Service & Maintenance / Extra Edition

Btu Buddy 51: Topping Off the Charge for a Fixed Bore Metering Device System

June 18, 2007
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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.

The dispatcher called Bob and asked him to go to a new customer who was reporting a system that was not cooling up to capacity.

Bob arrived at the job and looked over the system. The thermostat was set at 72°F and the house temperature was 78°. The unit was running all of the time during the day and not cooling the house down to a comfortable level.

Bob then went out to the outdoor unit and felt the suction line entering the unit. It was cool, but not cold like it should be. He thought, I will add a little refrigerant and be on to the next job. He went to his truck and got his gauges and a cylinder of R-22.

Btu Buddy appeared and began to question him. “What are you going to do?”

Bob said, “Just add a little refrigerant. It is only a little low.”

Btu Buddy then said, “I would suggest that before you remove the caps on the service ports that you check them for a leak. It is possible that they were left seeping refrigerant and that may be the source of the leak. There seems to be enough refrigerant in the system to leak check them.”

Bob turned the unit off to let the pressure equalize so there would be enough pressure on the low-pressure service port to leak check it also. He then went to his truck for his electronic leak detector and got set up to do the leak check.

Figure 1. This service tool can be used for changing or tightening Schrader valve service stems. They are much like tire stems, except for the material they are made of. Notice this tool has a place for extra valve stems.

Sure enough, the suction service port was leaking even with the cap on it. He removed the cap and used his Schrader valve tool and tightened the valve stem core, and the leak stopped (Figure 1).

Bob said, “Well, I guess the last person accidentally left this leak. If I had just put the gauges on the system and charged it, we may never have known where a leak was. This system is only a little short of refrigerant, so that should be the only leak.”

Btu Buddy responded, “Probably it was the only leak. The other valve stem was not leaking. Now, let’s just add enough refrigerant to get the system up to peak performance. How do you propose to do that?”

Bob said, “Just add refrigerant until the suction line is sweating is one way.”

Btu Buddy then said, “Let’s give this customer their money’s worth and do the job just like you would want the job done at your house where you are paying for the service call and the power bill. It is 85° outside temperature. Let’s get set up to charge the system just like it were a 95° day, so when it gets up to 95°, the charge will be correct and the system will perform at maximum efficiency.”

Bob asked, “Do you mean the superheat method? This is a system with an orifice for a metering device.”

“Yes”, answered Btu Buddy. “The only way to add refrigerant is to check the superheat at the outdoor unit and build up the head pressure as though it were a 95° day. What should the head pressure be for a 95° day for this system?”

Bob looked at the unit and said, “It is obvious that the unit has some age on it. My pressure temperature card shows that for a 95° day and a 30° allowance for a standard efficiency unit I think the head pressure should be about 278 psig.”

Btu Buddy said, “Good math, 95° outside temperature plus 30° condensing approach temperature would mean that the refrigerant should be condensing at about 125°, which corresponds to 278 psig for R-22.”

Bob then fastened a temperature lead to the suction line and fastened his gauges to the gauge ports and the gauges showed a head pressure of 225 psig  (a condensing temperature of 110°) and a suction pressure of 55 psig (a boiling temperature of 30°) and a suction line temperature of  60°.

Bob said, “The unit has a 30° superheat. The evaporator is starved for refrigerant.”

Btu Buddy agreed and said, “With a low head pressure and a low suction pressure, the system must be low on refrigerant. Let’s add some refrigerant and block some of the condenser air to get the head pressure to rise.”

Figure 2. This system is being charged on an 80°F day by blocking some of the air across the condenser to get the head pressure up to simulate a 95° day. (Figure is from Refrigeration & Air Conditioning Technology, 5th Edition, by William Whitman, William Johnson, and John Tomczyk, published by Thomson Delmar Learning.)

Bob used a piece of metal from his truck on the fan outlet to block the condenser airflow and added a small amount of refrigerant. The suction pressure began to rise and the head pressure began to rise. When the head pressure reached 275 psig, Bob began to uncover the condenser fan outlet and maintained the 275 psig head pressure (Figure 2). The superheat was now down to 20° with a suction pressure of 65 psig. The system was beginning to work now; the suction line was beginning to sweat.

Btu Buddy then said, “Let’s let the system run like this for awhile until it settles down. The superheat is not staying steady.”

After the system ran for about 15 minutes, the superheat was showing 20°. The system still needed some refrigerant.

Btu Buddy then explained to Bob, “We are taking the superheat at the outdoor unit. We are really shooting for a superheat of 8° to 12° at the air handler where the evaporator is, which is about 25 feet from the outdoor unit. There will be some pressure drop in the suction line and some heat will be added to the refrigerant before it reaches the outdoor unit, so we must compensate. With a suction line up to 30 feet we will charge the unit for 10° to 15°. If the line set is longer than 30 feet, we would charge for a superheat of 15° to 18°.”

They added a small amount of refrigerant again and after about 15 minutes, the superheat settled down to 13° (Figure 2). The actual boiling temperature at the evaporator should be about 40°, which is design temperature.

Bob said, “Well, there is a lot to getting the job done right. The homeowner will now have an efficient system.”

Btu Buddy said, “The last thing that you should do is check the gauge ports for leaks. Then if the system loses refrigerant again, you will know that the leak is somewhere else. It seems like technicians through the years have become careless about refrigerant leaks. Refrigerant became inexpensive for many years and technicians just got into the habit of adding refrigerant instead of finding leaks.”

Bob said, “With the laws like they are today, the technician that can search and find leaks, then repair them, is a good technician.”

Btu Buddy said, “Next, we will top off the charge for a high efficiency system that uses a thermostatic expansion valve (TXV). This procedure is a little different because the thermostatic expansion valve controls superheat and it cannot be used to charge the system.”

Publication date: 06/18/2007

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