Heat pump contactor cycles

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Old 01-08-15, 09:26 AM
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Heat pump contactor cycles

System: Split Carrier --> Furnace - FB4NANFO30, Heat Pump - 38YKC030300.

Contactor in outside unit begins to cycle after running for a short period of time (30 min). The contactor begins to cycle every 5 seconds. It opens and immediately recloses. The cycle is quick, the heat pump dips momentary each time when the main contacts cycle from on/off/on. The cycle continues as long as the heat pump is running. The sequence starts like this: Running normal --> off/on --> 5 seconds --> off/on --> 5 seconds --> off/on --> .......

After turning off the heat pump system for 30 min, it will turn on and operate correctly for a short period of time, the cycle will start again as noted above.

I have not changed any parts, yet. I am an electrical Engineer, 35 yrs. I was planning to change the outside unit contactor (24v coil, 230 v contacts) and 24v transformer feeding the contactor. Before changing anything, I will check voltage at the transformer and on the coil to see if I can verify the voltage input/output.

I am making the assumption, one of these components is bad.
Any other recommendations or tests I should consider to isolate the root cause?
 
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Old 01-08-15, 10:35 AM
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Sounds more like it's cycling on a low pressure switch. Can you trace the 24 volt line on the contactor and see if there is a pressure switch on the refrigerant line? If so, it might be time to call a pro and have the levels checked.

EDIT Don't allow it to keep cycling like that, it will shorten the life of the compressor.
 
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Old 01-08-15, 10:47 AM
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Sounds like a pressure problem,shutting it off for awhile the pressures equalize and your good to go until the pressure drops again.any signs of oil or wet areas on the piping either inside or out?
Geo
 
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Old 01-08-15, 11:43 AM
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According to the schematic..... there is a low pressure switch.
dms.hvac partners/docs/1009/public/07/38ykc-2w.pdf
 
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Old 01-08-15, 12:10 PM
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A dirty air filter or a dirty indoor coil can trip a high pressure switch on heat pumps in heat mode.

I would check the air filter.
 
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Old 01-26-15, 12:15 PM
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I have checked copper lines and inside unit, no leaks. I replaced the contactor, the contacts were basically burned away. While the contactor was turning "off/on" I checked the 40va transformer, the 24 vac was constant. So the transformer is good, the contactor is new and works well. I cleaned the inside furnace, no dust and new filter. I was incorrect in my original post, the relay does not start cycling until the end of the day or after a few days of use. Very intermittent. So in other words, it will run for maybe a few days with no problem. once it starts cycling, it does not stop. It started cycling last night and I turned off the entire system for 10 min, still cycling when I turned it back on. If I leave off 1 day, it will operate normally for a few days.
 
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Old 01-26-15, 12:17 PM
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Does the problem seem to be temperature related ?
It could still be a refrigerant charge issue.
 
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Old 01-27-15, 09:23 AM
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Does not appear to be temperature related. I have not excluded the low pressure switch (LPS). I was planning to put a switch across the LPS and when it started cycling, flip the switch to see if this eliminate the problem. Just a thought the pressure was boarding on the low side. This is one of the last items to test. Any other thoughts/proposals?
 
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