If your central heating works but you have no hot water, the boiler can still produce heat, but something may be preventing that heat from reaching your taps or hot-water cylinder. The likely cause depends partly on whether you have a combi boiler or a system that stores hot water.

With a combi boiler, possible causes include a diverter valve, hot-water flow sensor or heat exchanger problem.

If you have a hot-water cylinder, start by checking the hot-water programmer and cylinder thermostat before considering faults with motorised valves or the immersion heater.

Do you have a combi boiler or a hot-water cylinder?

Identifying your heating system makes troubleshooting much easier.

A combi boiler heats mains water when you open a hot tap, so there’s normally no separate hot-water cylinder. A valve inside the boiler directs heat towards the domestic hot-water supply when hot water is requested.

System and conventional boilers normally work with a hot-water cylinder, where heated water is stored until you need it. These systems can have separate controls and valves for central heating and hot water.

This means working radiators and cold taps can have different explanations depending on your system.

What should you check first?

Before assuming an internal component has failed, make a few basic checks:

• Try several hot taps to establish whether the problem affects the entire property or one outlet.
• Check the boiler for a fault code or warning symbol.
• Check that the hot-water temperature hasn’t accidentally been turned down.
• If you have a hot-water cylinder, make sure hot water is switched on at the programmer.
• Check the boiler pressure against the manufacturer’s specified range.
• With a combi, watch the boiler display while opening a hot tap and see whether the boiler responds.

Fault codes can be particularly useful because modern boilers may identify the type of problem they’ve detected.

If only one tap or shower is affected while other hot outlets work normally, the problem is more likely to be associated with that particular outlet rather than the boiler’s hot-water supply.

Combi boiler: could the diverter valve be faulty?

The diverter valve is an important possible cause when a combi provides central heating but no hot water.

When you open a hot tap, a combi needs to redirect heat towards the domestic hot-water supply. Once you close the tap, the valve returns to its central-heating position.

If the valve sticks or fails to move correctly, your radiators may continue working while the taps remain cold. 

A diverter valve can also cause the opposite symptom if heat isn’t being directed towards the radiators, resulting in hot water working but no central heating.

Another potential clue is radiators becoming warm when you’re only requesting hot water. That could indicate heat isn’t being directed where it’s supposed to go, although it isn’t enough to diagnose the diverter valve by itself.

Diverter-valve diagnosis and repair involves components inside the boiler and should be left to an appropriately qualified engineer.

What if the boiler doesn’t react when you open a hot tap?

A combi needs to detect that water is flowing before it knows you want hot water.

If you open a hot tap and the boiler doesn’t react at all, there could be a problem with the flow sensor or another component responsible for detecting hot-water demand.

Low water flow can also affect this process. The minimum flow required to initiate hot-water demand varies between boilers. One manufacturer specified approximately 2–3 L/min for some of their models.

That’s different from the boiler’s maximum hot-water output. Combi boiler flow rate determines how much hot water the boiler can potentially supply, while individual appliances may also require a minimum flow before they recognise a hot-water demand.

If several taps have unusually weak flow, the incoming water supply or a restriction may therefore be worth investigating before assuming the boiler itself has failed.

Could the plate heat exchanger cause no hot water?

A combi boiler normally uses a heat exchanger to transfer heat to the water you use at your taps.

If the domestic-hot-water heat exchanger becomes restricted by scale or debris, heat transfer can be reduced. Depending on the severity of the problem, you might experience water that is only lukewarm, fluctuates between hot and cold, or fails to reach the expected temperature.

The symptoms can overlap with sensor and diverter-valve problems, so the behaviour of the hot water alone isn’t enough to identify the faulty component.

Cleaning or replacing internal heat-exchanger components isn’t a homeowner repair.

Does boiler pressure affect hot water?

It’s worth checking the pressure gauge if you have a sealed heating system, particularly if the boiler is displaying a warning or fault code.

The correct pressure depends on the appliance, although many boilers operate at around 1–1.5 bar when cold. Always follow the manufacturer’s instructions rather than treating a generic figure as the required setting.

If you’re unsure what your boiler pressure should be, check it before attempting to repressurise the system.

However, because your central heating is already operating, low pressure isn’t necessarily the strongest explanation for an isolated loss of hot water. 

What if you have a hot-water cylinder?

