Gas and Oil Home Heating Furnaces - Central AC fan won't shut off

The AC has no difficulty cooling down the house. Yesterday I had it on but at a comfortable 25 degrees Celsius (77 F). A few hours later the outside fan came on and stayed on! Frost was building up on one of the copper pipes.

The only way to stop the fan was to yank out the fuses. I left them out all night. In the morning, there was no more frost on the pipe. When I went to put back the fuses, the fan started up again!

Oddly enough when this happened last weekend, the only thing that shut the fan off for good was a power spike during a thunder storm.

I haven't check the pipe leading into the drain to see if there is any blockage because it's not the easiest thing to do. I thought I'd check here first before doing that.

I live in Toronto and the house is detached, made of brick and about 30 years old. AC works fine otherwise.

Have ifs here. Did the inside fan come on also? Can the outdoor contactor be stuck on? Short in the low voltage wires? Can it be in the tstat ,have to look at it all.



ED ;)

Thanks Ed.

In answer to your questions. Inside fan comes on only when it needs to. Tstat appears to be working fine. Everything else appears to be working fine expcept for the outdoor fan kicking in after a couple of hours for no reason and then staying on.

Oddly enough, everything worked fine last night and is still OK this morning.

This doesn't always happen, just once in a while. When it does, I don't know what to do about it.

either the contactor outside is sticking, keeping the condenser running after the indoor section has been shut off by the tstat, or the tstat is bad. next time it happens, remove the Y wire at the tstat. if the unit continues to run, look at the contactor in the outside unit. it is probably sticking

Thanks guys.

Sorry for not posting in the AC section (newbie). Having read through that forum, it sounds like maybe the contactor.

I'll have a look at it and the tstat.