I have a Armstrong gas forced air furnace and a Armstrong AC unit, not sure of the model numbers. Anyway, the fan short cycles when the AC is on. I tried it with the heat and it runs fine. It will bring the temperature down in the house it just takes longer to do so. I am hoping this is just a thermostat problem, I just replaced the filter also. the short cycling differs, sometimes it runs for 30 seconds and then shuts down for 15 seconds and repeats,other times it will run for maybe 3 - 4 minutes and then shuts down for 15 seconds and repeats. Any help would be GREATLY appreciated!