Summary

Vaillant's aroTHERM range (and the R290 propane-refrigerant aroTHERM plus that has largely superseded the R410A original) is one of the most-installed air source heat pumps on UK Boiler Upgrade Scheme (BUS) projects, particularly attractive to installers already familiar with Vaillant's ecoTEC boiler platform. The aroTHERM shares its indoor control and display architecture — the sensoCOMFORT or sensoHOME wall controller, and the eBUS communication protocol — with Vaillant's boiler range, which is why some fault codes (particularly system water-side faults like low pressure) carry over with an identical meaning to the ecoTEC F-code system.

However, a heat pump has fault categories a gas boiler does not: refrigerant circuit protection, defrost cycle faults, outdoor unit communication, and compressor/inverter faults. These use codes outside the gas-boiler F.20–F.83 range that engineers familiar with Vaillant boilers will already know, and the exact numbering differs between the R410A aroTHERM and the R290 aroTHERM plus generations. This guide covers the confirmed shared water-side codes in detail, and describes the heat pump-specific fault categories generically where the precise code number should be checked against the specific unit's installation and service manual rather than assumed.

As with all UK heat pumps, refrigerant circuit work — recovery, leak testing, or recharge — requires F-Gas certification under the Fluorinated Greenhouse Gas Regulations. The R290 (propane) refrigerant used in current aroTHERM plus models is also flammable, which brings additional handling and ventilation requirements beyond standard F-Gas practice.

Key Facts

Quick Reference Table

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Code Category Meaning Source
F.22 System water pressure Pressure below minimum threshold — same meaning as ecoTEC boilers Confirmed — shared with boiler platform
F.75 Pump/pressure sensor No pressure change detected on pump start — sensor blocked, pump fault, or system sludge Confirmed — shared with boiler platform
F.76 Flow/return sensor or overheat protection Excessive temperature differential or overheat protection on the water side Confirmed pattern from boiler platform — verify exact trigger for heat pump application against model manual
Outdoor unit communication fault eBUS/communication Loose or damaged connection between outdoor unit and indoor hydraulic box Category confirmed; exact code varies by generation — check manual
Defrost cycle fault Refrigerant/defrost Excessive icing, failed defrost sensor, or reversing valve fault Category confirmed; exact code varies by generation — check manual
Compressor/inverter fault Refrigerant/electrical Compressor overcurrent, inverter PCB fault Category confirmed; exact code varies by generation — check manual
Refrigerant pressure protection Refrigerant Low or high pressure protection trip Category confirmed; exact code varies by generation — check manual

Codes marked "category confirmed, exact code varies" reflect genuine aroTHERM fault categories documented across Vaillant's heat pump range, but the specific numeric code differs between the R410A aroTHERM and R290 aroTHERM plus generations. Always confirm the precise code against the installation and service manual for the exact model and generation fitted before ordering parts.

