A combi boiler produces hot water instantly on demand. Unlike traditional systems, there is no hot water cylinder storing water — everything happens internally within the boiler itself.
Hot water faults on combi boilers should always be properly diagnosed on site, as there are several different components that can cause similar symptoms.
In most cases, the main hot water-related components to consider are:
Any combi boiler will have a means of detecting hot water demand, commonly through a flow sensor and flow turbine arrangement or similar flow-sensing device.
There is also one less common but important external cause that should not be overlooked:
Inside every combi boiler is a device that detects when you open a hot tap. This is called the flow sensor. When you open the tap:
If this sensor fails, the boiler will not realise you are asking for hot water, so the boiler may not fire at all.
Typical symptoms
Important note
The part itself is called the flow sensor.
However, when replacing the flow sensor, the flow turbine and the seals within the flow turbine adapter can sometimes be worn, brittle, or become compromised during removal. In some cases they can partially disintegrate when disturbed.
That is why engineers will often mention or replace the flow turbine adapter assembly at the same time, to ensure the repair is reliable and to reduce the risk of further hot water faults shortly afterwards.
The plate heat exchanger is the component that transfers heat from the boiler to the water coming out of your taps.
Over time, this part can become restricted by scale or debris, especially in areas with harder water.
Typical symptoms
If the boiler fires normally but the hot water is unstable, weak, or never gets properly hot, the plate heat exchanger is one of the most common causes. Click here to learn more about Plate Heat exchanger symptoms.
The boiler uses a hot water temperature sensor, often called the hot water NTC, to measure how hot the water is as it leaves the boiler.
If this sensor becomes faulty, the boiler may think the water is already hot when it is actually not.
For example, the boiler might believe the water is 55°C, when it is really only 35°C.
Typical symptoms
If the sensor is reading incorrectly, the boiler may reduce the flame too soon or stop heating before the water has actually reached the proper temperature.
The diverter valve directs heat either to your:
If this valve sticks, wears out, or lets by internally, heat can go to the wrong place.
Typical symptoms
A good diagnostic clue is when the radiators or heating pipework start getting warm while someone is only using hot water. That usually points towards a diverter valve problem.
Sometimes the problem is not the boiler at all.
A faulty mixer tap or thermostatic valve elsewhere in the house can allow cold water to push back into the hot water pipework. This is known as crossflow.
This is not the most common cause, but it is unusual and important enough that it should still be considered when diagnosing hot water complaints.
Typical symptoms
In this situation, the boiler may be producing hot water correctly, but the hot supply is being diluted elsewhere in the property by cold water crossing over through a faulty valve or tap.
On most combi boilers, the hot water temperature is usually set somewhere around 50°C to 55°C.
The boiler uses the hot water NTC sensor to monitor the temperature of the outgoing water.
When the water temperature rises above the target point, the boiler will reduce the flame or stop heating. When the temperature drops again, it will increase output.
If the sensor is faulty, or if heat transfer through the plate heat exchanger is poor, this can cause:
The most common causes of hot water faults in combi boilers include:
Any one of these can stop hot water completely or cause unstable temperatures.
If your hot water is not working but the heating still operates, the problem is often related to one of the hot water-specific parts rather than the entire boiler.
Common symptoms include:
These symptoms often point to a fault with the:
Hot water that goes from hot to warm to cold and keeps fluctuating is a classic sign of a blocked or restricted plate heat exchanger. This usually becomes worse gradually over time.
However, the same symptom can also be caused by:
That is why it is important not to assume the fault without properly checking the boiler and the wider hot water system.
Because several different components can cause similar symptoms, it is important that a qualified engineer checks the system properly before replacing parts.
During a diagnostic visit, an engineer will typically check:
Once these checks are completed, the correct repair can be identified.
A proper diagnosis helps avoid:
The main parts that control hot water in a combi boiler are the flow sensor, diverter valve, plate heat exchanger, and hot water temperature sensor. In some cases, plumbing crossflow elsewhere in the property can also affect hot water performance.
The most common fault points are the flow sensor, diverter motor, plate heat exchanger, and hot water NTC sensor. Less commonly, a faulty mixer tap or thermostatic valve can also cause hot water issues.
This is often caused by a faulty flow sensor not detecting water movement. It can also happen if the hot water temperature sensor is reading incorrectly and the boiler falsely believes the target temperature has already been reached.
This is commonly caused by a restricted plate heat exchanger, a faulty hot water NTC sensor, diverter valve problems, or sometimes plumbing crossflow from a mixer tap.
Yes. A faulty mixer tap or thermostatic valve can allow cold water to cross into the hot water pipework, making the hot water run lukewarm even if the boiler itself is working properly.
20th August, 2026
18th March, 2026
