Daikin Bill has jumpedwhat to check
The 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.
Take care
What this means
This is the most valuable thing in this category and it is not a code, which is precisely why nobody finds it. Daikin publish the mechanism in their own words: when the backup heater or booster heater takes over the heat load, electricity consumption will be considerably higher. What that means in practice is that a heat pump system can develop a real fault and give you no symptom at all — the house stays warm, the water stays hot, nothing displays, and the only evidence is that you are now heating your home with direct electric resistance at roughly three times the cost.
Daikin’s user reference guide even describes the setting that makes this silent: when Emergency is set to Automatic and a heat pump failure occurs, the backup heater automatically takes over the heat load and the booster heater in the tank takes over the hot water; set to Manual, heating and hot water stop until you confirm the takeover in the Malfunctioning menu. Owners are trained by boilers to treat cold as the alarm signal.
On a heat pump the alarm signal is financial. It is compounded by a second misunderstanding that runs the other way. A heat pump is designed to run long and low, so a household that has come from a gas boiler frequently concludes the new system is faulty because it never seems to switch off — and that one is usually nothing at all. The two beliefs together are exactly backwards: constant running is normal, and a warm house with a large bill is the thing to investigate. If you have a fault code as well, that narrows it. If you do not, it is still worth asking your installer to check whether the immersion or backup heater is being called on, and why.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| A fault has handed the load to the backup heater | Daikin publish the consequence explicitly. The house stays warm and the running cost rises sharply. | Common |
| Immersion heater left enabled | Switched on manually during a cold snap and never switched off. Very common and entirely undramatic. | Common |
| Weather compensation or flow temperature set too high | A commissioning setting rather than a fault. A higher flow temperature costs more for the same comfort. | Occasional |
| Legionella cycle running too often | Each cycle uses the immersion deliberately. Set too frequently, it adds up. | Occasional |
What to try yourself
Compare your electricity use against the same period last year rather than against last month. Heating varies with weather, so month-on-month tells you very little.
Check whether the immersion heater has been switched on manually. There is often a separate switch near the cylinder and it gets used and forgotten.
Look for any fault code, warning or maintenance message on the controller, including ones you have already cleared. Daikin distinguish errors from warnings, and a warning still means something.
Resist turning the setpoint up to compensate. Daikin publish the point directly: the space will not heat up faster, and a higher flow temperature costs more.
Ask your installer to check whether the backup or booster heater is being called on and why. Give them the consumption figures — that is the evidence.
Get an F-Gas certified heat pump engineer to look at your Daikin heat pump
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
£90–£250
Time on site
30 minutes
An investigation visit is typically £90 to £250. Set against a backup heater running unnoticed, which can add hundreds of pounds a quarter, it is usually the cheapest call you will make.
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
- 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
- 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: 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
- AHGet it checkedThe 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.Related: same brand
Own a Daikin heat pump? Save it and its codes come first next time.
Related guides
Common questions
My heat pump runs almost constantly. Is that the problem?
Almost certainly not. A heat pump is designed to run long and low rather than in short bursts like a boiler, and constant running at a modest output is how it is efficient. It is the bill rather than the runtime that tells you something is wrong.
My radiators are only warm, not hot. Is that related?
No, and turning it up will cost you money. Weather compensation deliberately runs radiators cooler in mild weather, and Daikin publish that a lower flow temperature means lower consumption and better performance. A radiator you can hold your hand on is normal heat pump behaviour.
There is no fault code at all. What do I tell the installer?
Tell them your consumption has stepped up, give them the figures, and ask them to check whether the backup or immersion heater is running and what is calling it. That is a specific question with a specific answer, and it is a much better starting point than "it seems expensive".
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 support portal
- 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.