Flue for garage wood burner

[Dave Baker]

I'm looking at making a wood burner for the garage out of an old 55 gallon oil drum. Plenty of plans and ideas on the Intergoogles. I'm currently thinking of having it horizontal with the door at one end so I can just chuck in logs 2 or 3 feet long to save sawing stuff into shorter lengths. The biggest issue is the flue and knocking a hole in the wall to get that outside. Is there any reason why I can't just stick a length of 5" diameter pipe up out of the back of the drum and then horizontally through the wall to outside and leave it like that with no vertical chimney going up above the roof line? I'm not bothered about smoke or where it goes. The garage is far enough away from the house and I live in the back of beyond with no near neighbours so emissions are not an issue nor are any stupid regulations that might otherwise prevent me doing something I want to. I just want the cheapest easiest way to vent the oil drum.

[Martin Brown]

I suspect it won't draw properly unless it has at least some section of flue that is least up at >45 degrees. I am assuming here that your garage is not hermetically sealed and ventilation isn't a problem.

Not sure how long a 45 gallon drum will last in that sort of usage - you might want to put a fire pit underneath it to catch the contents when as seems inevitable it disintegrates some time later.

Having said that half oil drums make reasonable BBQ gear provided you junk them before the rust eats through them.

[newshound]

I would have said that you *definitely* need a vertical run to get some "draw". Your proposed arrangement might work with the wind in some directions (low pressure area around the outlet) but in general I would say it is a recipe for filling your garage with smoke and fumes. You could make it forced draught (i.e. a fan blowing air in at the bottom) but of course would then need to have a sealed door, or induced draught (fan sucking on the flue) but this is exposed to the flue gases. You might not need to go above the roof line but I think it will all depend on local wind flow, shelter, etc.

Thinking really laterally, you could pressurise the garage with a big fan sucking air in so that the best way for the smoke to get out is via your flue.

You of all people ought to be able to visualise this sort of stuff :-)

[Dave Baker]

Why would a flue draw less well horizontally than vertically? The flue from the oil central heating boiler in the house just goes straight through the wall horizontally.

[Tim+]

Because hot air would far rather go up than along?

I'm pretty sure all oil boilers have a fans producing positive pressure within the boiler so that's going to help things along enormously.

For very short flues on non-fanned gas boilers, I would guess that there will be enough vertical height within the boiler to give the gases enough momentum to travel a short distance horizontally.

Tim

[newshound]

The hot air baloon principle

Non-room-sealed gas water heaters ("geysers") which sometimes didn't have much of an external vertical flue used to kill people regularly from CO poisoning.

[DICEGEORGE]

because in certain wind directions the flue would become an inlet and smoke and flames would come out of the air inlet.

[Harry Bloomfield]

Dave Baker formulated the question :

It needs to rise, so the hot gases can cause a vertical draw. Your boiler maybe has forced ventilation.

[Andrew Gabriel]

So do wood burners in garages/sheds.

Make sure you fit a CO alarm before using it.

[Dave Liquorice]

Aye, 2 to 3 ' long logs ain't light... and couple of them burning efficiently are going to chuck out a fair bit of heat. Expect drum to glow red...

A short stubby horizontal outlet is just asking for the wind to blow into it or even just draw air in if a particular wind direction makes the inside of the garage lower pressure than the outside.

You need some "draw" and in my experience(*) the only way to get that is with a vertical section of chimney/flu. This is obviously a "it'll do for a while" job, single skinned flue liner is cheap, cut a hole at the top back of the drum, stuff in and seal the end of the liner, take it through the wall and up to roof level. So the air passing over the top creates the draw. Remember it will get hot, very hot, so flammable things need to kept well clear, undergrowth etc...

(*) When we had central heating fitted when I was about 8 there was a

4' section of 5" flu liner left over. Used to build fire places with old bricks and jam this section of liner in the top. Worked well vertical, crap horizontal.

[F Murtz]

You can make a much more permanent one out of an old mains pressure water heater tank.

[Dave Liquorice]

Or a 15 kg butane cylinder, bit small but ample for a normal living room.

[Andrew Gabriel]

On my fire training course at the fire station for one company or another, one of the incidents covered was some guy who wanted to use one of these for something. It was empty, so he started to cut it open. Can't remember if it was with an angle grinder or cutting torch, but either way, it blow up when air mixed with the residual gas inside, seriously injuring the guy.

[Huge]

So, how do you empty them so you can convert them into a wood stove?

[Dave Liquorice]

Unscrew the valve? Even when "empty" I suspect ther might be still be a bit of pressure so I'd be cautious and just crack the joint and then let it sit and hiss... I think filling and rinsing with water is also recomended before taking gas axe or angle grinder to it.

Ho ho...

[Martin Brown]

Empty it of gas and then fill with water (or flush with an inert gas).

[Tim Lamb]

Perhaps it was an acetylene cylinder. I had an *empty* one go bang in a farm fire. Alternatively a scrap metal collector cut up a huge (

20,000l) diesel tank with an oxy/propane torch and no precautions whatsoever.

[Huge]

Of course. I are an eejit.

Purely coincidental.

[Dave Liquorice]

Empty with the valve off or empty with the valve on and closed?

The expansion of a cylinders contents due to the sheer heat of a good fire can make cylinder go bang. At 1000 C plus steel is getting soft...

Can diesel form an explosive mixture at normal temps and pressures? It spontaneously goes bang in an engine but not before it has been compressed at around 20:1 with that compression also heating the mixture.

[F Murtz]

No hassles at all just remove the valve completely from the cylinder. Fill with water if you want to feel safer and angle grind at will.(Even less dangerous than old oil drums). Just made a lead pot from one to make sinkers. Don't go on about the dangers of lead poisoning,I do it in a gale in the open. :)