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Correct flue liner size? Or two flue liners neede?


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In an old house with an unlined 7.5 x11.5 chimney, I need to know what is the proper metal flue liner size to use. There is an old gas furnace with a 4 " vent and old water heater with a 3 " vent. They both use the flue. Some contractors are saying that there must be two liners, one for each device. Another, says that one larger flue is enough. Does any one know what the normal code is?

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Why would you even want to screw around with this or get into the middle of something between a couple of HVAC contractors? Once it's referred off to an HVAC guy, let him tell them what they need, and if they call you up wanting your opinion of it just tell them that you're the family doctor and he's the brain surgeon; it's his call, not your's to make.

If you must know, you can use one flue but it needs to be equal to the area of the largest vent plus 1/2 of all others.

A 4 inch vent has 12.56 square inches.

A 3 inch vent has 7.065 square inches.

7.06/2 = 3.03 square inches

12.56 + 3.03 = 15.59 square inches.

A 5" vent has 19.62 square inches, so it should work fine, if it's ok to use a 3 inch vent on the water heater and a 4 inch vent on the furnace. Do you know that it is?

Unless you have years of HVAC work and experience specifying HVAC systems under your belt, it's really not very smart to get involved with specifying stuff like this - especially when it involves venting "old" gas appliances into a cold chimney stack. If you do that and either of them backdrafts into the home and sickens or kills someone, you could end up losing everything.

OT - OF!!!

M.

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Dear Mr hausdok,

Thank you for your quick response. The reason why I am interested in this is because I own the house and will be needing to do this work, and want a safe environment.

The one contractor that said I needed two flue liners had a price that was about twice as high as the one that was going to install only one liner. And he said that it was the code to have a separate flue liner for each appliance.

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Originally posted by Southbeach

Both the water heater and furnace are in the basement venting into the same flue. I am selling the house some replacing the them is not an option.

Sell the house "as is," and let the buyers worry about it. Or, get an estimate from a decent chimney guy, and give the amount of the estimate to the buyers as a credit.

Maybe it's just me, but if I'm selling a house, the last thing I want is the job of overseeing repairs. If something goes wrong, the buyer will want me to take care of it; and, he'd complain that I should've known about and disclosed this or that...

WJ

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Originally posted by Southbeach

In an old house with an unlined 7.5 x11.5 chimney, I need to know what is the proper metal flue liner size to use. There is an old gas furnace with a 4 " vent and old water heater with a 3 " vent. They both use the flue. Some contractors are saying that there must be two liners, one for each device. Another, says that one larger flue is enough. Does any one know what the normal code is?

One flue is fine.

Two flues are better.

In my house, I used one flue. No complaints.

- Jim Katen, Oregon

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