Detailed Guidance

Fault-Finding Decision Tree

START: aroTHERM showing fault code on sensoCOMFORT/sensoHOME
│
├─ STEP 1: Read the code and check the diagnostic/fault history
│   menu on the controller (installer access level)
│
├─ STEP 2: Is it a known shared water-side code?
│   ├─ F.22 (low pressure) → go to WATER PRESSURE branch
│   ├─ F.75 (pump/pressure sensor) → go to PUMP/SENSOR branch
│   ├─ F.76 (differential/overheat) → go to WATER-SIDE branch
│   └─ Not one of the above → go to STEP 3
│
├─ STEP 3: Identify general fault category from the manual
│   ├─ Communication fault → go to COMMS branch
│   ├─ Defrost-related fault → go to DEFROST branch
│   ├─ Compressor/inverter fault → go to COMPRESSOR branch
│   └─ Refrigerant pressure fault → go to REFRIGERANT branch
│
├─ WATER PRESSURE BRANCH (F.22)
│   ├─ Check pressure gauge; repressurise via filling loop to
│   │   1.0–1.5 bar cold if genuinely low
│   ├─ If pressure rises excessively on heat-up → suspect
│   │   expansion vessel fault
│   └─ If gauge reads normal but F.22 persists → suspect
│       pressure sensor fault
│
├─ PUMP/SENSOR BRANCH (F.75)
│   ├─ Listen/feel for pump running when system calls for heat
│   │   ├─ No vibration → pump seized or failed
│   │   └─ Pump runs, fault persists → pressure sensor blocked
│   │       with debris or electrically faulty
│   └─ Check system cleanliness — sludge is the most common
│       root cause of recurring F.75, exactly as on ecoTEC
│
├─ WATER-SIDE BRANCH (F.76 / differential faults)
│   ├─ Check circulating pump speed and operation
│   ├─ Check for air in the system, blocked strainer, closed
│   │   valves restricting flow
│   └─ Check flow/return NTC sensors if differential reading
│       looks implausible against actual pipe temperatures
│
├─ COMMS BRANCH
│   ├─ Check eBUS cable between outdoor unit and indoor
│   │   hydraulic box — terminals tight, no physical damage
│   ├─ Check for cable routing near unscreened power cables
│   │   (EMI risk)
│   └─ Power-cycle per manufacturer procedure, monitor recurrence
│
├─ DEFROST BRANCH
│   ├─ Is unit mid-defrost? (normal — wait for cycle to complete,
│   │   typically 2–10 minutes)
│   ├─ Check outdoor coil for excessive/abnormal icing pattern
│   ├─ Check condensate drain is clear and not refrozen
│   └─ If defrost cycle repeatedly fails to complete or recurs
│       excessively → F-Gas engineer to check refrigerant charge
│       and defrost/reversing valve operation
│
├─ COMPRESSOR BRANCH
│   ├─ Check outdoor unit isolator and supply voltage
│   ├─ Check outdoor unit airflow and coil cleanliness
│   ├─ Do NOT repeatedly reset a genuine overcurrent trip
│   └─ F-Gas certified engineer required for compressor
│       electrical and refrigerant diagnosis
│
└─ REFRIGERANT BRANCH
    ├─ Check airflow, clearances, and defrost status first
    ├─ Confirm refrigerant type fitted (R410A vs R290) before
    │   any further work — handling requirements differ
    └─ F-Gas certified engineer required to check pressures
        and charge

F.22 — Low System Water Pressure

This code carries the same meaning as on Vaillant's ecoTEC boiler range: system water pressure has dropped below the minimum operating threshold. The diagnostic approach is identical to a boiler F.22:

  1. Check the pressure gauge — repressurise via the filling loop to 1.0–1.5 bar cold if genuinely low.
  2. Investigate the cause of pressure loss — system leaks (radiator valves, pipe joints), PRV discharge (check the external discharge pipe for wetness, indicating expansion vessel failure), or a recent radiator bleed without repressurising.
  3. If the gauge reads normal but F.22 persists, suspect the pressure sensor/transducer itself.

F.75 — Pump/Pressure Sensor Fault

Also shared with the ecoTEC platform, F.75 indicates the controller expected a pressure change when the circulating pump started but did not detect one:

  1. Check whether the pump is physically running — listen and feel for vibration when the system calls for heat.
  2. If the pump is not running, check for seizure (try freeing gently via the bleed screw if accessible) or an electrical fault.
  3. If the pump runs normally, suspect the pressure sensor — magnetite/sludge fouling the sensor diaphragm is the most common cause on both boiler and heat pump installations, particularly on systems without a magnetic filter.
  4. As with the boiler platform, replacing the sensor alone without addressing system cleanliness (powerflush, magnetic filter, inhibitor) risks a recurrence within months on a heavily sludged system.

