Downdrafts :(

[Kelpie]

Recently acquired a Pansy charcoal heater and upon installing it, found that the draft is coming fairly strongly the wrong way down the flue. I've tried various combinations of hatches and vents open and closed, but unless it is dead calm outside, the draft coming down the flue is enough to blow out a match held up to it.
The flue currently just terminates at deck level. I knew that this was not ideal but I had no idea that it would cause such a downdraft.
Presumably the answer is to extend the flue above deck level- how much am I likely to need?

 

[Blueboatman]

(In haste). How much height? A lot. As a kid I used to sail with one for years, the flue height ( from coachroof top) was just less than the bottom of the boom. High is good.

I now have a drip feed diesel stove and the same applies, high flue away from dodgers etc.

Once it is hot the Pansy ' draws' fumes upwards.. Good bit of kit , deceptively simple.

You have the slotted top-hat bit on the flue top I assume?

 

[William_H]

You might consider trying a venturi device on top of the flue. This could be just 30mm high. You need an opeing facing windward I imagine about 180mm wide to windward which tapers down to about 100mm wide then tapers out again to 200 mm wide. These dimesions in plan view. (ie the opening would be 200mm wide and 30mm high) This is placed over the flue outlet such that flue hole is in the narrowest part but in the area where taper expands again to outlet.ie from the middle aft. A bit hard to explain without drawings. You might want a plate on the deck to protect from heat.
The whole would need to be rotatable unless you only use it with wind on the bow. the air comes through the venturi accelerating to the narrowest point then decelerates as it leaves the tight point. At this point there is a low pressure which is placed over the flue to suck air out. You might need to make it adjustable as it might suck too much in strong wind. If it sucks too much then an elbow in the flue so that outlet faces down wind might do the job.
Obviously ther eare eddies of wind off the mast and possibly the stacked mainsail which are blowing air down the flue in a wind. Maybe just a mve of the mainsial off to the side away from the flue would change everything. good luck olewill

 

[GHA]

Refleks drip feed recommend 400mm from memory.
I have one but haven't got round to sorting an extension yet, still at deck level so fitted a small computer fan instead which was easy as there's a nicely sized aperture to attach to. Works a treat but might not help you.

 

[chrisbitz]

I've heard a tip with open fires, about "warming the chimney first" or something like that? It kinda makes sense that if the chimney's warm then the hot air will continue to rise, rather than be cooled by a cold chimney and fall again?

 

[FWB]

If you download PBO No1 free from here http://www.pbo.co.uk/specials/530862/pbo-issue-no1
And turn to page 77 it recommends 6ft from ash pan to top of flue.

 

[Kelpie]

Thanks for the suggestions... I'll hunt around for the longest bit of pipe I have and go from there.
A Venturi device sounds ideal, maybe I could knock one up from two metal funnels...

 

[LittleShip]

Open flue appliances can be troublesome, if the air outside is just a few degrees cooler than inside the cold air will drop down the flue. The longer the flue the better chance you have of getting it to work correctly.

Be careful, you don't want products in the cabin.

Tom.

 

[Gordonmc]

I always had a problem with getting my charcoal stove to draw from cold until I got in the habit of warming the flue pipe first. That gets rid of the cold air "plug" in the pipe.
However, that doesn't appear to be your problem if there is a downdraft.
Is there nothing on the end of the pipe?
The venturi idea is good, but would need aligning to leeward. I used an "H" top made out of 28mm copper plumbing tube and three T pieces.

 

[huldah]

Whilst I am certain all the above is important, there are other factors that can influence flue flow. Having lived on a windy hillside, I have experienced the way room pressure can drop and suck air down a flue, even when the flue is hot.

A while ago a poster on this forum, and his wife, ended in hospital with carbon monoxide poisoning. This may have been caused by the boat swinging with the tide, so that the wind switched from the stern to the bows as they slept. This may have have reduced cabin pressure.

 

[oldvarnish]

When I had a pansy stove, the chimney extended 2ft above deck level.

 

[NUTMEG]

I have just extended the flue of my PA heater. 2' above the deck now with an 'H' on the top. 28mm copper plumbing pipe, three T pieces and jobs a good un.

The heater now works a treat, lights first time and the damper now regulates it, so I can turn it down to tick over or have it roaring away. I primed and painted the copper pipe with a smooth Hammerite copy and it looks rather nice in a trad sort of way.

 

[Kelpie]

A wee update.
Finally got the smoke blowing out the right hole and has the stove pumping out heat this evening. I have extended the flue up to about boom height, tonight was relatively calm, and finally I used a blowtorch to heat up the flue between the stove and deckhead. This last trick seemed to do the job and away it went.

Happy, hot boat owner!

 

[ribrage]

Recently acquired a Pansy charcoal heater and upon installing it, found that the draft is coming fairly strongly the wrong way down the flue. I've tried various combinations of hatches and vents open and closed, but unless it is dead calm outside, the draft coming down the flue is enough to blow out a match held up to it.
The flue currently just terminates at deck level. I knew that this was not ideal but I had no idea that it would cause such a downdraft.
Presumably the answer is to extend the flue above deck level- how much am I likely to need?

Not exactly the same , but i fitted my diesel drip feed from dickinsons, the recomended flue height was 6 feet , there was also reference to a barometric damper to stop what you are experiencing

see here :

http://www.boat-heaters.com/107-dickinson-marine-22-flue-pipe-barometric.html

 

[Iliade]

Could it just be that you have too high a vacuum being pulled by one or more of the boats ventilation devices?

e.g. companionway grille, especially if head into wind; dorades facing to leeward, Anchor well vents (sounds daft, but my old boat had 2*1' ones which provided a significant draft) &c, &c

It may well be that stopping off or rotating these while you get the fire going will avoid all the hassle of heating the pipe.

P.S. I left the woodburner at work burning fiercely after lunch two days ago. On our return at the end off the day the whole place was full of smoke! That's with a hundred plus year old two square foot cross section chimney albeit only about 15' above the fire. Sometimes it seems one just can't win with these devilish devices!

 

[Tormod]

See the before and after pics. Had major problems with down draft not helped by the shape of the wheelhouse! Contacted Meercat Exhausts in Kilbirnie and problem solved. 700mm of extension with an H cowl on top. Annulus between two stand pipes filled with exhaust matting. Great - able to grasp the stand pipe with your hand and barely warm with the H cowl very hot. Not cheap but very effective! FYI - Taylor diesel heater!