Fan assistance for wood stove chimney - possible?

[usenet]

We have a wood burning stove which works well in just about every way except for one thing. If you try and light it when there is a North wind blowing it smokes like crazy. Once it's alight and there's heat to get the draught going up the chimney it's fine.

The cause is very simple, a North wind produces a partial vacuum on one side of the house (well, a lowering in pressure) and that's where the ventilation for the stove is. So when there's a North wind air is sucked out of the ventilation and down the chimney. Thus changing the position of the chimney or increasing its height isn't going to help at all.

Can one get a fan to put in the chimney, it's a standard (?) metal tubular one, 6" diameter I would guess. The obvious place to put a fan would be at the top where the cowl is at present. Are there any fans designed specifically for this purpose?

I suppose the alternative is to work from the other end and have a fan blow air into the room where the stove is but that feels to me as if it would be rather more complicated somehow. At present the cure is to open the front door at the other end f the house until the fire starts drawing but while effective it's not very good for keeping the house warm.

[Tony Williams]

Yes you can get cowl-mounted fans for woodburners. Probably have to go to a woodburner specialist for one.

[jacob]

It sounds like your stove is only fired up intermittently so it could be a cold flue prob. Answer is either to improve draw with suitable cowl on top, or to line the chimney so it heats up quicker, or improve draw by making chimney airtight - i.e. big old chimneys often draw air in through cracks etc. needing pointing, inside and out. Never heard of fan in chimney - easier solutions available.

cheers Jacob

[The Natural Philosopher]

T effectve ption is to modifuy teh stack to take it out of a zone of positve pressure, wither by raising it, otr constructing baffles arond it.

Fans make a terrioble noise. Never successful IMHO.

surely you light teh bugger and shut teh doors? I have found use of firelighters to get the air column moving upwards smartly and preheat teh chimbly is a Good Thing.

[Dave Liquorice]

Is that ventilation on the same side of the house as the chimney? I'd have thought the venturi effect over the top of a chimney would be more than enough to defeat any lowering of pressure on the same side of house.

You can, normally placed at the top of the chimney but you shouldn't really need one. I wonder if double ventilation on opposite sides of the house possibly with flap valves that only allow air in would help? Though this might fall foul of "free air" ventilation regulations.

I fail to see how the interval between firings has much to do with this. If the flue is cold it's cold no matter if the last firing was yesterday or last month. The OP states once things have warmed up it's OK.

[usenet]

The house is T shaped.

------ | | | | | |

-------- ------C-- | |v | | | |

----------------------

C marks where the chimney is (actually a flue purpose built for the fire), v is where the ventilation is. North is 'up' on the above diagram, the fire smokes only when the flue is cold and when the wind is from a bit West of North (i.e from top left of above diagram).

As far as I can tell the smoking is caused by the wind causing a slight lowering on pressure at 'v'.

It would be easier in many ways to add a top-mounted fan on the flue.

[usenet]

A very expensive option surely.

So what about the noise, it would only need to be on for a minute or two.

Shutting the doors doesn't help, you just get lots of smoke in the biggest room in the house (7 metres square) which is pretty effective at filling the rest of the house with smoke afterwards.

At present we just open the front door at the other end of the house until the fire is drawing but I'm looking for a slightly less draughty solution.

[John Stumbles]

This has got the alarm bells ringing for me: sounds like the text book question about a body being found dead in a room with a gas fire yet ventilation and flue are to standard and the gas fire's not defective. The answer being that under certain wind conditions the vent is under negative pressure wrt to the flue so combustion products aren't being cleared resulting in incomplete combustion and carbon monoxide production.

In the OP's case I wonder if an additional source of ventilation could be arranged from the other side of the house (he says opening the front door helps) maybe via an under floor duct?

[usenet]

Yes, my diagnosis exactly, but fortunately the fire generates enough draught when hot to overcome the problem.

Well yes, but if we did that there would be cold air dragged right through the house. Ah, I see what you mean, a duct from somewhere else to provide air. Not very easy in the room in question, it has a solid floor.

I actually wish I'd enforced my original request to the installers to have an airbrick right by the fire (i.e. below the C on the diagram above). I suspect that would be quite effective.

[Dave Liquorice]

Certainly with the wind from the NW v will be in a relatively low pressure zone. Any chance of moving the vent to the same wall as the Chimney? For no particular reason having the vent and chimney across a corner just doesn't "feel" right.

