York Affinity (YZB) HeatPump perpetual defrost
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 ?
As above, the charge needs to be checked.
catmanacman, beenthere: thanks for your guidance.
It seems that there is no objective measure to know whether the charge is really low - just a best guess based on experience (not to be dismissed lightly!), and observed performance, so I will proceed to ensure that the charge is correct.
From all I read, the correct way to charge the system is to evacuate, purge, leak test, weigh in.
Reading the York documentation I find:
- Factory Charge: 9 lb 2 oz
- add, for indoor coil SV48D, TXV 904 : 10 oz
- add for 30 ft line set, .67 oz/ft over 15 ft : 15 * .67 = 10 oz
total = 10 lb 6 oz.
Is this the correct amount to weigh in or is there some residual amount that cannot be evacuated that needs to be allowed for ?
What about lubricant ? - Does it need to be replenished ?
As luck would have it, the HighPressureSwitch is defective and needs to be replaced. It is brazed in, so this would be the opportune time to recharge the system.
I'd like to get it done once, correctly; so any advice on how to find a technician - i.e. are there particular licences, affiliations that a qualified technician would require or otherwise have as indication of competence ?
That would be the correct way to charge it.
I have now had the system professionally (by an HVAC-TALK reference) evacuated, tested, freshly charged - by weighing in - to the exact York specification.
Originally Posted by catmanacman
After two days of careful observation: the "defrost exactly every 40 minutes of run time, needed or not" condition persists.
My next guess is a bad liquid line temperature sensor - except that defrost is terminated almost immediately when the defrost is not needed, and stays in defrost when it is actually needed; indicating that the exit condition: liquid line above 80 F, is indeed detected.
It seems that inadequate refrigerant flow would cause the problem. Since there is now definitely a full charge, what do you think are the chances of a malfunctioning TXV not allowing enough refrigerant flow ? Is a "bad" TXV difficult to diagnose ?
One might also suspect the defrost control board - except this is the third board, with two different firmware versions, that has exhibited this behavior.
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