I just purchased an older home in the winter with a middle aged gas furnace (2008) and an old AC condensing unit outside (1991). The furnace works great and does not have this issue when heating the house or when the fan is turned on. Here is a description of the issue:
When turning the thermostat to cool mode, everything works fine at first, and cool air comes out of our vents. After 10-20 minutes, the blower will cut out for about 10 seconds. During this time, the condensing unit stays on. It will then continually do this at random intervals between 1 and 20 minutes. Sometimes, I hear a relay trying to kick the blower back on multiple times for a few seconds. It does not do this when the thermostat calls for heat, or when I set the fan to on. However, if I set the thermostat to cool, and then set the fan to on, it will do this (I assume the call for cool overrides the fan control).
We have a Nest thermostat, which I installed. I have triple checked all wires at the thermostat to ensure there are no loose connections. I still have the old mercury switch thermostat; do you think I should try that thermostat to ensure it is not the Nest?
Has anyone ever heard of this? I have read somewhere that the blower may have 2 different lines (for different RPMs) between heat and cool modes coming out of its controller. I have also heard it may be the control board. Does anyone have any ideas? Is there anything I can check that would give me some insight into what is going on? I don't mind opening up the furnace and looking / testing things.
Thanks!
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