The Solution to 'The Case of the Baby Boomer's Heat Pump'
Their heat pump was just installed last year, near the end of the cooling season. The installing contractor was rushed for time, but the new, 2.5-ton, R-410A unit (plus indoor coil) was installed, charged, and operating quickly.
At the start of the 2006 cooling season, Kelly switched the thermostat over to cooling, and it works, but the fan always seems to be running. As the outdoor temperatures get hotter, the unit seems to run nonstop. Eventually it runs all day long and the 1,600-square-foot house doesn't cool down.
They call the installing contractor. Our hero, Tom, was not one of the installers of this unit. In fact, he has had to deal with a lot of dissatisfied customers.
When he gets to the house, the outdoor ambient is 85Â°F and the house is hot and humid. The pressure and temperature at the suction line, just where it enters the compressor, are 64 psig and 70Â°. The compressor amperage is low. The unit was covered, but Tom learns from Kelly that she covered it when she shut off the unit a day or so ago. The pressure and temperature at the evaporator outlet are 65 psig and 60Â°.
What was the true problem?
MYSTERY SOLVEDAs we said, there were more than 200 entries for this System Mystery - a new record! Answers ranged from speculation that the new 410A heat pump was charged as if it were a 22 system, to miswiring the reversing valve, and the fact that the cover was on the unit. We were also impressed with the number of entries that continued the fictional train of thought, even going so far as to kill off the homeowners.
However, the answers that best explained the operating conditions presented in our mystery involved the TXV and the reversing valve.
It was tough selecting just one winner. So, we selected several. Here are their responses:
"1. Not knowing the high-side pressure and liquid line temperature, the first thought I have is a leak in the system and way short of refrigerant. This would be an easy installer's error.
"2. Another possibility is a restricted liquid line filter-drier, especially if any drier installed is not specific for 410A
"3. Still another possibility is a restricted metering device for the indoor coil. Again, more info is needed to make a better judgment call on this operating condition. The story line did not mention whether or not the unit would operate properly in the heating mode but not the cooling mode.
"4. Low amperage draw by the compressor can be caused by any of the three items described above. Give me some more info and I might be able to nail down the problem a little better. I think the first thing the service tech did was leak check the system connections." - Steve Riebe
Sidebar: Instructor's ResponseWe received this detailed answer to "The Case of the Baby Boomer's Heat Pump" from an HVACR instructor. Roger W. Raffaelo, of Daytona Beach Community College, is receiving special publication of his reply in this bylined sidebar. He also will receive teaching aids from Emerson. Congratulations, Roger! Here is his reply:
"From what Tom checked, it would appear to be running a low suction pressure with high superheat. This would give the system low capacity, which would account for the increase in run time and insufficient cooling. To properly check a system, both low- and high-side pressures, as well as return air wetbulb and drybulb temperatures (true load on the evaporator) and outdoor drybulb, should be checked and compared to the system's charging information. Temperature difference across the indoor unit with these conditions would also be low. We'll assume the airflow is good due to the same-size system and proper performance with the old system. You would need to use a systematic approach to evaluate the system.
"To start, Tom should check to see what kind of metering device is in the indoor unit and put on his high-side gauge. If there was a TXV used (most likely)as the metering device, Tom would want to check to see if the liquid line was completely full of liquid to the metering device and the high-side pressure was near normal. If the liquid line contained a liquid-vapor mixture, it could account for the high superheat.
"Mass flow rate through the valve could be reduced due the vapor in the liquid line. Less liquid entering the indoor coil would mean low suction, high superheat, low capacity. To determine a full column of liquid, Tom would need to check for subcooling in the liquid line (an electronic sight glass would work also). If the liquid line was not full there may be a refrigerant leak. Make sure you check any liquid line drier for a restriction. This would cause flashing off of refrigerant through the drier causing the problem. If the drier is normal at this point, leak check any joints made by the installers and if no indication there check the installed equipment. If no leaks found there is a possibility the system may have never been charged properly when installed (there was a problem with the techs as well as installers).At this point charge to recommended subcooling and recheck superheat. Should be around 10Â° leaving the coil.
"If the liquid line is full, there is a problem with the metering device. The power assembly could be losing its charge. This is the opening force for the valve and would cause high superheat. You could take the bulb loose and see if the superheat goes down. Some power assembles are replaceable; if not, the valve needs to be replaced.
"Another problem could be trash in the orifice of the valve restricting the flow. This could be caused by poor piping practices allowing trash to get in the system plugging the strainer, or the refrigerant line set may not have been replaced and there was excessive residual mineral oil left in the system. If the valve could be taken apart and cleaned, it may be fixed, but it probably would not be cost effective. Best thing to do would be to replace the valve and install a properly sized liquid filter-drier before the new valve and charge to the manufacturer's specifications.
"If the system is a fixed orifice, compare the system pressures to the charging information. Check for any restrictions, same as a TXV system. If the high-side pressure is normal and according to the charging information, suction pressure is low and superheat is high, you may want to pump the system down and remove the piston orifice to make sure it's the right size and clean. I've have had small pieces of copper burrs get caught up in the orifice. Also, check the liquid strainer at the orifice body.
"To check a system's operation, the technician must use a systematic approach. Compare the manufacturer's operating specifications to the actual operating conditions. Be sure to check both low- and high-side pressures, subcooling and superheat, and adjust the system charge to what is necessary to achieve the energy efficiency, as well as Btu capacity the system is designed to achieve. Small adjustments made to the system to bring it to the manufacturer's specifications will greatly improve the overall performance of the system. It's up to the tech on the job to be qualified and competent to give the customers what they paid for."
- Roger W. Raffaelo
Publication date: 07/17/2006