graphic for Carbon Monoxide column by David Richardson

Carbon Monoxide: Return Grille Location and Combustion Safety

September 06, 2019

By David Richardson

David Richardson, NCI Trainer

When you diagnose combustion safety issues, there are various interactions that can cause unsafe conditions you must be aware of. One interaction occurs when return grille location interferes with proper flue gas venting of a drafthood appliance. This happens when a return is located too close to the equipment, such as in the same room or even in another portion of the building. Let’s look at two return grille location scenarios that could impact the safe operation of drafthood equipment and discover what the common factor is.

Return Grilles in the Same Room

One common issue is when a basement is converted into finished living space. To make the space comfortable, supply registers and return grilles are added. In many of these basements, you’ll also find enclosed mechanical rooms housing a gas water heater and furnace.

Forward-thinking contractors will often install combustion air grilles on the mechanical room wall to let air in for the water heater and furnace. The problem with this installation is a return grille is also often added in the adjoining room. The combustion air grilles can provide an unintentional pathway to pull air from the mechanical room when the blower is running. A competition for airflow is created that may cause the water heater to spill flue gases while operating.

Remember, air takes the path of least resistance and can pull from a basement return much easier than a return grille located in the upstairs living area. There is risk that the flue pipe will become a makeup air duct that causes any natural draft equipment to spill exhaust gases into the living space.

Central Return Grilles

Central returns are another common installation that present a safety risk if the right conditions exist. The return duct on this installation is often a hard-ducted single filter grille application or equipment mounted on a stand pulling air from the bottom.

In both applications they are typically accompanied by an extensive supply duct system with a supply register in every room. This installation could become a problem when interior doors are closed to rooms with supply registers in them.

Each room with a supply register has no airflow path back to the return side of the system when the door is closed. The central part of the home is depressurized while rooms with any closed doors are pressurized.
The depressurized area of the home may contain a gas water heater that is now in direct competition with the house for available combustion air. The house will usually win if the blower in the furnace is running. Just as with the return in an adjoining room, the flue becomes a make-up air duct that can cause flue gases to spill into the conditioned space through the water heater drafthood.

Discovering the Problem

To discover if a return installation like those listed above has the potential for spillage, you’ll need to perform a draft interference test. If the test passes, you have enough make-up air to account for return grille airflow. If the test fails, you’ll need to recommend correcting the installation.

By paying attention to the draft interference test results, you can isolate the circumstances that could cause flue gas to spill due to the return. Don’t forget that you’ll also need to verify stable combustion readings in addition to draft measurements.

Solving the Problem

These installations are found everywhere and have many opinions about how to correct them. One solution frequently mentioned for central returns is to undercut interior doors approximately an inch. Unfortunately, this small of an opening rarely provides enough area for air to circulate back to the central return.

Another solution for central returns is to use jumper ducts or transfer grilles to relieve room pressure. As doors are closed, airflow between rooms is maintained. I have used these occasionally with some success, but not as much as I hoped for.

The real solution is to pay attention to airflow and make the needed repairs, so it is as balanced as possible. Ducted supplies and returns in each room with balanced airflow go a long way toward assuring neutral room pressures.

Remember, any rooms with natural draft equipment should ideally be kept at a positive pressure. They should never be in a negative pressure.

About the Author

David Richardson serves the HVAC industry as a curriculum developer and trainer at the National Comfort Institute, Inc. (NCI). NCI specializes in training focused on improving, measuring, and verifying HVAC and Building Performance. If you’re an HVAC contractor or technician interested in learning more about adding combustion testing to your services, contact David at or call him at 800-633-7058. NCI’s website is full of free technical articles and downloads to help you improve your professionalism and strengthen your company.