With the weather getting cooler, I thought that now would be a good time to talk about combustion air. Don’t forget to check for proper combustion air.
Most codes provide detailed drawings illustrating where combustion air should
come from and how much you need, but there are still many furnace installations
that rely entirely on air from inside the building for combustion air. In days
gone by this was often considered adequate so long as the furnace was located
in a large enough space. In newer homes, combustion air should always be
provided.
Most 90% furnaces today can operate using sealed combustion. In the case of a sealed combustion furnace, the combustion air is being piped in from the outside. The combustion air is piped directly into the furnace. These are easy to spot, they have two pipes: one for combustion air and one for the vent. Also, their panels have no louvers for combustion air.
Traditional furnaces get their combustion air from the space where they are installed. Combustion air enters through louvers in their panels.Since the furnace is drawing air from the space it is in, fresh combustion air must be supplied to the room to keep the process going. Failure to supply the correct amount of combustion air can lead to negative room pressure, vent spillage, poor combustion, and CO production. All these things together can be disastrous.
When a technician checks a furnace that does not have sealed combustion, one of the first things to look
for is how the furnace receives combustion air. If the furnace is in a
ventilated crawlspace or attic, the ventilation for those spaces provides the
combustion air. However, even these can be a problem. A large furnace in a
small crawl space may not have adequate combustion air if the crawl space vents
are closed. I have also seen crawlspace vents clogged with debris, effectively reducing the combustion air.
The most troublesome
installations are furnaces located inside the house in a closet. They should
have a combustion air vent near the floor and another near the ceiling. Someone
asked me about a furnace installed in a closet off of a bathroom. When they
turn on the bathroom vent fan, they can smell gas! Another story involved a
fireplace and a furnace. When the furnace came on it sucked the smoke out of
the fireplace into the room. These types of stories indicate that the furnace
does not have adequate combustion air.
What if there are no obvious combustion air vents? Sometimes the vents were never provided,
other times they have been covered up. I have seen combustion air vents covered
with tape or plastic. Undoubtedly, someone noticed cold air coming in the vent
and “fixed” the problem – thereby creating a combustion air problem.
Occasionally insulation covers the grille into the attic. Another problem is
using the furnace closet for storage. This is dangerous in and of itself, but
it can also cause combustion air problems if boxes are stacked in front of the
combustion air grilles. For details on
combustion air requirements check your local code. Unit 53 Gas Furnace
Installation in Fundamentals of HVACR, 3rd ed also
has detailed drawings and specifications for the most common applications.
