Wood burner with back boiler

[HandyMart]

Any advice on using the back boiler in a wood burner to augment an existing expensive to run oil fired Rayburn heating system? Has anyone done this? What would be the best solution?

[Adrian Brentnall]

It's another of those 'it all depends' situations.....

Firstly - what sort of woodburner do you have - and what sort of heat ourput is claimed to the back boiler. Bear in mind that you'l only get the claimed outpur with the woodburner going 'flat out' - which may make it uncomfortably hot in the room which contains the woodburner.

AFAIK, there are two systems for 'coupling' boilers together. The problem to to make sure that the 'cooler' of the two boilers doesn't end up looking like a radiator, and taking heat from the other boiler, rather than contributing heat to the system.

One system is a metal tank that sits underneath your existing hot water cylinder - full details here....

a discussion about its use here

The other system was a cube, about 1ft per side, which contained a bunch of motorised valves - and acted as a kind of 'changeover switch' between the two heating systems - controlled by thermostats. Can't find a reference to it at the moment.... sorry

Both systems will involve a fair bit of re-plumbing to retro-fit....

If your central heating system is pumped (likely) then don't forget that your woodburner will need a gravity-fed radiator 'somewhere' that can dump heat in the event of a mains electricity failure. We lost the mains back in January - by the time we'd got back from walking the dogs the 'heat dump' radiator was far too hot to touch, the system had boiled over, and there was a very 'hot' smell about the place. Luckily, I've got one of those 12v to 240v inverters - so we were able to run that from the car until the mains came back.

ISTR that Dunsley were very helpful when I contacted them....

A Google on 'Dunsley neutraliser boiler' will give you hours of reading pleasure

Good luck !

Adrian Suffolk UK

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[HandyMart]

Many thanks Adrian, some good starters here.

Martin

Adrian Brentnall wrote:

[Adrian Brentnall]

No problem - if you need to chat about the project then feel free...

Adrian adrianambquality.co.uk

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[Rick]

Have a look at a Dunsley Baxter Neutraliser.

Rick

[noelogara]

Your back boiler takes the heat out ot your fire and you end up with a dead fire that you have to sit over to get warm. At the same time your radiators that are taking the heat from it are only lukewarm and never really have any serious heat output. Keep your oil fired system as is for heating the rads if you want real heat in them.

It would be essential to have your wood burner standing in the room rather than recessed in a fire place. If its in the fireplace you lose a lot of the heat while vice versa.

[Dave Liquorice]

Bung more wood in it and open the damper...

Any heat is better than no heat when the oil boiler breaks down or there is a power cut. And when the wood burner is lit you won't burn as much (expensive) oil.

Not quite sure I follow the logic. Yes the walls will take some of the heat to start with but once warmed up they give it back. Thermal inertia is wonderful, we have an 2' thick solid stone wall in the middle of our house, that really does help keep the house temp even in the long term (days rather than hours). Takes in heat during hot summer days to keep the inside cool and stops the place cooling out too much during the winter. The snag is that if you do let the house go cold it takes literally days to warm back up...

[noelogara]

speaking from experience Dave you would be bunging wood in every half hour and still getting little heat if you have a back boiler in it.

To answer the other matter, if the stove is placed within the chimney place rather than outside it, a lot of the heat just goes up the chimney. If the fireplace is sheeted over with tin flush with the wall, with a hole for the flue so that the whole stove is actually standing in the room, then all the heat is radiated within the room. another common mistake many people make is to light the fire on a grate and clear out the ash every day. I burn wood on a bed of ash and only clear it out when it rises above the front plate, that is about 4 inches. It has to be cleared out about every three weeks and burns better on a bed of ashes which hardens. There is an art to using wood burning stoves and they are a terrific fire place.

[robgraham]

It must be something to do with 'Noelog's' stove that he loses heat to the back boiler- my experience matches that of Adrian that driving the stove up to it's capacity output overheats the room it's in.

I've got a Dunsley Neutraliser to marry an oil CH system and the wood burner. An interesting plumbing exercise as it's like some octopus with

9 pipes going to it.

One thing that you do need to look at it is a black box that switches off/down the oil burner if the wood stove is hot. And do remember that you need to have a lot of wood continuously and the equipment to process it.

If you want anymore info and experience I'm more than happy to pass it on.

Rob

[noelogara]

I'm only speaking from my own experience. Thats why I dumped it. If you think that you have a box of cold water being pumped through the heart of your fire most of the time it takes the heart of the fire away. perhaps if the room is small you may not notice too much. I now have a lovely big wood burning stove and it heats quite a big room comfortably even on the coldest day of winter. Now if I had a box of cold water being pumped through it continuously I would probably have lukewarm rads everywhere and a poor fire. Of course there are variable factors like piling on firewood and adding coal, stopping draughts etc but for consistency the wood fire without a boiler is best by far. Then Handy Mart would have to spend a lot of money putting in a new cylinder with two loops and make the boiler. It would be a waste of a lot of money and he would curse the day he put it in.