If your radiators work but your cylinder isn’t producing hot water, troubleshooting is different from a combi.

First check the hot-water controls. Heating and hot water may have separate schedules, so central heating can operate even when hot water has inadvertently been switched off.

Check that:

• hot water is enabled at the programmer;
• the schedule covers the period when you expect the cylinder to heat;
• the cylinder thermostat hasn’t been turned down; and
• there hasn’t been a power interruption that reset the programmer.

If the settings are correct but the cylinder remains cold, a control, valve or other component may require investigation.

Could a motorised valve be faulty?

Many systems with hot-water cylinders use motorised valves to control where heated water circulates.

Depending on the design, these valves allow heat to travel towards the radiators, the cylinder coil or both. A fault can therefore allow the central heating to continue operating while preventing the cylinder from receiving heat.

This is effectively the cylinder-system equivalent of asking whether heat is being sent to the right place.

A faulty actuator, valve or associated electrical control requires proper diagnosis because several components can produce similar symptoms.

What about the immersion heater?

Many hot-water cylinders also contain an electric immersion heater. This heats the water electrically rather than using heat supplied by the boiler.

Depending on your setup, the immersion may be used as a backup or as another way of heating the cylinder.

If the boiler isn’t heating the cylinder but the immersion still works, you may temporarily be able to produce hot water electrically while the main fault is investigated. If the immersion itself doesn’t work, its electrical supply, thermostat or heating element may require attention.

Electrical faults should be handled by someone competent to perform the work rather than attempting to access the immersion heater yourself.

What can the symptoms tell you?

Looking at exactly what happens can help narrow down the possibilities:

What you notice	Possible direction to investigate
Heating works but every hot tap is cold	Hot-water controls or boiler/DHW fault
Combi doesn’t respond when hot tap opens	Flow/demand detection may need investigating
Combi responds but water stays cold	Diverter valve, heat exchanger or another DHW fault
Radiators warm when only hot water is requested	Diverter valve may not be switching correctly
Hot water is lukewarm or fluctuates	Heat transfer, sensor or flow problem
Only one tap or shower is affected	Problem may be local to that outlet
Cylinder remains cold but radiators heat	Hot-water controls, motorised valve or cylinder-related fault
Immersion heats cylinder but boiler doesn’t	Boiler-fed hot-water circuit may need investigation

These are diagnostic clues rather than confirmation that a particular component has failed.

When should you call an engineer?

If the simple controls and settings are correct but you still have no hot water, it’s usually time for professional diagnosis.

This is particularly important if the boiler doesn’t recognise hot-water demand, displays persistent fault codes, or you suspect a diverter valve, flow sensor, heat exchanger or another internal component.

Don’t remove the boiler casing to investigate internal components yourself. Gas work on domestic gas appliances must be carried out by a Gas Safe registered engineer who is competent for that type of work.

For a cylinder system, faults involving motorised valves, electrical controls or immersion heaters may require an appropriately qualified heating engineer or electrician depending on the problem.

Frequently asked questions

Why does my combi boiler run the heating but not the hot water?

Possible causes include a diverter-valve fault, a problem detecting hot-water flow or another fault on the domestic-hot-water side of the boiler. Check the boiler settings, pressure and fault display first.

Why doesn’t my boiler fire when I turn on a hot tap?

The boiler may not be detecting the flow of water. A flow-sensor fault is one possibility, although insufficient water flow or another control problem can produce similar symptoms.

Why is my hot water only lukewarm when the heating works?

Possible causes include restricted heat transfer, sensor problems or an issue with the flow of water through the boiler. The temperature and flow settings should also be checked before assuming a component has failed.

Can low water flow stop a combi producing hot water?

Potentially. Combi boilers require sufficient flow to detect a hot-water demand, and manufacturers specify minimum operating flow requirements for individual models. Low mains pressure can also reduce the hot-water flow available from a combi.

Can I fix no hot water myself?

You can check accessible settings, schedules, pressure and fault codes according to the manufacturer’s instructions. Don’t dismantle the boiler or attempt internal gas, electrical or component repairs yourself.

Sources

HSE, Gas safety – Home owners (https://www.hse.gov.uk/gas/domestic/faqownerocc.htm)

HSE, Gas safety check – who can do it? (https://www.hse.gov.uk/gas/landlords/safetycheckswhocan.htm)

Accurate as of: 22 September 2026