March, 2011
Inspection News and Views from the American Society of Home Inspectors

The Venting in Common of Multiple Gas Appliances


Over the years, we have received many questions regarding combined venting of furnaces and water heaters. The main concern has been whether or not a gravity-vented water heater flue can be connected to the fan-assisted flue of a
Plus 80 (AFUE-rated) forced air furnace.

Photo at top: a typical fan-assist Category I furnace. Photo © Mike Casey

A common misunderstanding of the National Fuel Gas Code (the basis for the gas piping sections of the UMC, IRC and entire IFGC) has led many inspectors to conclude that this is a prohibited connection. The National Fuel Gas Code, section 12. 4. 3. 4, states as follows: “Vent connectors serving appliances vented by natural draft shall not be connected into any portion of mechanical draft systems operating under positive pressure.”

At first glance, this appears to include all forced air furnaces with fan-assisted combustion systems. That conclusion has led many inspectors to make faulty disclosures in their reports. The purpose of this article, therefore, is to explain the meaning and intent of the pertinent code requirement and to clarify when vent systems can and cannot be installed in common.
To understand the venting requirements for forced air heating equipment, it’s essential to know the four vent categories of gas appliances. These can be found in the National Fuel Gas Code (ANSI Z223. 1).


Category I Furnaces are the type most commonly used for residential heating. They include old-fashioned gravity-vent furnaces and the newer fan-assisted Plus-80 models, also known as induced-draft furnaces. In a gravity-vented furnace, the heat of combustion provides sufficient stack effect to enable safe venting of combustion byproducts. Stack effect occurs because the hot gases inside the flue have lower density than the cooler atmosphere outside the flue.

A Plus 80 furnace is designed for greater fuel efficiency than a standard gravity-vented furnace. Sometimes, this is achieved by lengthening the heat exchanger to allow more heat transfer into the circulating air. Longer heat exchangers produce draft resistance, and they lower the temperature of the exhaust gases relative to atmospheric temperature. To enable proper venting, an inducer fan is built into the system. The fan applies a slight negative pressure on the heat exchanger to ensure that the products of combustion are evacuated upward. The fan, however, does not exert positive pressure into the flue pipe. The exhaust in the flue is gravity-vented. Therefore, its vent pressure is rated as “non-positive,” which is why it can be vented in common with a gravity-vented water heater.

Category II Appliances include those not typically intended for residential use. Exceptions are boilers and wall-vented water heaters. In those cases, the manufacturers’ specifications prohibit interconnection of the exhaust systems with other gas-burning appliances.

Category III Appliances
in residential use most often are tank-less water heaters that are vented with stainless steel. These appliances operate with positive vent pressure and, therefore, cannot be adjoined to a gravity-vented water heater.

Category IV Appliances
are mainly high-efficiency water heaters and furnaces that use plastic vent pipes. These appliances also operate with positive vent pressure and cannot share a common vent with a gravity-vented water heater.
Inspectors should not confuse fan-assisted furnaces with those that are power-vented. When a gravity-vented flue is connected to a power-vented flue, back-drafting can occur at the draft diverter of the gravity flue, exposing occupants of the building to noxious gases. With a Category I furnace, this is not a problem because both appliances are gravity-vented, even an induced-draft furnace.

The text box below, taken from an induced-draft furnace placard, clearly specifies common venting with Category I vents only. Many new furnaces now arrive with stickers stating that common venting is allowed because there has been so much confusion regarding these connections.


The illustrations above show typical specifications for common venting of gravity-vented and induced fan-vented appliances. Illustrations courtesy of Simpson Dura-Vent. Used with permission

Retrofit venting

Another potential problem with newer Category I vents involves furnace and water heater flues that are connected to masonry chimneys. In those cases, the cold masonry cools the exhaust gases, causing loss of draft as well as moisture condensation. The solution is to use the masonry chimney as a chase for a B-Vent.

When in doubt regarding any appliance type or installation, read the labels and, if possible, the installation manual. There are myriad appliances in today’s marketplace with many possible alternatives for proper venting.