Model #120.000 AMANA Countertop Microwave

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What does a draft hood do?


To help the burner see a constant air supply and constant draft, appliances with natural draft have a draft hood or a draft diverter. This opening at or near the top of the appliance allows room air to be drawn into the chimney. As the draft condi-tions change in the chimney, more or less air can be drawn through the draft diverter as necessary. This allows the burner to see a constant air supply regardless of outside temperature differentials or wind.

The hot gases heading up the chimney tend to create a strong draft, and can actually causes excess airflow through the burner, unless we put in the draft hood.

Mixing the room air with the exhaust products at the draft hood cools the exhaust. The exhaust products which approach 500F are cooled to about 300F or 350F after they've passed the draft diverter. We don't want to cool them too much, because we would start to get condensation. Furnaces are designed to have dis-charge gases in the 300F to 350F range, after they get past the draft hood.

Furnaces need both combustion air and dilution air. The requirements are about 15 cubic feet of combustion air and 15 cubic feet of dilution air for every cubic foot of gas burned. Since a cubic foot of natural gas contains about one thousand BTUs, a furnace that fires at the rate of 120,000 BTUs per hour, would use 120 cubic feet of gas if it fired constantly for 60 minutes. This means that it would use two cubic feet of gas per minute. We need about 60 cubic feet of air per minute (30 x 2) to ensure proper operation of this furnace. This is similar to the capacity of a typical bathroom exhaust fan. If we put the furnace in a closet and seal it off, it won't have enough air to work properly. Many authorities work on 50 cubic feet of air for every cubic foot of gas, building in a safety factor for unforeseen problems.

The available air in a closed room 5 feet by 5 feet by 8 feet tall is 200 cubic feet. This would be consumed very quickly by a furnace firing at this rate. If the room could not easily replace the air, the room and the furnace would be under negative pressure, relative to the chimney.

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expert -
Sears Technician
April 26, 2007