Perfect Weather for Defrost Problems

WE are experiencing  lot of cold, rainy days here in Georgia. The temperatures hover just above freezing into the mid 40's. This type of weather is perfect weather for heat pump defrost problems. Heat pumps have to defrost more in weather just above freezing than in much colder weather. If the temperature is so cold that water freezes out of the air, there won’t be much water in the air. If there is not a lot of water in the air, heat pump will not develop very much frost. On the other hand, if the temperature is 40 degrees and raining, there is a lot of water available. The coil in the heat pump will be below freezing even when it is 40 degrees outside, so the water will form frost on the coil. The system is more likely to need frequent defrosting at temperatures just above freezing than at much colder temperatures.  This is not to say heat pumps won’t got into a defrost cycle at very cold temperatures, but there is much less ice to defrost. The first thing to check on an iced over heat pump would be to make sure the outdoor fan motor is operating. If the outdoor fan motor is not moving air across the coil, it will frost very quickly in cold, wet conditions. The normal defrost periods would not be able to keep the coil clear. An undercharge can also make a system freeze up faster and keep it from clearing the ice when it does go into defrost. If water created when the coil is defrosted cannot drain away from the unit, a large ice floe can build up underneath the unit. This can create an ice chunk around the bottom of the unit. Heat pump condensers should be elevated enough to allow the water to drain. Sometimes the defrost thermostat just clips onto the coil. Occasionally they become loose and cannot sense the coil temperature, so they will not initiate a defrost cycle in weather that is above freezing. To check a defrost thermostat, just ohm it out. Generally, the defrost thermostat should close when it senses a temperature cold enough to initiate a defrost cycle. This can be checked by disconnecting the defrost thermostat from the circuit and ohming it out. If it is in the correct location, making good contact, and the coil is iced over, the defrost thermostat should be closed (0 ohms). If it is open (OL) it is bad. A thermistor defrost sensor is also checked by resistance, but these typically change resistance with temperature rather than opening and closing like a switch. You have to check the measured resistance against the manufacturer’s specs. Usually, these fail open, so if you have a measurable resistance, the defrost sensor is probably OK. Most defrost boards today have s test function. Jumping across the test pins accelerates time. If the defrost thermostat is closed, jump the test pins. If the board is good, it should initiate a defrost cycle. Do't automatically assume that the problem is a bad defrost board.Changing the board won't help if the problem is caused by one of the other causes.

My Heat Pump is on Fire!

Every tech that works on heat pumps will eventually get a service call from a distressed customer who believes their heat pump is on fire because they saw smoke pouring out. Typically, they just happened to catch the end of a defrost cycle and saw steam rising out of the unit as the defrost cycle finishes. They can see that the outdoor fan is no longer turning, yet they can hear the compressor still running. They may also hear loud compressor noises or the refrigerant whoosh as the system reverses at the conclusion of the defrost cycle. If they are near a register inside, they will most likely notice that the air is not very warm. All of this panics the customer into believing something terrible is wrong with their system. Simply telling them that this is all normal may sound like you are not interested in their problem. Or worse, you are hoping they will forget about the problem long enough for the warranty to expire. You should check the unit to see that it operates normally in first stage heat, second stage heat, emergency heat, and defrost. Then, explain the purpose of the defrost cycle. You probably don’t have time to completely explain heat pumps and the defrost cycle, but you can explain that ice buildup occurs as the unit operates and that it must be removed to keep the system operating efficiently. Once they understand that the defrost cycle is intended to heat up the coils to remove frost and ice, it is easy to see why you would not want the fan running while trying to heat up the coils. Heating the outdoor coil also explains the “smoke,” which if course is really steam created as the coil heats up. Finally, the loud compressor and whoosh noises simply signal that the defrost cycle is over as the reversing valve shifts. Taking the time to thoroughly check their system shows the customer that you are indeed interested in their concerns. Taking the time to explain what is happening will help reassure them and save you time down the road. Next time they will know what they are seeing.