With natural gas water heaters, combustion gases must be properly vented to avoid carbon monoxide issues. Atmospherically vented gas water heaters rely on the natural draft of air upwards into the flue to remove combustion gases. However, sometimes a backdraft—ambient depressurization that prevents the evacuation of gases—releases carbon monoxide in the immediate environment and poses a health and safety risk.

The carbon monoxide risk is slight in an unconditioned garage, but homes in the Northwest often have combustion appliances within a conditioned space. Placing a water heater next to another atmospheric or fan-assisted combustion appliance presents a safety risk. The two appliances will fight with each other over the same air mass, and one wins while the other backdrafts. Installing a clothes dryer near the water heater also impacts the unit’s effective exhaust.

The best solution to venting water heaters safely is to install a power-vented or a sealed combustion system, which does not rely on atmospheric venting. Another solution is to remove doors confining the space in order to use interior air for ventilation. An older solution was to bring in make-up air from outside the building envelope, which increases air leakage between the house and the exterior. It’s also common for homeowners to seal these holes without understanding the increased risk of carbon monoxide poisoning.

Heat pump water heaters also require special safety considerations. As an air-sourced model actively exchanges heat with the stored water, warm air enters the system and cold air is drawn out. This heat exchange tends to make the ambient space cold. Homeowners who have a conditioned space for their water heater seek units with ducting to vent the cold air outside the home. While great for efficiency and comfort, venting the cold exhaust air outside may depressurize the indoor air space. That can cause any atmospherically vented combustion appliances nearby to lose necessary air and release carbon monoxide.

To safely install a water heater, consider all aspects of a home’s system thoroughly. When installing a water heater in a specific location, consider water heater performance and impacts of the operation of other appliances. Problems and solutions will vary greatly from one home to the next, and well-informed decisions and carefully considered methods yield safer installations.