Summary

Air source heat pumps (ASHPs) are increasingly common on UK domestic properties following the Boiler Upgrade Scheme (BUS) grants introduced in 2022. Most residential installations use units from Daikin (Altherma), Mitsubishi (Ecodan), or Vaillant (aroTHERM), all of which share similar fault logic even though their specific error codes differ.

The majority of ASHP "faults" are not refrigerant or compressor failures — they are configuration issues, installer commissioning errors, or system design problems that cause the unit to operate outside its design parameters and lock out on a safety trip. Before assuming a refrigerant fault, work through the decision tree below systematically. Many call-outs can be resolved with a reset, a controls adjustment, or a system pressure check.

F-Gas Regulation (EU) 517/2014 (retained in UK law post-Brexit as the Fluorinated Greenhouse Gas Regulations) prohibits anyone without F-Gas certification from handling, recovering, or recharging refrigerant. If refrigerant work is needed, only a registered F-Gas engineer should proceed. MCS-certified installers hold the necessary qualifications for refrigerant handling on residential heat pump systems.

Key Facts

  • F-Gas Regulation — Fluorinated Greenhouse Gas Regulations [verify current UK statutory instrument]; only F-Gas qualified engineers may work on refrigerant circuits
  • Operating refrigerant pressure range — typically 7–15 bar on the high-pressure side during operation (R32 or R410A); not accessible by non-F-Gas engineers
  • Design flow temperature (UFH) — 35–45°C for underfloor heating; lower flow temps = higher COP
  • Design flow temperature (radiators) — 45–55°C for modern radiators; 55–65°C for older undersized rads (reduces COP significantly)
  • Design ΔT (flow-return) — 5°C for UFH systems, 10°C for radiator systems
  • Defrost cycle — normal — less than 15% of runtime in cold, damp conditions; 10–15 minutes per cycle
  • Defrost cycle — excessive — more than 20–25% of runtime; investigate refrigerant charge, outdoor coil fouling
  • COP at 7°C outdoor/35°C flow — should be 3.0–4.5 for a well-sized, correctly commissioned heat pump
  • Buffer vessel function — prevents short-cycling on low-load demand; minimum volume typically 10–20 litres per kW of heat pump output
  • Immersion heater backup — 3kW electric immersion usually integrated; activates when outdoor temps below design point or during defrost
  • MCS maintenance requirement — BUS grant conditions require annual servicing by an MCS-certified engineer; maintain records for grant compliance
  • Legionella setpoint — most ASHPs have a periodic DHW heat-up to 60°C for legionella prevention; check schedule is active
  • Outdoor unit clearance — minimum 300mm rear clearance, 1m front clearance; blocked airflow degrades COP and may cause high-pressure trips

Quick Reference Table: Common Fault Codes

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Fault Type Typical Code Likely Cause User Resettable?
Low refrigerant pressure E1 / P1 / F1 (varies by make) Refrigerant leak; low charge No — F-Gas engineer required
High refrigerant pressure E2 / P2 Blocked outdoor coil; insufficient airflow; high ambient Yes after resolving cause
Flow temperature too high E3 / over-temp Flow sensor fault; undersized buffer; blocked circuit Yes after resolving cause
Defrost lockout E4 / E6 Excessive icing; refrigerant fault; dirty coil Yes — investigate cause
Communication fault E5 / comms error Controller wiring; controller software Depends on make
Water flow fault E7 / flow switch Low system pressure; pump failure; air in system Yes after resolving cause
DHW overheat E8 / overheat DHW stat fault; poor DHW circulation Yes after resolving cause

Note: Error codes vary significantly between manufacturers — always refer to the specific installation and service manual for the installed unit.

