Closet Gas Furnace Installation

A gas furnace installed in a closet can become a hazard to the occupants if it is carelessly installed. Provisions for combustion air and sufficient air for the flue are critical. Return air ducts and grille placement can also be critical.

Here’s an illustration (straight from a training manual on duct installation) that attempts to show how a furnace can be installed in a closet. What it actually demonstrates is what NOT to do.

Note the the return-air filter does not pass through the closet wall. Return air is pulled through grilles on the closet door. If this were an electric furnace, the installation would work. However, because this is a gas furnace with a flue, this installation is can become deadly. In fact, it may be deadly the first time the furnace is fired.

The furnace depends on combustion air and dilution air to be available in the same air space (combustion air zone) as the return-air. If the grilles on the door are sufficiently large, enough air for both the flue and the return air might pass through the door, into the closet.

However, if the grilles become partially blocked with dust, the blower return-air will compete with the furnace burners and flue for available air, and the blower will win. When this happens, the flue gasses will be drawn down the flue and directly into the return air. As the burners operate and flue gasses fill the closet, the flue gasses will be re-burned by the furnace. This will create very high levels of carbon monoxide, very quickly.

Burners that normally produce less than 10ppm CO will be generating 200 to 1,200ppm CO because of re-burn. The high concentration of carbon monoxide is then pulled into the return-air filter and distributed throughout the structure. If a carbon monoxide alarm is not installed, the occupants will be overcome by carbon monoxide poisoning in a surprisingly short period of time.

The closet furnace style of installation has been used in thousands of apartments and small office buildings.  It is one of the reasons a furnace is always the first appliance identified as the culprit after a carbon monoxide incident.  Officials condemn the furnace, yet it is usually the application that is at fault.

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