Summary

Worcester Bosch is one of the UK's best-known heating brands through its Greenstar gas boiler range, but its air source heat pump (ASHP) line is a separate product family, built on the wider Bosch Thermotechnology group's heat pump platform (the same corporate group that also sells heat pumps under the Bosch and Buderus names in other European markets, and under IVT in Scandinavia). Worcester Bosch's UK domestic ASHP range is sold under the Compress name — monobloc outdoor units and split (outdoor + indoor hydraulic box) configurations — and has grown quickly on Boiler Upgrade Scheme (BUS) installations alongside Vaillant aroTHERM, Mitsubishi Ecodan, and Daikin Altherma.

Because the Compress range shares its refrigerant circuit, compressor, and PCB architecture with Bosch's wider European heat pump platform rather than with Worcester Bosch's own Greenstar boiler electronics, do not assume Greenstar boiler fault codes (the familiar EA, A1, D1, E9, C6 codes documented in Worcester Bosch's boiler error code reference) apply to the heat pump range — they don't. The heat pump uses its own code set, shown on the outdoor unit's diagnostic display where fitted, or more commonly on the connected room/hydraulic controller.

As with every heat pump brand in the UK market, a fault code that references pressure, refrigerant charge, or compressor behaviour sits inside the F-Gas regulatory boundary — only an F-Gas certified engineer may open the sealed refrigerant circuit to test, recover, or recharge refrigerant. A large proportion of real-world call-outs, however, trace back to causes outside that boundary: blocked outdoor unit airflow, a tripped isolator, a restricted water-side flow rate, or a loose communication cable. Checking these first avoids an unnecessary F-Gas engineer call-out and speeds up the fix when the fault genuinely is refrigerant-side.

Key Facts

Quick Reference Table

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Fault Category Typical Trigger Likely Cause F-Gas Required?
High-pressure protection High-pressure switch/transducer trips on refrigerant circuit Blocked outdoor coil, closed water valve, overcharge, poor airflow Possibly — check airflow and water-side flow first
Low-pressure protection Low-pressure switch/transducer trips Refrigerant shortage/leak, restricted expansion valve Yes, once non-refrigerant causes ruled out
Discharge temperature protection Compressor discharge sensor reads over threshold Refrigerant shortage, overload, blocked outdoor coil Yes, if refrigerant shortage confirmed
Compressor overcurrent/lock Inverter/compressor protection trips Compressor fault, incoming supply issue, blocked coil Yes, for compressor-side diagnosis
Outdoor air/flow/return thermistor fault Sensor reading out of expected range Sensor failed, connector corroded, wiring damaged No
Discharge/defrost pipe thermistor fault Sensor reading out of expected range Sensor failed or wiring fault No
Outdoor unit PCB fault Control board self-check fails PCB component failure, moisture ingress No
Indoor hydraulic module PCB fault Control board self-check fails PCB component failure, water ingress at connections No
Indoor–outdoor communication fault Signal loss between units Damaged/loose control cable, EMI interference No
Room controller communication fault Signal loss between controller and hydraulic module Wiring/connection fault, controller firmware No
Water flow/low delta-T fault Flow switch or delta-T monitoring trips Closed valve, air lock, failed circulating pump, blocked strainer No, but resolve before assuming refrigerant fault
Power supply/phase fault Undervoltage or phase-loss detection trips (3-phase models) Incoming supply issue, undersized cable run, DNO supply fault No

Category descriptions and likely causes are generic to the Bosch-platform Compress range and cross-referenced against the wider heat pump fault architecture used across Daikin, Mitsubishi Ecodan, and Vaillant aroTHERM. Exact code numbers are not listed here — confirm the precise code against the specific Compress model and generation's installation/service manual before ordering parts or scheduling refrigerant work.

