Heat pump not making hot water
A heat pump heats a cylinder rather than heating water on demand, so the questions are how much is in it, how warm it is set and what refilled it.
What it usually means
The first thing to establish is that a heat pump system has a tank. Unlike a combi boiler, which heats water as it passes through, a heat pump warms a cylinder of stored water using a coil inside it, then leaves it alone. That single difference changes every symptom in this category. Running out of hot water is a quantity problem: you have used more than the cylinder holds, and reheating it takes an hour or more rather than being instant. Water that is present but never hot is a temperature problem, and it is generally about the target the system has been given.
That target is usually lower than people expect, and for a good reason. Heat pump efficiency falls as the water gets hotter, so cylinders are typically set to around forty-eight or fifty degrees rather than the sixty a boiler might deliver. Water at fifty degrees is perfectly hot for a bath, but if the target has drifted down, or a schedule only reheats once overnight, the tank runs out in a way the household notices and reads as a fault. The other consequence is that most systems run a periodic high-temperature cycle for legionella control, using an immersion heater to reach a temperature the heat pump cannot manage efficiently.
Which brings up the fault that costs money rather than comfort. That immersion heater is the backup for the whole hot water function, and if a control setting or a failed sensor hands the job over to it permanently, the taps stay hot and the electricity bill climbs sharply. The reverse case exists too: an immersion running when it should not can push a cylinder over its limit and trip a high-temperature safety cut-out, which manufacturers publish as its own fault. Hot water that is fine but expensive, and hot water that has stopped after being unusually hot, are two ends of the same story.
If there is no hot water at all, work outward from the cylinder. The heat pump has to be able to send hot water to the coil, so a diverter valve stuck in the heating position, a circulating pump not running, air trapped at the top of the primary circuit, or low system pressure will all leave the cylinder cold with the heating apparently working. A cylinder that heats but loses its heat within a few hours is a different diagnosis: either the coil is fouled, the thermostat is misplaced, or hot water is escaping continuously up a poorly configured circuit.
Lastly, distinguish flow rate from temperature. A heat pump system generally serves an unvented cylinder, and complaints of a weak, cooling shower are often about the mains pressure and the pressure-reducing valve or filter feeding it rather than about the heat pump at all. Fill a bath tap and see how hot it gets in isolation, because that separates a system that is not making hot water from a system that cannot deliver it fast enough.
How to narrow it down
Work through these in order.
Is there no hot water at all, or does it run out sooner than it used to?
These are separate faults. None at all points at the primary side — a valve, a pump, air or pressure — or at a schedule that is not calling for water heating. Running out points at capacity and target temperature, which are settings rather than parts, and a cylinder reheats over an hour rather than instantly.
What temperature is the cylinder set to, and what does the controller say it is?
Compare the two on the controller. Heat pump cylinders normally run around forty-eight to fifty degrees, which is genuinely hot but leaves less margin than a boiler-fed tank. A target that has drifted down, or an actual reading that never approaches the target, tells you immediately which half of the system to look at.
When is water heating scheduled?
Look at the timer rather than the temperature. One overnight reheat is common and works well for a household whose use is predictable, and badly for one that baths children in the evening. Adding a second reheat in the afternoon usually solves the complaint outright and is cheaper than most people assume.
Has your electricity use jumped?
If it has and the water is hot, suspect the immersion heater doing the work instead of the heat pump. It is the most expensive way to heat a cylinder, it happens silently, and it is normally a control setting or a failed sensor rather than a broken component. That makes it worth chasing even though nothing appears wrong.
Is the heating working normally?
Heating that works with no hot water points at the parts of the system that are specific to the cylinder: the diverter valve, the coil, the cylinder sensor, or the schedule. Neither heating nor hot water points further back, at the heat pump, the primary circuit or the system pressure — and a pressure fault is the most likely of those.
Was the water unusually hot before it stopped?
That sequence matters. A cylinder taken above its limit will trip a high-temperature cut-out, and the usual cause is an immersion heater running when it should not have been. If the water was scalding last week and cold this week, say so when you call: it points at a specific and well-documented fault rather than a general one.
Does a bath tap run hot on its own?
Try it with everything else off. If a bath tap fills hot but a shower runs warm and weak, the cylinder is fine and the problem is delivery — mains pressure, a filter, or the pressure-reducing valve on an unvented system. That is a plumbing job and a different trade from the heat pump.
How old is the installation, and has it ever been satisfactory?
A system that has never quite delivered enough hot water may have a cylinder sized for a smaller household or a coil designed for a higher flow temperature than the heat pump provides. That is a design conversation with the installer rather than a repair, and it is best had with a fortnight of observations written down.
Codes commonly shown with this
- F.22VaillantWater pressure too low. Vaillant publish exact thresholds and two instructions: the indoor unit manual says inform your competent person; Vaillant UK’s fault page lets a competent owner top up to 1 to 1.5 bar.Get it checked
- L4Mitsubishi ElectricThe cylinder has overheated. Often the immersion heater running when it should not — which is also the fault that quietly doubles your electricity bill.Get it checked
- 7HDaikinA 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.Get it checked
- U4DaikinIndoor/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.Get it checked
- Bill has jumpedDaikinThe 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.Get it checked
- Water at the tap is too coldDaikinDaikin give two causes and a remedy that costs money. Before using it, check what your schedule is actually asking the cylinder to do.Get it checked
When it is urgent
Take care
What it costs
Typical repair cost
£0–£450
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Get an engineer to look at this
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.
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.