- Boilers
- Electric water heater
W25
Thermor W25 on an Aeromax water heater: compressor supply or operation fault
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
Medium
Quick answer
On Thermor heat pump water heaters, W25, Err25 or error 25 mean a fault in the heat pump compressor. The heat pump stops heating; in AUTO mode the electric back-up takes over. Check the mode and call a refrigeration technician.
Safety first
Stop and check this first
- Do not open the heater's cover: inside there is mains voltage and, on heat pump models, the refrigerant circuit. That diagnosis is for an installer or a refrigeration technician.
- When the electric back-up or emergency mode gives hot water again, test the temperature before showering, especially for children or older people.
Safe checks you can perform
- Operating mode and hot water
- Air entering and leaving the heater
- Compressor, capacitor and connections
Leave these tasks to a qualified professional
- Do not attempt this professional task: Compressor diagnosis with measurements
- Do not attempt this professional task: Heat pump test after the repair
What a technician must be able to justify
Technician visit
These are the steps they must carry out and show you. Tick them as they work: if steps are skipped, do not accept a part swap.
These are not instructions for opening or handling the equipment yourself. They are the checks that should support a technical conclusion.
Compressor diagnosis with measurements
What they doThe technician measures the voltage at the compressor terminals and the capacitor and reads the air temperatures in the expert menu before replacing parts.
They must show youthe measured voltages and air temperatures against Thermor's ranges.
In writingthe cause, the part replaced and, if the circuit was opened, the gas recovered and charged.
Heat pump test after the repair
What they doAfter the repair, the technician runs the heat pump for about ten minutes and reads the air temperatures again.
They must show youthe air temperature difference within Thermor's range.
In writingthe readings and that W25 has not returned.
Before accepting a part or repair
Nothing has been shown yet. Do not accept a part swap based on the code alone.
Follow these steps
Note: Follow the documented order. An error code identifies the affected system, but it does not prove by itself which part has failed.
Operating mode and hot water
- Who:
- you.
- How:
- check on the digital control which mode is active (AUTO, Manual ECO on or off, ECO) and whether hot water comes out of a tap. Take a photo showing W25.
- Result that matters:
- Thermor explains that in AUTO with Manual ECO off the electric back-up, fitted at mid height of the tank, comes on, while in Manual ECO on or ECO it does not (except on the Airlis) and there is no hot water.
- If you are in Manual ECO on or ECO:
- switch to AUTO, Manual ECO off or the emergency mode described in your model's manual to get hot water until the technician arrives.
- If you are in AUTO and there is less hot water:
- that is expected, because only the water above the element is heated.
- Evidence to keep:
- a photo of the code and the active mode, with the date.
Evidence: Comment résoudre un problème de chauffe-eau qui affiche une erreur 25 ou W25 ?
Air entering and leaving the heater
- Who:
- you.
- How:
- without opening the heater, check that the air inlets and outlets and any ducts are not covered, crushed or dirty.
- Result that matters:
- Thermor includes uncrushed ducts and clean air inlets and outlets among the installation checks.
- If you find a crushed duct or a covered outlet:
- tell the technician; it can change the diagnosis.
- If everything is clear:
- the fault is inside the heat pump; book the visit.
- Evidence to keep:
- photos of the inlets, outlets and ducts.
Evidence: Comment résoudre un problème de chauffe-eau qui affiche une erreur 25 ou W25 ?
Compressor, capacitor and connections
- Who:
- a refrigeration technician or qualified installer.
- How:
- from the expert menu runs a Boost, measures the voltage at the compressor terminals and the capacitor value, checks the pressure switch and hot gas valve connections and compares the air temperature difference with Thermor's figures.
- Result that matters:
- whether voltage reaches the compressor and whether heat exchange is within Thermor's figures: 8 to 15 °C between inlet and outlet air on integrated units, 3 to 8 °C below outdoor temperature on Split units.
- If no voltage leaves the board:
- Thermor says to replace the board.
- If voltage arrives and the compressor does not start or makes noise:
- Thermor says to replace the compressor.
- Evidence to keep:
- a report with voltages, capacitor value and temperatures read.
Mains voltage and refrigerant. Refrigeration technician or qualified installer only.
Evidence: Comment résoudre un problème de chauffe-eau qui affiche une erreur 25 ou W25 ?
Still not solved
If the error persists, stop here. See what the technician must do and show you.
Where this code applies
A code is meaningful only inside the right product and version context.
Primary scope
- System
- Boilers
- Brand
- Thermor
- Product type
- Electric water heater
Known code variants
- Err 05
- Err 25
- Err25
- Erreur 25
- Thermor W25
- W.25
What it means
Thermor links error 25 or W25, shown on some models as Err 05, to a supply or operation fault in the compressor of the heat pump water heater. The heat pump stops heating; depending on the mode, the electric back-up at mid height of the tank takes over or there is no hot water. Thermor gives as causes the compressor or its winding, the start capacitor and a connection fault; the first two are diagnosed by a refrigeration technician. The code does not tell them apart.
Warnings and stop conditions
- Higheditorial warning
Do not open the heater's cover: inside there is mains voltage and, on heat pump models, the refrigerant circuit. That diagnosis is for an installer or a refrigeration technician.
Evidence: Comment résoudre un problème de chauffe-eau qui affiche une erreur 25 ou W25 ?
- Mediumeditorial warning
When the electric back-up or emergency mode gives hot water again, test the temperature before showering, especially for children or older people.
Evidence: Comment résoudre un problème de chauffe-eau qui affiche une erreur 25 ou W25 ?
Symptoms and causes
Probable causes · in order
- Connection fault on a new unitPossible
Thermor says that on a new heater the connections should be checked in case they came off the board edge or the compressor. The technician checks it by opening the cover.
Evidence: Comment résoudre un problème de chauffe-eau qui affiche une erreur 25 ou W25 ?
- Faulty start capacitorPossible
Without a working capacitor the compressor cannot start. The technician sees it swollen or measures it out of tolerance.
Evidence: Comment résoudre un problème de chauffe-eau qui affiche une erreur 25 ou W25 ?
- Compressor or motor windingPossible
It is confirmed when correct voltage reaches the compressor and it does not start or makes noise. A refrigeration technician's diagnosis.
Evidence: Comment résoudre un problème de chauffe-eau qui affiche une erreur 25 ou W25 ?
Virtual technician
Does W25 still appear after these checks?
Answer a few questions to narrow down the likely cause and the safest next action. Guidance only - it never replaces a professional.
Sources and technical references
- 1
Comment résoudre un problème de chauffe-eau qui affiche une erreur 25 ou W25 ?
Thermor · 2025 · official_doc
- 2
Mon chauffe-eau affiche une erreur, que faire ?
Thermor · 2025 · official_doc
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