Nibe 155 Fault Code: Causes, Fixes & Repair Costs
What does the Nibe 155 fault code mean?
Fault code 155 is the Hot Gas Alarm. It activates when the discharge temperature sensor BT14 detects that the hot gas leaving the compressor has climbed above 135°C. At that point the heat pump shuts the compressor down to protect the refrigerant circuit from damage. The alarm clears itself automatically once the discharge temperature falls back below 90°C. Because the compressor is the most expensive component in the system, this self-protection trip should always be taken seriously — especially if it keeps coming back.
General guidance only — not a substitute for professional advice, and not certified by an heat pump engineer. Always have your specific appliance assessed by a qualified professional. Any heat pump work must be carried out by an MCS-certified heat pump engineer. If you notice a refrigerant leak, a burning smell or water leaking from the unit, switch it off at the isolator and call an MCS-certified heat pump engineer. Refrigerant work must be carried out by an F-Gas registered engineer.
Common causes
- Low refrigerant charge or leak in the sealed circuit Common
When the system is short of refrigerant — through a slow leak or an installation issue — the compressor has to work considerably harder to move heat, and the discharge gas temperature climbs rapidly. A refrigerant leak is the most common underlying reason for a persistent or repeated hot gas alarm on Nibe units. Leaks can develop at flare joints, brazed connections or on the outdoor coil due to corrosion or physical damage.
- Blocked or restricted airflow around the outdoor unit Common
The outdoor unit needs unrestricted airflow to release heat from the refrigerant circuit efficiently. A build-up of leaves, debris, snow or ice over the coil face or fan grille forces refrigerant pressures and temperatures higher than normal. This is the easiest cause to check yourself and should always be ruled out first.
- Faulty or stuck expansion valve Sometimes
The expansion valve controls the flow and pressure of refrigerant entering the evaporator. If it sticks in the wrong position or loses its calibration, the refrigerant balance across the circuit is disrupted, which can drive discharge temperatures to dangerous levels. A faulty expansion valve typically requires a qualified engineer to diagnose with manifold gauges and specialist tools.
- Defective BT14 hot gas temperature sensor Sometimes
If the BT14 sensor has developed a fault, corroded connection or a wiring break, it may report a falsely high temperature and trigger the alarm even when actual discharge temperatures are within normal range. An engineer can test the sensor's resistance values against the Nibe specification chart to confirm whether it is reading accurately.
- Repeated alarm triggering the lockout threshold Rare
Nibe's control logic can escalate the alarm to a full lockout if the hot gas threshold is breached twice within 60 minutes or continuously for 60 minutes. In this situation the underlying cause could be any of the above — the repeated pattern simply means the system has reached its safety threshold and will not restart without intervention.
How to fix it
- Check the area around the outdoor unit and clear any obstructions DIY safe
Walk outside and look at the outdoor unit. Remove any leaves, garden debris, snow or ice covering the coil grille or blocking the fan outlet above. Ensure there is at least 30–40 cm of clear space on all sides. Poor airflow is a quick win that is easy to rule out before calling anyone.
- Allow the unit to cool and wait for the auto-reset DIY safe
Fault code 155 resets itself once the discharge temperature drops below 90°C. Do not repeatedly press reset while the unit is still hot — this risks further stress on the compressor. Wait at least 15–20 minutes after the alarm appears before attempting any manual reset.
- Attempt a single manual reset via the controller DIY safe
On your Nibe display, select the fault message, choose 'I want to see more information about this alarm' and then select 'Reset alarm and try again.' The controller will restart and the heat pump will attempt to return to normal operation. If the fault clears and does not return within the next hour, continue to monitor it. If it immediately reappears or returns within the same hour, do not reset again.
- Check system water pressure DIY safe
While you are waiting, glance at the system pressure gauge on the indoor unit or buffer vessel. It should typically read between 1.0 and 1.5 bar when cold. A low-pressure reading may indicate a separate issue contributing to poor heat transfer. Top up via the filling loop if needed, following your system's instructions.
- Do not attempt further resets if the alarm keeps returning — call an MCS-certified heat pump engineer heat pump engineer
If fault code 155 returns after a single reset, the underlying cause almost certainly involves the refrigerant circuit, the expansion valve or the BT14 sensor — none of which are safe or legal for a homeowner to investigate. Contact an MCS-certified heat pump engineer, and note that any work on the refrigerant circuit (leak testing, pressure checks, topping up or recovering refrigerant) must be carried out by an F-Gas registered engineer. Nibe also has a network of Approved Repair Partners trained specifically on Nibe systems who can be reached via the Nibe homeowner care line on 0330 311 2201.
Parts you may need
- BT14 Hot Gas Temperature Sensor · from £55
- Electronic Expansion Valve (EEV) · from £180
- Expansion Valve Coil/Stepper Motor · from £65
- Outdoor Unit Fan Motor · from £140
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £150–£550, depending on the underlying cause.
Frequently asked questions
Will my Nibe heat pump restart on its own after fault code 155?
Yes, the alarm is designed to clear automatically once the BT14 sensor reads below 90°C, so the system will often attempt to restart by itself. However, if the root cause has not been resolved — such as a low refrigerant charge or a stuck expansion valve — it is very likely to trip again soon after. A single manual reset is reasonable, but if the fault returns within the same hour you should stop resetting and get an engineer involved to avoid putting repeated strain on the compressor.
Could a refrigerant leak cause fault code 155, and how much does it cost to fix in the UK?
A refrigerant leak is one of the most common reasons for a persistent hot gas alarm. When the circuit is low on refrigerant the compressor works harder and discharge temperatures spike. Diagnosing and repairing a leak typically costs between £200 and £500 for most accessible joints and connections, including an F-Gas compliant leak test and recharge. In more complex cases — where the leak is on the outdoor coil or requires significant pipework repair — costs can rise to around £800–£1,200. Only an F-Gas registered engineer can legally handle refrigerant in the UK.
What is the BT14 sensor and can it cause a false alarm?
BT14 is the discharge or 'hot gas' temperature sensor fitted to the compressor outlet pipe. It measures the temperature of refrigerant leaving the compressor and feeds that reading to the Nibe control board. If the sensor develops a fault, a loose connection or a broken wire, it can report a falsely elevated temperature and trigger fault code 155 even when the actual discharge temperature is fine. A qualified engineer can check the sensor's resistance against Nibe's reference values to confirm whether it is reading correctly. Sensor replacement including labour typically costs in the region of £130–£250.
Is fault code 155 dangerous — should I be worried about my heat pump?
The alarm itself is a safety feature, so the system is doing exactly what it is designed to do by stopping the compressor before damage occurs. There is no immediate danger to your home. However, ignoring a repeated hot gas alarm can lead to premature compressor failure, which is by far the most expensive repair on a heat pump — typically £1,200 to £2,500 or more depending on the model. Treating the fault promptly, rather than repeatedly resetting it and hoping for the best, is strongly advisable.