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Flue Liner

Terence McCann

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Hi Richard,

Well, sure, it might be the only appliance running all summer long. I think in a properly-sized flue it won't make that much of a difference but I think that when a flue is sized for both appliances and then you remove one you have a situation where the flue is waaaay oversized versus just a little bit or borderline or not at all.

We don't see them that much anymore but a decade ago we'd see a lot of large flues sized for oil burning furnaces with water heaters dumping into them. I think the HVAC guys are finally starting to "get" it.



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Yeah, sorry, I either misread the original post or got a little sidetracked by Bill. I wasn't referring to unlined masonry chimneys being used for gas appliances. That's an additional set of problems, and an easy call I've made numerous times in older homes. I don't remember actually seeing this, but I'm talking about a clay lined masonry chimney that is now being used to vent a gas furnace and water heater. It would seem that the main issue there is draw due to oversizing whenever the water heater is being used alone...summer or winter.

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I wasn't specifically commenting on unlined chimneys.

It would seem that the main issue there is draw due to oversizing whenever the water heater is being used alone...summer or winter.

In my experience, the "draw" isn't the issue, unless the flue size is way out of whack or there's some other issue combined with the orphaning thing.

The primary issue, in my experience, is condensation. During heating season weather, in my climate zone, the furnace/boiler cycles frequently enough to keep the chimney flue sufficiently warm. When the water heater fires during the furnace/boiler off cycle, the flue is still warm enough to keep condensation from forming. Another possible factor is when removing that furnace or boiler, there is less dilution air entering the chimney, further contributing to condensation.

In my experience, this results in the chimney being subject to continuous condensation throughout each heating season. This then shortens the life of the flue and rusts out the flue connectors. If the furnace/boiler that was removed was oil-fired, the condensation combined with the deposits in the chimney will destroy a terra cotta liner very quickly.

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Every improperly sized flue I see is reported as needing to be downsized; except mine! I upgraded the furnace more then 10 years ago, but didn't want to ugly-up my new chimney pots from Superior Clay with a metal cap sticking out of it.

Haven't had a problem. It's an outside chimney with three flues (dummy, fireplace, and water heater). When I sell, I expect the Inspector to report it as not right and I will correct it if the buyer requests, but for now, the pots look classy from the street. I've had people stop and comment on how nice they look.

My motto to my clients is: "do as I say, not as I do"

Ezra Malernee

Canton, Ohio

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