Hi folks, thanks for making your expertise available to the web, I've used results from this forum before and it's very valuable.
I have a 2 year old Carrier Infinity furnace, with A/C and electronic air cleaner. We're in a 50 year old bungalow with full basement.
During both heating and cooling cycles (the issue has happened for a while) the fan speeds occasionally get variable. The fan seems to move to high speed for 5 to 10 seconds and then slow down to medium. After another 5 to 10 seconds it moves back up to high speed and then back down to medium. The fan revs up and down like this for 15 minutes at least. It eventually evens out, but then occurs again shortly after. During this process, heat or cooling works fine. The compressor (or burners) aren't turning on and off, just the fan speed cycles up and down.
It doesn't do this all the time. I tried to determine if it only happened when the unit was calling for heat or cooling, but that's not always the case. I thought it might be when the unit is just below temperature and ready to turn on, (or just above temperature and ready to turn off) but that's not always the case. I can't predict when it's going to happen, but it happens a couple of times an hour, and it's quite annoying.
I've cleaned the filters, and even tried running the unit without any filters in for a short time, wondering if it was starved for air, but that had no effect.
We turn the fan to LOW at night because the cycling fan noise is disturbing.
Does anyone have any ideas that I can try as an advanced diy homeowner or is this one for the pros?