Another chimney liner question

Have a Harman Mag Stoker and it goes into an 8"x8" teracotta flue. When the days are warm above 50-60 deg I have a hard time keeping the coal going. Was thinking of putting a 6" single walled flue pipe down the terrcota flue with an elbow to hook up to my stoker. Would this give me a better draft, thus keeping the stove from going out? Maybe galv. pipe to keep it from rusting out? Thoughts? Coments?
Thanx Jimmy

Jimmy--

The rule as I understand it, it to use the same diameter flue as is the outlet of the stove works best. I also believe you will have to use a ss liner. I installed a 6" 316 ti ss flex liner. I saw substantial increase in draft. But I still have a difficult time burning coal above 40F. especially on damp, rainy days.

you may possibly, although unlikely experience an increase in draft, however, it is NOT worth the money to install stainless in an otherwise perfect size and condition flue, and should you decide to reline it will have to be stainless, even galvanized steel will rust out very quickly when used as a lining material and you could cause a dangerous situation with CO and exhaust gases becoming trapped between the leaky galvanized liner and the larger flue looking for a way out, possibly into the home. this is because normally there would be negative pressure in the chimney while the appliance is running but should you line improperly positive pressure could occur between the tile liner and the galv. liner. why waste the time/money on relining a perfectly good flue for likely no benefit. you will not have decent draft untill the outside temp is below around 40º F.

Berlin wrote:you may possibly, although unlikely experience an increase in draft, however, it is NOT worth the money to install stainless in an otherwise perfect size and condition flue, and should you decide to reline it will have to be stainless, even galvanized steel will rust out very quickly when used as a lining material and you could cause a dangerous situation with CO and exhaust gases becoming trapped between the leaky galvanized liner and the larger flue looking for a way out, possibly into the home. this is because normally there would be negative pressure in the chimney while the appliance is running but should you line improperly positive pressure could occur between the tile liner and the galv. liner. why waste the time/money on relining a perfectly good flue for likely no benefit. you will not have decent draft untill the outside temp is below around 40º F.

I disagree......Poor draft occurs when you vent a much smaller exit area stove into a larger flue area. If your stove has, for example, a 6" circular exit (about 28 sq inches) and you dump it into say, a 9" x 9" (81 sq inches) flue, the exhaust gases expand and slow down significantly (by about a factor of 3 in this example) and this reduces your draft significantly. The stove manufacturer made the stove of a certain exit area and you should probably maintain that area to the top of the chimney. I did this with my wood stove and I also insulated the liner to keep the hot gases hot and therefore rising. Insulation for a stove liner is important because if you have an exterior chimney (3 sides exposed to the outside) the cold flue tiles cool the hot gas in the liner and also reduce your draft. This is because a cooled gas takes up less volume and the velocity decreases to compensate, and this in-turn kills your draft.

Also, we should differentiate what people mean when they say their stove works best when it's colder outside.....what they mean is when the OUTSIDE air temp is colder it drafts better but you don't want the flue gas to cool until it exits into the colder outside air. An uninsulated liner means the gases can cool on their way to the chimney exit and that's why I insulated.....to keep the exhaust gas as hot as possible until it dumps into the colder atmosphere...

So...putting a liner into a flue solves the area ratio problem but you could also continue to have a draft problem because the cold flue is cooling the gases in the liner and also affecting your draft......and insulation may solve this....

Here is two cents worth, as I have a 6" stovepipe going to a short 8"x8" chimney.
1. Never use galvanized with coal, it is a no-no.
2. Insulate the stovepipe with Kaowool or similar, it will help your draft.
3. If you do anything to the chimney, make it taller and/or ext cap.

60*+ with a stove is near the end anyway.

Alot of times chimney height affects the draw. We have a large masonary chimney with a 8x12 clay liner, and it drafts just about always. Even cold, but the chimney is 30+ feet tall. I agree that it wouldn't be worth the cost to line the chimney if it is in good condition. And with the temps in the 50 to 60 degree range, I would let it go out and let the backup heat take over.

Since you only have a draft problem when the weather is mild I'd use a band aid approach. Install a Field Controls draft inducer. It's rated for coal use. See: http://fieldcontrols.com/draftinducers.php Only turn it on when needed.

There's just no good cure for warm outside temps. My Harman Mag vents into a 200-year-old fieldstone chimney that draws like a champ until the daytime temps hit roughly 45 degrees. Above that, and she just slows down and won't burn right no matter what I try. Same thing for my previous stove, an ancient hopper-fed using the same setup in the same place.
This "transition time" is the worst- the stove is ready to nap for the summer, but the wife wants heat in the morning, so the oil burner gets turned up...