Combi Boiler Not Heating Water: Causes & Fixes

If your radiators are nice and warm but your combi boiler isn’t heating the hot water properly, it often means a small but crucial part isn’t working as it should. Don’t worry, many common issues can be quickly identified with a few simple, safe checks you can do yourself at home. Running through these steps helps you understand the problem better and gives our engineer a head start when they arrive. This way, we can fix your hot water faster and get your system back to full strength without unnecessary delays.

Start here: heating works, hot water does not

With a combi boiler, central heating and hot water share the same appliance but use different controls and components. If the heating is fine yet hot water is intermittent, lukewarm or cutting out, the fault is almost always on the hot water side only.

Before you assume the boiler is “finished”, it is worth checking simple settings and conditions that can easily be put right. Think of it like a decision tree: rule out the basics at the tap, then move towards boiler pressure and internal parts.

Quick checks at the tap or shower

First, confirm the behaviour is the same on more than one outlet. Try a kitchen tap and a bathroom tap or shower, one at a time, on full hot flow.

• If all outlets are affected, it is likely a boiler or supply issue.
• If only one shower or tap plays up, you may have a mixer valve or tap cartridge problem rather than a boiler fault.
• If hot water goes cold when two outlets are used together, the boiler may be undersized or restricted by scale.

Also check that any thermostatic shower is not set too low and that the cold feed isolating valves under the bath or basin are fully open.

Boiler modes and temperature settings

A surprising number of “no hot water” callouts are actually caused by incorrect boiler settings. Most modern combis have separate controls for heating and hot water, plus seasonal or eco modes.

Summer / winter and hot water priority modes

On some boilers, summer mode disables the heating while leaving hot water active. If yours has been left in a different mode, it may affect performance. Check the front panel buttons for any “eco”, “comfort”, “summer” or tap symbols and refer to the user instructions if you are unsure.

Comfort or preheat settings keep a small amount of water warm inside the boiler for quicker hot water at the tap. If this has been turned off, you might have a longer delay, but it should not make the water permanently cold. If you are waiting ages for hot water, this is worth adjusting.

Thermostat vs hot water temperature control

Your room thermostat only controls the radiators. It has nothing to do with how hot the taps run. For hot water, look for the temperature dial or digital setting on the boiler with a tap symbol next to it.

Set the hot water temperature to a sensible level, typically around 50 to 55 °C. If it is turned right down, you may only get lukewarm water, especially in winter when the incoming cold mains is much colder.

System pressure, filling loop and resets

Low system pressure is a common cause of combi boilers misbehaving. Although it relates more directly to the heating side, some boilers will reduce or stop hot water output if pressure is too low.

Checking pressure and common filling loop mistakes

Look at the pressure gauge on the front of the boiler or nearby pipework. When the system is cold, it should usually be around 1.0 to 1.5 bar. If it is below 1 bar or sitting on zero, that needs attention.

If you know how to use the filling loop, you can carefully top the pressure up to the correct level according to your manual. Typical mistakes include leaving the filling loop taps slightly open, which causes pressure to climb too high, or opening the wrong valve and introducing lots of air. If you are unsure, it is safer to wait for an engineer.

When a reset is appropriate

If the boiler is locked out with a fault code but you cannot smell gas and there are no obvious leaks, one reset following the user manual is usually acceptable. Observe the boiler during restart to see whether it attempts to fire when you open a hot tap.

If the same fault code appears again, or the boiler keeps cutting out, do not keep resetting it. Repeated resets point to a genuine fault that needs proper diagnosis.

Key internal parts that affect hot water

If the controls and pressure seem fine, the issue often lies with a small internal component. You should not remove the boiler case yourself; these checks are for an engineer, but understanding them helps you know what is being discussed.

Diverter valve not sending heat to hot water

The diverter valve is the part that decides whether the boiler’s heat goes to the radiators or to your hot water. When it sticks or fails, typical symptoms include warm radiators when a hot tap is running, or tepid water even though the boiler appears to fire.

If your combi keeps losing hot water, it is usually a part that needs diagnosing quickly. Here is what we check on arrival: diverter valve movement, response to hot tap demand, and whether the boiler output temperature matches what comes out of the tap. If there is a mismatch, a diverter issue is high on the list.

Plate heat exchanger blocked with scale

The plate heat exchanger transfers heat from the boiler water to your tap water. Limescale or sludge can clog the narrow waterways, especially in hard water areas. The result is often good hot water for a minute or two, then it fades to lukewarm as the boiler struggles to pass heat through.

Engineers may clean or replace the plate heat exchanger and sometimes recommend scale protection such as a magnetic filter or limescale reducer on the incoming cold main.

Flow sensor and hot water sensors

A flow sensor tells the boiler that a tap has been opened and how fast the water is moving. If it sticks or fails, the boiler may not recognise low flow, or may cut the flame in and out, giving you that “hot then cold” effect in the shower.

Temperature sensors around the hot water circuit also feed information to the control board. Faulty readings can make the boiler think the water is hotter than it is, so it reduces power and leaves you with lukewarm taps.

Fault codes and when to call an engineer

Most modern combi boilers will show a fault code on the display when something is wrong. These codes usually point to a category of fault, such as ignition, fan, temperature or low pressure, rather than one specific part.

If a code appears, check your boiler manual for its meaning and any user-level steps allowed, such as topping up pressure or performing a single reset. If the code relates to gas, combustion or internal sensors, or if it reappears, you will need a Gas Safe registered engineer to investigate properly.

Very important: if you smell gas at any point, do not attempt resets or further checks. Turn off the gas at the meter if you can do so safely, open windows, avoid using switches or naked flames, and call the national gas emergency number immediately.

Frequently asked questions

Why does hot water run cold after 2 minutes?

This is often a sign of a partially blocked plate heat exchanger, a failing sensor, or a boiler that is modulating the flame down too far. The boiler initially heats well, but as restrictions or incorrect readings build up, it cannot maintain temperature and the water cools.

It can also happen if the hot water temperature is set too low, particularly in winter, or if there is a problem with a thermostatic mixer blending in too much cold. An engineer can measure temperatures at different points to pinpoint the restriction.

Why is the shower temperature fluctuating?

Temperature swings in the shower are usually caused by a combination of boiler modulation, water flow changes and the shower’s own mixer valve. If other taps are being used at the same time, the flow rate to the shower can vary, which confuses both the boiler and the mixer.

A worn thermostatic cartridge in the shower can also react badly to small pressure changes, causing you to feel sudden hot and cold bursts. If other outlets are stable but the shower is not, try servicing or replacing the shower valve.

Can low pressure cause no hot water?

Low system pressure is more closely linked to heating issues, but some combi boilers will refuse to run at all if the pressure is below a safe threshold. In that case, you may lose both heating and hot water together.

If you still have heating but hot water is intermittent, low pressure is less likely to be the only cause, although it is still worth checking and correcting. Ongoing pressure loss points to a leak or failing component that should be checked by an engineer.

Next steps and professional help

If you have checked controls, temperature settings and pressure, and your combi boiler is still not heating water correctly, the next step is a professional boiler repair. Parts such as diverter valves, plate heat exchangers and sensors require proper testing equipment and must be worked on with the boiler case safely in place.

Gas Wizard Heating Services Limited can carry out fast, engineering-led diagnosis and boiler repair to get your hot water stable again. To book a visit or discuss your symptoms, call Gas Wizard Heating Services Limited on 07890242485 or enquire through our Boiler Repair page and local location pages.