Detailed Guidance

Fault-Finding Decision Tree

START: Heat pump not heating / showing error code
│
├─ STEP 1: Check display for error code
│   ├─ No code → System not calling for heat? Check thermostat / zone valves / programmer
│   └─ Error code present → identify from manual
│
├─ STEP 2: Check system water pressure gauge
│   ├─ Below 1.0 bar → Repressurise to 1.0–1.5 bar; check for leaks; reset
│   └─ Above 3.0 bar → Expansion vessel fault; call engineer
│
├─ STEP 3: Check outdoor unit
│   ├─ Heavily iced over? → Is it in defrost mode (fan stopped, compressor running briefly)?
│   │   ├─ Yes → Wait for defrost to complete (10–15 min), then reset
│   │   └─ No → Excessive icing; check airflow clearance; call F-Gas engineer if icing persists
│   └─ No ice, airflow clear → Proceed
│
├─ STEP 4: Attempt user reset
│   ├─ Check manual for reset procedure (usually hold button or cycle power)
│   ├─ If fault clears → Monitor; if it recurs within 24 hours, call engineer
│   └─ If fault persists → Call MCS-certified heat pump engineer
│
└─ STEP 5: Check flow temperatures on controller
    ├─ Flow temp set above 60°C → Reduce; heat pump may be fighting itself
    └─ ΔT (flow minus return) > 15°C → Circulation problem; check pump speed, zone valves

Low COP Diagnosis

COP (Coefficient of Performance) is the ratio of heat output to electrical input. A heat pump producing 3kW of heat for every 1kW of electricity has a COP of 3.0. UK heat pumps are certified to their seasonal COP (SCOP) rather than instantaneous COP, but diagnosing low real-world performance requires checking operating conditions.

Step 1 — Check flow temperature setting The single most common cause of low COP on domestic installations is flow temperature set too high. Every 1°C reduction in flow temperature increases COP by approximately 2.5–3%. A heat pump running at 55°C flow when the radiators would be adequately sized at 45°C is dramatically underperforming.

  • Check the weather compensation curve on the controller
  • Verify that the flow temperature at design outdoor temperature (e.g., -2°C for most of England) does not exceed what is needed for room heating
  • If radiators feel lukewarm at the lower setting, the radiators may be undersized for heat pump use — a common issue on retrofit installations

Step 2 — Check outdoor temperature vs design point Heat pumps lose COP as outdoor temperature drops. At -5°C outdoor temperature, most domestic ASHPs have a COP of 1.5–2.5, compared to 3.5–4.5 at 7°C. If the homeowner reports consistently high bills in January but reasonable performance in October, this is expected behaviour — not a fault.

Step 3 — Check ΔT across the heat pump (flow minus return) The design ΔT for most ASHPs is:

  • 5°C for underfloor heating (low-flow-temp systems)
  • 10°C for radiator systems

If the actual ΔT is significantly higher than design (e.g., 15–20°C), the circulation rate is too low. Check:

  • Pump speed setting (often too low on installer commissioning)
  • Air in the system (bleed radiators and system high points)
  • Zone valves (partially closed or stuck)
  • Buffer vessel sizing

If the actual ΔT is much lower than design (e.g., 1–2°C), the system is short-cycling — the heat pump is heating a small volume of water rapidly. Check buffer vessel volume.

Step 4 — Check outdoor coil for fouling A heat exchanger coated in dust, leaves, or debris cannot extract heat from the air efficiently, reducing COP. Clean the outdoor coil with low-pressure water (not high-pressure which can damage fins). Check clearances — bushes or fences that have grown to reduce airflow will significantly affect performance.

Defrost Cycle Diagnosis

Heat pumps in cold, damp conditions will periodically defrost their outdoor coil. This is normal. During defrost:

  • The fan stops
  • The refrigerant cycle reverses briefly to melt ice on the outdoor coil
  • The indoor heating circuit may pause (some units divert heat from the buffer vessel during defrost)
  • Defrost typically lasts 5–15 minutes

Normal defrost frequency: In UK winter conditions (0–5°C, high humidity), defrost every 30–90 minutes is normal. Less than 15% of total runtime spent in defrost is acceptable.

Excessive defrost (>20–25% of runtime): Causes include:

  1. Low refrigerant charge — the system cannot maintain adequate suction pressure; requires F-Gas engineer to check and recharge
  2. Dirty outdoor coil — ice builds up faster if the coil is fouled; clean the coil
  3. Insufficient airflow — blocked clearances cause rapid ice build-up
  4. Defrost sensor fault — the temperature sensor that triggers defrost end is faulty; call engineer
  5. Incorrect defrost settings — some units allow defrost frequency/duration adjustment; check against installer settings

Buffer Vessel Function and Short-Cycling

Buffer vessels prevent "short-cycling" — where a heat pump starts and stops frequently due to low heat demand. Short-cycling is damaging to the compressor and reduces efficiency.

