Furnace exhaust question

A new oil fired furnace has a flapper valve installed in the exhaust line between the furnace and the wall. I can see the valve operates as a check valve, calibrated to swing in on a certain negative pressure in the exhaust.

The old furnace didn't have such a thing, and the new valve blows by, releasing exhaust gas into the space ... it appears to be designed to a "what's a little exhaust release among friends" standard, with a 1+ mm gap around the flapper plate! Why is the system set up this way, and what bad things might happen if this damn valve were capped?

(The furnace is on the ground floor of a three story house).

Reply to
Edward Green
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The valve is intended to set an lower limit on the firebox pressure. When the air pressure in the flue at the exit of the firebox is above the limit, the valve is closed. When the pressure goes below the limit (due to high flue temperatures, wind, etc.) the valve opens, admitting room air into the flue.

Mixing the flue gas with room air cools it, reducing the thermal head in the flue, reducing the draft. (The draft is minus the firebox gauge pressure.)

Burners work best at a specific firebox draft. Too much and the combustion may be inefficient, and in the extreme limit, the flame will blow out. Modern boilers should have functional draft control. Your boiler guy is supposed to set it at installation and at routine service intervals. If he doesn't, get another boiler guy. Don't readjust it, and don't cap it.

In normal operations, no flue gas should flow into the room. If you have positive pressure in your firebox, call your boiler guy.

hth, Fred Klingener

Reply to
Fred Klingener

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