Why Your Heat Pump Is Freezing Up

Winter here is the setup. Albuquerque averages about 113 freezing nights a year, per BestPlaces, and many nights hover around freezing, when a heat pump collects frost. Six things turn that normal frost into a real problem.

1. Failed defrost control. A bad board, sensor or relay means defrost never starts, or it stops too early.
2. Weak airflow outside. Dust packed into the coil, or a failing fan motor, lets the coil get too cold.
3. Low refrigerant. A leak drops the coil temperature, and frost builds faster. A low charge always means a leak.
4. Roof runoff. Water dripping from a canale, gutter or flat roof edge can freeze on top of the unit. Flat roofs and canales make this a real risk here.
5. Snow or ground ice. A unit sitting too low can block its own drain, and meltwater freezes around the base.
6. Low airflow inside. A clogged filter cuts the heat moving through the system and keeps it from running normally.

Summer freezing is its own thing. If ice shows up on the indoor coil or refrigerant lines while cooling, the usual cause is low airflow or low charge, as on any AC.

Placement decides how often causes 4 and 5 show up. PNNL's Building America guide says to install outdoor units well above snow height and away from spots where meltwater or sliding snow and ice can land, which favors a gable end over an eave. On a flat-roofed house, that means staying clear of every canale.

Neighborhood matters too. National Weather Service Albuquerque lists 187 days between freezes in the South Valley against 210 at the Sunport, so a South Valley unit can see more frost mornings than the airport figures imply.

The right fix depends on what you are willing to pay for, now and over the next few winters.

• Backup heat during defrost. Many systems run auxiliary heat during defrost so the vents do not blow cold. That protects comfort and uses energy. A Pacific Northwest National Laboratory report on DOE cold climate units says defrost energy can be significant when auxiliary heat is used.
• Emergency heat or a cold house. Emergency heat is a fair bridge for a day or two, and a poor way to spend a winter. Set a limit on how long you will run it before the repair gets done.
• Raising the unit. A stand keeps it above snow and ice. It also means line and electrical changes, and PNNL's guide notes wall-mounted units can vibrate, so check for that if you hang one.
• Hoods and splash guards. A guard can stop canale drip from landing on the cabinet. Put it too close and you choke the airflow the coil needs, so follow the manual's clearances.
• Defrost board or new system. A failed board on a sound unit is a repair. If the same old unit also has a leak or a tired compressor, price a replacement beside it.