ACHRNEWS

Btu Buddy 85: Superheat Problem With Three Coils

April 19, 2010

Bob is a service technician who is well trained and nationally certified. However, he sometimes suffers 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 something that no one else has. He recalls his long-time HVACR mentor and imagines him accompanying him as “Btu Buddy,” someone who reminds 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.

Bob was on his way to a job at a new customer for his company. He was anxious to make a good impression because this customer has several properties. He wanted to be part of helping build the customer base.

The job was a three-story office building with an air handler on each floor with a refrigerant coil in each air handler. A 75-ton condensing unit served the three 25-ton air coils. This was an old system that has still been doing a good job. The complaint was that the second floor unit was not holding the conditions that the thermostat was set for. It has been running 2°F higher than the set point on the thermostat.

Bob went to the building manager and they looked at the thermostat and Bob saw what he was talking about. The manager showed Bob where the air handler was and the roof hatch to the top of the building where the condensing unit was.

Bob asked the manager if there have been any other complaints about the system and the manager said, “I have been to both of the other floors and the thermostats are set at 72° and both of those floors are running at the set point.”

Bob then went to the room where the air handler was and looked around. Bob found a place on the suction line were he could touch it and it seemed plenty cool to the touch. He went to his truck and got an electronic thermometer and checked the temperature of the supply air and was surprised to find it was 59°. He was expecting it to be about 55°.

Bob was scratching his head when Btu Buddy showed up and asked, “What is the matter, Bob? You look confused.”

Bob told him what was going on and said, “I am going to one of the other air handlers and see what the discharge air is.”

Btu Buddy said, “That is a good idea. It might lead you to an answer.”

Bob went to the first floor and took a measurement and it was 56°. Then he said, “That is interesting, I think I will go to the third floor. I am confused.”

Bob went to the third floor and the temperature of the discharge air was 56°.

Bob said, “I don’t get it. There must be an excess heat load on the second floor.”

He went to the manager and asked him if anything had changed on the second floor and the manager said no. This temperature change just happened today.

Bob was now scratching his head for sure.

Btu Buddy then said, “Why don’t you look into what kind of heat this system has. Maybe the system is picking up heat out of the heating system. It is the season change and oftentimes calling for heat at night.”

Bob looked over the air handler and a building print that was in the room where the air handler was located and found that the system had a hot water heating system with a boiler on the top floor. He went to the boiler and found it up to temperature. He was sure he had solved the problem now. He went back to the second floor air handler and felt the inlet and outlet to the hot water coil. It didn’t seem to be warm at all. He shut off the valves just to be sure and waited about 15 minutes and checked the coil outlet temperature and the temperature hadn’t changed a bit.

Figure 1. This compressor is sweating at the motor housing and on the crankcase. It is a sure sign that liquid is getting back to the compressor. (Figures are from Refrigeration & Air Conditioning Technology, 5th Edition, by William Whitman, William Johnson, and John Tomczyk, published by Delmar Cengage Learning.) (©Delmar Cengage Learning.)

Bob then said, “I can’t imagine what is going on.”

Btu Buddy said, “If there is not excess heat, maybe the refrigerant coil is not performing. Let’s go and look at the compressor and see what the conditions are there.”

They went to the roof and removed the compressor compartment door and found the compressor sweating on the motor and the crankcase (Figure 1).

Bob said, “We have a problem here. This compressor is sweating all over the motor housing and crankcase. Liquid refrigerant must be coming from somewhere. I think we should start with the second floor air handler.”

Btu Buddy said, “I think you are on the right track now. You have found something that is wrong.”

Figure 2. This photo is of a nylon strap used to fasten duct insulation or flex duct. This is the correct application for these straps. (©Delmar Cengage Learning.)

They went to the second floor air handler and Bob took a close look at the expansion valve bulb mounting. He had to peel back the insulation. He then said, “Would you look at this; this bulb was mounted using two nylon straps (Figure 2) and both of them are broken. The bulb is loose on the suction line.”

Btu Buddy said, “Those straps can hold a bulb down tight, but they may eventually fatigue and break. You should make the repair using the correct fasteners.”

Bob shut the system down and it went through a pump down cycle and pumped the refrigerant into the condenser and receiver.

Btu Buddy said, “That was a good idea to shut the system down and let it pump down. If you had just shut off the breaker, when you started it back up, there was probably going to be some liquid slugging.”

Bob repaired the bulb fastening according to the manufacturer’s recommended method (Figure 3), then he said, “I bet the other coils have the same problem and it just hasn’t broken loose yet. I am going to check them while I have the system off.”

Figure 3. This is the correct method of fastening an expansion valve sensing bulb to the line. The bulb must accurately sense the suction line temperature. (©Delmar Cengage Learning.)

Btu Buddy suggested, “You may want to tell the manager what you are doing. He is surely going to have some calls.”

Bob told the manager what was going on and he suggested that Bob to repair it up to proper standards while he was working on the system.

The system had been off for nearly an hour when Bob turned it back on. The building had warmed up some. Bob then said, “I think we should go to the roof and see how that compressor is doing.”

They went to the roof and the compressor was running just fine. Bob felt the motor housing and the crankcase, and said, “This feels more normal. I believe we got to the bottom of the problem.”

Btu Buddy then suggested to Bob, “I think you should talk to the manager about a service contract now. I believe he would want to talk about it after this service call.”

Bob went to the manager and suggested that he let a representative call on him and give him a quote.

The manager said, “We have always had service when we needed it. The last service company never mentioned a service contract. I think that might be good.”

Bob explained, “The problem that this system was having would have shown up at a routine service checkup. The compressor probably has been operating with small amounts liquid refrigerant in the crankcase for some time. That is really not good. I believe a regular look at the system will be a good idea.”

As Bob and Btu Buddy were riding away, Bob asked, “Why didn’t that liquid getting back to the crankcase ruin that compressor?”

“Good question, Bob,” said Btu Buddy. “Let’s meet for breakfast in the morning and talk about it. Meanwhile, see if you have any more questions.”

Bob then said, “I think it was not explained why that didn’t effect the total system suction pressure and cause the other two coils to perform the same way.”

Publication date: 04/19/2010