How it works: The buffer vessel is a thermal store (typically 50–200 litres) that absorbs the heat pump's output when demand is low. The heat pump runs in longer, more efficient cycles to heat the buffer; the distribution system draws from the buffer as needed.

Signs of inadequate buffering:

  • Heat pump starting and stopping every few minutes
  • High number of compressor starts in the manufacturer's log (usually accessible via the controller)
  • Warm buffer vessel but cold radiators (zone valve or controls issue)

Minimum buffer volume: Typically 10–20 litres per kW of heat pump output. A 10kW heat pump needs at least 100–200 litres of buffering (this can be provided by the buffer vessel plus pipework volume).

Immersion Heater Backup

Most integrated ASHP/cylinder combinations include a 3kW electric immersion heater. This activates:

  • When outdoor temperature drops below the heat pump's rated operating range (typically -15 to -20°C for modern units, though efficiency drops significantly below -5°C)
  • During defrost cycles on some systems
  • When DHW demand exceeds what the heat pump can supply in time
  • As emergency backup if the heat pump faults

An immersion running for extended periods instead of the heat pump indicates either a heat pump fault or a controls misconfiguration. Check the controller's "emergency heat" or "backup heat" settings.

MCS, BUS Grants, and Maintenance Records

Homes that received a Boiler Upgrade Scheme (BUS) grant are required to maintain the heat pump in accordance with the manufacturer's service schedule, typically annually. MCS-certified engineers must carry out the service and sign off the maintenance record. Failure to maintain appropriate records could affect compliance with the grant conditions, though specific enforcement mechanisms are limited in practice.

For any heat pump covered by an MCS warranty or BUS conditions, retain all service records. If the original installer is no longer available, any MCS-certified heat pump engineer can carry out the annual service — they are not restricted to using the original installer.

Common Makes: Error Code Overview

Daikin Altherma: Error codes typically prefixed E (electrical), U (unit faults), or L (control errors). Key codes: E1 = low pressure, E3 = high temp, E4 = outdoor temperature sensor, U4 = communication between indoor/outdoor units.

Mitsubishi Ecodan: Error codes use letters + numbers. Common codes: P1 = low pressure protection, P2 = high pressure protection, E6 = communication fault, E7 = coil temperature sensor.

Vaillant aroTHERM: Error codes displayed in F series (fault) and S series (status). F.75 = water pressure fault, F.76 = heating flow sensor, F.22 = water pressure too low.

For all makes, the full error code list is in the installation and service manual. Always retrieve the full fault history (not just the most recent code) before making diagnostic decisions.

Frequently Asked Questions

Can I top up the refrigerant myself?

No. The F-Gas Regulations prohibit anyone without F-Gas certification from handling refrigerants, including topping up. Attempting to do so is illegal and voids the manufacturer's warranty. If you suspect low refrigerant (recurrent low-pressure codes, poor heating performance with no other explanation), call an F-Gas qualified heat pump engineer.

The heat pump is making a gurgling or hissing noise — is this serious?

Gurgling sounds are usually normal — the refrigerant cycle creates flow sounds, particularly during defrost when the cycle reverses. Persistent hissing from the outdoor unit (not defrost-related) could indicate a refrigerant leak; call an engineer. A banging or clanking noise from the outdoor unit fan is more serious — shut down the unit and call an engineer.

What outdoor temperature should the heat pump switch to backup heating?

Most UK ASHPs are rated to operate down to -20°C outdoor temperature, but COP drops significantly below 0°C. Backup immersion heating typically activates automatically below -7 to -10°C on most controllers. This threshold is configurable; some installers set it unnecessarily high, causing expensive electric backup use. Verify the backup temperature setting is set appropriately for the local climate.

How do I know if my heat pump is sized correctly?

Compare the heat pump's rated output (in kW) to the calculated heat loss of the property (from the MCS heat loss calculation, which should have been produced at installation). A heat pump sized at 100–110% of peak heat loss is ideal. An undersized heat pump (e.g., 8kW for a property needing 12kW at design conditions) will struggle in cold weather and run continuously, wearing the compressor.

Regulations & Standards