Blower keeps running, issue intermittent

Old 02-06-07, 08:44 PM
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Angry Blower keeps running, issue intermittent

Initial problem was the heat would not kick on, and we'd have to turn the unit off and on to get the heat to ignite. We have a Trane gas furnace, horizontal (18-CB15DD5-2) unit. Initial diagnosis was a bad control panel which was replaced. Heat then would kick on, but would not reach set temperature, then heat would shut off while blower continued to run. Code registered that it was blocked out - and that it was getting too hot. I had to manually turn the power off on the furnace in the attic to get the blower to stop and reset the thermostat. Contractors diagnosed the issue to be electrical, (i.e. bad connection in wall) so they re-ran a wire from the furnace to the thermostat. That worked for about 1.5 weeks, now it is doing it again. I'm concerned the overall furnace is bad (it's 12 yrs old - but we've already sunk $ into fixing it. Any suggestions on what the problem is? I want this fixed. Thanks.
Old 02-13-07, 01:36 PM
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When the blower keeps running and the furnace won't fire, this is sure to be a lockout situation where the board locks out the firing sequence usually after three tries. Resetting power remedies this but doesn't cure the problem.

Essentially once the gas comes on and heats the heat exchanger a sensor picks up the temp increase and turns on the circulating fan. When the stat is satisfied it turns off the gas, and when the heat exchanger cools enough then the fan stops. Note that the fan operation is independent.

So the problem could be a safety switch that keeps the furnace from firing.
Check the safety switches by looking at the wiring diagram and installing a jumper across the whole circuit. DO NOT leave the unit running with the jumper installed ... you could burn the freaking house down. This is for testing purposes only. If it now runs (i.e., a bad sensor somewhere) then check them one at a time by shorting around them.

If the sensors are all good it could be that the HE is overheating due to poor air circulation (i.e., not enough air to the HE to keep it cool, so it trips out on high-temp). This could be a voltage drop to the motor (bad connection) or it could be a plugged filter, fan covered with dirt and not running efficiently, return intakes blocked, etc).

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