It’s a fairly common service call in the HVAC business: the customer reports that the coils on their heat pump are frosted over and the unit isn’t working very efficiently. There are plenty of reasons why this might happen - some are pretty simple and others might take some troubleshooting and deduction.
Let’s start with some of the easier ones, some of which can actually be addressed by the homeowner.
First off, the unit is installed outdoors, which exposes it to all sorts of weather. In the winter, it might simply have been covered with ice from freezing rain or a gutter might have been dripping on the unit, causing it to be covered with ice. The coils might be blocked by leaves or snow drift, or the unit might be physically sunk into the ground to a point where there’s no place for ice and snow to melt away and drain.
Supply or return vents might be closed or blocked off, or it could even be something as basic as the AC filter being dirty and obstructing flow through the system.
Now that the easy stuff is out of the way…
1. Turning the thermostat below 70 degrees for a prolonged spell in the summer.
Running the system at too low a temperature can be enough to cause the indoor coil to frost or freeze up. It can also cause “sweat” and condensation on the duct work, which invites the growth of mold and mildew.
2. Problems with circulation.
The fan itself can be the cause of issues with circulation through the system and can lead to an iced-over unit in the summer. Check for problems like a failing fan motor that runs too slow, a loose or broken fan belt, a failed fan relay, failed indoor fan relay, a dirty blower wheel, undersized duct work or a problem with the thermostat that is preventing the blower from kicking on. .
3. Low refrigerant levels.
Remember that the problem is with the evaporator coil, the part of the system that absorbs heat from indoor air. It’s natural for the coil to sweat with condensation, but a depleted amount of refrigerant in the system can be enough to prevent the evaporator from working properly and can cause that condensation to turn to a layer of ice.
The evaporator should stay at about 40 degrees F, and a low refrigerant level means the temperature will drop below freezing. It also means that the system is leaking somewhere, which will mean leak detection and repair to address the problem.
4. Problems with the defrost cycle.
In winter conditions, a heat pump will automatically go into a defrost cycle by switching the system to air conditioning mode via the reversing valve. The outdoor evaporator switches roles and serves as a condenser, with the outdoor fan cycling off, and the high-pressure refrigerant circulates through the outdoor coil at high temperature and melts the ice.
Problems with the defrost cycle can point to the reversing valve solenoid coil, the defrost relay, thermostat, sensor or timer. In some cases, it can be the reversing valve itself sticking and not going into defrost mode.
5. Dirty indoor coil.
These parts often attract dirt, dust and pet dander that are circulating in the air, and if the evaporator coil has become too nasty with fuzz and dirt it can be enough to impede air flow and heat exchange, causing the coils to ice up. When cleaning the coil, be sure to use the correct cleaners and do not use a wire brush. The fins are delicate; be sure to brush only in the direction of the fins, as bent fins can be enough by themselves to impede flow and hurt performance.
Other things to bear in mind
When troubleshooting, be sure that the outdoor fan on the unit is working properly. Also be certain that the outdoor unit shuts off when the thermostat no longer calls for cooling. A bad contractor on the compressor can cause it to never shut off and run constantly, and with no indoor blower to move air into the house it can cause icing on the coil.
A home that’s equipped with a built-in humidifier should have the unit shut off and its damper closed for the summer months. In some instances, a blocked capillary tube, faulty expansion valve or blocked orifice have been known to cause icing. As for the homeowner blocking off or closing vents in order to control the distribution of AC to only a couple of rooms, remember there’s a formula for this.
Each register should supply 80 to 100 CFM of airflow. Each tonnage of air conditioning capacity requires 350 to 400 CFM of flow, so for a 2-ton unit 800 CFM will be required. In a home that has 12 registers, 8 to 10 of them should be open and unrestricted at any given time. When checking for flow from registers or intakes, be sure to look for things like a rug or furniture in the way. In other words, don’t forget the obvious things that are hiding in plain sight!
Interested in more heat pump troubleshooting, general HVAC learning, or starting a career in HVAC? Interplay Learning’s online, on-demand skilled trades course catalog has hours of HVAC training material at the beginner, intermediate and advanced levels. Start with fundamental HVAC basics, and work your way up to troubleshooting common equipment faults in life-like, 3D HVAC simulations.
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