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The Gas Code Defines "Confined Spaces"

by Gregg Harwood

Finishing basements is a very popular and cost effective way to add useable space to a dwelling. Many home owners feel comfortable in tackling this job themselves, where less-than-perfect craftsmanship is usually acceptable. However in building a partition around the heating equipment it is easy to create a confined space which may not have adequate air to supply the combustion equipment. And inadequate air means backdrafting and carbon monoxide.

The National Fuel Gas Code defines an confined space as "a space whose volume is less than 50 cubic feet per 1000 BTU per hour of aggregate input rating of all appliances installed in that space." This is also known as the 1 to 20 rule. It takes one cubic foot of room volume to supply twenty BTU per hour. OK, lets do some math. Let's say we have a 150,000 BTU furnace and a 40,000 BTU water heater in one room. This combination needs a whopping 9500 cubic feet of room volume. Your standard 8x10 mechanical room with an 8 foot ceiling only has 640 cubic feet. In fact, this combination needs a 30x40x8 room (or combination of interconnected spaces). This is about the size of a standard ranch house basement. If you have a larger furnace or a clothes dryer in the space of course you need more room volume.

The most common way of providing adequate air to confined mechanical rooms is to replace the solid door with a louver door or to put a couple of grilles through the wall to connect other interior spaces. Well, lets do the math on that. The National Fuel Gas Code calls for one square inch of free air space for each 1000 BTU in each of two openings. One opening has to start within 12 inches of the ceiling and the other within 12 inches of the floor. So for our hypothetical installation with a combined 190,000 BTU we need two openings with free areas of 190 square inches each. The Code says that if the louvers are not label otherwise to calculate metal louvers at 75% free space and wood louvers at 25%. So our installation needs two metal louvers at 253 square inches ( or approximately one foot by two foot). Now let's look at the wooden louver door option. At only 25% free area the door will have to be 1520 square inches in the louver portion. A common 3/0 X 6/8 door has a louver section just about that size or maybe a little bigger.

As home inspectors we are not out there in the field with tape measures and calculators, but my goal here is to illustrate that many gas appliances are installed in confined spaces and that fairly simple corrections can usually be made. Beware though! After we have done all of this math the Code goes on to state a number of conditions that could still lead to inadequate ventilation of the equipment. The operation of exhaust fans, clothes dryers and fireplaces can have an effect as can having an open cold air return in the mechanical room. The Code specifically states that when a furnace has supply ducts which carry circulating air outside of the space containing the furnace the return air shall also be handled by ducts terminating outside of the furnace space. The Handbook to the Code also brings up the point that modern construction methods may make houses which are tighter than what was anticipated in the Code and which may need additional make up air. This point was driven home to me a few years ago when after our firm remodeled a basement in a 25 year old house the water heater would back draft unless a window was open about an inch, even though it had unrestricted access to the entire basement. We ended up installing make up air from the exterior to this unit.

These issues are discussed in sections 5.3 and 6.3 in the National Fuel Gas Code Handbook.