Heat Pump Defrost Cycle Faults: Normal vs Excessive, Frost Sensor Failure and F-Gas Faults

Quick Answer: All air-source heat pumps frost their outdoor coil in cold, damp weather and periodically run a defrost cycle — briefly reversing to melt the ice. Normal defrost happens every 30–90 minutes in frosty conditions and lasts 2–10 minutes, producing steam and meltwater. Defrosting that is too frequent, too long, or never completes is a fault: usual causes are a refrozen/blocked condensate drain, a fouled coil, a failed frost/coil sensor, a stuck reversing valve, a fan fault, or low refrigerant charge. Refrigerant work is F-Gas certified only.

Summary

Defrost is normal and necessary, not a fault — but it is the single most misunderstood heat-pump behaviour, both by homeowners ("it's making steam, it's broken!") and by engineers who chase the wrong cause. When the outdoor air is cold and humid (worst around 0–7 °C with high humidity), moisture from the air freezes on the cold evaporator coil. To clear it, the heat pump briefly reverses the refrigerant cycle so hot gas flows through the outdoor coil and melts the frost, while a backup heater, buffer or system volume holds the heating up. You see steam, water, and a pause in heating — all expected.

The diagnostic question is proportion. A healthy unit defrosts occasionally and briefly, completes the cycle cleanly, and returns to heating. A faulty system defrosts too often, takes too long, builds ice faster than it can clear it, or ices in a strange pattern (the base, the fan, one side). Each of those points to a different cause, and getting it right matters because the wrong diagnosis is expensive: excessive defrosting can quietly halve the running efficiency, and severe icing can wreck the fan.

The boundary every engineer must respect is the F-Gas line. Many serious defrost faults — low refrigerant charge, a leaking circuit, a failed reversing valve — live inside the sealed refrigerant system, which only an F-Gas certified engineer may open. The water, drainage, sensor and control causes are diagnosable by any competent heating engineer, so work those first and bring in F-Gas competence only when the evidence points inside the circuit.

Key Facts

• Why frost forms — humid air below the coil's dew/frost point deposits ice on the evaporator; worst at ~0–7 °C with high humidity, less at very dry sub-zero temps.
• Normal defrost frequency — roughly every 30–90 minutes in frosty conditions (control-dependent).
• Normal defrost duration — typically 2–10 minutes; the unit reverses cycle or uses hot gas.
• Steam and water are normal — meltwater and visible vapour during/after defrost are expected, not a leak.
• Backup during defrost — heating is held by an immersion/backup heater, buffer tank, or system volume; brief flow-temp dip is normal.
• Demand defrost vs timed — modern units use sensors (coil temp + ambient) to defrost on demand; older/cruder controls use timed intervals (more wasteful).
• Frost/coil sensor — a thermistor on the coil initiates/ends defrost; a failed sensor causes too-frequent, too-rare, or endless defrost.
• Condensate drain — meltwater must drain away and not refreeze; a blocked/unheated drain causes ice to build at the base and the fan.
• Reversing (4-way) valve — switches the cycle for defrost; a stuck valve means the unit can't defrost or can't return to heating (F-Gas).
• Low refrigerant charge — reduces capacity, causes over-frosting and weak heating (F-Gas certified diagnosis only).
• Coil fouling / restricted airflow — leaves, fur, snow drift, tight clearances increase frosting and defrost demand.
• Excessive defrost = lost efficiency — every defrost takes heat from the house; too many cycles drives up bills and drops COP.

Quick Reference Table

Detailed Guidance

Normal vs excessive — the decision tree

HEAT PUMP DEFROST DIAGNOSIS
===========================
Is it actually cold & damp outside (≈ -2 to +7 °C, humid)?
   Frosting/defrost in these conditions is EXPECTED.

Count cycles & time them over an hour:
   ~1 defrost every 45-90 min, 2-10 min each, clears cleanly
        -> NORMAL. Reassure customer.
   Defrosting much more often / much longer / not clearing
        -> FAULT. Go to localisation.

LOCALISE THE FAULT
------------------
WHERE is the ice?
   Coil only, but too frequent  -> sensor / charge / airflow
   Base & fan, building up       -> CONDENSATE DRAIN refreezing
   Whole unit encased, fan jammed-> drain + airflow + run a manual
                                    defrost; protect the fan

Does a defrost cycle START?
   NO  -> frost/coil sensor failure, control fault, or stuck
          reversing valve (F-Gas if valve)
Does a defrost cycle END / return to heating?
   NO  -> stuck reversing valve, sensor stuck cold, control
          (valve = F-Gas)

Heating weak AND over-frosting in MILD weather?
   -> suspect LOW REFRIGERANT CHARGE  (F-Gas certified only)

Condensate drain — the most common fixable cause

Meltwater from defrost must drain freely and not refreeze. If the base tray drain is blocked (debris, sludge) or unheated in hard frost, water pools and freezes, ice climbs up into the coil and fan, and the unit fights a losing battle. Checks:

• Clear the base tray and drain outlet of debris and sludge.
• Confirm any drain trace-heating / tray heater is powered and working.
• Ensure the unit sits on feet/stand with clear drainage below (not flat on a slab that ponds).
• In severe sites, improve drainage falls and add gravel/soakaway so meltwater leaves the area.

