Heat Pump Defrost: Solve The Freezing or Icing Problems

I am quite familiar with the problem of a heat pump that is freezing or icing. In the normal course of making heat, your heat pump gets cold and WILL make ice and freeze depending on outdoor temperature and humidity.

The textbooks all say that most ice production is produced at 40 degrees F. But, this will vary with actual humidity and the temperature of the coils. When it is raining, snowing, or sleeting, the problem will aggravate itself tremendously. When it is very dry, you will not get any ice.

To get rid of this ice, most heat pumps have a time-temperature-defrost system. This system uses a timer, either a clock motor or an electronic timer. The timer will have mechanical stops or electronic jumpers with times at 30, 60, 50, 70, 90, etc. minutes depending on the manufacturer of the defrost equipment.

Some systems use a pressure switch to detect that the coils have become stopped with ice. I have seen this system on a 10 Ton York unit, but it is not very common on residential equipment. I have also added 300 PSI pressure switches to heat pump that turns into a block of ice to ensure proper termination.

How to defrost a heat pump?

Heat pump must regularly defrost when icing conditions occur and long enough to get rid of the ice and short enough to not waste energy. There is usually the best place for the sensor or thermostat on the outdoor coil just after the metering. Some manufacturers have a tubing stub for the thermostat, and others do as well. If you remove or change the sensor, try to mark where it was. Failure to place it in the correct place will cause the unit to either not defrost or the cycle will be too short, and the unit will build ice anyway. Place it at the wrong end of the coil, and you will get no defrost activity.

When the compressor runs, the clock runs, and time is accumulated. At the selected intervals, the clock will energize a defrost event through a thermostat or a sensing bulb If the temperature of the sensor (refrigerant-filled sensing bulb on the Ranco E-15) is below 28 degrees F. The defrost will take place. When the temperature of the sensor rises to 50 or 70 degrees F Or 10 minutes elapse, the defrost cycle is terminated.

A typical heat pump defrost cycle should be from 1.5-3 minutes.

When I use the word “sensor,” I am referring to an electronic input such as a thermistor or thermocouple. When I use the word “switch” or “thermostat,” I am referring to a bi-state device that is either on or off, depending on temperature.

Some technicians came up with the idea to terminate a defrost after 10 minutes, and every heat pump control has that time built in. I have found from working with functional units that after 3 minutes of defrosting in a fully functional unit, all the ice will be gone (from the coil, not the case) even if the unit hasn’t been defrosting in weeks. At 5 minutes, the high-pressure switch (you do have one, don’t you?) will cut out with a liquid line temp of 150F+. At 10 minutes, the compressor will have gone off on internal relief and, if you are lucky, tripped the internal overload after taking a few months off the life of the compressor.

If your unit has a GENERIC electronic timer board (used on Newer Heil/Tempstar, Rheem/Ruud, and others), the way the system should work is every 30, 60, or 90 minutes. The control board will put out a 24-volt ac signal for 10 minutes.

Then if the defrost thermostat is made (below 28 degrees), it will send a “24-volt ac high” signal to the defrost relay (a 3-pole relay) to reverse the system (go into air-conditioning mode), shut off the outdoor fan, turn on the electric heat. When the defrost thermostat gets above 50 or 70 degrees, it breaks the circuit to the defrost relay, and the unit goes about its business.

Some systems have a “reset” pin that restarts the time after the defrost thermostat opens. If there is a problem with this board, it can cause rapid cycling of the to defrost function.

You can test this board with the power (high voltage outside) off to the outdoor unit by making the thermostat call for heating (the contactor must be pulled in) and jumping out the test pins. This will speed up the cycle by 256 times.

If at the instant the unit goes into defrost, you can remove the jumper from the test pins, and the board should revert back to heating in 10 minutes (you will hear the relay click). You can test the output signal by connecting an 1819 bulb from the output terminal to 24 volts ac, not to “Common” (see the wiring page). The output terminal connects itself to “Common” on a defrost function and not to 24 volts ac like you might expect! When the system is running at any time, you should get a signal for 10 minutes every time the cycle comes around. This happens in summer too.

Other defrost schemes have relays (a total of 3 contacts) mounted on the control board. Some will have one relay mounted on the board and another one external. If the fan relay on your defrost board fails, add a 90-340 relay and share the reversing valve or the electric heat instead of replacing the board. A 90-340 is much more rugged than the fan relay on the board. These can also be tested with the line voltage power to the outdoor unit removed.

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About Mas Broto

Have been in the heating and air conditioning (HVAC) industry for over 20 years. He is person that will grow and thrive to learn more about the HVAC industry throughout his career. Mas Broto is also a blogger, who's dedicated to bringing you the best knowledge to get ahead in the game of life.