- Boilers
- Electric water heater
W21
Thermor W21 water heater code: air inlet or evaporator sensor
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
Medium
Quick answer
On Thermor heat pump water heaters, W21 or error 21 means the air inlet or evaporator sensor is reporting an impossible temperature. The heat pump stops; choose a mode that uses the element for hot water and have the installer check the sensors and fan.
Safety first
Stop and check this first
- Sensors, board and fan sit inside the casing next to mains voltage; only a qualified installer should open it, always with the water heater breaker off. Do not swap parts yourself.
- In ECO or Manual ECO active the unit will not heat at all while W21 is present; change mode if you need hot water and do not leave the error unchecked.
Safe checks you can perform
- Switch to a mode with electric backup
- Listen for the fan
- Write down the exact code and model
Leave these tasks to a qualified professional
- Do not attempt this professional task: Read the sensors in the Expert/Tests menu
- Do not attempt this professional task: Check sensor position and connector
- Do not attempt this professional task: Diagnose the fan
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.
Read the sensors in the Expert/Tests menu
What they doThe installer compares each sensor reading with the room temperature and replaces the sensor or its harness if the value makes no sense.
They must show youthe sensor reading next to the actual room temperature.
In writingwhich sensor or harness was replaced.
Check sensor position and connector
What they doWith the unit isolated, they check that each sensor sits in its holder, is firmly clipped and is plugged into the right board connector.
They must show youwhere the affected sensor is fitted.
In writingwhether it was loose or wrongly connected.
Diagnose the fan
What they doIf the fan does not start from the test menu, the professional tests the motor, capacitors and board output before repairing anything.
They must show youthe fan start test.
In writingthe faulty part and the part fitted.
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.
Switch to a mode with electric backup
While W21 is displayed the heat pump does not run. If you are in ECO or Manual ECO active, change to AUTO, Manual ECO inactive or the backup mode explained in your model's manual so the element heats part of the tank.
Evidence: Comment résoudre un problème de chauffe-eau qui affiche une erreur 21, W21 ou une erreur 22, W22 ?
Listen for the fan
Without opening the casing, listen next to the unit to hear whether the heat pump fan ever turns. If it stays silent, note it for the installer, because Thermor treats it as a separate lead from the sensor itself.
Evidence: Comment résoudre un problème de chauffe-eau qui affiche une erreur 21, W21 ou une erreur 22, W22 ?
Write down the exact code and model
Copy whether the display shows W21, W22, Err21 or Err22 and the model on the rating plate, for example Aéromax 4 or Split 2. Each number means something slightly different by model, so this tells the technician which sensor to test first.
Evidence: Mon chauffe-eau affiche une erreur, que faire ? · Comment résoudre un problème de chauffe-eau qui affiche une erreur 21, W21 ou une erreur 22, W22 ?
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
- 21
- Aéromax W21
- Err 21
- Err21
- Erreur 21
- Thermor W21
- W21
What it means
According to Thermor, Err21 or W21 refers to the air inlet (suction) sensor or the evaporator sensor, and Err22 or W22 to an upper or lower evaporator sensor depending on the model; on the Aéromax RT+ and Aéromax 4 the numbering differs. The message appears when a sensor sends the board a temperature that cannot be right, often above 70 °C or below -20 °C. The heat pump stops: in AUTO or Manual ECO inactive the element halfway up the tank heats and you get less hot water; in ECO or Manual ECO active there is no hot water. It concerns Aéromax, Aéromax Split, VMC, Access and Airlis.
Warnings and stop conditions
- Higheditorial warning
Sensors, board and fan sit inside the casing next to mains voltage; only a qualified installer should open it, always with the water heater breaker off. Do not swap parts yourself.
Evidence: Comment résoudre un problème de chauffe-eau qui affiche une erreur 21, W21 ou une erreur 22, W22 ?
- Mediumsource verified
In ECO or Manual ECO active the unit will not heat at all while W21 is present; change mode if you need hot water and do not leave the error unchecked.
Evidence: Comment résoudre un problème de chauffe-eau qui affiche une erreur 21, W21 ou une erreur 22, W22 ?
Symptoms and causes
Probable causes · in order
- Faulty sensor or sensor harnessCommon
From the Expert/Tests menu the technician reads the sensor temperature; if it does not match the room, Thermor says to replace the sensor or its wiring harness.
Evidence: Comment résoudre un problème de chauffe-eau qui affiche une erreur 21, W21 ou une erreur 22, W22 ?
- Sensor out of place or on the wrong connectorPossible
The sensors are held by clips or in a holder at a precise point; if they have moved or are plugged into another connector on the board, they read a false temperature.
Evidence: Comment résoudre un problème de chauffe-eau qui affiche une erreur 21, W21 ou une erreur 22, W22 ?
- Fan that does not startPossible
The fan moves air through the evaporator; if it does not spin, Thermor asks the installer to trace the cause to the motor, the capacitors or the control board.
Evidence: Comment résoudre un problème de chauffe-eau qui affiche une erreur 21, W21 ou une erreur 22, W22 ?
Virtual technician
Does W21 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
Mon chauffe-eau affiche une erreur, que faire ?
Thermor · 2025 · official_doc
- 2
Comment résoudre un problème de chauffe-eau qui affiche une erreur 21, W21 ou une erreur 22, W22 ?
Thermor · 2025 · official_doc
Was this entry useful?