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Compressor for air will not cycle on.

I have a Lennox G40UH(X) unit. ComEd the electical provider, came and installed an auto monitor and breaker on the feed line that enables the power company to turn off my units for 1 hour during peak time if necessary. One unit works the other does not. The contractor for the power company came out and could turn on the compressor manually from outside, he indicated that the signal from the furnance/thermostat was not triggering the compressor to turn on. We get the furnace fan to turn on, yet the compressor is still a no go. The thremostat is a Honeywell RTH6400, has 4 wire connection. I check the wires and all are properly installed and tight. Had no problems with the heat. Suggestions?

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Check and see if you have a common wire on the thermostat. If you do not, then you will need to replace batteries if applicable.

If you do have a common wire, then batteries are not necessary for thermostat operation.

Even though the thermostat wires are secured properly and the thermostat worked fine in the heat mode, it doesn't mean it will work for the AC mode.

If you want to check the thermostat in an easy manner, you will need to jump out the Red and Yellow terminals at the thermostat. Be sure to turn off power first to the furnace. After jumping out, restore power to furnace. If the unit comes on, then your thermostat is bad. If the unit fails to come on, you have another issue. You can repeat this process at the outdoor unit with the low voltage lines if you feel capable. If unit comes on, your problem lies between thermostat and outdoor unit. If your unit fails to start, your problem lies within the outdoor unit.

Be sure to check any drain pans, float switches, or cut-out safeties installed that may cause this. Many contractors will wire in drain safeties to the outdoor unit to shut down the compressor in the event a drain line has clogged and backed up into the emergency pan. Of a condensate pump has failed.


If you do not feel you are capable of this, you may need to contact a local professional to come diagnose the situation. But those are a few simple tasks that you can complete yourself to narrow down a problem.

Posted on May 26, 2010

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How many BYU is a Lennox G40UH (x) series furnace? And can it be vented with PVC? Thanks alot!


The Btu rating for this furnace is 40,000. Since the furnace has a 3 /34" metal flue collar on the top of the unit, you can not use PVC pipe to vent it. Only 90+ efficient gas furnaces can use PVC pipe for venting and the also come equipped with a condensation pump, which yours doesn't.

For more information on your furnace and how it's to be safely installed and vented, here is a link to the installation manual http://www.greenwoodheating.com/products/manuals/gasfurnaces/G40UH.pdf that you can download and print.

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With the Tstat off, the contactor should be open so the only way for the compressor to be trying to do anything is if the 110 volts from the unbroken side are traveling through the compressor to ground. Since 110v is not enough to run it, the overload is kicking it out, cooling off, and repeating the cycle. When you kick on the Tstat, the contactor is supplying voltage directly to ground without going through the resistence of the compressor windings and kicking the breaker.
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