chimeny question ? draft

[dookits13]

hi everyone , i know that the flue size must match your stove size for proper draft ... and if to small it will draft poorly.... but ive read that if it is way oversized it will also draft poorly --- i thought that to big of a flue would cause out of control drafting , but the theory was that because the flue was to big that it would never quite reach a good drafting temp. ?? is this true ---- trying to decide on how to tackle my chimeny??? i have a wood furnace with a 6 in flue and my chimeny flue size is approx. 9x9 ?? its the old clay rectangular type . once i get a good hot fire going it seems that it drafts just well enough to keep the furnace running properly ( i actually have the best results when i leave the ash door open, but then have no control of the draft, ) the real problem is when the house reaches temp. and the stove begins to "idle" , the fire will really start to smother and smoke usally starts puffing out of any crack it can find( like an old wet campfire), so could this be the problem or is it some other complicated draft issue.. also if i do need to reline it what type of pipe is suitable ---- size and kind .... does it have to be the double wall stuff being that it is already lined with the clay ????? thanks for any help

 

[Carl]

Can't tell you anything about your chimney but I sure wouldn't burn with the ash door open. If forgotten or left that way it could
overheat the furnace and as you said, there is no control of the draft. Very dangerous in my opinion.

As for the smoke, any fire will smoke when it is burning well and you choke it off as the furnace would when there is no call
for heat. Works but not very efficient.

You see those outside units smoking up the neighborhood for the same reason. They smolder till heat is called for then a
blower kicks on to fan the flames and bring them back to life. When the call for heat is done it is back to smoldering with
the draft shut off till heat is needed again. These units are not very efficient and do burn lots of wood ...er smolder lots.

 

[fossil]

Think of feeding a firehose from a garden hose. Your chimney flue is about 3 times the cross-sectional area of your stovepipe. When the flue gas stream gets to that junction, it slows waaaay down. That means it has plenty of time to cool down, get heavier, and condense out whatever creosote it might be carrying with it. The increase in density of the gas mass as it flows slowly up the chimney is probably enough to affect the draft through the stove beneath. I'm not an expert, but I think what would be the best thing for you to do would be to install a chimney liner (and block-off plate, if not already there) all the way from the woodstove to the top of the chimney and an appropriate cap on top. Lots of discussions here about doing that sort of installation. Good luck. Rick

 

[fraxinus]

Before comitting to a stainless liner, you might want to check out www.supaflu.com. There are a couple other pourd in place liner systems, but Supaflu is the only one I've had direct (and very positive) experience with. Cost competitive with stainless when I had a chimney done 5 years ago. The Supaflu former leaves a spiral pattern on the inside of the liner. This creates a venturi effect greatly enhancing draft.

 

[begreen]

Sounds like serious backpuffing. It might be caused by too green wood or damping down the furnace too soon. How dry is the wood being burned?

If the wood is very well seasoned, can you describe the flue connection between the furnace and the chimney? I'm wondering how long the horizontal run is and whether it is pitched uphill. Is this an exterior chimney?

Last, what else is in the basement competing for the combustion air used by the furnace? If there is negative pressure in the basement, this will affect draft.

 

[Gooserider]

Code for woodstoves says the chimney cross section should be no more than 2x the stove outlet with an exterior chimney, or 3x with an inside one. Your 9x9 flue has an 81sqin cross section, a 6" diameter outlet is about 28sqin, so you are marginal on an inside chimney, and over the limit on an exterior chimney.

Cheapest fix is to drop a stainless steel liner down the chimney, with appropriate blockoffs etc. on the top and bottom. It does NOT have to be a "class A" pipe like you would use for a stand-alone pre-fab chimney, but it does have to be a listed liner material. If the chimney is straight, you can use a rigid stainless pipe, or flex. If it isn't you have to use flex. If the existing chimney is clay lined, and is in good condition, insulation on the outside of the liner is a desirable option, if the chimney is in bad shape the insulation is required. - For more details do a search on chimney liners in the regular Hearth Room forum.

Gooserider