What it is

In heat mode the outdoor coil is the evaporator — it's pulling heat out of outdoor air, so the coil runs colder than the outdoor air around it. When that coil surface drops below freezing and there's moisture in the air, frost forms on it, just like on a window. Frost insulates the coil and blocks airflow, which kills capacity fast. The defrost cycle is the system's built-in way to melt that frost off so heating can continue.

The defrost board (or defrost control) is the brain that decides when to defrost and runs the steps.

How it works

To defrost, the system briefly runs the outdoor unit in cooling mode while it's actually trying to heat the house. Here's the sequence the board orchestrates:

  1. The board decides it's time to defrost (how it decides depends on the control type — see below).
  2. It energizes the reversing valve to flip the outdoor coil into condenser mode — now hot discharge gas flows through the outdoor coil and melts the frost.
  3. It shuts off the outdoor fan so it's not blowing cold air over a coil it's trying to warm up (and so it doesn't blow the melt-water around).
  4. Because the indoor coil is now temporarily acting as an evaporator (cold), the board brings on auxiliary/backup heat (electric strips or the gas furnace in dual-fuel) so the house doesn't get blasted with cold air during defrost.
  5. When the coil is clear, the board terminates defrost, flips the reversing valve back to heat, restarts the outdoor fan, and drops the aux heat. Back to normal heating.

That burst of "steam" rolling off an outdoor unit on a cold day is usually just a normal defrost — frost melting and flashing to vapor. Customers panic; it's normal.

Two families of defrost control:

  • Time/temperature ("timed") defrost. A timer initiates a defrost attempt at a fixed interval — commonly selectable like every 30, 60, or 90 minutes of compressor run time. It only actually defrosts if the defrost thermostat (a temperature sensor clamped to the coil) is closed, i.e., the coil is cold enough to possibly be frosted. It terminates on temperature (coil warms up) or on a maximum time limit (often ~10 minutes) as a backstop. Simple, but it can defrost when it doesn't need to (wasting energy) or wait too long.
  • Demand defrost. Smarter boards watch coil temperature and run time (some compare coil temp to ambient, track ΔT, or use accumulated conditions) to defrost only when frost has actually built up. Fewer unnecessary defrosts, better efficiency. More sensors, more to diagnose.

In the field

Force a defrost to test the circuit:

  • Most defrost boards have a way to speed up or force a test defrost — often a button, a "test" pin/header, or a way to short the speed-up terminals so the timer runs fast. Check the board legend. Forcing defrost lets you watch all the steps happen on demand instead of waiting.
  • When you force it, confirm: reversing valve shifts, outdoor fan stops, aux heat comes on, and after termination everything returns to heat.

Check the defrost thermostat (timed systems):

  • It's a temperature switch on the coil/tube. It should be closed when cold (below roughly the mid-30s °F, varies by part) and open when warm (often opens around the high-40s to ~70°F depending on the spec). If it's stuck open, the board never gets permission to defrost and the coil ices solid. If stuck closed, it can over-defrost.
  • Test by reading continuity cold vs warm, or substitute per the part's rated temps.

Normal values & targets

  • Timed defrost intervals: commonly field-selectable at 30 / 60 / 90 minutes of compressor run time (some add 50/70/etc.).
  • Defrost time limit (termination backstop): often ~10 minutes maximum per cycle.
  • Defrost thermostat close point: roughly low-30s °F (coil cold enough to frost) — verify the part.
  • Defrost thermostat open/terminate point: often somewhere from the high-40s up toward ~70°F — verify the part.
  • A normal defrost is short (a couple to several minutes) and ends with the coil visibly clear.

Common faults & what they mean

  • Outdoor coil iced over solid, no defrost happening — defrost not initiating or not completing. Suspect: defrost thermostat stuck open (timed), failed defrost board, bad coil sensor (demand), reversing valve not shifting during defrost, or the outdoor fan not stopping.
  • Defrosts way too often / runs aux too much — defrost thermostat stuck closed, board initiating on too-short an interval, or a demand sensor reading wrong. Wastes energy and runs up the electric bill.
  • Blows cold air in the house during defrost — aux heat isn't coming on when it should. Check the board's aux/defrost output, the strip/furnace, and the wiring. Defrost without aux = cold-air complaint.
  • Never terminates / runs the full time limit every time — coil isn't actually warming (low charge, weak compressor, reversing valve not fully shifting) or the termination sensor is bad. The time limit is a safety, not normal termination.
  • Reversing valve doesn't flip during defrost — frost never melts; see the reversing-valve article (signal, coil, or insufficient ΔP).

Tech tips & gotchas

  • A heat pump in defrost looks broken to a customer — steam, the outdoor fan stopped, a "whoosh" as the valve shifts, maybe a momentary cold draft. It's normal. Educate, don't replace.
  • Some frost on the outdoor coil in heat mode is normal. A light, even frost that the system clears on schedule is fine. Solid ice, ice on the bottom/pan, or frost that never clears is the problem.
  • Ice at the bottom of the unit or on the fan blades often points to a drainage problem or a coil that's been freezing/thawing repeatedly — look beyond just the defrost timing (low charge causes this too).
  • On demand defrost, a bad coil temperature sensor (or a sensor that's come loose from the tube) throws the whole logic off — it may defrost constantly or never. Check sensor mounting and resistance.
  • Don't forget the outdoor fan must STOP during defrost. If you force a defrost and the fan keeps running, the relay/board output for the fan is suspect — and the coil won't clear well.
  • When charge is low, a heat pump frosts more aggressively. If a unit "ices up every cold snap," verify the charge before blaming the board.

Safety / code notes

  • Defrost work that requires opening the sealed system (reversing valve, charge) is governed by EPA Section 608 — recover, don't vent.
  • Auxiliary electric heat strips draw heavy current; verify the circuit, sequencers/relays, and conductor sizing are correct before energizing — see the electric-heat/sequencer material.
  • Never pour hot water or chip ice off a frozen outdoor coil with metal tools to "help" — you'll damage fins or tubing. Diagnose why it's not defrosting instead.