Barometric damper


Old 01-29-03, 06:29 PM
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Question Barometric damper

Hi everybody,
All what I want to know is: what is the purpose of having a Barometric Damper on the flute of a furnace? Do all oil furnaces need Barometric Damper? If no, what type of oil furnace must have the damper installed? Your explaination will be highly appreciated. Thank you very much beforehand.

Tuan Le
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Old 01-29-03, 07:50 PM
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Here is a quote from another site that explains it better than I can:

"A TEE section is inserted into the stove pipe. In the side of the TEE, a weighted flap is installed. Whenever the chimney suction is greater than the stove or furnace needs, the flap rotates open and lets room air into the chimney, thereby spoiling the draft.

The benefit is that the stove or furnace will work more efficiently with a steady draft as opposed to constant changes in the chimney suction. For instance, a coal or wood stove may burn more steadily and for a longer period of time. An oil furnace will also be more efficient and less heat will be sucked up the flue when the unit is on AND off."
Old 01-30-03, 05:24 AM
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Hi Greh,
I am happy with your answer. Just one more question:
Do all oil furnaces need Barometric Damper?
Thank you.
Old 01-30-03, 07:25 AM
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No, Just the Oil furances that needs them.

The older style gas furnaces, (Not 80% and 90% eff) and gas water heater use a vent hood, or a draft diveriter. They are just on top of the unit or just off the side of the heat exchanger.

It act the same as the ones used on oil. Oil furance is under a slight pressure vs gas burning units. and that's why the damper is away from the oil unit itself. Nat gas is not under pressure in the burner area, and just works it's way out to the top or side, and is mixed in the vent hood/divitor. As the chimmey gets warm, it will "Pull" harder. so as it pulls, it will pull room air into the hood, and mixed in with the exhaust from the heat exchanger.
Old 01-30-03, 09:28 AM
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Mr. Wizard would be proud of this one.

As the boys have told you, it all depends on draft.

Most manufacturers instructions ask for .02in. w.c. draft over the fire. What that means is that if you have a U shaped glass tube partially filled with water and you connected a hose to one upright part of the U and put that hose inside the furnace, The draft inside the furnace should make the water rise up in that side and the difference between one side of the U and the other should be .02 in. As you can see it is a small amount. What should be at the breech (where the smoke pipe connects) is about -.04 in. w.c. Most chimneys make more draft than that and the draft would pull the heat through the furnace way too fast. Faster throughput means hotter temps going up chimney and that tends to increase the draft. Enter the draft regulator. The damper is balanced and adjustable. It should be closed when the unit is off and there isn't much draft in the chimney. Then as draft increases due to heating of the chimney, the damper opens in response to the higher draft and lets cool room air into the chimney. That tends to reduce draft and the damper will close slightly. It should be fluctuating most of the time. If you have minimal draft, it doesn't pay to have a regulator at all and it can be eliminated.

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