Btu Buddy 147: Low Performance Commercial Unit
Bob is a service technician who is well trained and nationally certified. However, he has sometimes suffered from the same confusion that all technicians occasionally do — the facts that he gathers may or may not point to the obvious cause of the problem or the best solution. But Bob has had something that no one else has. He recalled his long-time HVACR mentor and imagines him accompanying him as “Btu Buddy,” someone who reminded him to take time to stop and think before rushing to judgment, helping keep him on the right track, even with facts that are confusing.
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.
A new customer called and wanted their commercial unit serviced. The customer was a retail store and the manager seemed to think that the unit was not performing up to standard. The unit would run most of the time, even in mild weather, and did not seem to cool the store as well as it should.
When Bob and Tim arrived, they talked to the manager who said, “I don’t think the air conditioning unit is performing up to standard. I have been here three months and have seen no paperwork on when this system has been serviced. So we need a good general checkup and maintenance done on the system.”
It was a 10-ton split system with the condenser on the roof. Access to the roof was through a roof hatch in the stockroom.
Bob said, “Turn the thermostat down to about 60°F so the unit will not shut off and let’s do a visual check to see what we may need.”
They set the thermostat and went to the supply room where the air handler was located and began to examine the equipment when Tim said, “The suction line has a small amount of ice or frost on it. Is that good?”
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Bob said, “No, an air conditioning system should never have frost or ice on it. There must be a problem. Turn the system off. Look at the filter, the coil, and the blower wheel for signs of dirt.”
Tim examined them and said, “The filter is dirty, the coil face is dirty, and the blower wheel has a lot of dirt buildup.”
“Well,” said Bob, “there is part of the answer to the problem. If the blower wheel is dirty, the dirt had to come through the filter, then the blower wheel, and then into the wet coil in the summer (Figure 1). If the blower wheel is loaded with dirt, the coil must be dirty. We know that all three of these need service. Let’s go to the roof and look at the condenser and see what is going on up there.”
They went to the roof and examined the condenser. Tim said, “The condenser coil is really dirty. It has some kind of fuzzy stuff on the coil entrance.”
Bob looked and said, “See that cottonwood tree over there. It gives off a cotton-looking fuzz in the spring. It looks like there is a lot on the coil entrance. The fan sucks it up into the coil when it is in the air. Some of it is bound to be inside the coil. It looks like there are several years of fuzz on this coil. It must be cleaned. No wonder the system is not performing up to standard. It must be suffering from high head pressure and low suction pressure. That is a combination that will work the unit extra hard and not very efficiently. I think we know what we have to do. Let’s go and make the store manager aware of the work we will do.”
The manager gave the go ahead for doing the work and Bob suggested, “Let’s start with the air handler. Take the fan section out behind the store and let’s wash it with a pressure washer. We can change the filter at the same time. Spray the evaporator coil with evaporator coil cleaner and let it stand and soak in for about 10 minutes; then we will wash it down the condensate drain with the water hose. When we get through with cleaning the coil, we will wash out the condensate pan and the condensate trap and drain line. It will be dirty with all of that dirt coming off of the coil.”
They finished the evaporator, coil, fan, and condensate line cleaning and went to the roof.
Bob said to Tim, “Turn off the disconnect and take the fan and panel off of the unit to give us access to the coil. Then spray the coil with condenser coil cleaner and let it soak for about 10 minutes. While it is soaking, oil the condenser fan motors. Then examine the contacts on the contactor (Figure 2). We probably will need to change the contactor. I bet the contacts are pitted.”
Tim changed the contactor, cleaned the coil by washing it in the opposite direction to the airflow, and oiled the fan motors and was ready to start the system back up. Bob said, “Let’s put on the gauges and place a temperature lead on the suction line and the liquid line.”
Tim installed the gauges and the temperature leads and they started the unit up.
Tim asked, “This is an R-22 unit and it is 85° outside. What should the head pressure be?”
Bob said, “It is not a hard and fast rule, but the head pressure for a standard efficiency unit should correspond to a condensing temperature about 30° higher than the ambient air temperature. The air temperature is 85° plus 30°, so the condensing temperature should be 115°. For R-22 that would be a head pressure of 243 psig. The actual head pressure is 245 psig; that is close enough. The condenser seems to be functioning great.”
Bob then said, “Now tell me what the subcooling is on the condenser.”
Tim said, “The condensing temperature is about 115° and the liquid line temperature is 100°, so the subcooling is 15°. I would say that is about right. It should be somewhere between 10° and 20°.”
Bob said, “That is great. Now let’s see what the superheat is running at.”
Tim said, “The suction pressure is 71 psig, which corresponds to an evaporating temperature of 42°, and the suction line temperature is 54°, for a superheat of 12° (54 - 42 = 12). That is good, because it should be between 8° and 12°. It is on the high side. Maybe there is a little excess load on the evaporator coil because we had the unit off for awhile while we were performing maintenance.”
Bob then said, “Let’s put the system back together and reset the thermostat back to 75° where we found it and tell the manager what we did. When you put the panels back on, be sure to put a screw in every hole in the panels. I noticed that there were a lot of screws missing from prior service.”
They were back in the truck riding away when Tim said, “Why did you recommend putting all of the screws back? It seemed to be operating just fine with most of the screws holding it together.”
Bob explained, “Manufacturers spend a lot of time on design and are careful not to put too many of anything on a unit. Every screw is a key piece of a complicated puzzle. If you leave some of the screws out, it is likely at some point that the unit will set up a vibration which may cause the other screws to wear out their holes, then none of the holes will be the right size. It will be downhill from there to coils leaking from vibration and lines shaking from vibration causing leaks. Let’s just make a practice of putting equipment back together just like it was designed.”
Tim said, “Sounds good to me. I also believe the manufacturers would not put any extra parts on a unit. That would equate to a lot of money since they make so many units.”
Publication date: 6/22/2015
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