Air-to-air heat pump arrangement with refrigerant changeover; (a) is the cooling or defrosting mode, and (b) is the heating mode (from Refrigeration Principles and Systems: An Energy Approach, Business News Publishing Co.).
Welcome back to “TECH TALK,”The News’feature to encourage professional hvacr technicians to talk to each other about “live from the field” applications, problems, and solutions.

In the April 24 issue, we posed the problem of the homeowner who was having “nothing but problems” with a heat pump they had installed in July 1997. Considering the partial information given for this problem, we received some very logical, and some rather intuitive, responses; after all, you don’t learn everything from a textbook.

Here’s a quick recap of the customer’s complaint:

“We had a new heat pump installed in July 1997, and have had nothing but problems with it. Now [February 2000] it has begun to freeze up all of the time, even if we only leave it running for a short period of time. It’s not like it’s -20°F outside; in fact, it’s been in the 30s each time it freezes up.

“We have to turn the heat off, let it thaw and then turn it back on. Any suggestions? I’m pulling my hair out!

“The blower is still blowing out air, but the unit outside is still making a humming noise… kinda like it’s running, but the fan is not blowing. It goes into that every now and then. Is it OK to leave it running when that’s happening?”

Of course, your first response is to get more solid technical data, like running temps, and to observe this unit in operation. But a lot of the time you listen to customers like this on the phone first; you try to get a rough idea of what the problem could be, before you decide which of your service techs to send on the call, and what kind of supplies he should bring along. (And how about a tranquilizer for the customer?)

Your Replies

“I believe this is a case of a bad reversing valve or reversing valve (defrost) control,” writes Alan Sawyers of Texas Instruments Design Engineering.

“As we know, a heat pump defrosting is actually running in the air conditioning mode. This will allow the outdoor coil to use the heat in the house to heat up and melt the frost/ice on the unit. During the defrost action, the outdoor fan stops running to allow the coil to heat up as quickly as possible. After a period of time (10 minutes or so) or a certain temperature is met, the defrost control will switch the reversing valve back to heat pump mode and the outdoor fan will start running.

“Judging from the symptoms,” he concludes, “the unit is trying to defrost (the outdoor fan periodically stops, but the unit is making a “humming” noise — which is the compressor running), but most likely the valve is never switching over to a/c mode. In this case, the coil will never heat up and therefore never melt the frost/ice off of the unit.”

“It would be difficult to pinpoint a problem without having some idea as to temperatures,” points out Walt Bentz, Tech Serv Adviser & Service Coordinator for the Goodin Co., “but the description of ‘the blower is still blowing out air but the unit outside is still making a humming noise…kinda like it’s running, but the fan is not blowing; and the fact that ‘It goes into that every now and then,’ would indicate that the unit was entering defrost, and either the customer is shutting it down before it can complete a defrost, or the reversing valve is not going into the ‘cooling position’ for defrost, and is continuing to frost the coil in a defrost cycle.”

Now that he has a rough idea of the problem based on the customer’s description, what Bentz would like to see for himself at a visit to this customer’s house is, “Does the outside coil start to get warm when the outside unit ‘goes into that’?”

“I may have a solution,” writes Jim Baird, service manager for Quality Degree, Inc., Royersford, PA. “Given the information I have” — and he does point out that a site visit would be necessary to confirm it — “I want to make the following diagnosis:

“First let’s assume:

1. The system is heating the home to the t-stat’s set temperature;

2. The defrost cycle is initiating. The homeowner said ‘the outside unit is making a humming noise…kinda like it’s running, but the fan is not blowing.’

“The problem: The outdoor unit is freezing up during the heating cycle at moderate ambient temperatures.

“Diagnosis: The defrost thermostat is out of calibration. During moderate outdoor temperatures, the defrost t-stat is sending a signal to the defrost control that the liquid line is warmer than it really is, therefore not allowing the defrost cycle to initiate. During colder outdoor temperatures, the defrost thermostat, while still out of calibration, would still close, allowing a defrost cycle.

“This is the first time I’ve seen/tried your test, I’m looking forward to future issues to play, ‘Who Wants to be an Hvac Millionaire.’”

(Hmm. We may have to rename this column.)

Service tech Mark Ballard, from White House, TN, has a couple of ideas as to possible problems. “First,” he write, “the unit may be undercharged. Does it run OK during the cooling cycle? Second, is the outdoor coil clean?

“The outdoor unit sounds like it is running normal. It is running. The unit is going into defrost and the fan is not supposed to run at that time. They have had nothing but problems since the install. What are the other problems?”

Good question, Mark. Another one would be, have they called contractors to fix those other problems? What about the installing contractor? Have they called him? Has anyone else tried to fix this unit?

“The problem is in the defrost controls,” states reader Jerome Junker. “The customer saying that the unit made a humming noise and the condenser fan was off, tells us that it was trying to go into defrost but the four-way valve would not change over to allow defrost of the condenser coil. And if the customer is saying that it never worked right, the indoor electric heat is probably not coming on during defrost.”

