Carrier Airconditioner 38QCA Heating problem

Hi Everyone,
I have a 3 phase Carrier 38QCA065 and have some issues with the Heat Cycle. the Cooling Cycle seems to work fine.
I turn HEAT 'on' the unit at thermostat, after 3 seconds, the relay clicks in on the panel, the compressor and fan unit starts for 5-8 seconds then stops. You can then hear a hiss noise comming from the pump unit.
It is quite cold here at the moment, but wanted to try the COOL cycle. After selecting COOL, the compressor and fan starts and continues to run (only tested for 3 mins and seem to cool fine).
After reading some of the general forums, could the 'reversing valve' be faulty?
Thanks for your help, Gary
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I think you just have to ride out that cycle. My Carrier PTAC does the same thing, but if you wait it out it cycles through that and heats just fine. The first time it happened it freaked me out completely, I unplugged it and let it sit for a long time, then tried it again. I decided, go or blow, and let it have its head. It seems only to do that when a certain outdoor temperature is reached, it may be a deicing thing
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