I have a 3.5 ton (YZB04211A) with matched handler, 4 years old.
1) Complaint: defrost cycle every 40 minutes, whether needed or not, whenever ambient is less than 50F.
2) defrost cycle lasts from 1 minute to 6 minutes. the shorter times are when a defrost wasn't necessary - typically at higher ambients or lower humidity. On the longer defrosts, the defrost action: steam etc. seems correct.
3) according to the York install document, the defrost is inhibited for temperatures over 50F, so I think the ambient temperature sensor is correct.
4) according to the York install document, the defrost is initiated when the LiquidLine temperature is below the defrost control curve (this is control curve #3 for this unit), for a given ambient.
5) It appears that the defrost is always waiting for the 40 minute 'minimum time between defrosts' timer to expire; this would be so if the liquid line sensor was reading low, or the liquid line is actually low.
6) Because the defrost cycle seems to terminate correctly, I think maybe the liquid line temperature sensor is ok, and the liquid line temperature is indeed low.
7) Another clue is that the hot heat pump feature, which basically causes the airhandler blower speed to increase, is to be activated when the liquidLine temperature is above some particular value; this rarely happens - indicating that the liquid line temperature is always low (or the sensor reads low) .
8) Another clue is that the heatpump performance has always been poor; my measurements of building heat loss etc. show that the heat pump is roughly 5000 btuh shy of specification ...( maybe not a surprise since York uses a nominal 3 ton compressor ).
9) the liquid line at the airHandler feels warm to the touch, the vapor line is hot.
10) the liquidLine temperature sensor is located after the TXV before the evaporator; it is not readily observed - at least until I figure out how to remove the corner access panel - so don't know for certain that it is correctly mounted. I did measure the resistance of the sensor - it was around 70 k-ohms at ~45F ambient.
11) it seems that the choices for a culprit are:
- liquid line sensor
- bad TXV ... or maybe improperly installed
- wrong amount of refrigerant
- contaminated refrigerant
- bad control board (this is the 4th control board, but I observed the problem with the previous board)
Does this sound familiar ?