The Consequences of Supply Logs in Attached Garages


If you pay attention to local or national news, you’ve probably seen reports of vehicles with remote start capabilities contributing to CO (carbon monoxide) poisoning.

In most stories, the car is accidentally turned on with the remote control while parked in an attached garage.

The risk of CO poisoning increases in homes with supply registers that condition a garage attached to the living space. It’s easy to assume that air will only blow through these openings, but that could be a dangerous misconception.

The dampers and the ducts connected to them can become unintended passages for automatic exhaust and other harmful vapors to enter the living space. Let’s look at how supply registers in an attached garage can have these deadly consequences if the wrong conditions occur.

Three airflow rules

It would help if you had some airflow basics to understand what happens when supply registers are in a garage. These three simple airflow rules will help you understand why this is a problem and how it can occur. They are:

1. Air always takes the path of least resistance

2. Air always moves from higher pressure to lower pressure

3. Heated air rises while cooler air falls – often referred to as the stack effect.

Keep these principles in mind as we look at what happens when the HVAC fan is on and what can happen when it is off.

When the HVAC fan is on

We typically focus on the HVAC fan running in the “on” position with power registers in a garage. The system provides air conditioning to the garage during equipment operation to temper the air in the garage. Many homeowners want to prevent their water lines from freezing or comfortable beloved antique show car.

Unfortunately, this practice can have unintended consequences. Let’s say you have a three ton system moving 1200 cfm (cubic feet per minute) of airflow. If you add 200 cfm of supply air to the garage with no return air, the house will have 1200 cfm of return air but only 1000 cfm of supply air.

A airflow imbalance now exists between the supply and return airflow, creating negative pressure inside the house. The fan will try to squeeze the “missing” 200 cfm from wherever it can. This reaction can lead to backdrafting, excessive equipment run times, IAQ (indoor air quality) issues, and comfort issues. Remember that air takes the path of least resistance.

When the HVAC fan is off

Unless the HVAC fan is running, the supply damper openings in the garage are simply pathways that connect the garage to the living space. They can shoot from the air in the house as easily as blowing it. Whenever there is an opening and a pressure difference across it, you will have airflow moving through that opening.

Automobile exhaust is the primary concern in this situation. The CO produced by a car is always excessive and dangerous. Once the garage door is closed, it can drag on for a long time. If the customer backs into the vehicle, the remaining CO levels in the garage are usually even higher.

When another fan inside the house is turned on to exhaust air outside – such as a clothes dryer, bathroom fan, or kitchen exhaust fan – an imbalance in airflow air is created. Air is drawn from the path of least resistance to compensate for the air exhausted to the outside.

Unfortunately, opening a supply damper could be a pathway for air to flow back into the house. fumes can be brought back into the living space if an automobile or combustion equipment, such as a gas grill or generator, passes through or near the garage.

Antennae up

Be aware of the danger and keep your eyes peeled for any ducts that run into a garage or other attached building outside the main house. If you notice this installation defect, you have a professional obligation to make your customers aware of the potential dangers.

The best action is to eliminate all connections between the garage and the living room. If the client wants to cool the garage, offer an upgrade to a ductless mini-split or a separate heating and cooling system and be sure to isolate it from the living space.

Many codes prohibit installing supply registers in attached garages, but that doesn’t mean you won’t see it. This is usually an afterthought, added after the original install.

The principles of airflow imbalance in a building apply to hundreds of field conditions. Be proactive and keep your antennas open to offer your customers a higher level of security.

David Richardson serves the HVAC industry as Director of Training for the National Comfort Institute, Inc. (NCI). NCI specializes in training focused on improving, measuring and verifying HVAC and building performance.

If you are an HVAC contractor or technician interested in learning more about adding carbon monoxide testing to your services, contact David at or call 800-633-7058. NCI website is packed with free technical articles and downloads to help you improve your professionalism and strengthen your business.

Photo captions

Photo 1: Air can flow back through a supply damper under the right conditions.

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