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Consider furnace type, BTUs weighing air supply needs

QuestionNot along ago, a furnace inspector said that my furnace room does not have a large enough vent going into it. He said I would have to leave the furnace room door ajar until I could add more ventilation in order to avoid the buildup of carbon monoxide. The current vent grill is about 1 foot by 2 feet, but he didn't say how much more was needed. What are the guidelines for the correct ventilation of a furnace room?

AnswerInterestingly enough, I'm in the process of framing a mechanical room to house my gas furnace, water heater, and water softener. As I researched an answer to your question, I was able to answer my own question on the same topic—you just beat me to it!

I'm not sure that there is an easy answer to your question because several factors come into play. A major consideration is the type of furnace you have, i.e., is it a gravity warm-air furnace (draft hood) or fan-assisted or a direct-vent appliance (which brings fresh air back into the home). Will there be another appliance in the same room such as a gas water heater? Also, how old is your house?

According to the Uniform Mechanical Code, in unusually tight construction, vented appliances can be installed in a mechanical room if the room contains at least 50 cubic feet of volume for each 1,000 BTUs of fuel input. A combustion air opening can be sized at 1 square-inch per 3,000 BTUs of fuel input. This opening must be in the upper 12 inches of the room and needs to freely communicate with the outdoors.

Direct-vent appliances are vented directly to the outside, usually through a wall and they get combustion air the same way.

In ordinary construction where gravity warm-air or fan-assist furnaces will be installed in a mechanical room, then two required openings of 1 square-inch per 1,000 BTUs of fuel input are required, one located 12 inches off the floor and the other within 12 inches of the ceiling.

Gravity warm-air or fan-assist furnaces are those appliances that vent out through the roof—normally through a chimney—and do not bring in combustion air from the outside. The requirements for combustion air are the same under the Uniform Mechanical Code as under the International Fuel Gas Code.

Where other appliances, such as a gas water heater with draft hood, share the same room, then the BTUs of both appliances need to be added together to get the total BTUs of fuel input for this room.

It's not clear to me if the furnace inspector you mention was someone who worked for the company that installed the furnace or if the inspector worked for the city or county building department. A mechanical permit should have been issued for this project when the new furnace and water heater were installed and any deficiencies in venting and combustion air should have been identified by the inspector and corrected prior to the final inspection. Because carbon monoxide is involved, this is a safety issue, so I suggest that you contact your building department for specific guidance.

Copyright © 2004, 2006, & 2007 LAF/C.R.S., Inc. All rights reserved.
Question answered by Leon A. Frechette.

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