My thinking is either the contactor is beginning to fail, or the thermostat and/or wiring is not energizing the contactor relay.

If it happens again see if you are getting 24 volts at the contactor terminal. This will determine whether the problem is the contactor itself or the control wiring.

This post was edited by mike_home on Sat, Jun 29, 13 at 23:19

Should the control voltage at the contactor be 24 volts? Since it is now working, and rain looked imminent, I just replaced the cover panel and moved on to other things.

Probably is 24v. Whatever it's supposed to be will be indicated on the relay itself.

SR

24 to 27 volts at the contactor's low voltage coil. Being that it's a heat pump, there should be a 5 minute timer built into the outdoor circuit board. The 5 minute delay starts every time the thermostat cycles the a/c off & after line voltage (240vac) is restored if lets say a breaker was turned off then back on again.

Hope this helps

You are right it should be 24 volts.

My thinking on this is that the thermostat did start the air handler-- and as far as I know, only one set of "contacts" in the thermostat is involved in controlling both the (indoor) air handler and the compressor for AC operation.
Yes, the system has a 5 minute delay. No power outage that day. But the room temperature was at 80 with an (undisturbed) thermostat setting of 75. It takes hours for that kind of rise and it takes hours for the AC to cool from 80 to 75. The 5 minute delay does not, for me, explain the situation.
Intermittent problem with the relay? Perhaps. Intermittent problems are the most difficult to diagnose. The system has not malfunctioned again since the one incident of Friday June 28.
Thanks for the responses.

You seem to be building a strong case for a defective contactor relay. You could disconnect the compressor wires from the relay and check what the voltage is across the energized contacts or disconnect the power leads altogether and check what the resistance is across the various relay contacts with the coil energized for a comparison. They should all be VERY low resistance, well below 1-ohm.

If your meter doesnâÂÂt have a method of canceling out the lead (probe) resistance, measure the shorted lead resistance, then subtract that amount from subsequent resistance readings across the relay contacts.

SR

There is one set of 'contacts' for the fan in the furnace, another to activate the compressor, and another for the reversing valve (if a heat pump), This last one is active in heat or cold depending on manufacturer.

"There is one set of 'contacts' for the fan in the furnace, another to activate the compressor, and another for the reversing valve (if a heat pump), This last one is active in heat or cold depending on manufacturer."
Meaning in the thermostat?
So if the thermostat is in COOL mode, fan is AUTO and the temperature rises above the set point, the thermostat closes how many sets of contacts (or equivalents)?

at least 2, perhaps 3.

Interesting. Sounds possible that the thermostat could be the problem. It is a Robertshaw 9620 which has not been programmed.