# Odd carrier problem



## clehndymn (Nov 14, 2011)

Carrier gas furnace was not starting inducer motor on call for heat so we put it into test mode, inducer runs, blower runs, ignitor gets hot. Checked all rollout and limit switches (four total) and all have 24v+ to ground on both sides when it is running normally on a call for heat but inducer fails to start. The sequence is: furnace sees power, call for heat, blower starts and runs. Inducer never starts. If I disconnect a limit switch after the call for heat (doesn't matter which one, rollouts or high limits in flame box) the inducer starts and runs but the furnace will not proceed any further into the sequence. (ie: no ignitor). The board was just replaced with the same results. Pressure switch tests fine as well as all rollouts/ limits. Inducer runs on the bench and in self test mode. I am just curious why the inducer runs if I disconnect any limit switch temporarily (breaking 24v)... any ideas?


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## flashheatingand (May 3, 2008)

Blower shouldn't start on a call for heat..unless the fan is in the on position. Limit switch and pressure switch are on two seprate circuits. Don't focus on the limit switches, they aren't the culprit, don't get a board, that isn't the culprit either. Read the wiring diagram, and trace all the low voltage circuits. You will then find the culprit


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## clehndymn (Nov 14, 2011)

I thought some carriers run the blower for 90 seconds on start up. Either way why does the inducer start when I pull a 24 lead to a limit switch?. My pressure switch is fine. I don't see the gas valve causing this. What else could it be? I have the thermostat dis connected and I'm jumping the red to the white on the board to call for heat.


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## flashheatingand (May 3, 2008)

The reason the blower is running is becuase you are likely cutting out the high voltage circuit (door switch) and when you re-energize the circuit with the r to w, the blower runs for 90 seconds.

How many volts from pressure switch to common?


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## acefurnacefixer (Dec 7, 2006)

How old is this unit? can you post a picture with the front doors off? Sounds like you have a bad ignitor or ignition control.


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## flashheatingand (May 3, 2008)

It is probably the vent safety switch by the flue pipe. I suspect he found help at another site.

To those who frequent the site, there are usually two separate safety circuits with most furnaces. One of of the safeties is in regards to the high temps. There are several high temp limit switches, usually all wired in series. The other safety circuit has to do with the venting of the gasses. The reason, the inducer motor probably did not energize in heat mode is because the vent safety switch likely tripped.

I post this because there may be some young hvac dogs out there, and hopefully this may be of use to them. Also, maybe some of the regular ct guys dabble in a little hvac, and might find this tidbit useful.

I am not a fan of giving out free advise to strangers because it's taking money from another hvac outfit. However, for friends and family, that is another story. For those CT regulars, feel free to post a question, as I will try to help out, and there are a few other guys that are quite smart (present company not included).


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## clehndymn (Nov 14, 2011)

got it fixed... ending up being a bad connection, after disconnecting and re-connecting everything it works. Not sure what wire it was exactly but after a tear down and bench test of all the major components (again) it worked after re-assembly. thanks for the help/ideas. Just goes to show it helps to trace EVERY wire even if you think they are good or have tested them before.


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## flashheatingand (May 3, 2008)

Glad you got it running. Thanks for the follow-up. 

I suspected the area in question was a tripped switch which can be reset. But, I was wrong... again. The most important thing I learned in hvac school was to be able to read a wiring diagram and use a multi meter.


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## Dr Heat (Dec 25, 2008)

Flash I do not believe you were wrong. When the wires were being checked I think he inadvertently reset the spill switch.


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