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Ventilation and air supply for furnace room

QuestionI have been a home inspector for 18 years and your article about the inspector and furnace room reminded me why home inspectors try to stay away from quoting codes. We have to be generalists as it is practically impossible to be an expert in every area. I believe gas appliances require a minimum of 100 sq. inches per opening for combustion air or 1 square inch per 1,000 BTUs, whichever is greater. Also, these openings are usually covered with metal or wood louvers or grilles in which case multipliers of 75% and 25% must be used, respectively.

Please correct me if my interpretation is wrong. Since it is so difficult to keep up on all the codes, I rely heavily on the code check field guides written by several very talented home inspectors. They have bailed me out many times when I needed to flag an item and direct my client to an appropriate technician. Keep up the good work!

Respectfully, William M., Inland Home & Building Inspection, Inc.

AnswerWilliam, there are two things I never wanted to do when I entered the construction field: own rental property and be a home inspector. However, I guess when I got my contractor's license back in '76, I soon learned what it meant to be a home inspector whether I wanted to be one or not. In order to bid projects, you do indeed have to inspect what you build, and over the course of time you end up learning a lot about the codes involved: construction, electrical, heating, and plumbing. But you are right; trying to remember all these codes is a lifetime commitment. Today I still inspect projects, but normally they are for court cases, and that is another can of worms!

The article you mentioned, "Consider furnace type, BTUs weighing air supply needs," appeared March 4, 2004. Technically you are correct, but it depends on the type of furnace in this room, 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). Also to be considered are the age of the home and whether another appliance will be in the same room such as a gas water heater.

The Uniform Mechanical Code states that 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 sq. 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, two required openings of 1 sq. 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-assisted furnaces are those appliances that vent out through the roof—normally through a chimney—and do not bring in combustion air from the outside.

Washington State (where I live) has defined any house built after July 1986 as being "unusually tight construction." Homes built before that time are normally considered "ordinary construction," unless the house was upgraded with weatherstripping and insulation and air leaks were sealed. Washington has adopted, and will soon be enforcing, the International Mechanical Code. Under this code, the requirements for combustion air are the same as those under the Uniform Mechanical 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.

Grills or louvers that cover these openings will restrict the required vent openings, so yes, the multiplier should be used; unfortunately many inspectors don't consider it even though they should. The required opening size should be based on the net free area of the louver or grille to be used to cover that opening. The percentage you refer to assumes that wood louvers will have about 25 percent net free area whereas metal louvers and grilles have about 75 percent net free area. I should also point out that if wire mesh is involved, the minimum mesh size shouldn't be smaller than 1/4 inch. I hope this clears up any confusion, and I appreciate your kind remarks!

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

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