If your home utilizes natural gas or propane, chances are you have a gas water heater. Most of us take for granted the reliable and seemingly endless supply of hot water it provides. And, if periodically checked and maintained, a properly installed gas hot water heater will continue to provide a reliable and safe source of hot water for many years to come.
However, if your gas water heater is not getting adequate combustion air, or if the exhaust is not venting properly due to blockage or incorrect installation, combustion gases could be back-drafted into the home — exposing the residents to carbon monoxide (CO) and other harmful gases. The following application note reviews operation of the tank-style gas water heater and the potential for CO exposure from incorrectly installed hot water heaters and exhaust systems.
Tank-Style Gas Hot Water Heaters and Carbon Monoxide — The Basics
A tank-style gas water heater is the most common type of gas hot water heater found in the home due to its affordable purchase price and lower cost of ownership. It burns natural gas or propane to heat cold water and the reservoir inside the cylindrical tank holds the heated water until it is needed by the appliances and fixtures in the home.
The hot water tank consists of a cylindrical outer steel jacket and a specially coated, rust-resistant inner metal water holding tank. Running through the center of the tank is an exhaust flue which channels exhaust gases from the gas burner at the bottom of the tank to the exhaust vent at the top of the tank. A metal baffle inside the flue is used to maximize the conversion of heat from the exhaust gases to the water to be heated in the tank.
However, the combination of gas water heaters and carbon monoxide can present some serious safety issues for the home or business owner.
The Potential Hazards of Carbon Monoxide (CO) from Atmospheric Vent Gas Hot Water Heaters
Gas water heaters that release CO into the environment most likely have an issue with the venting of the exhaust, especially for atmospheric gas water heaters. Atmospheric gas water heaters draw combustion air from the space in which the appliance is installed and vents into exhaust piping or a chimney running vertically through the roof.
This type of venting relies on the heat of the gas and piping to draw the fumes up and out of the house. In contrast, power and direct-vent gas water heaters use a fan or blower to push the exhaust out the side of the building, making CO issues less common unless installation is poorly done.
Typical problems with atmospheric vent gas water heaters include:
- Incorrectly installed exhaust piping – If the exhaust piping has a horizontal or downward slope, multiple bends, or diameter reduction, it can result in the exhaust gases being released directly into the room.
- Plugged vent pipe or chimney – An obstruction in the vent pipe or chimney can prevent exhaust gases from rising up and out properly, causing CO and other fumes to be released into the home.
- Insufficient combustion air – When a gas water heater is installed in a small room without proper door vents (i.e one on the top of the door and one on the bottom of the door), there will not be enough fresh air for the gas hot water heater to vent properly.
- Low or negative indoor air pressure – When exhaust fans and dryers are pushing air out of the home, there is a potential for low indoor air pressure which can create a back-draft of the gas hot water heater exhaust. Essentially, the combustion fumes from the hot water heater will be sucked back into the home.
Signs that there may be a venting issue:
- Moist air around the draft hood of the gas hot water heater can signify a backdraft or exhaust venting issue. A venting or back-draft issue will cause a smooth surface such as a mirror or piece of glass to fog up when it is positioned near the draft hood.
- Corrosion or condensation on the top of the water heater can be caused by a venting issue as moist exhaust gases from a gas hot water heater are acidic.
- Soot residue can also be an indicator of a venting or backdraft issue.
- Slightly melted plastic due to hot gases around the cold inlet and hot outlet pipes could signify a back-draft or venting issue.
If you are experiencing issues with carbon monoxide (CO) due to a gas water heater in your home, you should immediately call a gas plumber, plumber, HVAC technician, or home inspector to investigate.
This professional can inspect the appliance to check for correct installation and venting as well as diagnose and correct any problems due to faulty parts, improper maintenance, or other issues.
To learn more about the effects of CO poisoning, please review Understanding the Effects of Carbon Monoxide