Adding boiler stove to system?

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Hi
I have an existing old gas boiler, pump, and vented cylinder. I am looking to feed in a small multifuel boiler to add heat to my 7 radiators to enable me to cut down on gas bill. The boiler I am looking at is a 7 kw to water (5kw to room). My existing total radiator capacity is 8kw, which is warm enough.
My question is, if I have a 1.5kw heat sink radiator in the bathroom above the multifuel stove, could I have it so there is an `option` to turn the pump on when required when the rest of the house needs warming,,, or would the pump need to be constantly running to distribute the heat. my objective is to not have the pump running all the time due to the cost of running it.
Thank you for any advices.
 
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My question is... can heat be exchanged without the need for a pump with reference to the above set up?
 
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Adding a solid fuel stove to an existing heating system isn't as simple as lobbing a couple of pipes and a heat leak radiator in. It's a bit more involved than that. Have a look at www.h2panel.co.uk for a suitable control system. Not only can heat be exchanged without the need for a pump, the regulations say that it MUST be able to operate without using a pump, to prevent an uncontrolled heat source boiling in the event of a powercut.

Worth noting that your 7kW to water / 5kW to the room might not be suitable. 5kW is a lot to be pumping into one room, and you'll need to burn it hard in order to get the 7kW to water as the outputs are broadly linear. Drop the fire to 50% (2.5kW to the room) and you only get 3.5kW to water. Also, woodburners are often more expensive to run than gas boilers unless you have a ready supply of free, dry wood
 
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Thanks Muggles. I dont use my gas fire so I agree I would not want a lot of heat to the room however the location of the stove would direct heat 3 rooms so I agree the 5kw output is ample... oddly most boiler stoves have no less output to the rooms. Interestingly most boiler stoves have a similar ratio (room v water) so I am unsure about how that works. I saw your link and I really dont want anything complicated, only something to add heat to radiators and to the rooms, maybe just a thermostatic pump to kick in. Thank you for advices regarding pump.
I have a supply of dry wood and I would save a fortune on gas, its only the pump that would be costly to run hence my query. However if there is background heat I might save more money by using the pump less.
 
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Sorry but you really need to do it properly or not at all. It's complicated because it needs to be for safety. If you end up turning the water inside the stove to steam, the rapid rate of expansion can cause a steam-powered explosion. The loft tank needs to be replaced with one that can handle boiling water so it doesn't split and scald you if it overheats, and so on and so forth, and as above you're not allowed to rely on an electrical pump to circulate the water in the majority of stoves because that won't work if there's a power cut. The very few that do permit it have a mains pressure cold water supply plumbed into them via a quenching valve which can allow fresh cold water in to rapidly cool the stove in the event of overheating.
 
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Running radiators direct from a woodburner can be a bit of a pain. The return water has to be fairly hot (if you use a pump rather than thermal syphon) to prevent condensation in the firebox. Best way to capture the heat (and to link the systems) is to use a thermal store (big water cylinder with multiple coils and tappings). Significant space required, also significant installation costs.
 

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