Question about Heating & Cooling
The exhaust blower starts, but the pilot igniter does not cycle
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Posted on Jan 02, 2017
You have a limit switch located in the furnace (a little bigger than a dime and located near the center of the front panel, sometimes the switch is mounted to a small fiberglass board) . There will be two wires going to it. They sometimes trip so often that they fail to automatically reset(due to dirty filters or undersized ductwork). You can SHUT OFF POWER TO THE FURNACE, then remove the two screws holding the switch in the furnace, pull the switch out of the furnace and tap on it and wipe it off.
The switch may work a few more times but should be replaced with the same size and type. You will see some rating printed on the switch somewhere. Write down all the numbers and get a replacemet from a furnace/AC supply house. If you have a meter and know how to use it you could check the switch for continuity.
Sometimes just a tap on the switch will get it going. There will also be a couple of rollout switches that could be tripped. They have little buttons on them to reset them. If they trip it may have other issues to be solved.
Hope this helps!
Posted on Feb 01, 2009
Keep in mind all furnaces and circuit boards are not the same. Look on the back of the door to the blower compartment. You should find the list of diagnostic codes. Look up four blinks it should narrow down the problem. Although from the discription you gave it sounds lie the high limit control. When a high limit appears to be acting up it DOES NOT mean that the high limit needs replaced. The high limit just may be doing its job. This means that the furnace is overheating. Check your filters. Are all of your registers open? Is your blower set on the right speed? Is your blower going bad? Is the blower motor capacitor bad? Is your evaporator coil restricted? Just changing a high limit switch does not always solve the original problem!
Posted on Apr 03, 2009
On a call for heat, the 24 volt thermostat sends a signal to the control module. The control module will indicate a call for heat with a light on the control either blinking or remain solid depending upon model. The inducer (exhaust) blower will purge all gasses from the furnace and pressurize a pressure switch. Once the pressure switch tells the module to continue, the electronic ignition will energize and send 120 volts to the ignitor. The ignitor will glow and you will be able to see it if viewed thru the small inspection port. Once the ignitor gets hot enough, it sends a signal to the module opening up the gas valve (24 volts). Either a pilot will come on or the burner tube will ignite then spread the flame to all burners. Lastly a safety sensor will be looking for a certain temperature within a few seconds and the furnace will continue to operate and the room air blower will turn on in a minute or two.
What could go wrong? The unit will not run if there is no signal from the thermostat (bad thermostat or broken wire), the control module does not sense a signal from the thermostat (bad control), the inducer does not energize (bad motor), the pressure switch does not close (blocked vent piping, bad switch, plugged condensate hose), the ignitor does not energize (bad control, bad ignitor), the gas valve does not open or there is no gas (bad gas valve, broken wire, no gas), the pilot does not lite (dirty pilot), the burner does not lite (bad burner, plugged orifice, not enough combustion air), the flame does not spread to each burner (bad flame spreader, dirty flame spreader, more bad burners), the flame safety sensor does not detect flame (dirty or bad flame spreader, bad flame sensor, broken wire, bad control), or the room air blower does not energize (bad fan motor, bad control).
Most newer furnaces will have a diagnostic center or control built into the control module. You may be able to view it as it is running thru a small viewing port. Some models will require the removal of an access panel prior to finding it. On the reverse of the access panel, there will be a diagnostic chart that will aide you in understanding any error code recorded (usually a blinking light).
Posted on Oct 18, 2009
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