Btu Buddy 104: Bob and Tim Continue Heat Pump Repairs
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 have arrived back at the apartment building where they have been working for several days inspecting and making necessary repairs to the building’s split-system heat pumps.
Bob said, “Check the list for another unit that seemed to have a problem and let’s take a closer look at it.”
Tim said, “This list has been really handy. We looked over all these units for just a visual inspection and for signs of trouble. I think that we only have one more unit that seems to have a problem. We noticed that unit number 58 was running and shutting off after a short running time. The tenants are out of town so we removed the red low voltage wire until we could get back to it. You said that it was important to keep the crankcase heat on so the unit can’t run with the red wire removed. Let’s see what the problem is.”
They went to unit 58 and shut off the disconnect and replaced the red low voltage wire and closed the disconnect and the unit started. The unit began to sound like it was running under a strain and Bob said, “Feel the gas line and see what it feels like.”
Tim touched the line and jumped back and said, “It is hot, really hot.”
Bob said, “Shut off the disconnect.”
Tim shut off the disconnect and said, “That unit is trying to operate in the heating mode. No wonder it was shutting down the other day. It was overloaded.”
Bob explained, “These units only have a motor winding temperature thermostat (Figure 1) for a safety control. It serves as a low charge protector, high pressure protector, and motor overload.”
“How can that be?” asked Tim.
Bob explained, “First, if there is a low charge condition, the motor will overheat because there is not enough refrigerant to cool the motor (Figure 2). Second, if there is a high pressure condition, the motor will draw high amperage and the motor temperature will rise and the winding thermostat will shut the motor off. Third, if there is a high amperage condition, such as the motor won’t start, there will be high amperage and the motor winding thermostat will shut the motor off.”
Tim then said, “There is no telling how many times that motor tried to start before we discovered it. It seems like the control would fail with so many attempts to run.”
Bob said, “That is called a duty cycle. The manufacturers test these controls through thousands of duty cycles before they make a manufacturing run of the controls. They have proven very reliable over the long haul.”
Tim now said, “Let’s find out why this unit is running in the heat cycle on a summer day. It is 90°F out here. What should we do next? We can’t operate the unit in this temperature.”
Bob said, “Let’s remove the compressor compartment door and see what we see. We need to remove the wires to the compressor and tape them to make them safe. We can check the four-way valve and see if it is getting power to the coil. These units have the four-way valve energized in the summer cycle.”
They removed the cover and removed the common wire and taped it off safely and turned the power back on.
Bob said, “Check from the common wire to the yellow wire on the terminal board (Figure 3) to make sure the unit is calling for cooling, then check common to the ‘O’ wire and see if it is calling for the four-way valve to be energized for starters.”
Tim said, “The ‘Y’ (yellow wire) terminal is hot and the ‘O’ terminal is hot. The unit is calling for cooling all the way. Now what?”
Bob said, “Place your screwdriver tip next to the four-way valve coil and see if it is magnetized.”
Tim said, “It is not attracted to my screwdriver tip.”
Bob then said, “Let’s be sure it is getting voltage. Check the voltage at the coil.”
Tim checked the voltage and said, “It is getting 25 volts.”
Bob said, “Turn off the power and disconnect the coil wires and let’s see if there is any continuity through the coil.”
Tim checked the coil with his ohmmeter and said, “The coil is open circuit. Here is the problem.”
Tim changed the coil, reconnected the compressor common wire, and was ready to start the unit with the disconnect.
Bob said, “Go ahead and start the unit and let’s feel the gas line to see what happens.”
Tim started the unit and it sounded normal and as the pressure built up the unit made a swishing sound. He then said, “What was that sound?”
Bob explained, “That was the four-way valve changing from heat to cool.”
Tim said, “The gas line is cool, not cold.”
Bob said, “There is a very big load on this unit because the apartment is really hot. Let’s go to lunch and come back and come back and see what we see.”
They returned after lunch and Tim did a touch test of the unit and said, “The air leaving the condenser is really hot and the suction line is now cold. It is cold all the way back to the compressor and there is some sweat on the side of the compressor. Do you think the unit has too much refrigerant in it?”
Bob said, “No, because the unit is really loaded up, the head pressure is higher than normal and pushing more refrigerant through the orifice metering device than normal. Under these conditions, the superheat at the compressor is at the very lowest. When the head pressure comes down, the sweat line will fall back to normal. Just let it run. It is really doing the job now.”
Tim said, “It is really good to know these little tips. I probably would have fastened gauges to the unit but now I see that there was no need.”
Bob said, “The work we have done on these units has all been pretty basic. Soon we are bound to run into a tough service call. Fall is coming on and the nights are getting cooler. People will begin to start up their heating systems and a new set of circumstances will occur.”
Publication date: 11/21/2011