graphic for Carbon Monoxide column by David Richardson

Carbon Monoxide: Keep Supply Registers Out of Attached Garages

May 27, 2019

By David Richardson

David Richardson, NCI Trainer

If you pay attention to the local or national news, you’re probably seen reports about vehicles with remote start accessories that have contributed to CO (carbon monoxide) poisonings. In most stories, the vehicle was accidentally turned on with the remote 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. These registers and ducts may become unintentional pathways for auto exhaust and other harmful fumes to travel into 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

To understand what happens when supply registers are in garage, you need a foundation in airflow. Three simple airflow rules will help you understand why this is an issue and what takes place. They are:

  1. Air takes the path of least resistance.
  2. Air goes from higher pressure to lower pressure.
  3. Heated air rises as cooler air falls – often referred to as stack effect.

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

When the HVAC Blower is On

The HVAC blower operating in the “on” position is the typical condition we focus on with supply registers in a garage. The system delivers supply air into the garage during equipment operation to temper the garage air. Many homeowners simply want to keep their water pipes from freezing or keep a beloved antique show car comfortable.

Unfortunately, this practice has an unintentional consequence that results in more return air being pulled out of the living space than supplied to it.

Let’s say you have a three-ton system moving 1200 cfm (cubic feet per minute) of airflow. If 200 cfm of supply air is added to the garage with no return air, the house will have 1200 cfm of return air and only 1000 cfm of supply air.

An airflow imbalance now exists between supply and return airflow that creates a negative pressure inside the home. The blower will try to pull the “missing” 200 cfm from wherever it can and cause backdrafting, excessive equipment run times, IAQ (indoor air quality) issues, and comfort problems. Remember, air takes the path of least resistance.

When the HVAC Blower is Off

Unless the HVAC blower is operating, supply register openings in the garage are simply pathways that connect the garage to the living space. They can pull air into the home just as easily as blow it out. Anytime there is an opening and a pressure difference across it, you will have airflow through that opening.

Auto exhaust is the main concern in this installation. CO produced from an automobile is always excessive and dangerous. It can hang around in garages for a long time once the door is shut. If the car was backed in, CO levels remaining in the garage are usually higher.

When another fan inside the home that exhausts air outside – such as a clothes dryer, bath fan, or kitchen exhaust fan – is turned on, an airflow imbalance is created. Air is pulled from the lowest resistance pathway to make up for air exhausted outside.

Unfortunately, a supply register opening could be a pathway for air to flow back into the home. If an automobile or some other fuel-burning equipment runs in or near the garage, their fumes can also be pulled into the living space.

Antennas Up

Be aware of the dangers and keep your eyes open for any ducts run into a garage or other building outside of the main house. If you see this installation defect, you have a professional obligation to make your customers aware of any potential dangers.
The best course of action is to eliminate any connections between the garage and living area. If the garage must be conditioned, offer an upgrade to a ductless mini-split system and make sure to isolate it from the living space.

The principles of airflow imbalances in a building apply to hundreds of field conditions. Be proactive and keep your antennas up for opportunities to offer a higher level of safety to your customers.

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 [email protected] 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.