- Boilers
- Electric water heater
W15
Thermor W15 water heater code: date and time not set or lost
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
Medium
Quick answer
On a Thermor heat pump water heater, W15 or error 15 means the date and time are not set or have been lost. Enter them again in the Settings menu; if they are wiped every day, an electrician should check how the unit is wired to the off-peak supply.
Safety first
Stop and check this first
- Checking the off-peak contactor and the water heater wiring means working at the consumer unit on mains voltage: leave it to an electrician and do not open the panel or the unit's electrical cover yourself.
- On the PAC Prog/Elec Prog schedule, W15 can leave the home without hot water; set the clock promptly or temporarily choose a schedule that does not rely on programming.
Safe checks you can perform
- Set the date and time again
- Think back to any power cut
- Check the operating schedule
- Watch whether it returns daily
Leave these tasks to a qualified professional
- Do not attempt this professional task: Check the wiring against the tariff
- Do not attempt this professional task: Replace the digital control if it persists
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.
Check the wiring against the tariff
What they doAn electrician checks at the consumer unit that the water heater's permanent supply does not pass through the off-peak contactor and that only the off-peak signal cable goes to it.
They must show youwhich breaker and which contactor each water heater cable goes to.
In writingthe correction made at the consumer unit.
Replace the digital control if it persists
What they doIf the wiring is correct and the clock is still wiped, the installer fits the replacement control listed for that model.
They must show youthat the permanent supply stays on and the time is still lost.
In writingthe part number of the control 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.
Set the date and time again
Open the Settings menu on the control as your model's manual describes and enter the current date and time. Thermor says this is how W15 is acknowledged; when you finish, check that the message has gone from the display.
Evidence: Comment résoudre un problème de chauffe-eau qui affiche une erreur 15 ou W15 ?
Think back to any power cut
Recall whether there was a blackout, a storm or work on the consumer unit in the last few hours. A cut of over five minutes is enough to wipe the clock, so in that case W15 is expected and does not mean a fault.
Evidence: Comment résoudre un problème de chauffe-eau qui affiche une erreur 15 ou W15 ?
Check the operating schedule
See which schedule the control is set to. On PAC Prog/Elec Prog the message can stop hot water production until the clock is set; on the 24h or HC schedules the unit keeps heating even with W15 showing.
Evidence: Comment résoudre un problème de chauffe-eau qui affiche une erreur 15 ou W15 ?
Watch whether it returns daily
After setting the clock, watch for a few days whether W15 comes back right when off-peak hours end. If so, note it and do not touch the consumer unit: it points to a wiring mistake an electrician must correct.
Evidence: Comment résoudre un problème de chauffe-eau qui affiche une erreur 15 ou W15 ?
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
- 15
- Aéromax W15
- Err 15
- Erreur 15
- Info 15
- Thermor W15
- W15
What it means
Thermor treats W15 as information rather than a fault. It appears on the Aéromax 5, Aéromax VMC 4 and Aéromax Split 2 Inverter and Split 3 when the digital control has no date or time, either because they were never set at first start-up or because they were lost. Electric tanks without a heat pump do not show it. Its effect depends on the operating schedule: with PAC 24h/Elec 24h, PAC 24h/Elec HC or PAC HC/Elec HC heating continues, whereas with PAC Prog/Elec Prog you can end up short of hot water or with none. The message clears once the date and time are entered again.
Warnings and stop conditions
- Higheditorial warning
Checking the off-peak contactor and the water heater wiring means working at the consumer unit on mains voltage: leave it to an electrician and do not open the panel or the unit's electrical cover yourself.
Evidence: Comment résoudre un problème de chauffe-eau qui affiche une erreur 15 ou W15 ?
- Mediumsource verified
On the PAC Prog/Elec Prog schedule, W15 can leave the home without hot water; set the clock promptly or temporarily choose a schedule that does not rely on programming.
Evidence: Comment résoudre un problème de chauffe-eau qui affiche une erreur 15 ou W15 ?
Symptoms and causes
Probable causes · in order
- Date and time skipped at first start-upCommon
At first power-up the control opens a guided menu that includes date and time; if that step is skipped, W15 is shown on the display.
Evidence: Comment résoudre un problème de chauffe-eau qui affiche une erreur 15 ou W15 ?
- Power cut longer than 5 minutesCommon
Thermor states that an electricity cut of more than five minutes makes the water heater lose the date and time it was given.
Evidence: Comment résoudre un problème de chauffe-eau qui affiche une erreur 15 ou W15 ?
- Permanent supply wired to the off-peak contactorPossible
If the cable meant to be live at all times goes to the off-peak contactor, every switch to peak hours leaves the unit unpowered and wipes the clock again.
Evidence: Comment résoudre un problème de chauffe-eau qui affiche une erreur 15 ou W15 ?
- Faulty digital controlLess common
Once all the points above have been checked and corrected and W15 stays or keeps returning, Thermor suggests having the installer replace the digital control.
Evidence: Comment résoudre un problème de chauffe-eau qui affiche une erreur 15 ou W15 ?
Virtual technician
Does W15 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 15 ou W15 ?
Thermor · 2026 · official_doc
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