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This is a temperature log from a reverse cycle air conditioning system that has run fine for getting on for 30 years. It has suddenly started to blast out cold air on days when it should be heating.

reverse cycle air conditioner with rogue cooling cycles

The temperature was measured at a point just inside a ceiling outlet vent - hence the wide fluctuations.

What would cause the cooling cycle to start in this manner?

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  • If it worked well for 29+ years, I suspect you have gotten your money's worth and it's time for a major rebuild or complete replacement. "A control system error" or "system failure resulting in no-heat production" is the vague answer that's not worth making an answer, thus the comment.
    – Ecnerwal
    Commented Jun 30, 2015 at 14:12
  • Thanks for your comment. An air-conditioning serviceman (who was unsuccessful in finding the fault) checked out the system and pronounced it otherwise healthy.
    – rossmcm
    Commented Jun 30, 2015 at 23:31
  • 30 yrs is almost 2 times normal life span.
    – Ceaser Oreo
    Commented Sep 2, 2015 at 7:50

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Nice chart. This looks like abnormally frequent triggering of the defrost cycle. Typically this will occur if the unit is severely low on charge or when the outdoor thermostatic expansion valve (TEV) has failed and is restricting flow of refrigerant through the outdoor coil so severely that the coil freezes in an abnormally short period of time, triggering frequent defrost cycles. You would have a similar problem if your outdoor coil was exceptionally dirty, but I assume you would have noticed that.

To positively diagnose this, a technician would connect a manifold to the service ports when the unit is heating, and verify that the subcooling of the refrigerant returning from the interior coil is at least 8 C to confirm the unit is not too low on charge. If the charge is adequate, then check the saturated suction temperature (SST) and also the superheat of the vapor in the suction line returning to the compressor. If the SST is much less than 6 C below ambient and the superheat in the line is much more than 11 C, then the outdoor TEV has likely failed in a closed position or become plugged. Depending on the unit this may be a fairly major repair. In a 29+ year old system it may not be worth it versus replacing the unit.

EDIT: Some defrost control boards have a manual setting that forces a defrost cycle at a programmed interval. If you have one of these control boards, you should verify that it has not been set to force a defrost cycle every 30 minutes. Also, someone should confirm that the "freeze-stat" that signals the defrost board when the outdoor coil needs defrosting has not become disconnected or damaged. You can eliminate these by looking at the outdoor coil after the system has been heating for 15 minutes. If it is covered with frost and really does need defrosting in that short a time, then this problem is not merely a control malfunction and is more likely a refrigerant charge / TEV problem.

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  • thanks for your detailed and informed comments. Our thinking is that the problem is unlikely to be related to the coil freezing as the unit starts cooling the instant it is powered on. It then seems to switch to heating when it reaches some lower set-point (corresponding to a temperature of around 10C at the outlet).
    – rossmcm
    Commented Jul 14, 2015 at 0:34
  • There is not usually a "lower set-point" on a heat pump defrost control, only a defrost timer that ends the defrost cycle or a temperature sensor that terminates the cycle when the outdoor coil is sufficiently warm. I missed that your chart shows the cold air on startup. This situation points to a defrost control issue where the control always thinks your outdoor coils are iced over, such as the outdoor frost sensor being disconnected from the board, a sensor malfunction, or possibly a bad defrost control board. Happily, these are much less costly to fix than a stuck outdoor TEV. Good luck.
    – user39367
    Commented Jul 14, 2015 at 2:19

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