Most UK wall splits heat by reversing the refrigeration cycle. The indoor unit should blow warm air and the outdoor unit blows cold. If yours stays cool indoors, check heat mode, a setpoint above the room temperature, and clean filters. In cold, damp British weather the outdoor coil frosts and the unit defrosts. That pause is normal. It is not normal for heat to never return, for the outdoor unit to stay silent, or for ice to encase the condenser after defrost should have finished. Do not cover the outdoor unit. Do not open the gas circuit. Call an F-gas engineer when a real heating demand produces no heat after a full defrost.
Cold outdoor air means heat is being collected
In heating, the indoor coil is the warm coil and the outdoor coil collects heat from outside air, even on a cold day, until the unit reaches its limit. Energy Saving Trust describes these systems as air-to-air heat pumps when they heat as well as cool. The outdoor unit will feel like it is blowing chilled air. Neighbours sometimes think that means it is broken. In heating, cold outdoor air is the heat being taken from outside. Hot outdoor air while you are in heat mode is a sign the cycle has not reversed properly.
Sun icon, setpoint above the room, then time
The remote must be in heat, often a sun icon, with a setpoint higher than the room. Auto on a mild day may never select heat. Fan-only never heats. A schedule from an app can turn heat off at the time you sit down. Set the mode yourself and give it time. The first minutes can feel lukewarm while the coil warms. Describe the outdoor weather in plain words, such as a hard frost or a mild wet day.
Frost, a pause, then water from the base
Frost on the outdoor coil in heating is expected in damp cold. The unit reverses briefly, the indoor fan may stop, and water drains from the base of the condenser. That is defrost, not a leak of refrigerant. It should finish and heat should return. Defrost water should be clear. Oily residue is not normal. Do not taste it and do not open valves to investigate.
The outdoor range printed for that model
Every model has an outdoor temperature range in its data sheet. Below that range, output falls away and some units stop. A single split is rarely the whole-house heating strategy for a Victorian terrace in a hard frost. It heats the room it is sized for, with doors and heat loss taken into account. Cold bedrooms with no indoor unit are a design limit, not a remote fault. If the room never warms on a typical UK winter day inside the stated range, that is a fault or a sizing problem. Read the range in the manual. Do not invent a lockout from a flashing light you have not matched to that model. HSE temperature guidance is workplace law, not a domestic setpoint for this split.
Leaves off the grille, shelter left off
Clear snow and leaves away from the outdoor unit so air can move, from the ground, with the fan stopped if you need to lift debris off the grille. Do not build a shelter that blocks airflow. Do not pour hot water over the fins. If the isolator is off, turn it on only if you understand the handle and there is no damage. Then wait several minutes.
When another defrost wait is the wrong move
Stop if heat never returns after defrost, if the outdoor unit never starts, if ice encases the coil and stays, or if a breaker trips. An F-gas engineer, certified as GOV.UK requires for refrigerant work, should test the reversing valve behaviour, airflow and, only if measurements justify it, the refrigerant circuit. Do not add gas because heating feels weak. A low charge can harm heating, but so can a failed valve, a sensor, or a unit that was never sized for the room's heat loss. Leave flare nuts alone. Leave the electrical board alone. If nothing heats after one proper wait, book the visit rather than repeating the experiment all evening.
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How this guide was prepared
This guide was written by the Local AC Installers Editorial Team. We compare official guidance, legislation and established consumer information, separate general information from project-specific advice, and show publication dates and sources. We do not accept installer accreditation claims at face value.