- Boilers
- Electric water heater
W7
Thermor W7 on a water heater: ACI Hybrid protection circuit fault
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
High
Quick answer
On Thermor water heaters with ACI Hybrid, W7, Err 07 or error 7 mean the protection loop between the board and the anode is broken and the heater stops heating. First check that the tank is full of water.
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.
- Do not power an empty heater. The ACI protection cuts out to avoid heating dry and damaging the element and heating body, according to Thermor.
Safe checks you can perform
- Tank full of water
- Type of water supplying the heater
- ACI plug, wire and board
Leave these tasks to a qualified professional
- Do not attempt this professional task: ACI Hybrid loop check
- Do not attempt this professional task: 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.
ACI Hybrid loop check
What they doThe technician checks water, ACI plug, wire and board in the order Thermor gives before replacing parts.
They must show youthe plug and wire checked, with photos.
In writingthe cause found and, if the heating body or board is replaced, why the rest was ruled out.
Test after the repair
What they doWith the tank full, the technician restores power and checks that the heater heats and W7 does not return.
They must show youthe heater heating with no code.
In writingthe date and the test result.
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.
Tank full of water
- Who:
- you.
- How:
- cut the heater's power at its switch or the consumer unit, open a hot tap and check that water flows steadily without air; check that the cold water inlet valve is open.
- Result that matters:
- Thermor says that if the tank is not full, filling it clears error 7 or W7.
- If air came out or no water flowed:
- run the tap until water flows without air, restore power and wait; on heat pump models the error can take a minute to clear.
- If the tank was already full:
- the ACI loop is broken for another reason; go on to the water type.
- Evidence to keep:
- a note of any water cut or drain-down and a photo of the code.
Evidence: Comment résoudre un problème de chauffe-eau qui affiche une erreur 7 ou W7 ?
Type of water supplying the heater
- Who:
- you.
- How:
- find out whether the heater gets mains, rain or borehole water, and whether you have a softener and what hardness it is set to.
- Result that matters:
- Thermor explains that mineral-free water does not conduct enough for the ACI protection and recommends not softening below 8 °TH.
- If you use rain or borehole water:
- mixing it with mains water clears error 7 according to Thermor; discuss it with your installer.
- If the softener is set below 8 °TH:
- have it adjusted.
- Evidence to keep:
- the hardness set on the softener or the water bill.
Evidence: Comment résoudre un problème de chauffe-eau qui affiche une erreur 7 ou W7 ?
ACI plug, wire and board
- Who:
- an installer or qualified technician.
- How:
- with power off, opens the cover, checks that the ACI plug contacts are fully inserted and that the ACI wire is not oxidised or cut; if all is fine, checks the board or thermostat.
- Result that matters:
- where the ACI loop is broken.
- If the plug was loose:
- reseating it clears the error and confirms the cause.
- If the ACI wire is damaged:
- Thermor says to replace the heating body.
- Evidence to keep:
- photos of the plug and wire and a job sheet.
Under the cover there is mains voltage. Opening it and handling the ACI plug or the heating body is technician work.
Evidence: Comment résoudre un problème de chauffe-eau qui affiche une erreur 7 ou W7 ?
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 07
- Err7
- Erreur 7
- Error 7 Thermor
- Thermor error 7
- Thermor W7
- W.7
What it means
Thermor defines error 7 or W7 as a fault in the ACI Hybrid protection system: a small voltage runs in a loop through the thermostat board or main board, the titanium anode, the water and the tank. If the loop breaks, the heater cuts heating instantly so it does not heat dry. Thermor gives as causes a tank without water, the ACI plug poorly inserted, water that is too soft, a damaged ACI wire or the board. The code does not say which.
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 7 ou W7 ?
- Highsource verified
Do not power an empty heater. The ACI protection cuts out to avoid heating dry and damaging the element and heating body, according to Thermor.
Evidence: Comment résoudre un problème de chauffe-eau qui affiche une erreur 7 ou W7 ?
Symptoms and causes
Probable causes · in order
- Tank not filledCommon
Thermor explains that without water the dry heating protection trips and that filling the tank clears the error; on heat pump models it can take a minute.
Evidence: Comment résoudre un problème de chauffe-eau qui affiche une erreur 7 ou W7 ?
- Water too softPossible
Rainwater, borehole water or over-softened water lacks enough conductivity. Thermor recommends not softening below 8 °TH and mixing with mains water.
Evidence: Comment résoudre un problème de chauffe-eau qui affiche une erreur 7 ou W7 ?
- ACI plug poorly inserted or damaged wirePossible
A loose connection at the plug or an ACI wire oxidised inside the sleeve breaks the loop. It is told apart by opening the cover, which is technician work.
Evidence: Comment résoudre un problème de chauffe-eau qui affiche une erreur 7 ou W7 ?
- Thermostat or main boardLess common
Thermor says to replace them only once everything above has been checked and corrected.
Evidence: Comment résoudre un problème de chauffe-eau qui affiche une erreur 7 ou W7 ?
Virtual technician
Does W7 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 7 ou W7 ?
Thermor · 2025 · official_doc
- 2
Mon chauffe-eau affiche une erreur, que faire ?
Thermor · 2025 · official_doc
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