Detailed Guidance

Fault-Finding Decision Tree

START: Worcester Bosch Compress heat pump showing fault code /
       lockout indication on control panel or room controller
│
├─ STEP 0: Confirm this is the HEAT PUMP range, not a Greenstar
│   boiler — the two use entirely different code sets. Check the
│   data plate / model name (Compress = heat pump).
│
├─ STEP 1: Read the code from the display + check the fault
│   history/service log in the installer menu (note last several
│   faults and run hours, not just the current one)
│
├─ STEP 2: Which fault category does the code belong to?
│   (confirm exact category against the model's manual — the
│   groupings below are the general cross-manufacturer pattern)
│   ├─ Refrigerant/pressure category → PRESSURE/PROTECTION branch
│   ├─ Sensor/thermistor category → SENSOR branch
│   ├─ PCB/electronic category → ELECTRONICS branch
│   ├─ Communication category → COMMS branch
│   └─ Power supply category → POWER branch
│
├─ PRESSURE/PROTECTION BRANCH
│   ├─ STOP — check these before suspecting refrigerant fault:
│   ├─ Outdoor unit airflow — coil clogged, fencing/planting too
│   │   close, snow/leaf blockage, clearances per install manual
│   ├─ Outdoor unit isolator on + supply voltage within spec
│   ├─ Water-side flow rate — closed valve, air lock, failed
│   │   pump, blocked strainer (restricted flow overheats the
│   │   refrigerant-to-water exchanger and can trip pressure
│   │   protection that looks like a refrigerant fault)
│   ├─ Mid-defrost? — heavy icing during active defrost is normal
│   │   in cold, humid UK winter conditions — wait and recheck
│   └─ If airflow, power, water flow and defrost all check out
│       → F-Gas certified engineer required for pressures,
│       superheat/subcooling, and compressor electrical checks
│
├─ SENSOR BRANCH
│   ├─ Inspect thermistor connector for corrosion/loose pin
│   │   (common on outdoor-unit-mounted sensors exposed to weather)
│   ├─ Check resistance against the manufacturer's
│   │   temperature/resistance chart if accessible
│   └─ Wiring and resistance both correct → suspect PCB input
│       circuit rather than the sensor itself
│
├─ ELECTRONICS BRANCH
│   ├─ Check for visible moisture ingress at PCB enclosure seals
│   ├─ Check incoming supply voltage and any recent power event
│   │   (surge, brownout) that could have damaged the board
│   └─ If external causes ruled out → board replacement likely;
│       confirm against manual before ordering the part
│
├─ COMMS BRANCH
│   ├─ Check control cable terminals at indoor/outdoor units and
│   │   room controller are tight and undamaged
│   ├─ Check cable run for physical damage, especially where it
│   │   shares a duct or conduit with mains cabling
│   └─ Intermittent fault → suspect EMI from an unscreened power
│       cable run parallel to the control cable
│
└─ POWER BRANCH
    ├─ Check incoming supply voltage at the isolator against the
    │   unit's rated tolerance
    ├─ On 3-phase models, check for phase loss/imbalance
    └─ Undersized or long supply cable run can cause undervoltage
        under compressor starting load — check cable sizing
        against the installation manual, not just continuity

Before Calling an F-Gas Engineer: Checks Any Heating Engineer Can Do

Most fault codes that reference the refrigerant circuit are triggered by conditions outside the sealed system far more often than by a genuine refrigerant fault. Before booking an F-Gas certified engineer, check:

  1. Outdoor unit airflow and clearances — confirm the coil is free of debris and that installation clearances (check the specific model's manual — these vary by output and cabinet design) are met.
  2. Isolator and incoming supply — confirm the dedicated isolator is on and supply voltage is within tolerance; undervoltage on a long or undersized cable run is a genuine, recurring cause of protection trips.
  3. Water-side flow rate — a restricted flow (closed valve, air lock, failed pump, blocked strainer/magnetic filter) overheats the refrigerant-to-water exchanger, tripping pressure or temperature protection that presents identically to an actual refrigerant fault.
  4. Defrost cycle status — check whether the unit is mid-defrost before assuming a fault. Heavy icing during active defrost is normal in cold, humid UK conditions; a fault only exists if defrost is excessively frequent or fails to clear the coil (see the general defrost cycle diagnosis article for what "excessive" looks like).
  5. Fault history pattern — an isolated code that cleared on reset and hasn't recurred is less concerning than one recurring at a consistent operating condition (every cold snap, every DHW heat-up).