[Tony Williams]

I was leaning over a gate the other day, commenting on the number of chimney pots on my pal's 1840's cottage.... twice as many pots as fireplaces.

Apparently every room with a fireplace has two chimneys, one for the fire and one to provide the ventilation. Because the two chimney pots are near to each other they see the same external air pressures. A Victorian version of the balanced flue maybe.

Perhaps you could extend 'v' upwards so that it is near the top of the chimney. Simple plastic drainpipe, or something?

[usenet]

If installing again as it were I'd definitely agitate for ventilation on the same wall as C. However it's not really an option now, there are no windows or other openings in that wall so adding ventilation there would involve making a hole in a 13" cavity wall.

Hence the original question.

[Dave Liquorice]

Which is not difficult(*). Hire or find some one with a core drill and it's less than a mornings work.

(*) Though loose fill insulation might make it "interesting". Blown fibre, rockwool batts or celotex type should be any trouble as they don't flow...

The question would be how big does the ventilation hole need to be? Would a standard 110mm dia air vent be enough, though I guess if you keep the other one it will be.

[usenet]

Yes, less than a morning to make the hole but rather more to make a good job of thre resulting mess. I do have an SDS drill so actually making the hole isn't a big issue but there would be quite a bit of making good required.

Then, in addition, it's not a guaranteed fix.

I could always put a fan in the hole! :-)

[Pete C]

Hi,

How long is the chimney outside the building and does it clear the roofline? Also which way does air flow through the vent under backdraft conditions? And does opening a window elsewhere of the house stop the backdraft?

Either the chimney is getting pressure and/or the vent getting a vacuum due to the wind conditions, or air is being sucked out elsewhere in the house.

You could extend the chimney if it is too short or duct the vent to somewhere more suitable if the wind is causing a vacuum through the vent.

Annother alternative is to fan assist the vent ensuring it's performane is not compromised during normal operation, but a passive solution is far better.

What you really really need is a good CO alarm, it will protect you from when the chimney is partially blocked, the wood damp, it's windy, you fall asleep at the fire....

cheers, Pete.

[Andrew Heggie]

Nofumo do a couple of anti downdraught cowls, the one that spins is a sort of savonius rotor like those used on dog vans.

There are 4 suppliers of cowl mounted exhaust fans, I can only remember Exhausto atm. Expensive though it will operate automatically as soon as it senses a hot gas.

Our pellet stoves have an externally mounted induced draught fan as an option but this is for a sealed appliance so may not cope with the volume of an open fire.

A google on nofumo, fluesystems or exhausto will find you other suppliers.

AJH

[Dave Liquorice]

A core drill makes a nice neat round hole by just removing a relatively small amount of material, most of the "hole" is one cylindrical solid chunk with a small guide hole up the middle. Unlike a normal drill that mashes up the entire volume of the hole or a bolster taking out bricks. "Making good" shouldn't be much more than sealing the hole liner to the wall and fixing the facias.

Sorry if your Grandma has learnt about eggs again. B-)

This is true but you do say you could put a fan in the hole. Maybe a small extractor fan kit is the way to go, modified to blow instead of suck of course. Swap the wires to the motor round?

[usenet]

Hardly useful as the moment there is 'hot gas' at the top of the flue the problem is over!

Thanks for the pointers though, I will take a look.

Hmm, more than £400 for the smallest ones, eek!

[Andrew Heggie]

Depends where the sensor is placed.

OK if the effect is that transient how about a hair dryer fan rigged as an ejector in a Y as the fire is started, it would need a spring loaded flap to prevent loss of the thermo syphon once the ire was running. It will need to develop enough pressure that the kinetic energy in the nozzle does not pressurise the flue, i.e. high velocity low volume.

AJH

[N. Thornton]

Hi

2 things strike me.

1. Wood stoves produce a lot of CO, and it sounds like you may be in quite a dodgy situation with that. CO is lethal at small concentrations, and bad for health in lower amounts.

2. The problem has been caused by the layout, and the one effective solution is to fix it, and this could be done by running a large dia tube up the outside wall from the intake to near the chimney - if the intake feeds the fire and not the room.

If instead it feeds the room, you'd need to get it above the roofline so its exposed to the same winds, but keep it well away from the chimney. This could perhaps be achieved by rising at an angle, plus adding some extra height to the existing chimney, which could perhaps be done with a real big chimney pot.

Regards, NT