Btu Buddy 24: Looking At A Heat Pump Drawing A Lot Of Power
The dispatcher calls Bob and reports that a good customer wants their home heat pump checked out. The power bill has taken a dramatic rise in the last two months.
It is 28 degrees outside. Thank goodness it isn't snowing, Bob thinks. He then arrives at the house and talks to the housewife. She shows him her power bill and it is at least 40 percent higher than the month before. She says that the house has been comfortable, but that the light on the thermostat seems to stay on a lot more. In fact, it is on now. Bob walks down the hall for a look at the thermostat and it is on. He goes outside to the outdoor unit and looks around. The large copper line going to the house seems normal. There was a little ice on the outdoor coil. He goes to the basement and looks at the indoor unit. All seems normal there. He removes the electric heat access panel and uses his ammeter to check the electric heat. Two out of three heating elements are drawing current.
Bob decides to fasten gauges to the unit to see what the high and low side pressures are operating at. The unit has R-410A as the refrigerant. The evaporator in the outdoor unit should be boiling at about -7 degrees F, which is 35 degrees less than the 28 degree F outdoor temperature. The suction pressure should be about 41 psig. He installs the gauges and discovers that the suction pressure is 62 psig, which corresponds to about 10 degrees F. The suction pressure seems too high. The head pressure should be about 340 psig and it is 275 psig. He then takes an amperage reading and discovers that it is lower than normal.
Bob is scratching his head over these figures when Btu Buddy appears and asks, "What do you think the problem may be, Bob?"
Bob says, "I think the compressor is not pumping correctly. The suction pressure is high, the head pressure is low, and the amperage is low. That sounds like an inefficient compressor to me."
Btu Buddy then says, "If this were a cooling unit, I would agree with you. But we have another factor in this unit; we have a four-way valve. You may want to test it before you change the compressor and find that you have the same symptoms when you start the unit with a new compressor. Let's check the four-way valve for leak through next."
Bob then goes out to the outdoor unit to check the temperatures. Since the suction pressure is 62 psig for the boiling refrigerant of 10 degrees F, the suction line from the evaporator should contain about 10 degrees of superheat. So the line should be about 20 degrees F and it is 23 degrees F - very close. However, the suction line leaving the four-way valve going to the compressor should be no higher than 3 degrees F more than the evaporator suction line. It is reading 50 degrees. Bob then says, "This four-way valve is really leaking through, causing the system to be inefficient, not the compressor."
Btu Buddy responds, "Now you are getting the picture. The valve is definitely defective. This would cause the auxiliary heat to operate for longer periods and cause the power bill to rise. We can try reversing it several times to see if we can free it up. It may have a piece of trash under the seat or it may have been damaged when it was installed. It isn't likely that it would be worn out because it doesn't get much activity."
Bob uses the outdoor wiring to cause the four-way valve to change over several times and then checks the temperature again. It is still leaking through, so he makes the decision to tell the homeowner that the valve must be changed. The unit is only eight years old and in very good shape, so changing the unit is not the best economical solution for the customer. He gives her an estimated price and she says to change it out.
Btu Buddy tells Bob, "Drop some of that ice off of the coil onto the discharge line from the compressor and see what you see."
Bob then drops a little ice on the line and it sizzles and boils. Bob asks, "What was that all about?"
Btu Buddy explains, "That compressor is running too hot because of the leak through. It is probably shutting off because of internal overload from time to time. The discharge line should never be hot enough to boil water like that under normal conditions. Have the owner switch the unit to emergency heat until you can change the valve to prevent compressor damage. When a compressor runs that hot, it will decompose the oil over time. Let's not take a chance with this compressor; it is still running."
Bob arranges to change the valve out the next week because there seems to be a break coming in the weather.
As they are riding away, Btu Buddy says, "Changing a four-way valve is a big and intricate job. I will meet you here next week and walk you through the procedure."
Note: In the next installment of the Btu Buddy series, Bob and Btu Buddy install the new four-way valve.
Bill Johnson has been active in the HVACR industry since the 1950s. He graduated in gas fuel technology and refrigeration from the Southern Technical Institute, a branch of Georgia Tech (now known as Southern Polytechnic Institute). He taught HVAC classes at Coosa Valley Vocational & Technical Institute for four years. He moved on to become service manager for Layne Trane, Charlotte, N.C. He taught for 15 years at Central Piedmont Community College, part of this time as program director. He had his own business for five years doing installation and service work. Now retired, he is the author of Practical Heating Technology and Practical Cooling Technology, and continues as a co-author of Refrigeration & Air Conditioning Technology, 5th Edition, all published by Delmar Publishers. For more information, he can be reached at 704-553-0087, 704-643-3928 (fax), or firstname.lastname@example.org.
Publication date: 03/21/2005