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My home is only a few months old and the furnace had been making grinding / clicking noises. A call to the HVAC guy solicited an immediate response....sure enough the condensate drain was plugged. I tried a little methyl hydrate, some diluted CLR, flushing as much as possible with water but it will not unplug from the top. I was reluctant to try compressed air but I did apply both suction and blowing from a fairly big shop vac, all to no avail. Interestingly the suction does pull a little air back up (bubbling sounds) but blowing is met with strong resistance. There is only one sewer drop through the belly bag, i.e. no easy access to the condensate drain itself. Any suggestions? Chemicals?

My dad ran his own HVAC company for many years. He had a little Co2 bottle he would use to blow the drains out. Worked every time. If I recall correctly the end of it was ran through a rubber stopper so when he held it up against the drain it was a tight fit.

Where are you located? I just had to un-freeze one the other day hear in Delaware. Check where it goes outside it might be frozen, especially if it has been windy.

Ron

Northern Alberta. It could very well be frozen but the only portion outside the belly bag is the main sewer drop, which has heat trace and has never had a problem with flow. Also I tried some methyl hydrate into the top of the condensate drain to no avail.

I guess you checked the heat tape?

Only the main 4" sewer drop below the belly bag has heat tape and this is not plugged. The condensate drain is a 5/8" rubber line that connects to the main drop inside the belly bag. The condensate line has no heat tape, but yes absolutely it can't hurt to check the heat tape on the main drop.