Vince Gingras, owner of Vincent’s Aire-Care, Lady Lake, FL, comments, “The biggest clue to this mystery is the customer’s statement that the outdoor coil is freezing up in a short period during moderate conditions. They also describe what sounds like a normal response to a need to defrost.

“I would lean toward refrigerant charge, but pay special attention to the operation of the outdoor metering device (TXV or fixed orifice) and also check for proper operation of the reversing valve. The system could be properly charged, but could also have debris floating around the system, interfering with metering and switchover functions.

“I’d also look to see if the driers were of the right type and directional orientation.

“Reaching even further into past experience,” writes Gingras, “I would check to make sure the air handler is actually equipped with a bypass for heat pump operation. Maybe the check valve is stuck. Maybe it was never installed. Stranger things have happened.

“Since most systems do not have low-pressure cut-outs, problems could go undetected by the unknowing homeowner until the high winter electric bills start coming in.”

My Favorite Answers

While reading this response, it occurred to me that both of the TECH TALK columns that have run so far could be dealing with two-speed motor problems. So I’m wondering if the introduction of this technology into the residential market is creating problems with “less-experienced” installers.

“The scenario that is given (30ü, indoor blower running, compressor apparently running, and intermittent outdoor fan) leads me to believe that the outdoor unit either has a two-speed motor that is controlled by temperature, or a motor that is overheating and tripping on overload when the coil begins to ice, reducing the amount of air across the motor,” says Lawrence Hoyle, field service representative for The Trane Company, Charlotte, NC.

“When the motor stops, since there is no air moving across the coil, the icing condition will be made worse,” Hoyle explains. “And since the motor operates occasionally, this is most likely the problem.

“If the motor is two speed, it is probably controlled by thermostats or sensors, and when the temperature drops, the motor should be in high speed. If the thermostat or sensor that controls the speeds of the motor has failed, the motor won’t run in the higher speed. The motor will then stop and again, the icing condition will be made worse.

“In either case, the defrost control will probably work,” according to Hoyle, “but since the fan doesn’t come on, the icing condition will continue.”

Wayne Shirley, of Wayne Shirley Service Co., Inc., Randolph, AL, points out, “The homeowner provides data that is both objective and subjective (‘the unit has had nothing but problems since installation’).” Shirley also picks up on the customer’s tone, stating, “It’s difficult not to sense some degree of emotional exaggeration in that statement.

“Nevertheless, we can make some reasonable observations from what the homeowner has told us:

1. The unit has been in operation approximately 2 1/2 years and is in its fourth heating season.

2. This is apparently the first time the owner has noticed a frosting problem.

3. The defrost controls appear to be working (‘the outside unit makes a humming sound but the fan is not blowing [I assume not running]…it goes into this every now and then’).

4. The homeowner has not been educated on the operation of a heat pump by the installing contractor (or anybody else; I would like to know if this is his first heat pump system).

5. Insufficient refrigerant charge, liquid line restrictions, or TXV problems will generally cause bands or stripes of frost on the outside coil in the heat mode because the coil is not fully loaded with refrigerant. Since the homeowner makes no mention of the ‘zebra’ effect, we can eliminate refrigerant charge as a problem.

6. The homeowner states that the unit freezes up in a short period of time. We need to know how short.

“Now, some physics and heat pump basics,” Shirley continues. “The warmer the outside air, the more capacity it has for moisture. Heat pump coils generally run 2° colder than the outside air temperature to facilitate heat exchange. So if the outside air temp is 35ü, the coil is 15° or so — plenty cold to lower the air passing through to dewpoint temperature and then convert the condensation to frost.

“Moderate outdoor air temperatures accelerate the frosting phenomenon, other things being equal. Turning a system off will, of course, halt the frosting process, but it doesn’t do a hell of lot for defrosting what has already formed, especially if the outdoor air is 35° — but that’s what everybody does, except for the more creative ones who drag the hose to the unit and wash the frost off.

“From all of this, I could/ would make a few ‘over-the-phone’ possible diagnoses:

1. The homeowner hates heat pumps and is looking for any excuse to complain and bad-mouth the system and really does not have a problem. He should allow the system to run without interfering and note if the frost comes and goes, which would be a normal operation.

2. If the unit does frost up in a short period of time, say 10 or 15 minutes running time, I would suspect the outdoor coil has become substantially restricted with grass cuttings, dog hair, or some other foreign debris. If the airflow through any evaporator coil is substantially reduced, the coil frosting rate is considerably accelerated.

3. If the outdoor fan motor is not running or running intermittently (assuming my earlier assumption is in error), this would cause rapid frosting.

“In conclusion, I’m inclined to think, at this point, the problem is neither refrigerant charge nor defrost controls, but more likely a lack of education on the part of the homeowner, or an airflow problem at the outside unit.”

And if the unit was installed in 97, I guess two years’ worth of grass clippings and/or dog hair — don’t even think of the air filter — would be enough to accelerate the defrost.

Stump your peers! Send in your best service conundrums to Troubleshooting/Maintenance Editor Barb Checket-Hanks by e-mail ( or fax (248-244-6444). Those we publish will earn the sender a gift from The News.