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I need a chimney liner the furnace is 5” converted down to 4” and the hot water heater 3”

0 votes

From Pat:


I don’t even want start with — our gas furnace and hot water heater were venting under the house for years because the vent pipes went into a 2′ brick (that’s the outer dimension) unlined chimney. Heard a noise in chimney in the winter went up to take look and a raccoon was living about half way down the chimney. (old two story home approx 26′ chimney) I was wondering how she was surviving in the chimney with the furnace running… hmmm no heat coming form chimney at all. Check it out and the pipe was buried with debris a raccoon feces about 2′ worth.

I need a chimney liner the furnace is 5” converted down to 4” and the hot water heater 3” connects into the 4” pipe this is all double walled until it reaches the chimney which is only 4′ away then it is 4” single wall a elbow and a short piece to just go up into the bottom of the chimney about 3′ then it vents into the unlined brick.

It looks like I need at least 6” – 7” as the chimney liner right? 4” that is there is to small?
As the opening is very small into the chimney what is the smallest liner that will work with a 4” water and 4”furnace….?

Shouldn’t the furnace be 5” as it is converted down to 4” ?

asked in House Chimney by darekrudy (21,730 points)
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1 Answer

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Hi Pat,

Using The Seven Times Rule:

"When two or more appliances are connected to a chimney, the flow area of the largest section of that chimney cannot exceed seven times the smallest flue collar, or draft hood outlet area, unless designed with approved engineering methods."

the largest diameter of the chimney flue liner would be 7″. Assuming that the furnace’s vent pipe has been improperly downsized (you’d have to check with the manufacturer / some types of furnaces permit 1″ downsizing), the smallest diameter would be 6.5″ (largest diameter + 50% of the smallest).

Since they don’t make 6.5″ diameter vent pipes the closest one is 7″ or 6″ if the downsizing of your furnace vent pipe would be permitted. If by any chance you’re in a cold climate, make sure you have insulated liner installed.

You can find more information about this subject by visiting chimney flue size post


answered by darekrudy (21,730 points)
Thank you, how cold is cold climate? southern Ohio
The larger the temperature differential between the outside air and chimney flue gasses, the more heat / energy will be required to create sufficient draft inside your chimney. By installing an insulated flue liner inside the chimney your are dramatically improving its draft / venting characteristics, and minimizing possibility of condensation and back-drafting.

Insulated liner will have more impact on an exposed chimney (attached to the side wall or free standing) and less on one that is partially concealed within the house structure and only top section exposed above the roof.

Ohio is definitely experiencing cold winters and while the furnace and the water heater burning fuel at the same time will have no problem with quickly warming up cold chimney, water heater operating on its own can use some help.