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One of the most common furnace problems that I see is when the furnace hot surface ignitor will not glow. The furnace may have a bad ignitor, loose wire connections, open limit switch, open rollout switch, open pressure switch, or bad control board. This is what you might see in the furnace sequence of operation:
1. Thermostat calls for heat when you turn up the thermostat.
You should first check to make sure you are getting 24 to 28 volts ac between the W (white) and C (com) terminals on the control board.
Make sure furnace blower door safety switch is pressed in when testing. If the control board has a fuse on it, make sure the low voltage fuse is not blown. You might have to temporarily tape the blower door safety switch closed for temporary testing. If the fuse is blown on the control board, nothing will happen. If the fuse is blown, you might have a short in the low voltage thermostat wires. Check to make sure the wires are not pinched anywhere.
2. Draft inducer motor starts.
If the draft inducer does not start, then you either have a thermostat problem, thermostat wiring problem, loose wire connection, transformer problem, bad draft inducer relay on the control board, or a draft inducer problem. Make sure you can spin the draft inducer wheel freely. Sometimes if the draft inducer sits for a long time (like over summer) without operating, the draft inducer wheel can become tight.
3. Pressure switch attached by a small plastic or rubber tube to the draft inducer senses the negative pressure produced by the draft inducer and closes.
You should have 24 volts to ground across both terminals on the pressure switch.
If the pressure switch is not closed with the draft inducer running, check for a stopped up vent or a stopped up condensate drain line if you have a condensing furnace.
4. Limit switch and rollout switch/switches should all be closed.
Press in on the reset button in the center of each rollout switch to make sure each rollout has been reset. You should have 24 to 28 volts from each terminal to ground. On most furnaces the ignitor will not glow unless all the safety controls (limit, rollout switch/switches, pressure switch) are closed. If a limit or rollout switch is open, you can have an overheating problem and a possible dangerous condition. You may need to replace the limit switch or rollout switch.
5. Draft inducer runs for 30 seconds to a minute before you hear a gas hissing sound.
If you hear a hissing gas sound with no gas ignition, then more than likely either the ignitor is broken or the relay on the control board that sends power to the ignitor is broken. If the ignitor did not glow, the flame sensor (a small metal probe about 1/8-inch in diameter, with a white porcelain base) does not sense the flame, so after eight to 10 seconds the hissing sound stops with no ignition of gas to heat the home. The furnace will shut down, and try ignition again. Most of the time after three tries the furnace will go into a lock-out condition until you turn the power switch back off and on again.
Publication date: 2/3/2014