A service call comes in- the coils on the heat pump are frosted over and the unit isn’t working very effectively. This is a scenario you’re going to run into time and again as an HVAC tech, and there are several reasons why it might occur. Some of those reasons are pretty straightforward and direct, while others are going to take some heat pump troubleshooting and critical thinking.
What Does It Mean When A Heat Pump Freezes Up?
If there’s a thin layer of frost over the coils of a heat pump, that’s not cause for alarm - light frost is normal. What we’re talking about is a complete icing-over of the coils or the unit, preventing it from operating. In extreme cases, the whole outdoor unit might be encased in a block of ice, collapsing the coils and potentially ruining the entire system. A few common causes can include:
- Rather than starting on its own timer, the heat pump defrost cycle is not working
- Restricted airflow at the filter, vents, or around the outdoor unit itself
- Infiltration of water from a gutter or the roof, which then freezes
- A poorly-maintained unit, possibly with an undercharge or excess charge of refrigerant in the system.
What To Do When A Heat Pump Is Frozen?
Regardless of the circumstances, it’s a bad idea to continue to try to operate the unit in a freeze-up condition. Instead, try the following:
- Turn off the unit at the breaker box so there’s not a chance of it coming back on. If it continues to try to operate, it can result in damage. Additionally, you don’t want the unit trying to kick back on while you’re servicing it.
- Remove the ice around the interior coils with a brush, blow dryer, or heat gun. Do not attempt to chip away at the ice with a screwdriver or any kind of object that can damage the coils and fins.
- Check the simple stuff - look for a buildup of leaves and plant matter around the outdoor unit, and inspect the condition of the filter.
What To Do If The Outdoor Coil Freezes Up In Winter?
For the heat pump’s evaporator to do its job properly, the air has to be able to circulate around the fins and coils. A buildup of ice can impede that airflow.
Did it rain or snow recently, with subfreezing temperatures? Sometimes that can be all it takes to create an ice accumulation on outdoor components. In some cases, heavy rains can soften the ground to the point where the heavy outdoor unit can sink into the soil and leave no way for ice and snow to melt and drain off. In these cases, the unit will need to go on a pad or special feet to prevent that from happening again.
What To Do If The Indoor Coil Freezes Up In Summer?
If the airflow around the vents, ductwork and filter check out, and the indoor blower is clean, chances are the unit is icing over due to low refrigerant levels. That’s why you’ll need to check the refrigerant and start diagnosing a possible leak in the system.
In many cases in the summer, icing can occur because the thermostat is set too low (below 70 degrees) for too long. This low heat load on the coil can not only cause the indoor coils to ice up but can cause “sweat” and condensation in the ducts and on registers, promoting the growth of mold and mildew.
What Are Heat Pump Troubleshooting Options?
When you can’t identify the source of the problem, it’s time to start heat pump troubleshooting. The following are common areas that can cause heat pump problems and how to fix them:
Problems with Circulation
Along with checking the filter and leaves or debris around the outdoor unit, have a look at the fan.
- Is the fan speed too slow?
- Is the belt intact?
- Is the ductwork undersized?
- Is there any furniture or rugs blocking registers?
- Check the blower’s squirrel cage wheel for dust and dirt, and check the indoor fan relay.
If the fan speed is inadequate, remember there are calculations you can use to come up with the proper speed setting. The factory default speed for the blower may not be the right speed.
Blower speed is tied in with factors like the size and type of filter being used, total BTU rating of the system, and the size and total run length of the ductwork. 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.
Low Refrigerant Level
We mentioned above that an overcharge of refrigerant can lead to coil icing, but a depletion of refrigerant is more likely. The problem is with the evaporator coil, and a low charge of refrigerant can be enough to prevent the evaporator from working properly and can cause icing.
Evaporators should stay at about 40 degrees F, and a low charge can be enough to drop it below freezing and turn normal condensation into ice. If your equipment shows a low charge, your next task will be to track down where the leak is occurring and repair it.
Defrosting will occur when the reversing valve switches to A/C mode (in winter) and sends high-pressure, high temperature refrigerant, back through the outdoor coil, which is now serving as a condenser.
The high-pressure, high temperature refrigerant will then be hot enough to melt any ice buildup. If you suspect a problem with the defrost cycle, have a look at the reversing valve solenoid coil, defrost relay, timer, sensor, or defrost thermostat. In some cases, it can even be the valve itself sticking.
Dirty Indoor Coil
If you find that the filter is saturated with dirt and fuzz, check the indoor coil. An accumulation of dust and dirt can be enough to prevent air circulation and heat exchange around the coil.
Remember that removing dirt and fuzz from the coil is a delicate process. It’s important to use the correct cleaning solution and do not use a wire brush. Brush only in the direction of the fins; all it takes is a bent fin to impede performance.
Other Things To Keep In Mind
Important things to keep in mind when a heat pump freezes up:
- Check to make sure the outdoor unit kicks off when signaled by the thermostat. A bad contactor on the compressor can also cause it to run nonstop, This can be enough to cause the outdoor coil to ice up in heating mode and indoor coil icing in cooling mode
- If the home has a humidifier, it shouldn’t be running in the summer months and its damper should be closed.
- Inspect the capillary tube, expansion valve, and orifice. Problems with these have been known to cause icing.
- If the homeowner has closed registers in rooms that aren’t being used, refer to the formula mentioned above for circulation through the system.
Diagnosing and repairing a frozen heat pump can be tricky, but don’t miss out on the obvious things that are hiding in plain sight. If you want to learn more about the diagnosis of common heat pump problems from home, the SkillMill™ Heat Pump Troubleshooting course does a really good job at breaking it all down and shows you areas to inspect and how to fix them.