Vaillant Heat Pump F.823 Fault Code Explained

If your Vaillant aroTHERM heat pump is displaying F.823, it's flagging a hot gas temperature switch issue, the sensor monitoring refrigerant temperature at the compressor outlet has detected a reading outside its expected safe range. This isn't a fault to reset repeatedly and hope it clears. It's connected to the refrigerant circuit and the compressor's protection system, and resetting without finding the cause risks masking a problem that gets worse, not better.

At a glance

What F.823 is actually telling you

The hot gas temperature switch exists to protect the compressor from operating outside safe limits. Three genuinely different things can trigger it, and they call for different responses.

Restricted or reduced refrigerant flow. This is the most common cause in practice. A partially blocked filter drier, a refrigerant charge that's dropped below the correct level, or a faulty expansion valve not metering refrigerant correctly can all cause hot gas temperature to climb. Each of these is refrigerant circuit work, meaning it legally requires an F-Gas certified engineer, not something to investigate yourself.

A genuinely faulty sensor or switch. Sometimes the fault is with the monitoring component itself, not an actual overheating condition. A competent diagnosis needs to confirm whether the reading is accurate before anyone starts working on the refrigerant circuit based on a false alarm.

The compressor working harder than it should, for reasons outside the refrigerant circuit entirely. This is the part that gets skipped in most fault-code guides, including our own first draft of this one. If a heat pump is being asked to produce unusually high flow temperatures because of undersized emitters, poor system design, or settings that were never adjusted from default, the compressor works under more strain than intended. That added load can show up as a heat-related fault like F.823, even when the refrigerant circuit itself is functioning correctly. Our article on why some rooms stay cold with a heat pump covers a related pattern we see often, where the heat pump gets blamed for a fault that actually traces back to system design decisions made before it was ever switched on.

What to check before calling an engineer

Photograph the exact fault code and note when it appeared before resetting anything, this single step makes diagnosis meaningfully faster, whoever ends up doing it. If the fault appeared immediately after work on the system, bleeding radiators, valve adjustments, a power interruption, a single occurrence isn't automatically serious. If it's recurring without an obvious trigger, that's a stronger signal something genuine needs investigating rather than resetting away.

Checking your system's water pressure gauge and confirming no isolation valves have been left partially closed is safe and worth doing. Going further isn't. Refrigerant circuit diagnosis requires specific tools, F-Gas certification, and manufacturer-level knowledge, and should always be left to a suitably qualified heat pump engineer.

Why this fault sometimes points beyond the refrigerant circuit

We've reviewed systems where a recurring heat-related fault kept returning despite repeated resets and even engineer visits, because the visits addressed the fault code itself without ever checking whether the underlying flow temperature and emitter sizing were correct for the property. Treating the symptom without checking the design behind it means the same fault often returns, sometimes within weeks. If F.823 keeps recurring after it's supposedly been fixed, that's worth a second look, not another reset.

If it keeps coming back

Our Fix My Heat Pump service reviews your system settings, flow temperatures, and operating behaviour independently of whoever installed or last serviced it, to establish whether a recurring fault is a standalone refrigerant issue or a symptom of a wider design problem. We're also happy to work directly alongside your existing engineer or plumber if you'd rather keep them involved.

FAQ

Is F.823 a dangerous fault? It relates to the refrigerant circuit and compressor protection, so it should be taken seriously and investigated by a qualified engineer rather than repeatedly reset.

Can I fix F.823 myself? No. Anything involving the refrigerant circuit requires F-Gas certification and specialist tools. Checking your system pressure and photographing the fault code is safe; going further isn't.

Why does F.823 keep coming back after an engineer has already looked at it? If the underlying cause wasn't the refrigerant switch itself but a wider system design issue causing the compressor to overwork, a repair that only addresses the fault code symptom won't necessarily prevent it recurring.