I have a Bryant Furnace 394GAD from about 1991. About 2 or 3 weeks ago, I had the air conditioning unit replaced and since that time, I had a problem begin with the furnace. When I adjust the thermostat high enough for the heat to come on, the flames come on for about 2 minutes. Then the fan kicks on and simultaneously the flames turn off (the pilot remains lit). The blower runs for about 45 seconds blowing heat through the vents and stops. When the blower stops, simultaneously the flames kick back on. This will continue over and over until the temperature set on the thermostat is reached. Until my A/C unit was replaced, the flames stayed lit with the blower running until the desired temperature was met. I called the company that installed the A/C unit and they said that there is nothing that they did wrong and said I can pay them a fee to come out and find out what's wrong with my furnace. I am wondering if there is anything that they could have done to "coincidentally" have this happen. They installed an outdoor Lennox unit with the coil on top of the furnace.
I've replaced the filter. I tried replacing the limit switch since that was pretty inexpensive and there was no change. The limit switch on this furnace has no settings (no dials or anything to adjust the limits). I also replaced the control board this summer, so it's pretty new. I cannot find a flame sensor on my furnace. Is it possible that there isn't one? Is it possible that they ran any wires incorrectly? From what I have read, it sounds like I should be looking for blocked vents, air returns, etc. but I can't find anything blocked and not sure where all I should be looking. Can anyone point me in the right direction? It's not taking that many cycles to heat the place right now, but I'm not confident that this won't be a real problem once the real cold weather season begins. Thanks for any help.