Daikin AH fault codewhat it means and how to fix it
The tank disinfection cycle did not complete: the setpoint was not reached within six hours or not held long enough. The unit keeps running, and Daikin’s first cause is hot water drawn mid-cycle.
Take care
What AH means
AH is Daikin’s legionella code, and it is a warning rather than a stop. The service manual gives the trigger as the disinfection setpoint not being reached within six hours or not being kept for the required time; the unit will not stop operating, and the code resets itself when a disinfection completes. The disinfection function is the Altherma’s version of the anti-legionella cycle every heat pump cylinder runs — periodically heating the tank well above its everyday setpoint, usually with the backup or booster heater doing the last part, so that bacteria which multiply at lower temperatures are killed. AH means it tried and did not get there.
Daikin’s published causes are mostly about timing. The disinfection was interrupted by domestic hot water tapping, so their advice is to programme it for a time when no hot water is expected for the coming four hours. A large draw of hot water shortly before the programmed start had the same effect, and here Daikin get specific: with the tank in Reheat only or Schedule and reheat mode, start the disinfection at least four hours after the last large tapping, and in Schedule only mode programme an Eco action three hours before it to preheat the tank. Or the cycle was stopped manually — tank operation switched off during disinfection, which Daikin say not to do.
The service manual adds the settings that stop the heater doing its part: the backup heater restricted during disinfection or not allowed at all, and the booster heater settings. So the everyday picture is a cycle scheduled for early evening in a house that showers then, or a backup heater that has been limited to save money and can no longer lift the tank the final few degrees. Neither is a broken part, but a cycle that fails every time is a cylinder that is not being disinfected, and that is the reason to fix it.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Hot water drawn during or just before the cycle | Daikin’s first two causes. A shower mid-cycle or a bath shortly before the start leaves the tank unable to reach and hold the setpoint in time. | Common |
| Backup heater restricted or not allowed | Daikin’s service check on the backup heater settings. The heat pump alone may not reach the disinfection temperature, so a restricted heater means a cycle that never finishes. | Common |
| Cycle stopped manually | Daikin’s third published cause: tank operation was turned off during disinfection. Do not stop tank operation while it is running. | Occasional |
| Booster heater settings or a genuine heater fault | The last item on Daikin’s list, and the point at which an engineer checks the heater actually works rather than whether it is allowed to. | Occasional |
What to try yourself
Find out when the disinfection is scheduled. If it falls in a period when the household uses hot water, that is Daikin’s first cause, and moving it — normally to the small hours — is the fix.
Do not switch the tank off while a disinfection is running. Daikin publish this as a cause in its own right.
If AH appears every time regardless of timing, ask your installer whether the backup heater has been restricted or disallowed during disinfection. It is a settings check, not a repair, but it is their menu.
If the settings are right and the cycle still fails, the backup or booster heater may have a fault of its own. Book the installer and say the disinfection is not completing.
Get an F-Gas certified heat pump engineer to look at your Daikin AH
We do not pass enquiries to a repair firm, so there is no form here. When you book an F-Gas certified heat pump engineer, tell them the make, the model and the exact code or message on the display, and ask whether they carry parts for this make — that is what turns one visit into a repair rather than a quote.
What it costs to fix
Typical repair cost
£0–£250
Time on site
15 minutes
Nothing if it is the schedule. An installer visit to review the backup heater and disinfection settings is typically £90 to £180; a heater fault is more.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Daikin fault codes
- Water at the tap is too coldGet it checkedDaikin give two causes and a remedy that costs money. Before using it, check what your schedule is actually asking the cylinder to do.Related: hot water and cylinder
- Bill has jumpedGet it checkedThe most important heat pump fault has no code. If your electricity cost has stepped up with no change in habits, the backup heater is probably doing the work.Related: same brand
- Ice on the outdoor unitGet it checkedNormal in cold damp weather, and the unit clears it itself. Daikin specifically forbid helping — no hot water, no de-icer, no scraping.Related: same brand
- 7HGet it checkedA water flow problem. The sub-code matters more than the code: 7H-01 clears itself, 7H-04 to 7H-06 need a manual reset, 7H-08 is the pump, and 7H-07 is the unit freeing a stuck pump.Related: same brand
- U0Get it checkedShortage of refrigerant in the Altherma outdoor unit. Daikin’s own checklist ends with a leak test, the circuit is sealed, and on an R32 unit this is F-Gas certified work by law.Related: same brand
- U4Get it checkedIndoor/outdoor unit communication problem. Daikin’s checks start with the power supply and the wiring between the two boards, and the published reset is a power reset at the outdoor unit.Related: same brand
Own a Daikin heat pump? Save it and its codes come first next time.
Related guides
Common questions about Daikin AH
Is AH urgent?
Not today, but do not leave it. The unit keeps running and your hot water is unaffected. What is failing is the periodic high-temperature cycle that keeps the cylinder safe, and a cycle that never completes means a cylinder that is never disinfected.
Why does Daikin want four hours without hot water?
Because disinfection needs the whole tank at temperature for a set time, and every litre drawn is replaced with cold. Daikin’s advice is to programme the start for a period when no tapping is expected for four hours, and in reheat modes to leave at least four hours after the last large draw.
Can I just turn the disinfection off?
It is an installer setting and it exists for a reason: legionella multiply at the temperatures a heat pump holds a cylinder at for efficiency. Fix the timing or the heater settings instead. If you want it off for a specific reason, that is a conversation with your installer.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.