Btu Buddy 163: Suction Line Restriction
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.
Bob and Tim were at a customer’s house and the customer was explaining to them what was going on with his air conditioner. He said, “The unit was running fine yesterday morning and then the house started warming up and it was warm last night and it is still warm this morning. The thermostat is set at 73°F and the house temperature is 78°.”
The system was a gas furnace with an electric air conditioning system and they could hear the indoor fan running. Bob went to one of the registers and put his hand over it and it was moving air, but the air felt like room temperature; the system was not cooling.
Bob said, “Let’s go to the furnace area and see what we see there.”
When Bob and Tim approached the furnace, the fan was still running, so Bob felt the suction line leaving the evaporator coil and he said, “This line is not cool like it should be. Let’s go to the condensing unit and see what is going on there.”
They went out to the condensing unit and the fan was running, but the compressor was not running. They took the top off of the unit and found that the compressor was hot. Bob said to Tim, “Get the gauges and tools and let’s see what the pressure is in the system.”
Tim got the gauge set and the tools and they installed gauges on the system, and the system had full pressure on it, normal for a unit that was shut off. Bob then said, “It seems as though the compressor is overheating and the over-temperature control has the compressor shut off. Let’s turn off the disconnect and get a water hose over here and cool this compressor down.”
They ran water over the compressor housing for about 15 minutes. Meanwhile they changed filters in the inside unit and checked the fan for dirt and the condensate line to make sure everything looked as good as it could (Figure 1).
They started the unit and the suction pressure immediately started down quickly and was down to about 20 psig with the compressor running and Tim said, “It looks as though the unit is out of refrigerant or very low on refrigerant.”
Bob said, “That pressure went down awfully fast from normal to low. I thought when we first saw the gauges and it had full pressure that there must be enough refrigerant to build pressure. When we turned the unit on and the pressure went down so fast, I was surprised. I think we should try to add some refrigerant and see if the suction pressure comes up. I’m suspicious that we are on the wrong track but I don’t know why.”
They measured in one pound of refrigerant and the suction pressure did not rise at all. Now they were both suspicious. Bob said, “Let’s go under the house and follow the suction line and liquid line and see if we find anything different. Let’s look for restrictions in the liquid line or the suction line coming out of the unit under the house. Wherever the restriction is, if there is one, it should get very cold right after the restriction.”
They crawled and looked at both lines and they discovered that the line set had a long loop in it to take up the extra length because it was too long. They felt the suction line all along the insulation but could find no problem; however, it looked like the line may have a kink under the insulation. Tim took out his knife and cut a slit in the insulation to look for a problem and found a kink in the suction line (Figure 2).
Bob said, “I wonder how that kink got in the suction line. This system has been running all summer with no problems. Let’s go upstairs and explain this to the customer and tell him what we have to do and see if we can find out what has happened here.”
They went upstairs and talked to the customer and the customer told them that he had the cable television people out yesterday morning installing some more cable under his house. He thought that more than likely that is when the kink got in the line.
Bob said, “We will have to remove all of the refrigerant from the system into a vacuum, then charge it back with nitrogen to atmospheric pressure, put a splice in the line, then leak check it and evacuate the system and recharge it.”
The customer asked, “Can’t you just straighten the line out?”
Bob responded, “No, if we try to straighten the line out we will create a very weak spot in the line that is very likely to start leaking and causing more problems than we have now. It should be repaired correctly.”
Bob and Tim went to work and pulled the refrigerant out of the system with their recovery unit (Figure 3). Then they added a coupling in the line where the kink was and added a small amount of refrigerant and pressured the system with nitrogen and a small amount of R-22 and checked for a leak. It was leak free. Then they connected their vacuum pump and let it run until it was into a deep vacuum of about 200 µm. They measured the line set to see what length it was and it was 50 feet. They totaled up the amount of refrigerant required by adding the evaporator refrigerant, the line set refrigerant, and the condensing unit refrigerant and came up with the total. Then they connected the refrigerant cylinder and added liquid until the liquid refrigerant stopped flowing. They then switched over to vapor and started the unit. Since they were weighing the charge in all they had to do was charge vapor into the suction side until they had charged the correct weight. The unit was cooling and the gauges were reading just as they should.
Tim asked, “Why didn’t we feel liquid at the point of the kink in the suction line? It seems like there should have been liquid at that point and the liquid should have been seeping through the kink and expanding and making it cold at the kink outlet.”
Bob said, “This unit uses a thermostatic expansion valve which would only allow vapor to be in the suction line maintaining 10° of superheat at the end of the evaporator. If this had been an orifice or capillary tube metering device, there would have been liquid in the suction line.”
They went back in the house and talked to the owner and explained what they had done and the owner asked, “What should I do about the cable company that caused this problem?”
Bob said, “I would call the company and explain to them what happened and tell them if they want to talk to us about it to call me at this number.” And he gave the customer his cell phone number.
As they were riding away, Tim said, “Well, that was a new one. I think we did a service by fastening that line set up to the floor joist better so it would be out of the cable people’s way and probably less likely to be damaged again.”
Bob said, “Yes, that line set was not installed like we would have installed it. It’s good to go now though.”
Publication date: 10/24/2016