Daikin Altherma hot water faults: two duties competing for one machine
A Daikin Altherma heats the house and the cylinder with the same heat pump. Learn how that sharing produces complaints that look like faults.
What this means
An Altherma serves space heating and the hot water cylinder from one heat pump, and it cannot do both at once. Priority settings decide which duty wins, so heating that pauses during a cylinder reheat, or hot water that takes longer than a boiler did, is the system working as designed rather than reporting a fault.
Equipment and model context
Daikin Altherma air-to-water heat pump systems serving space heating and a domestic hot water cylinder
- Altherma 3 H HT
- Altherma 3 R
- Altherma 3 M
- Altherma integrated cylinder units
Altherma covers split, monobloc, and integrated cylinder configurations across several generations, with different controllers and different parameter names. The installation and operation manual for your configuration governs, and hot water behaviour in particular depends on commissioning choices as much as on the hardware.
What you may notice
- Space heating pauses for a period and then resumes, with no fault displayed.
- Hot water takes far longer to reheat than the previous boiler and cylinder arrangement did.
- The immersion element engages during hot water production and electricity consumption rises.
Likely causes
- Hot water priority suspends space heating while the cylinder is reheated, which is a configured behaviour rather than a fault.
- A heat pump reheats a cylinder at a lower temperature difference than a boiler, so the same cylinder takes longer.
- Legionella or disinfection cycles raise cylinder temperature beyond what the heat pump alone can deliver, bringing in the immersion element.
Diagnostic paths
Heating pauses and then comes back
Radiators or underfloor circuits go cool for a period, then warm again, without anything being displayed on the controller.
Check whether the pause coincides with hot water production. Priority switching between duties is designed behaviour and accounts for most of these reports.
The immersion runs during hot water heating
An electric element engages while the cylinder is being reheated, rather than the heat pump doing the work alone.
Ask what cylinder temperature is set and whether a disinfection cycle is scheduled. Both decide how much of the reheat sits above what the heat pump can deliver.
Hot water recovery is slower than the old boiler
The cylinder takes noticeably longer to come back up to temperature than it did before the heat pump was installed.
Note how long a full reheat takes and at what cylinder temperature. A heat pump works across a narrower temperature difference, so slower recovery is expected rather than faulty.
The heat pump produces a single flow of hot water and a diverting arrangement sends it either to the emitters or to the cylinder coil. Nothing splits it, so the controller alternates rather than sharing.
- Heat pump outputOne flow at one temperature
- Diverting valveSends it to heating or to the cylinder
- Priority ruleDecides which duty wins and for how long
- The other dutyWaits until the first releases the machine
| Aspect | Boiler and cylinder | Heat pump and cylinder |
|---|---|---|
| Flow temperature to the coil | High | Lower, limited by the heat pump |
| Reheat time | Shorter | Longer for the same cylinder |
| High cylinder temperatures | Reached by the boiler | May need the immersion element |
| Effect on heating | Usually independent | Suspended while the cylinder reheats |
Designed behaviour that reads as a breakdown
Households arriving from a gas boiler bring expectations that a heat pump cannot meet in the same way. A combi boiler produced hot water instantly and heated the house at the same time. A heat pump with a cylinder does neither instantly nor simultaneously, and the pauses are structural rather than symptomatic.
Knowing that in advance prevents a run of service visits that find nothing. It also makes the genuinely faulty case easier to spot, because a pause that coincides with hot water production is normal while one that does not is worth investigating.
The immersion element is not a failure
Cylinder temperatures above what the heat pump can deliver are reached by an electric element, and disinfection cycles deliberately go there. Seeing that element run is therefore expected at certain times rather than evidence that the heat pump is underperforming, though it does make the cost of those cycles visible.
What is worth questioning is how often it runs and why. An element engaging during ordinary reheats, rather than only during scheduled disinfection, suggests a cylinder temperature set higher than the installation needs. Lowering a cylinder set point by a few degrees can move a large share of the reheat back onto the heat pump, and the saving compounds across a year. For the underlying decisions, the hot water cylinder sizing guide covers how a cylinder and its coil are matched to a heat pump rather than to a boiler, and the Altherma flow control page covers how the leaving water temperature is arrived at in the first place.
What to give the technician
Give the installer the Altherma configuration, whether heating pauses coincide with hot water, the cylinder temperature setting, the reheat time you measured, and the disinfection schedule.
Safe homeowner checks
- Note whether space heating pauses coincide with hot water production, which distinguishes priority switching from a fault.
- Time a full cylinder reheat and record the cylinder temperature setting alongside it.
- Check whether a disinfection cycle is scheduled and at what time, since that is when the element is expected to run.
Is a reset appropriate?
Priority switching and disinfection cycles are scheduled behaviour with nothing to reset. Confirm that a genuine fault code is displayed before treating the pattern as a failure.
When to call a technician
- Cylinder temperature settings, priority parameters, and disinfection schedules affect both safety and running cost, and the installer who commissioned the system should change them.
- Anything on the sealed refrigerant side of an Altherma needs a certified handler, regardless of whether the symptom appears on the hot water side.
Questions people ask about this
Why does my Altherma stop heating when the hot water runs?
The heat pump produces one flow of hot water and a diverting arrangement sends it to either the emitters or the cylinder. Priority settings decide which duty wins, and the other waits rather than sharing.
Is the immersion element supposed to run?
It reaches cylinder temperatures above what the heat pump can deliver, including during scheduled disinfection cycles. Running at those times is expected; running during every ordinary reheat is worth questioning.
Why is hot water recovery slower than my old boiler?
A heat pump delivers a lower flow temperature to the cylinder coil, so the same cylinder takes longer to come back up. The behaviour reflects the technology rather than a fault in the installation.
Evidence recordHow this page was checked
official manufacturer support article, operation manual · checked 2026-09-15
Every technical claim above was written from primary documentation held in the HVAC Bench evidence record: Daikin UK and Daikin technical literature. Where a source limits a definition to certain models, test conditions, or product classes, that limit is repeated here rather than generalised.
- Documentation class
- official manufacturer support article, operation manual
- Scope of the definition
- Daikin Altherma air-to-water heat pump systems serving space heating and a domestic hot water cylinder
- Last checked
- 2026-09-15