Mysterious problem

10 years ago

HVAC is not my trade. I am a competent electrician.
2- ton Trane XR-12 heat pump about 10 years old. Set to cool at 75 deg, thermostat was set to OFF for about an hour the night before. This one serves the second floor. The unit for the first floor was and is working normally.
The inside air handler fan was running, room temperature had risen to 80. Nothing running at the condenser unit. Remove cover and look at the capacitors-- which look OK. Push the contactor on the relay and the unit starts, both fan and compressor. But the contactor does not hold. Go to get my voltmeter. Phone rings in the house, so some delay in getting back to the condenser. When I get back with the voltmeter, the unit is running and cooling!!!! No problem since then.
No power outages here for the past several days.
Any ideas about what caused this condenser unit to stop working for several hours and then to resume operation as explained above?

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Comments (11)

mike_home
10 years ago
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
bus_driver
Original Author
10 years ago
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.
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fsq4cw
10 years ago
Probably is 24v. Whatever it's supposed to be will be indicated on the relay itself.

SR
fluffybunnysui
10 years ago
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
mike_home
10 years ago
You are right it should be 24 volts.
bus_driver
Original Author
10 years ago
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.
fsq4cw
10 years ago
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 doesnÃ¢ÂÂt 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
weedmeister
10 years ago
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.
bus_driver
Original Author
10 years ago
"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)?
weedmeister
10 years ago
at least 2, perhaps 3.
bus_driver
Original Author
10 years ago
Interesting. Sounds possible that the thermostat could be the problem. It is a Robertshaw 9620 which has not been programmed.

Mysterious problem

Comments (11)

mike_home

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Mysterious problem

Comments (11)

mike_home

bus_driverOriginal Author

Related Discussions

fsq4cw

fluffybunnysui

mike_home

bus_driverOriginal Author

fsq4cw

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bus_driverOriginal Author

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GET HELP

CONNECT WITH US

bus_driver
Original Author

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