A stumper for the HVAC technicians?

Question

So I'm in an HVAC class. My lab was a unit change out. Everything was going smooth and easy, until I tried to raise the head pressure to 278 psig.

The first time I got my head pressure up around 278 psig. While measuring my subcool(which was fluttering around 20 degrees) the high pressure switch cut out. After that It cut out at around 160psig. I bypassed the hi/lo pressure switches, because they kept tripping, and raised head pressure to 200 psig. Above that and the compressor cuts out while the condenser fan keeps blowing.

I weighed in the charge of 5.8 lbs of R-22, like the label on the unit called for, if anything it is overcharged. I pulled a triple evac. It is not on me yet to troubleshoot this unit, but I hate failure. This one has two of my instructors suspecting different causes; bad compressor or undercharge. Any ideas?

To raise the head pressure, we wrap the coils with plastic and limit airflow. I say it is overcharged because 5.8 lbs is a complete charge with 25' of lineset. This unit has approximately 19' of lineset. The evap coil was not replaced with the condenser. The TXV works fine, it maintains 10 degrees superheat.

I think I got everything.

Additional Details

The filter drier is just a standard issue. About the size of an orange. The sight glass has a few stray bubbles, which I have received mixed information on. A short time instructor said a proper charge should show no bubbles, another instructor said a proper charge would show some bubbles.

The subcool number does show an undercharge...either a coolant loss or I had the scale set to kilos?

I think it was a potential relay. All those little black boxes look the same, and I could not see the wiring diagram without removing it from the unit.

discharge temp. was around 85-110, the higher number with higher pressure.

hermetic, scroll, 208 volt, high temp, used residential unit. We have dozens of A/C's, I swapped out a 13 SEER unit for an antique 12 SEER Rheem.

I don't remember the amp draw, but it did increase as pressure rose.

This problem developed near the end of the day. So we didn't have time to start troubleshooting. School is closed till Monday, but I have been pondering this since last night.

From the questions you have led me with, I tend to suspect the compressor.

The compressor was extremely hot to the touch. I think it was a potential relay. I could not read the schematic on it.

I admit I am a rookie with no practical field experience.

evap pressure stays steady at 70 psig. The pressure rises to around 80 psig when head pressure is forced up to 200 psig.

Answer 1

Best Answer - Chosen by Asker

If you bypassed the high and low pressure control then it wouldn't matter if you were over charged or undercharged. The pressure controls are bypassed.

What was the AMP draw of the compressor. What was the compressor discharge line temperature? Discharge line temperature is a good indication of how hot the internal of the compressor is getting. This temperature should normally stay under 200°, but never over 220°. This sounds like it's tripping on the compressor thermal overload. The compressor cuts out and the condenser fan keeps running.

You left some important details out. What kind of motor is this? Semi-hermetic, hermetic, scroll, rotary, screw??? Is this a low, medium, or high temperature. Is it single phase or three phase. Does it have a current relay or a potential relay. Is it a PSC type compressor? There are a lot of factors to consider.

If you have an instructor and he never told you to put an amp clamp on that motor then you need to find a new instructor. Have you even checked the capacitors with a meter? Have you meg-ohmed the compressor windings? Have you checked to see if the compressor is shorted to ground? A tiny short can cause intermittent problems.

I know you are a student, but if you have to remember the details. You and your instructor are so worried about that refrigerant charge being the problem that you are overlooking the obvious electrical possibilities. If your refrigerant charge is even close you will not get trips on high and low pressure. If your pressure controls are jumped out then move on to the next option.

A COMPRESSOR CAN STILL RUN HOT EVEN IF THE HEAD PRESSURE IS NORMAL.

Faulty or improper size starting components can cause this. The pick up rating on the potential relay has to be correct or it might not open the start winding (if applicable). The compressor may keep running drawing high current, but may not high enough to trip your breaker. If your start relay contacts are sticking the first thing to check is the bleed resistor on the start caapacitor. If it is not present or not defective than the start capacitor wil discharge through the relay contacts as they close. The resistor allows the discharge to happen a lot faster and prevents arcing of the contacts. The contacts can get stuck or welded together.
This will eventually cause the motor to overheat and trip on the thermal overload too. Maybe you have it plugged into an oversized breaker.

If you are working on a PSC compressor then all that start component stuff is irrelevent. You still need to check the run capacitor for the proper MFD rating and voltage rating. Also make sure it's not shorted to the case.

Check these links for some simple help. When you get to the link right click to view them in full screen mode because they are othrwise hard to read. I have a lot more information I can share if you need help getting through school.

http://good-times.webshots.com/photo/280…

http://good-times.webshots.com/album/569…

Source(s):

Commercial Refrigeration Mechanic for Hussmann 20 years

Asker's Rating:

Asker's Comment:

Thanks for the help. I am convinced my refrigerant charge is at least close to right. The TXV is performing right. So that does narrow the suspect to electrical. I hate leaving a job unfinished...

Answer 2

You may still be undercharged. What is the size of the supply and return lines? And have you calculated the capacity of the dryer in your weight?

My (albeit limited) experience with units going off under high-head is that they are undercharged - leading to excess gas pressure in the system.

Do you have a sight-glass in the system? Perhaps a bit old-fashioned as compared to gauges but in a situation like this quite useful.

Answer 3

im sorry i need more info is this a new condenser?13 seer ?theres3 reasons why it will trip on high head is the fan right rpm my experiance with 13 seer systems is a much lower head pressure like 215 and what is your cutoff rated at 160 if so its faulty is comp pulling the right amount of amps? could be bad valve and or overcharged troubleshooting in class you have steps to go by go back to basics retrace your steps if used condenser check fan position if to high or low it will be high head and also how many elbows are in line set that adds to amt of refrigerant what seer is txv alot to try to fix over pc

Answer 4

maybe it wasnt the high pressure switch but how about a bad contactor or something on the electrical side

Answer 5

First you don't say what the low pressure side is at. Second no amp draw. You are looking at this backwards. The condenser don't do the work it only gets rid of the load. What is the Evaporator doing Pressure, Temp Delta T and compressor amp draw. That is where the load is at. Head pressure will follow evaporator pressure, You don't adjust charge by head pressure alone. What was the Delta T across the condenser? A rule of thumb is 30 deg F Delta T Air enter to air leaving the condenser. Need more Info.