Will a 6-inch insulated liner cause excessive draft in a tall chimney?

[pm01]

I have a Jotul Oslo in my basement, connected through a thimble into an exterior masonry chimney that extends 40 feet above the thimble. The flue tiles are nominal 13x13, with an interior measurement of 11.25x11.25. The flue is larger than Jotul calls for and it's a cold exterior masonry chimney, but it has drafted well enough to make us love our stove for the past three years. I hate having to mess with things that are working...

That much said, flue tile cracks make a reline important for safety. I think I can fit either a 6-inch or an 8-inch insulated liner down the flue. It seems that the standard liner recommended for an EPA stove is 6 inches, and that's my preferred size for cost and ease of installation. Will that size liner cause an excessive draft in a 40-foot chimney, or will the Oslo likely perform okay with it?

Thanks for any insight.

 

[BrotherBart]

I am in your neck of the woods and have a 5.5" un-insulated liner running 33 feet up a outside masonry chimney. 40 feet if you include the connector pipe to the stove and the run through the thimble. Two 90 degree turns. It drafts like a vacuum cleaner. But is controllable with a key damper in the connector pipe above the stove. The stove on the pipe is a Jotul F3 CB.

I am actually convinced that my two 5.5" liners draft harder than a 6" would because they stay hotter. No scientific evidence of it though. But without insulation they make the two EPA stoves walk and talk and don't get crapped up with creosote.

The long and short of it is I think you will be fine. If it drafts too hard a Simpson key damper section at the stove top is a good answer to it if you have single wall connector pipe inside the house.

 

[laynes69]

Yes, hands down you will have excessive draft. I do think it is important though to reline. Not only for safety, but also a way to keep those gasses in the chimney warmer. Installing a simple key damper in the flue will allow for better control of the overdraft if you decide to reline. With a flue that size, it would only benefit to reline.

 

[Green Energy]

Yes, hands down you will have excessive draft. I do think it is important though to reline. Not only for safety, but also a way to keep those gasses in the chimney warmer. Installing a simple key damper in the flue will allow for better control of the overdraft if you decide to reline. With a flue that size, it would only benefit to reline.

+1 Laynes

I have a 25' insulated 6" flex liner. Pulls like a banshie. I certainly would not want another 15' on top of this without a damper.

 

[pm01]

Thanks for the replies. It appears unanimous that a 6-inch insulated liner will create excessive draft. I'm not sure I can fit a damper because my single wall pipe is short. I have only 6 inches vertical out of the stove, then a 90 degree elbow, then another 6 inches horizontal to the thimble. I think I might also prefer to try controlling air to the stove by restricting the intake if necessary. The Oslo's intake is fairly easy for me to reach, and I might be able to rig up an adjustable restrictor plate to slow things down.

That much said, I'd rather not have to mess with a damper or restrictor plate if I can avoid it - especially since the EPA stoves weren't designed to use those. I've read that larger flues create weaker drafts. An 8-inch liner is about 75% larger than a 6-inch liner. I don't particularly enjoy the thought of spending more for a larger liner, or wrestling a larger, heavier liner down the flue, but I think I would make that trade-off if it would allow the Oslo to operate without other modification.

If I installed an 8-inch insulated liner in my flue, would I end up with something that's closer to a "normal draft," or at least something less excessive?

 

[btuser]

Probably not. I'm not an expert by any means but if your stove was drafting well in such a large flue before any liner is going to seriously improve it. The larger liner would give you the option of a larger stove down the road. That's the only reason I'd consider it myself.

 

[mellow]

Have you had a camera run down your chimney to check for cracked tiles? If not I would default to running an insulated 6" liner down that chimney to make sure it is up to code. 6" is usually what most stove companies want you to use, plus an 8" insulated might be a tight fit down that chimney.

I was curious to see if anyone came out with a mini flue damper but couldn't find one.