Possibility of using a radiator as a backboiler on wood burner.

[Steve shuff]

Hi All ,
I am contemplating "hanging" a radiator on the rear of our 12kw Stovax woodburner then plumbing this into the CH system.
Our setup comprises an oil fired open vented system serving several rads and DHW tank , we live in the middle of knowhere and our firewood is free.
we are not looking to circulate a great amount of heat round the rads , however a small amount would be enough just to keep the cooler rooms of the house comfortable without having to run the oil boiler.

To me it makes sense if we could pump a small amount of heat around the system and save our oil , the wood burner is lit most days .
Anybody got any thoughts on this id be very interested the hear them , Thanks .

 

[Madrab]

There would be very little thermal transfer between the 2 TBH. They would need to be in direct contact with each other with a thermal conductor in between and even then I'm not sure how the mild steel of the radiator would cope with the excessive temps of the wood burner.

 

[Steve shuff]

Hi Rob,
Thanks for the reply,
I see your point and its an interesting one ,
Something I had noticed is the rear of the woodburner is cooler than the other surfaces and I assume this is because of the firebrick stacked in the rear of the stove casing. So, in theory it may not get too hot for the steel radiator even with a thermal conductor in place .
I think its a project that you could never be sure of unless it was tried.
What would you suggest to use as a thermal conductor I wonder ?

Thanks

 

[Madrab]

I think its a project that you could never be sure of unless it was tried

I would certainly agree with that

What would you suggest to use as a thermal conductor I wonder

The usual would be a paste/liquid or other high thermally conductive medium. In that large specific application though, you would need a flat to flat surface contact with a thermal compound in between to maximise transfer. A ribbed steel radiator may not be the best material, though larger copper tubing, covered in insulation may be better.

 

[cozzmic]

Why not put an approved heat exchanger in the woodburner and link to system if it is open vent?

 

[Steve shuff]

Thanks, that's certainly an option. I have seen a matrix of copper pipe secured to the rear which looked appealing. Tho not as efficient as a heat exchanger..

One would assume the return from the backboiler connects to the main return somewhere near the boiler and the flow connects just before the pump with a thermostat near the backboiler pipework to switch pump on when the woodburner is burning.

 

[ericmark]

At work our boilers are pressure tested and have fusable plugs which will melt if it gets too hot.

Any boiler needs some safety considerations, and I know my parents AGA the copper boiler failed a lot, but the quary tiles kitchen floor would allow the water to flow out of back door and not go further into the house, in the end replaced with stainless steel.

Wood burners need to burn very hot to stop particular emissions, which requires air adding after main burn, then cooled to around 150ºC so as not to waste energy, water cooling is used to do this, but it is rather complex.

 

[Steve shuff]

Update ,
Ok Ive had chance to provide a drawing of my intended addition to the system .
As you can see the backboiler (orange area) will go in parallel with the boiler and the circulating pump will be controlled by a pipe thermostat near the woodburner , This will switch the pump on when the woodburner is lit. The Oil fired boiler will have the isolation switch off most of the time to conserve oil.
The new plan for the backboiler is to have a 22mm copper pipe matrix similar to the attached photo .
If anyone has any thoughts on the circuit i would be happy to hear them , just in case I have missed something. Thanks

 

[JohnD]

I used to have a cottage with a Multifuel stove, plumbed to heat HW cylinder and bedroom radiator by gravity flow. I think it was a Morso.

More efficient and cleaner burning than a woodburner.

Unless you are doing it for the fun of tinkering, I think a purpose-built stove would give much much better results.

I have a relative with a multifuel sized to run whole-house CH and HW, and it is pretty huge

 

[Steve shuff]

Yes a multifuel stove would be more efficient, I agree.
The purpose of my project is to percolate some of the heat around the house , as long as it just takes off the chill in the non lived in rooms that will be fine.
The main bedroom above the woodburner room is currently around 14~15 degrees through the night , whilst the other rooms are around 8~9 degrees, So the intention is to make them a little more comfortable.
When winter really kicks in and we have snow or sub zero temps, thats when we would benefit from the project , I hope .

 

[Harry Bloomfield]

ood burners need to burn very hot to stop particular emissions, which requires air adding after main burn, then cooled to around 150ºC so as not to waste energy, water cooling is used to do this, but it is rather complex.

Yep. Incinerators use a 'afterburner' compartment, to burn of the the emissions.

 

[Madrab]

As suggested ... You just need to ensure there are safety devices and a heat sump - constant flow - when using an uncontrolled heat source like a wood burner, especially if the system is to be sealed

 

[Steve shuff]

Madrab,
Thanks for your input,
It will be an open vented system and am curious as to your term "safety device" are you referring to a pressure relief valve which would be beneficial in a sealed system. Yes, I could put a heat sink radiator in the circuit in case of pump/power failure.

 

[lostinthelight]

Theres a lot of potential there for air locks I would have thought, you may have better results with buying or getting a baffle boiler made or having the loops run horizontal.

 

[Steve shuff]

Yes that dawned on me earlier today.. I could change the design so to remove airlocks..