- Boilers
- Electric water heater
W28
Thermor W28 on a heat pump water heater: defrost cycle fault
Risk level
Low
Usually safe to check without specialist tools.
Urgency
Medium
Quick answer
On Thermor heat pump water heaters, W28 or error 28 mean a fault in the heat pump defrost cycle. While it is repaired, you can get hot water from the electric back-up in AUTO mode or with the electric-only mode.
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
- Hot water until the technician arrives
- Installation and air inlets
- Gas charge and fan
Leave these tasks to a qualified professional
- Do not attempt this professional task: Defrost diagnosis
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.
Defrost diagnosis
What they doThe professional checks installation and settings, maintenance, fan and gas charge, the four causes Thermor gives.
They must show youwhat they checked on the fan and the charge.
In writingthe cause found, what was corrected and, if gas was recharged, where the leak was and the amount.
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.
Hot water until the technician arrives
- Who:
- you.
- How:
- check on the digital control whether you are in AUTO mode, or choose the electric-only mode (emergency mode, FELC or another name depending on your model's manual).
- Result that matters:
- Thermor says the electric back-up comes on in AUTO and that you can force it with the electric-only mode.
- If hot water returns:
- keep that mode until the repair.
- If there is no hot water even in electric mode:
- tell the technician; the problem is not only the defrost.
- Evidence to keep:
- a photo of the code and the mode chosen.
Evidence: Mon chauffe-eau thermodynamique affiche une erreur 28 ou Err W28, que faire ?
Installation and air inlets
- Who:
- you.
- How:
- without opening the heater, check that the air inlets and outlets are clear and look in the manual to see whether the heater is installed and set as it describes.
- Result that matters:
- Thermor lists installation outside the manual and lack of maintenance as causes of error 28.
- If inlets are covered or maintenance is overdue:
- tell the installer; it may be the cause.
- If everything looks right:
- the remaining causes, gas and fan, are for the professional.
- Evidence to keep:
- photos of the air inlets and the date of the last maintenance.
Evidence: Mon chauffe-eau thermodynamique affiche une erreur 28 ou Err W28, que faire ?
Gas charge and fan
- Who:
- an installer or refrigeration technician.
- How:
- tests the fan and checks the refrigerant charge with suitable equipment, with support from Thermor's technical service if needed.
- Result that matters:
- whether the fan works and whether the gas charge is correct.
- If the fan does not work:
- that cause of the defrost fault is confirmed.
- If gas is missing:
- the leak must be found before recharging.
- Evidence to keep:
- a report of the tests and, if gas was handled, the amount charged.
Refrigerant and mains voltage. Thermor leaves this to an installer or refrigeration technician.
Evidence: Mon chauffe-eau thermodynamique affiche une erreur 28 ou Err W28, que faire ?
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
- Err28
- Erreur 28
- Thermor W28
- W.28
What it means
Thermor links error 28 or Err W28 to a malfunction of the defrost cycle of the heat pump in its water heaters. It gives four possible origins: an installation or setting that does not follow the manual, lack of maintenance, lack of refrigerant gas or the fan. Thermor says that diagnosing the fan and the gas charge requires a professional, an installer or a refrigeration technician. The code does not distinguish between these causes.
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: Mon chauffe-eau thermodynamique affiche une erreur 28 ou Err W28, que faire ?
- 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: Mon chauffe-eau thermodynamique affiche une erreur 28 ou Err W28, que faire ?
Symptoms and causes
Probable causes · in order
- Installation or settings outside the manualPossible
Thermor lists it first. It is checked by comparing the installation and parameters with the model's manual.
Evidence: Mon chauffe-eau thermodynamique affiche une erreur 28 ou Err W28, que faire ?
- Lack of maintenancePossible
A dirty evaporator or air inlets make defrosting harder. Thermor lists it as a cause.
Evidence: Mon chauffe-eau thermodynamique affiche une erreur 28 ou Err W28, que faire ?
- Lack of refrigerant or the fanPossible
These are the two causes Thermor leaves to a professional, because they require measuring the charge and testing the fan.
Evidence: Mon chauffe-eau thermodynamique affiche une erreur 28 ou Err W28, que faire ?
Virtual technician
Does W28 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 thermodynamique affiche une erreur 28 ou Err W28, que faire ?
Thermor · 2026 · official_doc
- 2
Mon chauffe-eau affiche une erreur, que faire ?
Thermor · 2025 · official_doc
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