What you'll see

It's cold out, the heat pump's running, and the outdoor coil is frosted or fully encased in ice. Some frost on a heat pump in winter is normal — the outdoor coil is the evaporator in heat mode, it runs below freezing, and it collects frost that the defrost cycle is supposed to melt periodically. The problem is when defrost doesn't clear it (coil buries in ice) or the unit gets stuck in defrost (constant cold blow indoors). A solid block of ice over the whole coil and base means defrost isn't doing its job, or something is dumping water onto the coil.

Decide first: is this normal frost that'll clear on the next defrost, or a failure?

Walk it in order

  1. Assess the ice. Light, even frost on the coil that's actively running heat is likely normal between defrost cycles. A thick, solid sheet of ice covering the whole coil, the fan blades iced, or ice mounded at the base means defrost is failing or water is being added (gutter drip, runoff, a leaking line above it).
  1. Force or observe a defrost. Many boards have a way to initiate defrost (a test pin, or by jumpering the defrost sensor). Trigger it and watch: the reversing valve should shift to cooling, the outdoor fan should stop, and the hot gas should start melting the coil. If nothing shifts, the defrost board or reversing valve is suspect. If it shifts but the coil doesn't clear, suspect charge or the valve not fully porting.
  1. Check defrost initiation. Defrost starts on a combination of time and coil temperature (timed/temperature, demand, or pressure-based depending on the system). If the defrost sensor/thermostat on the coil is open, shorted, or loose, the board never starts a defrost and the coil ices up. Check the sensor's resistance against temperature and confirm it's clamped tight to the coil/line.
  1. Check defrost termination. The cycle ends on temperature (or time as a backup). A sensor reading wrong can either end defrost too early (ice never fully clears) or never end it (stuck in defrost, constant cold air indoors with aux running and a high bill). Watch whether defrost terminates on coil warm-up like it should.
  1. Confirm the outdoor fan stops during defrost. If the fan keeps running in defrost, it blows cold air across the coil and fights the melt — usually a board or relay fault. The fan must be off during defrost for it to work.
  1. Rule out airflow and drainage. A dirty/blocked outdoor coil, leaves packed in the fins, low fan airflow, or the unit sitting in standing water/snow all promote icing and overwhelm defrost. So does a coil that can't drain meltwater because it's mounted too low or the base is plugged — the melt refreezes. Confirm the unit is up on stand-offs and draining.
  1. Read the charge in heat mode. Low charge or a restriction drops coil temperature and pressure, frosting the coil harder and faster than defrost can keep up. Evaluate by the heat-mode method (subcooling/manufacturer spec, accounting for low ambient). Overcharge or a partially stuck reversing valve can also throw it off.

What "normal" looks like

  • Frost: light, even frost between defrost cycles in cold weather is expected.
  • Defrost interval: periodic — varies by control type and conditions; a board may allow defrost every ~30–90 minutes of accumulated run time when the coil is cold enough.
  • During defrost: reversing valve in cooling, outdoor fan OFF, hot gas melting the coil, aux heat ideally covering the indoor cold blow, terminating on coil temperature in a few minutes.
  • After defrost: coil clears, valve returns to heat, normal heating resumes.
  • Defrost sensor: good resistance-vs-temperature curve, tightly mounted to the coil/line.

Common faults & what they mean

  • Failed defrost sensor/thermostat: never initiates or never terminates. Coil ices solid, or unit sticks in defrost. Check resistance and mounting.
  • Defrost board fault: no defrost initiation, wrong fan behavior, or stuck cycle. Confirm the board sequences valve and fan correctly.
  • Outdoor fan runs during defrost: blows the melt away, coil never clears. Relay/board fault.
  • Reversing valve not fully shifting: hot gas doesn't fully reach the coil in defrost; partial melt. Check the solenoid and valve.
  • Low charge / restriction (heat mode): coil frosts faster than defrost can clear. Read and correct charge in heat mode.
  • Airflow/drainage: plugged coil, low mounting, standing water, snow drift. Meltwater refreezes. Clean and elevate/drain the unit.

Tech tips & gotchas

  • Light frost in winter is not a fault. Don't condemn a board for normal between-cycle frost. Watch for an actual failed defrost before you sell parts.
  • The outdoor fan MUST be off during defrost. A fan that keeps spinning in defrost is a dead giveaway of a control fault — easy to spot, easy to confirm.
  • Water dripping onto the coil from above will beat any defrost system. Check gutters, roof runoff, and lines above the unit. The defrost board isn't broken — physics is.
  • Stuck-in-defrost shows up as a high bill and cold air. Aux heat carries the load while the heat pump uselessly runs in cooling. Check termination, not just initiation.
  • Always check the sensor mounting. A defrost sensor that's fallen off the coil/line reads ambient and ruins the timing — five-minute find.
  • Read charge in heat mode, not cooling-mode habits. Frosting from low charge needs the heat-mode method to confirm.

Safety / code notes

  • De-energize before servicing; bleed capacitors first.
  • Never chip ice off the coil with metal — you'll puncture the fins or the coil. Thaw it (defrost cycle, heat mode, or time).
  • Recover and handle refrigerant per EPA 608 (40 CFR Part 82, Subpart F) if you open the system for charge or restriction work.
  • Make sure the unit is mounted and drained so meltwater doesn't ice over walkways — a code and liability issue in winter.