In the past, I have written a post on compressor replacement in air conditioners, but is there anything special that should be done when replacing a compressor in a heat pump?
We need to keep in mind that Heat Pumps have refrigerant that flows in both directions, depending on the mode of operation. Most heat pumps will also have a suction line accumulator in the unit to prevent “slugging” the compressor. So, what should we do when replacing a compressor in a Heat Pump?
A lot of the change-out procedures are the same as an air conditioner (since a heat pump is also an air conditioner).
- Suction line accumulator should absolutely be changed out, carte blanch, anytime there is a burnout –period. Keep in mind that the accumulator will contain contaminated oil and we do not want to leave that in the system.
- Add suction-line filter – If the refrigerant system was opened to replace a compressor burnout, then the 1st suction drier has to be activated charcoal HH type. This should be left in the system for 72 hours of run-time and periodically checked for acid with a kit and the pressure drop to see where you are.
- So, now the compressor is changed and the system is operational, additional steps are required to run the system until contaminants are removed.Then, if necessary, change it to another HH drier if acid is still present or now go to a 100% activated alumina suction-line filter drier.
- Run the system for 10 hours, then monitor the suction line drier pressure drop. If the pressure drop exceeds 3 psig, replace suction line filter drier and re-run the system for another 10 hours. If the pressure drop is less than 3 psig, remove the suction line drier, and perform an acid test. If the test shows no presence of acid, remove the suction filter dryer and close system.
- If the acid test shows acid still present in the system, replace suction line drier and re-run the system for another 10 hours. Repeat this process until acid is not present in the system. Note that the suction filter drier can be run for more than 10 hours, but never leave the suction line drier in the system without replacing the cartridge for over 50 hours of operation as this could lead to further contamination and compressor failure.
- And remember — suction line driers should NEVER be left in a system once it is cleaned up.
- The bi-flow liquid line drier can stay in only until you have cleaned up the system with the first HH suction drier install and no acid present. Once the acid is gone — CHANGE THE BI-FLOW DRIER. Of course –this should be done any time a system is opened. Too often, a compressor gets replaced and the drier does not. The drier could be partially plugged and now the new compressor is showing “strange pressures” and the contractor thinks he has a bad compressor again.
If all of this is done properly, chances are pretty good that you will not have to do anything with the reversing valve. However, if you find a lot of “contaminants” in the oil or “blackening” inside the tubing, since you are going to have the system open, you might want to change out the reversing valve as well.
To me, determining the original cause of the compressor failure and correcting it is probably the MOST IMPORTANT step in any compressor replacement. If the original cause of failure is not fixed, what do you think will happen to this new compressor? Is there power problems? Are all the wires and termination in good condition? Is the condenser coil clean. does the condenser fan motor work? Are the indoor filters clean? Is the blower moving enough air? Is the indoor coil clean? And on, and on, and on. In a number of posts I have said — the only thing that can go wrong on the refrigerant side of the system is OVER-CHARGE, UNDER-CHARGE, AND NON-CONDENSIBLES –all other problems are external to this side of the system. All need to be checked to make sure this compressor does not fail prematurely.
Keep in mind that “best practices” still go a long way. Just as with an air conditioner, we need to looks at a few other things. Let’s take each item and examine it:
- Proper brazing has to be followed with nitrogen trace to prevent contamination inside the line set.
- “Evacuated the System? ” This has always been a critical step in any air conditioning service when the refrigerant side of the system is opened but even more so when replacing a compressor. All manufacturer’s recommend a vacuum down to 500 microns. Then seeing if it holds or rises back up. If it rises back up and stops, it is an indication that you still have moisture in the system and need to continue to evacuate. If it rises and keeps rising — there is a leak. What better time to address this that when you have the system empty and open. Pressurize with a trace of refrigerant and nitrogen. Find the leak and repair it and then re-evacuate the unit to 500 microns and see if it now holds. This is especially critical with systems that have P.O.E. oil since that oils is very hygroscopic (meaning it absorbs moisture) and needs to be evacuated well.
- “Install the proper size capacitor?” Probably one of the most overlooked items on single phase air conditioners. This, along with any start capacitors and start relays should always be replaced when replacing the compressor. If the compressor had an “electrical failure”, these components are part of that “side” if the system. One of the biggest mistakes I have seen contractors make over the years is replacing a start capacitor and NOT changing the start relay. The major cause of start capacitor failures is a bad start relay. This is “cheap insurance” when replacing any single phase compressor. This also goes back to finding out why the original compressor failed in the first place.
- Checked line voltage from the contactor?” How often have you found that the points on the contactor are corroded or pitted causing voltage drops and problems? Probably more often that you would like. A compressor is a motor and it runs on electricity. It needs proper voltage to operate correctly just as any other electrical component requires proper power to work.
- “Wired power leads correctly?” This applies to both single phase and 3-phase compressors. On single phase, do we have C (common), S (start), and R (run) to the correct terminals on the compressor and are they properly wired to the capacitor? Did you realize that if you reversed start and run, you would reverse the rotation of the motor in the compressor? Problem is it will only run like that for a short period of time and then the windings will fail and you will be replacing the compressor again. Very important to make sure the compressor is properly wired. On 3-phase scroll compressors, you need to verify that the compressor is turning in the right direction. 3-phase scroll compressors can only pump in one direction unlike a piston type compressor that does not care which way the motor is turning.
So there you have it. There is more to replacing a compressor — and doing it right — than just removing the old and brazing in the new. Then there are those few extra steps when doing a heat pump compressor. It still all goes back to proper diagnostics and installation practices.