back boiler capacity for underfloor heating

[edintheshed1]

hello.

i've searched lots but no luck so hopefully someone with more knowledge than me can help!

I have:

30 litre capacity underfloor heating system
14kw wood burning stove
72msq floor area x approx 2.5metre floor to ceiling

What capacity of back boiler will i need for this to work efficiently?

i am not using any other type of heating sysyem.

I live in france too so the rules and regs are pretty much non existent! so no need to worry about them.

cheers, ed

 

[possum_jenkins]

Wont happen im afraid! the system needs to be vented if using a solid fuel back boiler as there is no effective way of controlling the temperature of the water.

 

[edintheshed1]

there must be a way around it? any suggestions? could an expansion vessel or pressure release valve not be used to release the pressure created by expansion. surely the temp could be controlled in a similair way to a boiler?

any help very muchg appreciated.

 

[edintheshed1]

just a quick thought. the underfloor heating is basically a giant radiator, can it not be trated in the same way as a back boiler heating central heating?

 

[muggles]

You'll need to do it on a low pressure system with a tank in the loft, as has already been (kind of) suggested. However, you do need a way of dumping excess heat, otherwise you could boil the water in your system, as back boilers aren't terribly controllable. If you have a hot water cylinder, consider connecting the back boiler into that as well to heat your water, you'll need zone valves to control where the heat's going but properly set up you should be able to get a warm room and hot water. I'm assuming you're heating water with an immersion at the moment?

 

[Deleted member 105431]

just a quick thought. the underfloor heating is basically a giant radiator, can it not be trated in the same way as a back boiler heating central heating?

no. underfloor htg operates max 50 deg c!

 

[Corgigrouch]

just a quick thought. the underfloor heating is basically a giant radiator, can it not be trated in the same way as a back boiler heating central heating?

Try standing on a radiator in your bare feet!... As has already been said, vented system and blending manifolds for the underfloor to keep it down to 50c.

The French have standards too, though I'm not sure if they would care if one of those idiotic "Roast Beefs" was to blow himself to bits.

 

[edintheshed1]

thanks for your help. what you have said already is the conclusions we have come to, but it helps to have them confirmed.

were going to use a sealed sysem with an adjustable pump so we can tweek the flow rate, expansion vessel and pressure release valve just in case. The manifold has adjustable valves so each room can be independantely tweeked. (only 3 circuits)

why do we have to use a vented system?

we were then going to have thermastatically controlled off take to re route the hot water to a heat exchanger (big tank full of cold water) if the system goes above 50 degrees. If it produces far more heat than expected this heat exchanger will be used to heat water. if not it will be used to dump heat (seems a shame to do that though).

at the moment the hot water will be run off an immersion heated as a seperate system.

I'm assuming by the lack of info available this is not a normal way to do things, but it surely can be done?!

what effect does the size of the back boiler have on the heat output?

i have a 30 litre system and a 14kw stove. should a 15L back boiler work? or is it better to have it smaller or larger one?. i will make the back boiler myself.

ed

ps. what makes you think i'm roast beef. i said i was french!!

 

[Norcon]

edintheshed1 wrote

why do we have to use a vented system?

Its normal practise in the UK to vent a solid fuel system as they are classified as uncontrolled burning devices.
However many of the gasification wood boilers are installed as sealed systems here in the UK as they have sophisticated controls in place to prevent overheating.

 

[Agile]

Which part of France are you living in?

UFH output is usually about 50 watts per m² ! Its unlikely to be enough heat output, many smaller rooms need 100w/m²

Tony

 

[Norcon]

Agile wrote

UFH output is usually about 50 watts per m²

Output is based on 4 factors. Pipe spacing, flow temperature, floor covering and target room temperature. A combination of these can vary outputs between 18w - 216w per sq/m as specified in BS EN 1264 performance tables for 16mm (OD) PE-PEX/PE-RT uf pipes embedded within 75mm floor screed.

 

[Corgigrouch]

ps. what makes you think i'm roast beef. i said i was french!!

If you would care to read what you have written, you stated that you live in France.. Forgive me if I am wrong, but I don't think that you have to be French to live there.

Sealed system = Bad idea, but then again we have advised the best we can, on your own head be it.

 

[edintheshed1]

yeah yeah yeah, your right. i am roast beef really!!

i think i'm going to have to do some serious pondering on this one! thanks for all your help you've been extremely helpful.

cheers, ed

 

[Onetap]

why do we have to use a vented system?!

It's much safer. If it overheats (quite likely) the excess heat is dissipated in producing steam that blows out of the open vent.

In a sealed system, you're relying on mechanical devices (the safety valve) to operate to allow steam to be discharged. Safety valves can fail closed; I've seen several that had in my career. The excess heat then cannot be dissipated and goes into increasing the pressure in the system until something is overpressurized and fails. This is an explosive release of pressure as in demolish walls, kill people, etc..

Some wood burners are (I think) cheap grey cast iron and will not withstand any pressure.

i will make the back boiler myself.

You shouldn't; there're companies who have been making boilers for years and your best efforts will not compare. It's nothing to do with the boiler capacity in litres, more with the heating capacity in kW and surface area.

You'd be best advised to link the back boiler to a large thermal store and then have a secondary variable temperature heating circuit serving the UFH. The UFH must have low flow temperatures, the wood burner must have high (56+ degC) return temperatures or you'll get back-end corroison and excessive tar deposits, chimney fires, etc. It's the only way to go.