Only once these are ruled out should diagnosis move to the sealed refrigerant circuit — pressures, superheat, subcooling, and compressor electrical characteristics — which requires F-Gas certification.

Communication and Controller Faults

Communication faults between the outdoor unit, indoor hydraulic module, and room controller are among the most common non-refrigerant call-outs across every heat pump brand, and Worcester Bosch's Compress range is no exception. The indoor and outdoor units typically communicate over a dedicated control cable separate from the power supply cabling.

Sensor and Thermistor Faults

Sensor faults generally point to a specific thermistor — outdoor air, flow, return, discharge pipe, or defrost sensors. These are low-cost, straightforward checks that do not require F-Gas certification (replacing a confirmed-faulty sensor does not involve opening the refrigerant circuit, though care is needed where a sensor is strapped to refrigerant pipework):

Reset Procedure and When Not to Reset

Most non-pressure faults (sensor, communication, controller) clear with a power-cycle reset of the outdoor unit isolator — switch off, allow several minutes for the PCB to fully discharge, then restore power. A single reset to confirm the fault clears is reasonable practice for these categories while further diagnosis continues.

For pressure, discharge temperature, and compressor-related faults, do not repeatedly reset without addressing the underlying cause. Cycling power on a genuine compressor or pressure fault risks further damage to the compressor and is poor practice — diagnose properly, including an F-Gas engineer where the refrigerant circuit is implicated, before returning the unit to service.

Frequently Asked Questions

Can I reset a Worcester Bosch heat pump fault code myself, or do I need to call an engineer?

For sensor, communication, and controller-category faults, a single power-cycle reset to see if the fault clears is reasonable, provided you also investigate the likely non-refrigerant cause (loose connector, damaged cable, blocked airflow) rather than resetting repeatedly. For pressure, discharge temperature, and compressor-category faults, stop after one reset if the fault recurs — repeated resets on a genuine refrigerant-side fault risk compressor damage and should be diagnosed by a qualified (and where the refrigerant circuit is implicated, F-Gas certified) engineer.

Where do I find the exact fault code list for my specific Compress model?

Worcester Bosch's heat pump fault categories follow the general Bosch Thermotechnology group pattern described above, but the exact numeric codes vary between models and generations within the Compress range (monobloc vs split, and across product revisions). Check the installation and service manual for the specific model, or the commissioning paperwork, before relying on a specific code number. This article deliberately does not list precise code numbers for that reason — treat any code number quoted elsewhere with the same caution until confirmed against the model-specific manual.

I keep seeing Worcester Bosch boiler error codes (EA, A1, D1, E9, C6) online — do these apply to the heat pump?

No. Those codes belong to the Greenstar gas boiler range's own electronics and fault architecture, documented separately in Worcester Bosch's boiler error code reference. The Compress heat pump range uses a different platform (shared with Bosch's wider European heat pump line) and a different fault code set. Confirm which product range is actually fitted — the data plate and model name will show "Compress" for the heat pump — before consulting any code reference.

My Worcester Bosch heat pump shows no fault code but isn't heating properly — where do I start?

If there's no fault code, the unit believes it's operating normally, so the issue is more likely a settings, commissioning, or system-design problem than a component fault. Check the flow temperature setpoint and weather compensation curve against the MCS commissioning record, check DHW priority settings if hot water and space heating compete for run time, and check the actual system flow rate and delta-T. See the general heat pump fault-finding article for a broader diagnostic approach covering non-fault-code symptoms such as low COP and short-cycling.

Does F-Gas certification need to be Worcester Bosch/Bosch-specific, or is any F-Gas card valid?

Any engineer holding a valid F-Gas certification appropriate to the refrigerant type and equipment category (typically Category 1 for stationary refrigeration, air conditioning, and heat pump equipment) can legally carry out refrigerant-circuit work on a Worcester Bosch Compress unit — there is no brand-specific F-Gas qualification requirement in law. However, manufacturer-specific product training is valuable for efficient fault diagnosis and may be a condition of maintaining warranty cover on parts and labour — check the specific warranty terms before undertaking refrigerant-circuit work on a unit still under warranty.

Regulations & Standards