How They Work
A gas water heater heats the water inside the tank using a burner assembly. The pilot light stays lit, and when the thermostat turns on in the gas valve the gas control valve opens to supply gas through the fuel tube into the combustion chamber. The pilot lights the gas, and a flame begins to heat the bottom of the tank.
As the air inside the combustion chamber is heated the hot air begins to slowly rise and make its way through the center baffle to the top of the water heater. This draft causes air to be drawn into the bottom of the water heater to supply the needed air for the burner assembly to burn efficiently. As the hot air rises, it radiates into the tank helping to heat the water.
Burner Assembly
The burner assembly is a term for the burner plate and all attached components. The burner itself requires no maintenance under normal conditions, and only needs to be clean to operate efficiently.
When replacing a burner assembly, the important thing to be aware of is the placement of the components which are clipped to either the burner or the fuel tube. The igniter, pilot, thermopile, and fuel tube placement is critical.
To remove the burner assembly, you first remove all components from the gas valve by unhooking the two wiring connectors and unscrewing the pilot and fuel tubes. The pilot tube connector typically has reverse (left-hand) threads, and strips easily if you turn it the wrong way. Next, you take out the screws from the corners of the combustion chamber cover door. After this, you can lift the entire assembly up and it slides straight out.
Burner assembly. From left to right, the components pictured in the foreground of the cover door are: igniter wire, pilot, thermopile flame sensor, fuel tube, and viewing window.
Igniter
Thermal switch (left), igniter (center) and pilot (right).
The igniter has a single wire that runs from the rod next to the pilot to a push button mounted on the gas control valve. Pushing the button emits an electric arc or spark that ignites the gas coming out of the pilot.
Test the igniter when the water heater is turned to "off" by looking through the viewing window and pushing the igniter button several times. You should see a spark coming out of the rod.
If you are having trouble seeing, you can disconnect the igniter wire at the harness at the cover door. With the end of the wire approximately 1/8"-1/4" away from some metal, such as the fuel tube, press the igniter button several times. You should see an electric arc. If you can't get it to arc, the igniter is bad and needs to be replaced.
Pilot
The pilot is always on and lit when the water heater is working normally. The only time the pilot should not be burning is when the water heater is set to "off". To light the pilot, you rotate the knob on the gas control valve to "pilot" setting, press and hold it down for 90 seconds, and press the igniter button 3 times. Release the knob, then rotate the knob on the gas valve to the desired temperature setting. If you are troubleshooting, look through the viewing window while you press the igniter button to observe the pilot. If the pilot will not light at all, there is a gas supply problem.
If the pilot light lights, but goes out when you release the pilot knob, there is an electrical issue. Either there is no current getting to the gas valve (likely), or the gas control valve is malfunctioning (unlikely).
Thermal Safety Switch
The thermal safety switch is located on the cover door. A typical setup is that the thermopile is hooked to the the thermal safety switch. The voltage that is generated by the thermopile must travel through this switch on the way to supply power to the gas control valve. It acts as a cutoff switch to stop electrical supply to the gas valve so that the gas control valve can't open to supply gas through the fuel tube to the burner assembly.
To test if the thermal safety switch has been activated, press the reset button and listen and feel for a click. A click indicated the switch was activated, which indicated that the combustion chamber overheated, or the switch is failing. Overheating is caused by improper air supply or air flow. Check the exhaust section for troubleshooting air supply issues. If you rule out air supply as the problem, then the switch may be failing and require replacement.
If the pilot will not light and the switch does not click when you press the button then test the switch using a multimeter. To do so, first disconnect the wiring connectors from the switch at the door. On the continuity setting, place one probe from your test leads on each of the wire mounts on the switch. The switch is normally closed, so if there is continuity this indicated the switch is working and allowing voltage through. If there is no continuity, the switch has failed.
Thermal safety switch. Reset by pressing the red button.
Thermopile
Most modern water heaters have replaced the thermocouple with a thermopile, which is just a bundle of 10 or more thermocouples in one unit. The thermopile generates electricity from heat, which is what supplies the gas control valve with the required power to operate. The thermopile is placed near the flame so it can be heated sufficiently to generate more than the 350 millivolts required to power an electronic gas control valve. Typically the thermopile is hooked to the thermal safety switch, and then two wires connect from the thermal safety switch to the gas control valve through a wiring harness or clip. If the pilot will not stay lit, and you have already tested the thermal safety switch, the next step is to test the thermopile.
To test the thermopile you need to put a flame to the end of the thermopile, and use a multimeter set to detect millivolts DC, or just on the DC voltage setting if you are using an auto-ranging multimeter. Normally you apply flame by simply holding down the pilot light button while it is lit. First, disconnect the wires of the thermopile from the gas control valve. Next, with the flame applied, put a probe from each test lead on the two separate wires and observe for 90 seconds. The millivolts should slowly climb as the probe heats up, and exceed 350 millivolts within 90 seconds or the thermopile is bad.
Exhaust
The exhaust must be able to vent to the roof or through the wall to outside in an approved area. In addition to the problem of not heating properly, if there is inadequate make-up air or blocked vents the combustion process may produce excess carbon monoxide during the burning of natural gas. This would create a potentially hazardous or deadly environment. This is a rare situation, but still care must be made to avoid this problem.
Figuring out how much make-up air is needed for combustion can be calculated using the water heater BTU's and cubic feet of open space in the room.
Make-up air required = BTUs * .05 cu. ft.
A standard residential gas tank water heater is approximately 40,000 BTU.
Example: 40,000 BTU x .05 cu. ft. = 2,000 cu. ft. of air needed
To calculate the cu. ft. of air available, take the Length x Width x Height of the room, in feet.
Example: Laundry Room is 12 ft x 8 ft x 9 ft tall = 864 ft.
Because in the example there is inadequate make-up air available, the door would require a louvre on the door or another source of ventilation to provide additional make-up air. If a grill or louvre is required additional calculations can be made from the following formula:
Louvre/Grill opening required 1 cu. in. per 1,000 BTUs. 40,000 BTU water heater / 1,000 = 40 sq. in. louvre free area.
To calculate the free area of a louvre, use the following formula:
Louvre size: Length (inches) x Width (inches) x coefficient,
Where coefficient is .75 for a steel louvre or .25 for wooden louvre or louvre doors.
Example: 40,000 BTU / 1000 = 40 sq. in louvre required.
Wooden louvre installed on top and bottom of door: 14" x 12" x .25 = 42 sq. in. free area.
Tools
Troubleshooting
All of the detailed steps you need to properly test an element and thermostat are already listed above, so this section will serve as a guide to help diagnose the problem.
No hot water
Attempt to light the pilot
If pilot does not light make sure that the igniter is sparking. If the igniter is sparking, troubleshoot gas control valve to determine why there is no gas.
If pilot lights, but does not stay lit when you release pilot button check the thermal reset switch. If it clicks, relight the pilot and try again. If it does not click, test the reset button to see if it has failed.
If the thermal reset is not the problem, test the thermocouple
If the thermal reset button and thermocouple both test good, the problem is likely the gas control valve. Replace the gas control valve.
Water is warm, but not hot
Check the temperature setting. If needed, turn it up to see if this helps the problem.
If adjusting the temperature setting has no effect, check the dip tube. If the dip tube is broken incoming cold water will mix with the hot water. If damaged, replace the dip tube.
If the above steps do not work, replace the gas control valve due to a faulty thermostat.
Odor problems
If the hot water comes out smelling like rotten eggs, or hydrogen sulfide, this is caused by bacteria growing on the anode rod. Flushing will not fix the problem. Replace the anode rod.