F.76 and Water-Side Differential Faults

On the boiler platform, F.76 relates to primary heat exchanger overheat protection. On the aroTHERM, a code in this category more typically relates to an excessive flow/return temperature differential or overheat protection on the water side of the refrigerant-to-water heat exchanger — check the exact trigger condition against the specific model's manual, since the underlying cause investigation is similar either way:

Defrost Cycle Faults

The aroTHERM outdoor unit periodically defrosts its coil in cold, humid UK winter conditions — this is normal and not a fault in itself. A genuine defrost fault presents as: icing that never clears, defrost cycles that run excessively long or excessively often, or ice building in an abnormal pattern (base of unit, fan blades specifically). Before assuming a refrigerant fault:

  1. Confirm the unit isn't simply mid-defrost (fan stopped briefly, some visible ice/steam, compressor running quietly) — this resolves itself within roughly 2–10 minutes.
  2. Check the condensate drain is clear and not itself frozen, which can cause water to back up and refreeze around the base of the unit, mimicking a coil defrost fault.
  3. Check outdoor unit clearances and coil cleanliness — restricted airflow increases frosting and defrost demand.
  4. If defrost cycles are genuinely excessive (more than roughly 20–25% of runtime) after the above checks, refrigerant charge and the reversing valve are the next things to check — both F-Gas certified activities.

Communication and Compressor Faults

Outdoor-to-indoor communication faults on the aroTHERM typically point to the eBUS cable connection between the outdoor unit and the indoor hydraulic box — check terminals for tightness and the cable route for physical damage or interference from nearby power cabling, exactly as with any eBUS-connected Vaillant appliance.

Compressor and inverter faults require checking the outdoor unit's electrical supply and airflow first (undervoltage and restricted airflow both being common non-component causes of protection trips), before escalating to an F-Gas certified engineer for compressor electrical testing and refrigerant charge verification.

R290 (Propane) Refrigerant — Additional Considerations

Current aroTHERM plus models use R290 (propane), a flammable A3-classified refrigerant, replacing the non-flammable R410A used in the original aroTHERM range. This has practical implications for fault-finding and any refrigerant-circuit work:

Frequently Asked Questions

Are aroTHERM heat pump fault codes the same as ecoTEC boiler fault codes?

Partially. The two platforms share their controller architecture (sensoCOMFORT/sensoHOME, eBUS), and confirmed shared codes like F.22 and F.75 carry an identical meaning across both. However, the heat pump has fault categories a boiler does not — refrigerant pressure protection, defrost faults, outdoor unit communication, and compressor/inverter faults — which use codes outside the familiar boiler F.20–F.83 gas-ignition-focused range. Do not assume every boiler F-code applies unchanged to the heat pump; confirm against the aroTHERM-specific manual.

Can I reset an aroTHERM fault code myself?

For water-side codes like F.22 (after confirming and correcting the pressure) and communication faults following a cable check, a single reset while monitoring for recurrence is reasonable. For refrigerant pressure, compressor, and persistent defrost faults, do not repeatedly reset — these point towards the sealed refrigerant circuit or compressor, and repeated resets on a genuine fault risk further damage. Diagnosis by an F-Gas certified engineer is required before returning the unit to service.

The aroTHERM shows no fault code but isn't heating efficiently — what should I check?

Absence of a fault code means the controller believes it's operating within normal parameters, so the issue is more likely a settings or design problem: check the flow temperature setpoint and weather compensation curve on the sensoCOMFORT/sensoHOME controller, and check the actual ΔT (flow minus return) against the design value for the emitter type (radiators vs underfloor heating). See the general heat pump fault-finding guide for the broader low-COP diagnostic approach applicable across all manufacturers.

Does the aroTHERM's R290 refrigerant change how I should approach a suspected refrigerant fault?

Yes — beyond requiring F-Gas certification, R290 is flammable, so any suspected refrigerant leak should be treated with the same caution as a gas leak: ensure adequate ventilation, avoid ignition sources near the outdoor unit, and do not attempt any refrigerant-circuit intervention without certification covering flammable refrigerants. This is an additional safety consideration beyond the non-flammable R410A used in the original aroTHERM range.

Regulations & Standards