Carbon Monoxide from Hot Water Heaters
Summary:Natural gas and propane fired hot water heaters pose a substantial risk of emitting high (potentially dangerous) levels of carbon monoxide gas into the living space of any home or building in which they are located. The primary cause is typically one or both of the following issues: 1. Inefficient combustion. 2. Improper venting of the exhaust gas. Every homeowner, property manager, maintenance technician, and building superintendent should conduct or arrange for periodic inspections of their hot water heater systems – and this should include a carbon monoxide emissions test for the flue system as well as a CO concentration analysis of the flue gas. The CO Inspector from Sensorcon is the ideal tool for accomplishing such inspections. |
Measuring the Carbon Monoxide in the exhaust coming from a hot water heater.
The Carbon Monoxide Inspector Kit, (which includes a hand aspirator pump & case) is used in this photo. It can be purchased for $179.00. |
Key things to look for:
Carbon monoxide concentrations near a hot water heater shouldn’t exceed 5ppm if ventilation is proper.
Exhaust from a hot water heater normally should not have carbon monoxide concentrations exceeding 100ppm, (except perhaps during 1st 30seconds after firing up). 10-50ppm is probably optimum for steady state operation.
Identifying CO Emission Problems with Fuel Burning Hot Water Heaters
Generally speaking, you should not read CO levels of more than 5ppm outside of the hot water heater, and exhaust from most hot water heaters should not be more than 100ppm. If you are going to do a comprehensive inspection, your detailed checklist for conducting a CO emissions inspection for a fuel burning hot water heater system should include the following:
- With the CO Inspector in-hand, enter the building and proceed to the room in which the water heater is located. If the carbon monoxide concentration in ppm (parts per million) increases as you get closer to the hot water heater, then there is a problem to search for, most likely improper venting (e.g. a leak). However, if the water heater burner has not fired for some time, a problematic situation may have diminished – so, even though an elevated CO level may not be detected at this time, the possibility of a CO related problem may still exist.
- If the burner is firing, turn the control knob to a colder water setting until it goes out. Remove the hatch at the base of the water heater and inspect the combustion chamber. Look around the area for excessive dust, debris, kid’s toys, dead bugs, clutter, etc. Make certain the area is clean. Vacuum away anything that shouldn’t be there. Look at the roof of the chamber – the part that the flames of the burner come in contact with. If the roof is black and sooty, then that’s a sign of trouble caused by poor combustion, drafting trouble, and fume problems. Among other things, this can be a fire hazard. If the roof is rusty with signs of white scale, the tank is corroding and the water heater should be replaced soon. A normal roof should appear clean and grey with perhaps some white condensation marks.
- Let the burner fire up by turning the control knob toward a hotter water setting (you may need to run the hot water a while to engage the burner). Take a look at the color of the flame, it should be mostly blue. If there is a lot of yellow, there is a combustion problem, generally caused by improper fuel to air mixture – the cause of which can be a maladjusted or dysfunctional regulator or contamination problem. Poor combustion can increases the concentration of carbon monoxide in the exhaust gas – and if the system is back-drafting or poorly vented this spells double trouble.
- With the burner firing, hold the CO Inspector near the bottom of the water heater and check for elevated readings. If the CO level in ppm is higher as you get nearer to this area, then there may be a back-drafting problem. It is best to use the pump while in MAX mode to take readings from the hot water heater.
- With the burner firing, hold the CO Inspector near the draft diverter at the top of the water heater and check for elevated readings. If the CO level in ppm is higher as you get nearer to this area, then there may be a clogged vent or drafting problem. You can check the drafting to verify if exhaust is being vented properly by lighting a cigarette or an incense stick and holding the smoking stick near the draft diverter. The smoke should be sucked up the pipe. If not, there is a problem. (*note that the smoke will contain high concentrations of CO, as it too is from an incomplete combustion process).
- With the burner firing, hold the CO Inspector near the exhaust duct at each and every connection point or seam, including where the duct may enter the chimney flue or exit the building. If the CO level in ppm is higher as you get nearer to any of these areas, then there is a leak in the exhaust path as well as a possible clogged vent or drafting problem.
- Install the sample pump with probe onto the CO Inspector. Insert the probe into the exhaust stream at the draft diverter at the top of the water heater. Squeeze the bulb 6 to 10 times to draw a gas sample. While viewing the digital display on the CO Inspector, take notice of the maximum ppm level achieved by operating in MAX mode. A CO level in excess of 100 ppm is indicative of a combustion problem and corrective action is recommended. (Note that higher CO levels will likely be observed immediately after the burner first fires up, as there will be excess fuel present briefly. CO levels of several hundred ppm should only occur during start up, then the CO levels should be in the 10s of ppm after the combustion process is in a steady state.) Proper combustion tuning is necessary to ensure safety and efficiency of the hot water heater. If the ventilation is proper, excessively high CO concentrations will be exhausted safely from the building. However, even if the ventilation from the hot water heater is adequate, high CO readings indicates a rich mixture/ poor combustion efficiency, which will increase your heating costs/energy consumption in the long run, and may even result in soot buildup that can lead to other problems.
Causes of Carbon Monoxide Emissions with Fuel Burning Hot Water Heaters
Carbon monoxide gas is a byproduct of incomplete hydrocarbon fuel combustion. Under safe and proper operating conditions, the CO gas is vented outside of the building. The operation of a fuel burning hot water heater becomes unsafe when the exhaust gas is insufficiently vented. Listed below are situations which can lower the venting performance of a hot water heater system:
- Obstructions in the vent path, including draft diverter, vent pipes, and chimney. Items to look for include dirt, dust, bee hives, insect nests, bird nests, animals and squirrel nests, crushed or crimped pipes, leaves, sticks, snow or ice buildup, improperly installed vent caps, roofing materials, water that has settled to a low point in the vent pipe path (if a low point exists, it is an improperly designed vent path).
- Check vents for corrosion, rust, gaps, and holes.
- The vent should be of the same diameter as the draft diverter for the water heater.
- Atmospherically vented water heater vents should generally be directed up and out, never down. Direct vent types should go out a side wall.
- Segmented vents should be screwed together with 3 screws at all joints to prevent the potential of becoming disconnected.
- Atmospherically vented vents should be tall enough to create adequate draft and should always be taller than the building so that carbon monoxide and other exhaust gasses will not be sucked into an open window.
- Notes about back-drafting: Water heaters are atmospherically combusted appliances. They draw air in to aid with combustion and exhaust spent gas out. Every cubic foot of exhaust gas leaving a building requires a cubic foot of fresh air to enter the building. If the building is airtight, the exhaust will not escape the building, rather it will be contained within the building envelope and create a health risk. Direct vent water heaters overcome this problem by pulling air for combustion from outside the building. Furnaces are also atmospherically combusted appliances. When a furnace is running, an inadequate supply of fresh air entering into the building can be supplemented by air being drawn down the flue of the water heater – a condition termed reverse stacking. When the water heater turns on, this condition creates back drafting which: a). causes inadequate fuel combustion in the water heater, leading to increased carbon monoxide levels in the combustion gas and b). causes this gas to remain within the building envelope since it cannot escape up the flue. Here again, a direct vent furnace will overcome this problem.
Further resources for troubleshooting problems with hot water heaters can be found here:
