FAULT FINDING GUIDE

Heat Pump Electrical Fault: Wiring Problems and Fault Finding

Your heat pump has stopped working or is showing an electrical fault. This guide covers control board issues, compressor starting problems, power supply faults, dedicated circuit requirements, and when you need a qualified engineer.

Start 7-day free trial

Free for 7 days · No charge until day 8 · Cancel anytime · Used by 1,000+ UK electricians

12 min readUpdated 2026-05-18Andrew Moore, Founder of Elec-Mate

Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.

ShareXinW
Follow

1,000+

UK electricians

“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”

Daniel Palmer — DP Electrical

Key Takeaways

  • 1Heat pumps draw significantly higher starting currents than their running currents — a 5 kW air source heat pump may have a compressor start current of 25 A or more. An undersized supply cable or MCB rated too close to the running current is a common cause of electrical faults.
  • 2Heat pump control boards govern the entire system including compressor sequencing, defrost cycles, and communication with the domestic hot water cylinder. A failed control board typically causes the system to be completely inoperative or stuck in a fault or defrost loop.
  • 3Dedicated circuit requirements under BS 7671:2018+A2:2022 apply to heat pumps as high-load fixed equipment. The circuit must be sized for the maximum continuous current of the unit and protected by an MCB of appropriate type and rating.
  • 4Low or fluctuating supply voltage is a common cause of compressor starting failures. The compressor motor requires its full starting torque to overcome the refrigerant pressure difference — voltage below approximately 207 V can prevent reliable starting.
  • 5Any electrical work on the heat pump supply circuit, including fault finding, cable replacement, or protective device changes, must be carried out by a qualified electrician registered with a competent person scheme. The work must be notified to Building Control unless carried out by a scheme member.
01 · Fault Finding Guide

Heat Pump Control Board Issues

The control board is the brain of the heat pump system. It manages compressor sequencing, defrost cycles, heating and cooling mode switching, domestic hot water priority, and communication with the room thermostat and hot water cylinder sensor. A faulty control board can present in many ways — from complete system failure to subtle misbehaviour such as incorrect defrost cycle timing or inability to switch between heating and hot water modes.

  • Fault codes and lockout — modern heat pump control boards log fault codes with timestamps. A heat pump that has locked out after a fault will not restart until the fault is acknowledged, either via the controller display or by a power cycle. Note the displayed fault code before resetting — it is the primary diagnostic information.
  • Stuck defrost loop — a heat pump that appears to be constantly defrosting, or that produces minimal heat output despite running, may have a faulty outdoor coil temperature sensor feeding incorrect data to the control board. The control board believes the coil is frosted when it is not, and runs the defrost cycle continuously. Replace the suspect sensor.
  • Communication faults — many heat pump systems use proprietary two-wire or CAN bus communication between the indoor controller, outdoor unit, and hot water cylinder. A break in this communication link, or a mismatch in firmware versions following a software update, can cause the control board to log a communication fault and disable outputs.
  • Control board replacement — control board replacement is a manufacturer or specialist installer task. The replacement board must be the correct part number for the unit and may require configuration via the service menu. Incorrect board installation or configuration can damage the compressor inverter drive.
Free download

Get the BS 7671 A4:2026 Cheat Sheet — free

Every key change in the 2026 amendment on one page. AFDDs, TN-C-S protection, new schedule columns, model forms. Pinned on your van dash.

  • Every regulation change summarised
  • New model forms (EIC + MEIWC)
  • Free PDF — no subscription

We'll email it once. No spam — unsubscribe any time.

02 · Fault Finding Guide

Compressor Starting Problems

The compressor is the highest-power component in a heat pump and the most demanding load on the electrical supply. Starting problems — where the compressor fails to reach running speed — are among the most stressful events for the electrical installation and can damage both the compressor and the supply circuit if the fault is not diagnosed and corrected.

  • Inverter-driven vs. fixed-speed compressors — most modern heat pumps use variable-speed (inverter-driven) compressors that have a soft-start characteristic and lower starting currents. Fixed-speed compressors have a hard start with inrush currents of five to eight times the running current. If the supply impedance is high, a hard-start compressor will cause a voltage dip that can exceed the motor's operating tolerance.
  • Equalisation period — after shutdown, the refrigerant pressure difference across the compressor needs time to equalise before a restart is possible. Starting against a pressure difference causes very high starting current and torque. Heat pump control boards enforce a minimum off-time (typically 3 to 5 minutes) for this reason. A power cycle that bypasses this delay can cause repeated starting failures and compressor damage.
  • Failed inverter drive — in inverter-driven heat pumps, the inverter PCB converts the AC supply to DC and then synthesises variable-frequency AC for the compressor motor. A failed inverter drive causes a fault code and complete compressor failure. Inverter drive failure is often caused by voltage transients on the supply — SPD protection is recommended.
03 · Fault Finding Guide

Power Supply Problems

Heat pumps place a continuous, high-current demand on the property's electrical installation. Supply-side problems that are invisible to other loads can manifest dramatically when a heat pump is running, particularly during cold weather when the heat pump operates at maximum capacity for extended periods.

  • High supply impedance — older rural properties with long service cable runs from the DNO transformer can have high supply impedance. Under the heat pump's running current, the voltage drop across the supply impedance may reduce the terminal voltage at the consumer unit below 207 V. Measure voltage at the consumer unit under maximum heat pump load. If below 207 V, report to the DNO.
  • Corroded or undersized consumer unit connections — the meter tail connections, main switch, and circuit breakers in an older consumer unit may have higher resistance than specified due to age, corrosion, or insufficient tightening torque at installation. Under heat pump load, this resistance causes localised heating. Thermal imaging of the consumer unit under heat pump load can identify high-resistance connections before they fail.
  • Neutral fault — a high-resistance or broken neutral connection anywhere in the supply chain causes voltage asymmetry across the installation and can result in voltages outside equipment operating limits. Heat pump control boards are sensitive to supply quality and will fault on abnormal voltages.
04 · Fault Finding Guide

Dedicated Circuit Requirements

Heat pump manufacturers and BS 7671:2018+A2:2022 both require a dedicated final circuit for the heat pump supply. This is not merely good practice — it is a mandatory requirement of most manufacturer warranties and the Building Regulations compliance package.

  • Cable sizing — the supply cable must be sized to carry the heat pump's maximum continuous current with the voltage drop within BS 7671 limits (typically 3% on a final circuit, under Appendix 4). For a 5 kW to 8 kW heat pump running at 32 A, 6 mm² or 10 mm² twin and earth is typically required depending on the route length and installation method.
  • MCB type and rating — heat pump compressors require Type C MCBs to tolerate the starting inrush current without nuisance tripping. Use the manufacturer's specified MCB rating — do not uprate the MCB beyond the specified value as this removes the short-circuit protection for the supply cable.
  • RCD protection — a 30 mA RCD is required on the heat pump circuit for additional protection. Type A is generally sufficient for inverter-driven heat pumps but confirm with the manufacturer. Some heat pump manufacturers specify Type B RCDs for specific models.
  • Isolation and switching — a means of isolation must be provided adjacent to the heat pump (accessible for maintenance) in addition to the MCB at the consumer unit. This is typically a 45 A double-pole isolator or a suitably rated rotary isolator.

Try Elec-Mate free for 7 days

16 certificate types, 70+ calculators, RAMS, quoting, invoicing, AI agents, and 46+ training courses — from £6.99/mo.

Start free trial
Download on the App StoreGet it on Google Play
05 · Fault Finding Guide

Common Wiring Problems on Heat Pump Installations

Wiring errors on heat pump installations can cause intermittent faults that are difficult to diagnose without systematic testing. Many heat pump installations are carried out by plumbing contractors who may subcontract the electrical work to electricians who are unfamiliar with heat pump-specific requirements.

  • Reversed live and neutral — reversing L and N at the heat pump connection terminals can damage the control electronics and inverter drive. Some units will appear to function but will have reduced protection and may fail prematurely. Verify polarity with a voltage tester before energising.
  • Undersized cable for the route length — cable routes that pass through insulation or are buried in thermal insulation dramatically reduce the cable's current-carrying capacity. A 6 mm² cable installed in thermal insulation may be derated to less than 20 A continuous capacity, insufficient for a heat pump running at 25 A.
  • Poor earth connection — the heat pump enclosure must be connected to the protective earthing conductor. A missing or high-resistance earth connection will cause the heat pump's internal earth fault monitoring to operate and will prevent the unit from starting.
06 · Fault Finding Guide

When to Call an Engineer

Some heat pump issues can be resolved by the owner — checking for a tripped MCB, acknowledging a lockout fault on the controller, or waiting for the pressure equalisation delay. However, any fault that persists after basic checks requires a qualified engineer.

