats the house whether through the rads or direct).

Annoyingly, blending heat sources isn't that cheap (it is low tech though).

pipes wrapped around a potbellied stove returned 6,260,000 results in 0.56 seconds

 

[oldbutnotdead]

I found several being used as pool heaters. Which will work- very large mass of water, open vented, no fiddly gas boiler in the way. Again highly inefficient and it'd need a pump but if your fuel and time are free.....