Btu Buddy 146: No Cooling Call on a Heat Pump
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 no cooling call, the first of the year. The temperature was 85°F outdoors and 76° indoors at the residence. The homeowner was talking to them and explained, “I noticed it was warm in the house yesterday afternoon, but it cooled down later in the evening.”
Bob explained, “The cooling down in the evening was because it got down to 50° during the early evening, just after dark. Let us look at the unit and see what we can see.”
They turned the thermostat to cooling and pushed the lever down to a low setting so the heat pump would come on and stay on. They went outdoors to the outdoor unit and felt the leaving air and the piping and Tim said, “The unit is running in the heating mode. I wonder what is going on?”
Bob said, “Turn off the unit at the disconnect. Let’s see what happens.”
They turned the unit off. Bob then said, “The four-way valve is controlled by 24 volts, so I it should stay in place while we check it out. Check the voltage from the common (white) wire to the four-way valve terminal ‘O’ orange wire.”
Tim removed the control panel cover and checked the voltage and said, “There is 24 volts. What should it be?”
Bob said, “The ‘O’ terminal should be hot in the cooling cycle to energize the four-way valve, so that is normal. I wonder why the valve is not shifting over. Check to see if the four-way valve solenoid is warm. If it is, the coil is OK.”
Tim touched the valve coil and said, “It is plenty warm. Let’s disconnect it and see if it changes over, then we can ohm it out to be sure.”
Tim disconnected the wire from the valve coil and they both heard it click, and Tim said, “The solenoid part of the valve is good. I wonder why it is not changing over when the unit is running?”
Bob then said, “There is not enough pressure difference for the valve to change over now.”
Tim asked, “What do you mean pressure difference?”
Bob explained, “The little solenoid is called a pilot-operated solenoid. It does not do the work to change the valve position. That is a rather large valve and would require a large solenoid. The pilot solenoid merely directs the high pressure gas to the side of the real valve which causes the shift (Figure 1).
“During the heating cycle, the valve is shifted so that hot gas from the compressor moves into the indoor coil, heating the structure (Figure 2). During the cooling cycle, the valve is shifted in the opposite direction and the hot gas is directed to the outdoor coil and the unit is in the cooling mode (Figure 3). The key to following the cycle is to follow where the hot gas is going. It is easy to follow because it is hot. There has to be a pressure difference from the low pressure side to the high pressure side to shift the valve piston.”
Tim said, “Then we know that the pilot valve is working, or at least making a noise. How do we know if the pilot valve is not shifting, or the main valve is not shifting?”
Bob said, “The pilot valve is shifting or functioning; you can hear it click when it is energized or de-energized. We can tell when the main valve is shifting by which direction the hot gas is moving.
“It is very warm today and we should not let the system run long in the heating mode because tremendous pressures will build up in the system. Get the soft face hammer and what we are going to do is start the system, let the pressure difference build up, and gently tap each end of the main valve and see if it will change over. Sometime, a valve may stick and with a little help change over. Also, let’s put the ammeter on the compressor circuit and not let the amperage climb too high.”
They had everything in place and started the unit. The compressor full load amperage was 17A. The unit started running and the amperage started climbing, and Tim said, “The discharge line is getting hot and the gas is going toward the indoor coil, so it is still in heating. I am going to tap the end of the four-way valve.”
When Tim tapped the end of the valve, the system made a loud noise. Tim said, “That sounded like a toilet flushing.”
Bob said, “The valve changed over. Feel where the hot gas it flowing now.”
Tim said, “Sure enough, it is now flowing toward the outdoor coil. It is in the cooling mode.”
Bob said, “Now remove the 24V wire to the four-way valve. You can just take it off of the ‘O’ terminal. The system should change back to the heating cycle.”
Tim removed the wire and it changed back to the heating cycle, and Bob said, “Touch the wire back to the terminal again and change it back to cooling.”
Tim touched the wire to the terminal and it changed back again, and Bob said, “Change back and forth several times. This is to get the piston in the valve to slide back and forth several times.”
Tim forced the system to change back and forth several times and it worked and he asked, “What are we doing?”
Bob responded, “Hopefully a particle was causing the piston to stick in the valve. If we can get the particle to keep moving in the system, hopefully the filter will pick it up and take it out of circulation. All of this is hopefully because that may or may not solve the problem. If we don’t hear any more from the customer, it is repaired. If this reoccurs, we will have to change the valve. This is an old heat pump and it would probably make sense to change the system out. Changing the four-way valve is major surgery. Either choice is a big expense. Let’s take the chance and see if this works.”
Tim said, “That is great to think of the customer like that.”
Bob said, “I try to think just as though I were the customer and that is how I would treat me, so that is how I treat the customer. Remember, a good customer kept is better than finding a new one.”
Tim said, “I guess that is part of learning to be a professional service technician.”
Bob said, “Always keep that in mind. It will serve you well.”
Publication date: 5/18/2015