Btu Buddy 103: Bob and Tim Begin Heat Pump Repairs
Now, Bob’s company has promoted him to help train a new employee, right out of a school specializing in HVAC, just like Bob was. Bob is now Tim’s Btu Buddy. Tim is anxious to travel with Bob. Tim realizes that he is right out of school, with the theory and lab work that he accomplished in school, but still needs help. He knows that he worked with many of the components of the systems in the school, under ideal conditions with good light and air conditioning. Now it is into the field, sometimes under the house with poor lighting, or out on the rooftop in the sun, where the real action is. He is naturally and normally reluctant, but he has Bob to help guide him.
Bob and Tim have arrived at the apartment house that they have been working at for the last several days. They’ve gone to the roof where the outdoor units are located. There are 60 heat pumps that use R-22 as the refrigerant. These units are about 10 years old.
Tim asked, “Where are we going to start?”
Bob said, “Let’s just work down the list from the notes you have made while inspecting the units. It looks like Unit No. 2 is a good place to start. You made a note here that the suction line was not sweating very much and didn’t seem cold. You said that the unit may be undercharged. Let’s look at it first.”
The unit is running, because it is 90°F outside. Tim felt the suction line and said, “The line is still only cool. I believe we should fasten gauges to this one.”
Bob agreed, so Tim fastened his gauges to the unit, then said, “Look at this. The suction pressure is 85 psig and the head pressure is only 200 psig. The head pressure is too low and the suction pressure is too high. I guess the compressor is bad.”
Bob said, “The four-way valve could be leaking from the high side to the low side. Let’s check it before we suggest changing the compressor (Figure 1).”
Tim said, “Boy, I forgot about that possibility.”
They got set up and checked the four-way valve and it proved to be good.
Bob then suggested that they check the amperage of the compressor.
Tim said, “The compressor is supposed to be pulling 18 amps and it is only pulling 8 amps. What do you think?”
Bob said, “That proves it to me. Let’s get set up to change the compressor.”
They got set up and changed the compressor then started the unit up after adding a measured charge to the unit.
Tim said, “The suction pressure is now 70 psig and the discharge pressure is 275 psig. That is more like normal. Why was there no complaint from this apartment about not enough cooling?”
Bob said, “With a suction pressure of 85 psig, the evaporator temperature was about 50°, so there was some cooling. Remember, the normal suction pressure would be about 70 psig with an evaporator temperature of 40°. The apartment was on the first floor between two other apartments so it didn’t have a huge load. The unit probably ran all of the time and the space temperature probably did not get too high. Some people probably would not really notice or complain about the conditions.”
Tim asked, “Should we remove the gauges?”
Bob said, “Yes, be sure to close the valve at the end of the high pressure gauge line and pull as much of the refrigerant into the low side of the system as you can. This unit has a measured charge and it can be critical to get all of that measured charge back into the system. If that high side gauge hose has some condensed liquid in it, it could be enough to affect the system performance. Be sure to leak check the gauge connections after the gauges are removed. We don’t want to leave a leak at the gauge connections.”
Tim said, “You sure are careful about the charge and leaks.”
Bob noted, “When you get a callback on a system, it is in warranty and the cost falls on the company. If you spend an hour on a service call and then have to spend another hour on a callback, it is expensive. When you get a few of those, the company will start to wonder if you are worth keeping on the payroll. It is best to do it right the first time.”
Tim then said, “We have time before lunch to look at one more unit. Let’s look at No. 12. It was very noisy.”
When they approached Unit No. 12, they could hear it rattle.
Tim asked, “What do you suppose is the matter with that unit?”
Bob said, “It sounds like the fan is off balance. Let’s shut it off and look at the fan.”
They shut the unit off and removed the top of the unit where the fan was mounted to the underside of the top cover (Figure 2).
Tim looked at the at the fan and its mount. It had three mounting bolts that fastened the bracket to the top cover. One of the bolts was loose allowing the bracket to move. He then said, “It looks like if we tighten the fan bracket this fan will be good to run.”
Bob said, “This unit has another problem. The screws have been removed and tightened so many times that the screw holes are oversized. If there is any shake in the components of this unit, it will be amplified because the panels cannot be tightened up securely.”
Tim asked, “What can we do about that?”
Bob said, “Go to the truck and get that box out of the side compartment labeled #9 screws.”
When Tim got back, Bob had tightened the fan bracket and replaced the top on the unit. Tim opened the box labeled #9 screws and asked, “What’s the difference with these screws and the ones that are in the unit?”
Bob explained, “The screws that are in the unit are #8 sheet metal screws. The next standard size screw is #10. Years ago, the company found some #9 screws that are not a typical size and they bought several boxes of 1,000 and we still have some left. I am not sure you can even get them anymore.”
Tim asked, “What is wrong with using #10 screws?”
Bob responded, “The #9 screws are just right for an oversized screw hole. The #10 screws are just a little large and hard to tighten. Try this #10 screw in that cabinet.”
Tim said, “You are right. It is very tight. It probably would be best to re-drill the hole to a larger size.”
Bob said, “Any time you get a drill bit near a coil cabinet, you run the risk of creating a coil or line leak. Now try the #9 screw in that hole on the other side.”
Tim said, “The #9 screw is the best answer. Surely, someone still makes that screw.”
They fastened the cabinet back together and started the unit.
Tim said, “This sounds great now.”
Bob said, “The longer the unit is allowed to operate with a loose cabinet, the larger those oversized screw holes will get. These cabinets are designed for all of the screw holes to have a proper screw that is tight.”
Tim then said, “This is all good to know. It is lunchtime now. After lunch, let’s go on to another unit.”
(Continued next month.)
Publication date: 10/24/2011