No hot water but heating works: what to check

Start with the symptom, not a replacement part

People often describe this fault as a diverter valve problem. That is one possible explanation on some combi systems, but the description alone does not establish a diagnosis. A useful enquiry says what happens, where it happens and what type of equipment supplies the water.

Write down when the fault started and whether it is constant. Water that never warms, turns cold after a short period, or only warms while radiators are running gives the professional different information. Include recent maintenance and any changes to the controls, without assuming the previous work caused the fault.

If you have a combi boiler

A standard combi heats tap water on demand, without a separate domestic hot water cylinder. It needs to recognise water demand and deliver heat correctly. The hot water side can fail while the boiler continues to heat radiators; an internal valve, sensing problem or restricted heat transfer may be involved.

You can record the externally visible display or fault code during ordinary use, provided there is no safety concern. Note whether it changes when a hot tap is opened. Do not remove the casing, bridge a sensor or manipulate internal components. The observation helps an appropriately qualified engineer choose tests; it does not identify the failed part by itself.

If you have a separate cylinder

An indirect cylinder receives heat from a separate heat source through a coil. Radiators working shows that some heating function is available, but does not prove the cylinder is receiving heat. The hot water programme, controls or associated circulation need their own assessment.

Check the ordinary user-facing hot water schedule against the instructions, especially after a power interruption. Do not force motorised valves or alter wiring. If an immersion heater is fitted, tell the engineer whether it is normally used and what you have observed; do not remove its cover or repeatedly reset a safety cut-out.

A direct electric cylinder has a different heating arrangement. Some homes have electric space heating, so “the heating works” may say nothing about the cylinder supply. A photograph of equipment labels and the whole cupboard is often more useful than guessing the system name.

One shower, every tap, or no flow at all?

If there is hot water at a kitchen or basin tap but not at one shower, the local mixing control or outlet may require attention. A shower-specific problem should be described as such. If hot taps barely flow while cold taps work normally, report the lack of flow separately from the water temperature.

You do not need to test every outlet repeatedly. One or two ordinary observations, where safe, are enough to describe the pattern. Stop using an outlet that becomes unexpectedly scalding or varies dangerously, and keep children and anyone with reduced sensitivity away from it.

What you can safely record

Keep checks at the level of normal user controls and visible information. Compare settings with the correct model instructions rather than a generic online reset routine. If an appliance keeps locking out, repeated resets can obscure the pattern without resolving the fault.

• Boiler make, model and the exact displayed code.
• Whether a cylinder is present and its visible make or model.
• Whether water flows normally and which outlets are affected.
• Whether the fault is constant, intermittent or linked to a schedule.
• Any visible leak, unusual discharge or recent service history.

When to stop checking and seek help

Do not investigate further if water is near electrical equipment, the cylinder is steaming or hot water is discharging unusually. Keep clear and request urgent professional advice. Never block a relief discharge or bypass temperature controls in an attempt to restore hot water.

If you suspect a gas escape, a carbon monoxide alarm sounds or you suspect carbon monoxide, leave the property and call the National Gas Emergency Service on 0800 111 999 from outside in a safe place. Avoid flames and operating electrical switches. Seek urgent medical help for suspected exposure, and call 999 if anyone has severe symptoms. Do not wait for an online booking response.

What a repair visit should resolve

The assessment should identify whether the problem sits at the appliance, cylinder controls, distribution pipework or individual outlet. Ask for an explanation of the finding, the proposed repair and any parts or further testing required. A working radiator circuit is useful evidence but is not a complete test of the whole system.

Before authorising replacement equipment, ask why a repair is unsuitable and whether the proposed change resolves the actual fault. A new boiler would not correct a failed shower mixer; a new cylinder may not be necessary for a control fault. The answer should connect the proposed work to the diagnosis.

Getting help in London

This guide explains a symptom that occurs nationwide. Loyal Plumbers provides the corresponding hot water repair service across London and inside the M25, with 10 active vans and 24/7 operation. Send your postcode and the observations above in your initial enquiry.

For a rented flat or managed building, say who can authorise work and whether hot water comes from communal plant. Arrange access to the relevant cupboard and controls. If other flats are affected, tell the managing agent as well, because the fault may extend beyond your individual home.

Start with written details in the enquiry form. If photographs would help, the team can request them by email after your initial enquiry; the form does not accept image uploads. Only take photographs where access is safe.