Central Air Experts: Cannot turn AC off without removing the fuse

[RDKLL]

To this awesome group...
We have 2 AC systems in our house, an upstairs unit and a downstairs unit. We turned off both and opened the windows last night and my wife noticed the outside unit stayed running. She also noticed that it had a bit of a different sound. So this afternoon I made sure it was turned off (even cycled on and off a couple of times). I then went out and put the big fuse thing back in and the fan came on again. Any thoughts on what it might be?

Thanks in advance

[Oss]

not an ac guy but I had my 2 ton unit at my office go on opening a damper to outside on its own.  It was winter and like 30 degrees too  

The tech that came said it cant go on like that ...until he saw that it did

I think it was a short in the thermostat but it was like 10 yrs ago

[Challenger]

Mine was a sticking relay for the outside unit, Flicking the power on and off several times would release it, but would stick on the next time it ran. New relay and all is well.  Although "Oss" could be onto something also. 

[Sorcerer]

Mine did the same thing this summer. I could get the relay to unlatch but would hang the next  Cycle. $7.00 relay was all that was needed.

[Dino]

If it is a strait A/C unit sounds like the points on the contactor may have welded shut . If it is a Heat Pump ,could be something to do with the defrost cycle or a post purge delay !  That would likely have a computerized circuit board a little more complicated than if it is A/C only.

[Jess Tolbirt]

contactor is stuck,, new one should be around 10 bucs

[Patrick]

Side story. Got a call about a similar thing, AC was running on the Florida place a couple months ago.  Now it had been running fine and I cleaned it before leaving for the summer. So an 'expert' was called to fix it.  He stated the coils needed re-cleaning and a new 'contactor'. I looked at the contactor and the points looked nice and smooth/clean before we left.
This 'expert' charged $322 for the contractor replacement and $250 to re-clean the coils. Cleaning the coils takes me maybe an hour and the contactor even at Lowes or Home Depot is $9-10 and takes about 10 minutes to change.

[Wizzard]

happened to me,, was gone for a weekend and came home to it stuck on. Went out to the unit and noticed a strip of bark off a nearby tree stripped off from way up to the bottom. Knew it was lightning and sure enough, contacter welded shut. Replaced and all was well.

[Forge]

I am a licensed HVAC contractor and Dino is correct. If it is a straight A/C unit, then it is a stuck or fused contactor (relay) and it will need to be replaced. If you have a heat pump, it can either be a stuck/fused contactor, or a bad relay on the control board in which case the entire control board needs to be replaced. Depending on brand, some of those control boards can be expensive.

[RDKLL]

Thanks everybody! Andrea has a co-worker who works on the HVAC systems of Apache helicopters and residential units who will be coming over to take a look. It is a heat pump and consensus is contactor  or circuit board...

[Patrick]

My contacter/point set was a dual point open unit and I told the friend looking after the place to have the 'expert' save and leave the points so i can look at it.  The Packard replacements are enclosed/sealed and seem pretty light and cincsy.