I've got a couple of disused fireplaces I'm going to bring back into use with woodburners. I'll need to fit stainless steel flues. Do all flues & burners have a standard gauge or do I have to worry about matching them up? I'll probably be going for quite small burners, about 3-4kW at most. Recommendations welcome.
In message , Big Les Wade writes
The manufacturers site will tell you about flue sizing. Just as important is insulation.
To meet the regulations you either need to involve building control or use a Hetas registered installer.
There are standard sizes for flue (5, 6 or 7" are the most common) and yes it does have to match the stove you fit.
to do a "legal" job you have to either pay a HETAS bod to do it for you OR DIY & meet the regs & involve Bldg Ctrl..
Jim K
Can you not fit the largest flue and them put a spigot plate at the bottom to match your stove?
You can as in physically but I don't think the BCO would like it as it is probably against the installation instructions of the stove.
A large flue will slow down the flow of gases, the longer they take the cooler they get and more chance of tar/soot deposition in the flue.
It will also affect the draw. I can't decide if it won't draw enough, large column of air that will be hard to get moving, or draw too much, wind over chimeny pot sucking a 7" column of air due to the venturi effect being rather more than a 5" flued stove would want. Or a combination of both depending on conditions...
I don't think he can complain as long as it is greater than 150mm id
Yes but to some extent this will depend on heat loss to the sides, if the flue is insulated or the brickwork has reached a constant temperature then there may not be a lot of difference. I imagine many brick chimney are 9" and would still pass.
The draw is dependent on the buoyancy of the air in the column, if it's cooler then less draw.
I still cannot figure why a sophisticated bit of kit like a modern gas boiler, which is fueled by a gas that is readily combustible, has a fan, yet a wood fire which has a low cv fuel which is difficult to cleanly burn and would benefit from the turbulence and air control is resisted.
AJH
In message , snipped-for-privacy@sylva.icuklive.co.uk writes
Maybe because the gas can be shut off whereas a charged log grate can't?
The fan in most gas gas boilers is working on a mixture of gas and air. The effect of this is to thoroughly mix them so promoting efficiency.
Obviously not an option with solid fuel. With solid fuels you have two options, either suck or blow the combustion air/products
The problem with blow (forced draught)(air) is that if the chimney blocks,combustion gases are forced into the room. This means the boiler has to be in a non-occupied space. Eg a wood pellet boiler
The problem with suck (induced draught) (combustion gases) is that the fan becomes unbalanced due to tar buildup. So not suitable for wood burning stoves. But often used with coal. gas and oil.
Large industrial boilers often have both suck and blow (balanced draught.)
There can be ways around that, generally natural draught can provide for a safe burn out as long as primary air is limited in case of fan failure. With a single id fan , as on many pellet stoves, this becomes self limiting.
AJH
I don't follow that, I though it was just an induced draught fan after the heat exchanger? If so then it would be no different in effect on the log burner, primary air causes a combustible gas to be evolved which then burns out in much more secondary air.
Which is why I mentioned a fan would aid wood combustion, it can provide more turbulence than natural draught.
All the larger wood fired systems I worked on had id fans and many had primary air fans too. The better ones had wide band lambda sensors to control secondary air and I have never had problems with fouling of id fan blades. I did have a big problem from a chimney being terminated with a conventional gas cowl which sooted up.
AJH
In message , snipped-for-privacy@sylva.icuklive.co.uk writes
As usual Harry's confusing things
In most conventional boilers, the fan works on the air or the exhaust gasses, in modern condensing boilers, quite often, the fan is blowing premixed air/gas into the combustion chamber
In message , snipped-for-privacy@sylva.icuklive.co.uk writes
It is because most boilers are room sealed, they are not drawing air in from the room, it has to be sucked in from the outside via the outer tube in the flue and the products of combustion pushed out down the inner tube of the flue
Re. gas boilers. In days of yore that was true but things have moved on. The first thing you have to do for efficiency is to thoroughly mix the fuel and air, premixing it does this better so lowering the unburnt fuel in the combustion gases.
In a solid fuel system the fan just overcomes the resistance to the passage of air/gases meaning the airways/chimney can be smaller/ longer. Also the boiler can be more compact for any given heat output. Also the combustion process can be boosted/cut back more quickly, ie there is better control. It enables the "pyrolising" process you get in moost wood pellet boilers.
I think that's what I said. Whether it's condensing or not is neither here nor there.
Also the existing chimney (into which the flue pipe will be inserted) is quite narrow. The builder thinks it will only take a 5in pipe.
Then it will not comply with part J unless burning smokeless fuel, even the exempt devices under the clean air act only allow for coal and not wood.
There appears to be no exception when burning wood pellets even though smaller than 150mm and balanced flues are allowed in other countries.
AJH
That explains why a gas boiler has a fan.
I therefore imagine that because a woodburner has to have a chimney and that creates its own draught fans aren't used other than in pellet stoves.
My experiments with small burners is they do benefit from fans
AJH
Actually that's much what Harry said.
I hadn't realised blown premixed boilers were in use, I did see that modern pellet burners had moved from being an id fan drawing air into a burn pot to a fan firing into a blast tube.
AJH
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