This is a non-refrigerant, non-electrical-internal job any heating engineer can do, and it solves a large share of "icing up" callbacks.

Frost / coil sensor faults

The defrost control relies on a coil thermistor (and often an ambient sensor). A drifted or failed sensor causes classic symptoms:

• Reads too warm → unit under-defrosts, ice accumulates, heating fades.
• Reads too cold → unit over-defrosts, wasting heat and short-cycling.
• Open/short circuit → controller fault code, erratic behaviour.

Check the sensor resistance against the manufacturer's thermistor curve and confirm it is correctly clipped to the coil. Sensor replacement is usually a non-F-Gas task.

Airflow and fouling

Restricted airflow makes the coil colder and frostier than it should be:

• Clear leaves, pollen, fur and snow drift from the coil and fan path.
• Comb out bent fins; never jet-wash hard or bend fins.
• Confirm clearances around the unit meet the manufacturer's spec.
• Check the fan runs at the right speed (a failing fan motor reduces airflow and increases frosting).

Refrigerant-side faults (F-Gas territory)

When the water, drain, sensor and airflow checks are clean but the unit still over-frosts with weak heating — especially in mild weather — suspect the sealed circuit:

• Low refrigerant charge (a leak) lowers coil temperature and capacity, causing heavy frosting and poor heat.
• Stuck reversing (4-way) valve prevents defrost starting or prevents return to heating.
• Compressor/expansion-device faults affect capacity.

All of these require an F-Gas certified engineer to diagnose and rectify. Do not vent, top up, or open the circuit without certification — it is illegal and unsafe (and dangerous with R290).

Frequently Asked Questions

Is it normal for my heat pump to steam and stop heating for a few minutes?

Yes. In cold, damp weather the unit periodically runs a defrost cycle — reversing briefly to melt frost off the outdoor coil — which produces visible steam and meltwater and pauses heating for a few minutes. This is normal and the house is held by the system's stored heat or backup heater. It becomes a concern only if defrosts are very frequent, very long, or the unit ices over and can't clear it.

How often should a heat pump defrost?

In frosty, humid conditions, roughly once every 45–90 minutes, for about 2–10 minutes each time, clearing cleanly. Frequency depends on temperature, humidity and the control strategy (demand defrost is more efficient than timed). Defrosting every 10–20 minutes, or cycles that run for 20+ minutes or never finish, indicate a fault — typically a refrozen drain, a sensor problem, restricted airflow, or low refrigerant charge.

Why is the bottom of my outdoor unit a block of ice?

Almost always a condensate drainage problem. Defrost meltwater is failing to drain and is refreezing at the base, then building up into the coil and fan. Clear the base tray and drain, confirm any tray/drain heater is working, make sure the unit is raised with free drainage below it, and improve the falls/soakaway in bad cases. Ice at the base (rather than just light frost on the coil) is the tell-tale.

Can a normal engineer fix excessive defrosting?

Often yes — the most common causes (blocked condensate drain, fouled coil, restricted airflow, failed frost sensor, control settings) are all diagnosable and fixable by a competent heating engineer. Only when the evidence points inside the sealed refrigerant circuit — low charge, a stuck reversing valve, compressor issues — does it become an F-Gas certified job. Work the accessible causes first.

Could low refrigerant cause my heat pump to ice up?

Yes. A low refrigerant charge (from a leak) lowers the evaporator temperature and reduces capacity, so the coil frosts more heavily while delivering weak heat — and you may see icing even in mild weather where it shouldn't occur. This is a sealed-circuit fault: an F-Gas certified engineer must leak-test and rectify it. Never attempt to top up refrigerant without certification.

Regulations & Standards

• GB F-Gas Regulations — only certified engineers may work on the refrigerant circuit (recovery, leak test, recharge).

• BS EN 14511 / BS EN 14825 — heat-pump performance and defrost effects on seasonal efficiency.

• BS EN 378 — refrigeration system safety.

• MCS / MIS 3005 — installation standards (correct siting, drainage and clearances reduce defrost problems).

• Manufacturer technical manuals — model-specific sensor curves, defrost logic and fault codes.

• GOV.UK — F-gas guidance — who can work on refrigerant circuits.

• MCS — heat pump standards — siting, drainage and installation requirements.

• BSI — BS EN 14825 / BS EN 378 — performance and refrigeration safety standards.

• [Heat pump manufacturer technical documentation] — model-specific defrost logic and fault codes.

• heat pump faults — broader heat-pump fault diagnosis

• heat pump servicing — annual servicing that prevents defrost faults

• f gas regulations heat pumps — the F-Gas boundary for refrigerant work

• heat pump cop scop explained — how excessive defrost hits efficiency

• heat pump noise planning — siting that affects airflow and frosting