First cold Monday morning in October. Staff walk into the office and it’s 14°C inside. The heat pump is running — you can hear the outdoor unit humming — but the air coming out of the ceiling cassettes is cool. Not warm. Not lukewarm. The system was cooling perfectly all summer, and now it won’t switch to heating. Everyone’s working in their coats.
This is one of the most common autumn call-outs we handle across London. Heat pumps are designed to do both jobs — heating and cooling — using the same refrigerant circuit. But the changeover between modes relies on several components working in sequence, and when any one of them fails, you get exactly this: a system stuck blowing cold air when you need heat.
How Heat Pump Reversal Works
Understanding the mechanics helps narrow down the fault. In cooling mode, the system absorbs heat from inside the building and dumps it outside through the outdoor coil. In heating mode, it does the opposite — pulls heat from the outdoor air and delivers it inside. Same compressor, same refrigerant, same pipework. The only thing that changes is the direction of flow.
The component that makes this possible is the four-way reversing valve, mounted inside the outdoor unit. This valve redirects refrigerant flow so the indoor coil switches from evaporator to condenser and vice versa. When the reversing valve or anything controlling it fails, the system stays locked in whichever mode it was last running. Since most systems spend the summer in cooling, that’s where they get stuck.
Failed Four-Way Reversing Valve
The most common mechanical failure behind this symptom. The four-way reversing valve uses a sliding mechanism inside a brass body to redirect refrigerant. Over time — especially after months of continuous operation in one mode — the slider can seize. Corrosion, particulate contamination in the refrigerant, or simple mechanical wear can all prevent it from shifting position when the system calls for a mode change.
The telltale sign is that the system runs and pressures look reasonable, but the outdoor coil is acting as a condenser when it should be an evaporator. A thermal imaging camera makes this immediately obvious — the outdoor coil reads hot when it should be cold in heating mode. Replacing a reversing valve means recovering the refrigerant, brazing out the old valve, brazing in the new one, pressure testing, evacuating, and recharging. It’s a significant repair but a definitive one.
Reversing Valve Solenoid Coil Failure
The solenoid coil is the electrical pilot that triggers the reversing valve to change position. These coils sit on top of the valve body, exposed to weather on outdoor units. They burn out, suffer water ingress through cracked insulation, or lose their electrical connection at the terminal block. When the coil fails, the valve never receives the signal to shift.
This is actually the best-case scenario. Solenoid coils are cheap, readily available, and can be replaced without touching the refrigerant circuit. An engineer tests the coil with a multimeter — no continuity means the coil is dead. Swap it out, power up, command the system into heating mode, and the valve should shift. Ten-minute fix once diagnosed.
Thermostat or Controller Not Set to Heat Mode
We include this because it accounts for more call-outs than anyone wants to admit. The wall controller is still set to “Cool Only” from summer. The BMS hasn’t been switched from summer to winter schedule. The thermostat is in manual cooling mode rather than auto changeover. Some systems require a deliberate manual changeover at the controller or through the building management system — they don’t switch automatically.
Before assuming a mechanical fault, check every controller on the system. Check the BMS setpoints and schedules. Verify whether the system is configured for auto changeover or manual. This is a five-minute fix that saves an engineer visit and a diagnostic charge.
Outdoor Unit PCB Fault
The outdoor unit’s main printed circuit board controls the reversing valve output. If the relay or triac that drives the solenoid coil has failed on the PCB, the board runs the compressor perfectly but never sends the electrical signal to energise the reversing valve. The system cools fine because the default valve position is cooling. But heating requires the PCB to actively switch the valve — and if that output circuit is dead, nothing happens.
Diagnosis involves measuring for voltage at the reversing valve solenoid terminals when the system is commanded into heating mode. No voltage present with a confirmed good solenoid coil points directly at the PCB. Replacement boards for most commercial heat pumps are available, though lead times vary by manufacturer and model.
Defrost Cycle Stuck
Heat pumps in heating mode periodically run a defrost cycle to clear ice from the outdoor coil. During defrost, the system temporarily reverses back to cooling mode — sending hot refrigerant gas through the outdoor coil to melt the frost. Once the coil is clear, the system switches back to heating. If the defrost cycle fails to terminate — due to a faulty defrost sensor, a stuck timer, or a controller logic error — the system stays in cooling mode permanently. It thinks it’s still defrosting when it isn’t.
Check the defrost thermostat or sensor on the outdoor coil. Verify the defrost termination temperature setting. On many systems, you can manually force a defrost exit through the service menu or by cycling power to the outdoor unit. If the sensor has failed, replace it and the defrost cycle should terminate normally.
Low Refrigerant Affecting Heating Capacity
A heat pump with a slow leak might cool adequately during summer when ambient temperatures are higher and the cooling demand is moderate. But heating mode is a harder job — the system has to extract heat from cold outdoor air, which requires the full refrigerant charge to work efficiently. A system that’s 15-20% low on refrigerant may manage cooling but produce barely noticeable heat output. Suction pressure drops too low, the compressor runs inefficiently, and the supply air stays lukewarm.
If the system heated fine last winter but struggles this year, a refrigerant leak with gradual charge loss is a strong possibility. An engineer checks operating pressures and compares them to the manufacturer’s expected values for the current ambient conditions. If the charge is low, the leak has to be found and repaired before topping up — otherwise you’re back in the same position next autumn.
What ADK Does About It
We carry reversing valve solenoid coils, common PCBs, and refrigerant on every van. For a solenoid or controller fault, most heat pump changeover issues are resolved on the first visit. For a seized reversing valve, we diagnose on-site and schedule the replacement with minimal downtime. If your heat pump is blowing cold when it should be heating, call ADK on 0207 801 0808. We’ll get your system switched over properly.