  • Call immediately — burning smell or visible scorching on the heat pump supply cable or connections, MCB that trips immediately on reset, or supply voltage significantly outside BS EN 50160 limits. Switch off the dedicated circuit at the consumer unit.
  • Call promptly (within 24 to 48 hours) — in cold weather, a heat pump that is not working is a risk to vulnerable occupants. Most heat pump manufacturers and installers offer emergency call-out services. Contact the original installer first as they will have system documentation.
  • Electrical vs. refrigerant engineer — an electrician can diagnose and repair supply circuit faults, control board wiring faults, and sensor faults. A refrigerant engineer (F-Gas registered) is required for any fault involving the refrigerant circuit, compressor, or heat exchanger. Many heat pump specialists hold both qualifications.
07 · Fault Finding Guide

For Electricians: Heat Pump Electrical Work and Certification

Heat pump electrical installation and fault finding is a growing specialist area. As the UK government targets 600,000 heat pump installations per year by 2028, the demand for electricians competent in heat pump supply circuits, controls wiring, and certification is expanding rapidly.

Minor Works and Installation Certificates

Heat pump supply circuit work requires an Electrical Installation Certificate (EIC) or Minor Works Certificate depending on scope. Use the Elec-Mate EIC app to complete certificates on site with test results, circuit details, and instant PDF export.

EICR and Heat Pump Circuits

When carrying out an EICR on a property with a heat pump, verify the dedicated circuit MCB type, cable sizing, RCD provision, and isolator arrangement. Record the heat pump circuit in the schedule of inspections. Use the Elec-Mate EICR app to generate compliant reports on site.

Certificate heat pump installations with Elec-Mate

Join 1,000+ UK electricians using Elec-Mate to certificate heat pump supply circuits, complete EICRs…

Try it free for 7 days
Download on the App StoreGet it on Google Play

Frequently Asked Questions About Heat Pump Electrical Faults

What electricians say

Verified reviews from the UK App Store.

One App for Everything!

Elec-Mate is my go to app for business and electrical work. It's feature rich without feeling cluttered. A true all in one app for quotes, certs, calculations, RAMS, EICRs, and more. I use it every day without fail, and it makes my workflow much smoother since I'm not jumping between apps anymore. The price-to-feature ratio is excellent. Any issues I've had, the developer responds within the hour and usually fixes them the same day. 100% recommend.

Apple App Store · GBR

Fantastic app for electricians

I've used the app and the web based version for a while now and it's well worth the investment. If you're an apprentice or experienced Spark give it a go, you won't be disappointed.

Apple App Store · GBR

Absolutely amazing

I've been using Elec-Mate for a while now, and honestly, it's one of the best apps I've ever downloaded. Every aspect of it feels thoughtfully designed, from the clean and intuitive interface to the powerful features that make everything so easy to manage. It's clear that a lot of care and attention went into building this app, and it shows in every detail.

Apple App Store · GBR

Trusted by electricians across the UK

Real feedback from real sparks

“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”

Daniel Palmer

Sole Trader · DP Electrical

“I've won two contracts this month because I could turn quotes around same-day with the AI cost engineer.”

Nathan Perry

Electrician · NP Electrical Services

“The study centre got me through my AM2. Mock exams and flashcards are brilliant.”

Jake Pizey

3rd Year Apprentice · Apprentice

7-Day Free Trial — Cancel Anytime, No Hassle

Certificate Heat Pump Installations on Your Phone

Join 1,000+ UK electricians using Elec-Mate for heat pump installation certificates, EICRs, and BS 7671-compliant documentation. 7-day free trial, cancel anytime.

“Replaced three separate apps with Elec-Mate. Certs, quotes, and scheduling all in one place.”

Daniel Palmer, DP Electrical

From £6.99/mo after trial — less than a coffee a week

or download the app
Download on the App StoreGet it on Google Play
7 days free, then from £6.99/moCancel in one tap — no calls, no hassleiOS, Android & WebBS 7671 compliant
16
Certificate Types
70+
Calculators
46+
Training Courses
8
AI Agents

1,000+ electricians · From £6.99/mo after trial

App Store

We use cookies to improve the app and measure what works. Cookie Policy