[Dave Liquorice]

I suspect that you are expecting too much from the back boiler, to supply

15kW of heat you are going to be bunging wood in fairly often, the energy has to come from somewhere. You also mention pumped rather than gravity, I can see that overcooling the fire as well.

I'll be putting in a wood burner at some stage here with a boiler but I don't expect it to heat the whole house (our 38kW oil jobbie struggles when it's -5C and blowing a gale) but it should be able to keep the chill out and will be completely passive, no pumps or motorised valves.

Ah an open based chimney, that seems like a bad installation to me. A properly installed wood burner really needs an insulated flue to stop the gases condensing inside the chimney. This implies that the base of the chimney is "sealed" from the room.

Agreed wood burns best on a bed of ash. Coal on the other hand doesn't. I've just cleaned out our open fireplace after burning the cut up remains of two old sofas. Lots of heavy wire staples, castor sockets, screws etc left on the grate enabling a nice bed of ash to build up. Fire burned great on that, doesn't now. B-(

[HandyMart]

Thanks Adrian. It is possible the project may simplify to replacing the DHW cylinder with a twin coil one and only using the wood burner to heat the water via the second coil. This still gives rise to some questions: Would a heat dump radiator be required or advised? Would a simple gravity system work OK? Should I have a separate header tank or could the existing one for the oil fired system be used? What are the considerations when selecting a twin coil cylinder?

Martin

[Adrian Brentnall]

HI Martin

Only trouble there is that you'll quickly get to the point where your DHW is 'hot enough' - and then what are you going to do with the 'surplus' heat....? ((On our system, with a fairly crude control, the first 30 mins to 1 hour after the stove coming up to 'pump on' temperature (about 45C) has the boiler output directed to DHW heating, via a stat on the tank. After that, the circulating water runs through the radiator circuit.))

What's the rated output of the boiler ?

It's somewhere for the 'spare heat' to go - after you've got the DHW tank as hot as you want it....

If you do that you'll maybe find it difficult to 'turn off' the circuit....?

You might want to work on the two systems independantly...? Could use one tank and a stop-c*ck on the feed to each system..? maybe..?

Don't know. For our system I just specced the largest possible tank to fit into the space available - and the manufacturers made the tank up for me with 2x copper coils.... Solar goes to the lower coil, woodburner goes to the upper coil.

Hope this helps

Adrian Suffolk UK

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[Dave Liquorice]

Agreed, even a small wood burner is quite capable of boiling a cylinder of water in a few hours.

Aye, one thing you don't want is you boiler boiling, you must be able to get rid of the heat and it must be fail safe. ie the heat must still be capable of being dissipated when mains power has failed and backup power systems have run flat or out of fuel. When the power goes is when you really need the wood burner to be on to provide heat for the house. Having to shut it down to stop it exploding strikes me as just plain silly.

IMHO the only way to reliably achieve this fail safe mode is with a gravity loop.

Isolation valves that can be locked open on the wood burners gravity loop would enable the different primary loops to be isoloated for maintenance if required.

A Dunsley Naturaliser is the way to go but from my investigations of the device it is nothing more than a sealed empty box where all the loop feeds and returns (oil boiler, wood burner, cylinder and CH) come together.

[AJH]

I think it's also the only legal way to go with a batch loaded wood burner.

AJH

[Dave Liquorice]

FECK, are the bar stewards putting "comon sense" regulations on wood burners now.

[AJH]

Part J isn't it?

As I drove to work I realised I was in error, I have been "familiarised" with a domestic, batch loaded, pressurised, wood burning system with a CE mark. I know it cannot legally have been installed and self certified but I did not see, or ask to see, the building control officer's certificate ( and I wouldn't know what one looked like if I were shown one). I'm happy it was fairly safe as it had the thermal inertia of two tonnes of water to absorb any runaway.

AJH

[HandyMart]

The proposed wood burner is of the "chuck it in one at a time" variety and would not be pressurized. Batch loaded is something else I think and I suspect it's the pressurized aspect that gives rise to regulatory controls?

Martin

[AJH]

That's somewhere between batch loaded, where you fill the stove, light it and let it burn out, and metered like a pellet stove or wood chip boiler with an augered feed.

I was referring to a pressurised primary circuit from the boiler to the central heating. The problem as I see it is that if there is sufficient energy in the wood and if a thermal runaway happens then the pressurised circuit has less ability to absorb the heat, but I have no experience of the plumbing side of these things. Either way it is probably non trivial to get a pressurized water system approved for a wood burner in single domestic premises.

AJH

[Dave Liquorice]

Agreed, I wouldn't be surprised if a pressurised primary fell under all the regulations pertaining to boilers as in steam engine...