No hot water but heating works? Causes and quick checks

If your radiators are warm but the hot taps remain cold, it’s frustrating — and common. Knowing what to check first can save time and stop you calling out an engineer unnecessarily. This guide explains the most likely causes, straightforward checks you can make safely, and when to book a professional visit.

Why the difference between heating and hot water matters

The same boiler can heat radiators and provide domestic hot water, but it does so using different components and controls. When one function works and the other doesn’t, the fault is often with controls, valves, sensors or secondary components rather than the boiler burner itself. Identifying whether you have a combi, system or regular (conventional) boiler is the first step to diagnosing the issue.

Step 1 — identify your boiler type

• Combi boiler: Heats water on demand from the mains, no hot water cylinder. The boiler usually fires up when a hot tap is opened.
• System or regular boiler: Uses a stored hot water cylinder (typically in an airing cupboard). There may also be a small header tank in the loft on some older systems.

Mixed symptoms — radiators working but no hot water — often point to diverter or motorised valves, cylinder controls, or sensors rather than a total boiler failure. Once you know the boiler type, use the relevant checks below.

Step 2 — basic controls and thermostat checks

Always start with the simplest controls before assuming a mechanical fault.

• Check the programmer/time clock: many system and regular boilers have separate settings for heating and hot water. Ensure hot water is set to 'On' or 'Continuous' for testing.
• On combi boilers, check any hot water temperature dial and make sure the boiler isn’t set to an eco or heating‑only mode that limits domestic hot water.
• If you have a cylinder, inspect the cylinder thermostat — typically set between 55°C and 65°C. If it’s been knocked down, raise the setting slightly and listen for a click indicating it’s calling for heat.
• Look at the boiler pressure gauge: many combi boilers need a system pressure in the region of 1–1.5 bar when cold to operate correctly.

Combi boiler faults: diverter valves, heat exchangers and flow

Combi boilers rely on a few internal parts to switch between heating and hot water. Problems with these parts commonly cause hot water faults while heating remains functional.

Diverter valve

The diverter valve directs flow either to the radiators or to the domestic hot water. If it gets stuck on the heating position, radiators will get hot but taps remain cold or only mildly warm. Other signs include hot radiators whenever you run a tap, or intermittent hot water. Diverter valve replacement is an internal boiler job for a qualified engineer.

Plate heat exchanger and limescale

The plate heat exchanger transfers heat from the boiler circuit to the domestic water. In hard water areas limescale can accumulate, reducing heat transfer and flow. Symptoms include lukewarm water, hot water that cools quickly or good heat at low flow but cold at high flow. Descaling or replacement requires a professional; regular servicing and scale protection help prevent recurrence.

Flow sensors and low pressure

Combi boilers use flow sensors to detect a hot water demand. If a sensor fails, the boiler may not recognise the tap being opened. Low system pressure can also prevent a combi firing for hot water. Both issues are typically diagnosed and rectified by a gas engineer.

System and regular boiler faults: motorised and zone valves

On systems with a hot water cylinder, motorised valves or zone valves route hot water to the cylinder or radiators. If the valve serving the cylinder fails, you can have excellent heating and no hot water.

• Typical signs: the programmer shows hot water is on, the boiler fires for heating, but the pipework to the cylinder stays cold.
• You may hear the valve trying to move, or the valve lever may feel loose or stuck.

Valve internals and wiring are inside the boiler or near electrics and should be handled only by a qualified engineer.

Everyday scenarios and what they usually mean

• Lukewarm hot water: Often a temperature setting turned down, a scaled plate heat exchanger, or a cylinder thermostat issue. Test a different outlet to see if the problem is local to a single tap or shower.
• Hot water cuts out after 30 seconds: Could indicate a restricted heat exchanger, faulty sensors or poor flow rate caused by limescale or debris.
• Only one shower is cold: If the kitchen tap is piping hot but one shower is cold, the issue is often the shower mixer valve rather than the boiler.

Safe checks you can do now

Do not remove boiler covers or touch gas components. These simple checks are safe and will often point you in the right direction before calling an engineer:

1. Confirm the boiler type (combi or system/regular) and note which outlets are affected.
2. Check programmer settings, room thermostat and cylinder stat positions.
3. Look at boiler pressure and any warning lights or error codes on the display.
4. Try several hot taps/showers at different flow rates to spot patterns.
5. Note whether hot water is only available when the heating is on — that’s a useful symptom to report.

Modern boilers often show fault codes; consult the manufacturer’s manual or website for the code description and pass that information to the engineer.

When to call a professional

If your checks suggest problems with diverter valves, plate heat exchangers, motorised valves, flow sensors or wiring, book a qualified gas engineer. These components are inside the boiler or involve live electrics and must be handled by a registered professional. A proper diagnostic visit will test controls, valves and safety devices and then repair or replace the faulty components.

For expert fault finding and repair, consider our boiler repair or boiler servicing options to restore reliable hot water and prevent repeat problems:

• Boiler repair services
• Boiler servicing and maintenance

Get your hot water back working reliably

Work through the safe checks above, note any error codes and symptoms, then contact a qualified engineer if the fault points to internal boiler parts or electrics. If you’d like help diagnosing or repairing the issue, Summit Plumbing & Heating Solutions Ltd can assist — call 07947 403269 or 020 3355 7452 to arrange a visit from an experienced gas engineer who will track down the fault and get your hot water running again.

For related advice on central heating issues, see our other guides on radiator problems and radiator bleeding in the blog.