I am the lucky owner of a Trane XR14 (2.5T) heat pump installed in June 2008. During the first winter, I called the installer because of weird defrosting behaviour: it was defrosting very often and for 10 minutes every time (it also had the timed defrost error code on the defrost board). So the installer came and said the defrosting control board was bad so he replaced it. I didn't pay too much attention to the unit afterwards but it seemed to defrost less frequently so I thought it was all good.
One year later... Since a couple of weeks, it looks like the trane is defrosting very often (sometimes 2 defrost cycles in less than 1 hour), eventhough there is no frost on the outside coil. When the unit defrosts, everything looks normal: I hear the reversing valve (tsishhh), the compressor starts with the fan off and the unit outside gets hot. The defrosting cycle lasts between 5 and 8 minutes.
So I called my installer and told him everything. In short, he's saying that he never heard of customer having unnecessary defrost cycles and don't really know what to do. To help him a little bit, I placed a temperature sensor on the freon line near my handler to see how often the HP is acting up. Here are the results for the last 12 hours. Dips around 10oC are defrost cycles. Spikes to 50oC are heat cycles. As you can see, it is almost defrosting after every heating cycle or so since this morning. The outside temp this morning is around 0oC. I went outside when I heard the reversing valve at around 11h20am this morning and there was no ice on the unit. Are these defrost cycles normal?? Any help would be really appreciated!