Baxi boiler radiators hot but no hot water

If the radiators heat normally but taps stay cold, the boiler can produce heat but the hot-water side is not completing its job. The useful next step depends on whether you have a Baxi combi boiler or a boiler working with a separate hot-water cylinder.

A combi heats tap water on demand. A system or heat-only boiler normally heats stored water in a cylinder through separate controls and valves. The same symptom therefore has different likely causes, and identifying the system avoids irrelevant or unsafe checks.

First: is it a combi or a cylinder system?

A combi normally has no hot-water cylinder and starts when a hot tap opens. A system or heat-only boiler usually works with a cylinder in an airing cupboard or plant space. Do not assume that every wall-mounted Baxi is a combi.

Find the model name on the closed appliance and use Baxi’s literature library to open the matching user guide. It will show the correct symbols, pressure range, controls and restart procedure for that model.

If it is a Baxi combi

Open one hot tap to a normal flow and watch the boiler display without removing any panel. Record whether a tap or flame symbol appears, whether the boiler makes a normal start attempt, whether a fault code is shown and whether the water is completely cold or briefly warms.

Try one other hot outlet. One failed tap or shower can point towards a local mixer, cartridge, isolation valve or restricted outlet. Every hot outlet staying cold while cold-water flow remains normal makes the boiler’s hot-water demand path more relevant.

Does the boiler recognise the hot tap?

If the display does not change when a hot tap opens, the boiler may not be detecting sufficient hot-water flow. The cause could be low flow at that outlet, an installation issue or a fault in the boiler’s flow-detection system. This cannot be confirmed by sound or guesswork.

Run the tap at a sensible, steady flow rather than a trickle, then stop. Do not remove tap restrictors, force a sensor or open the boiler. Note whether another outlet produces a demand symbol, as that comparison will help an engineer.

If there is a demand but no hot water

A visible hot-water demand with cold water at the tap suggests that the request is reaching the boiler but heat is not being transferred or directed correctly. Possible professional diagnostic areas include the changeover mechanism, temperature sensing, circulation and the domestic hot-water heat exchanger.

Those are not homeowner repair tasks. The fact that radiators work does not identify one failed component, and replacing parts from an online list is unsafe and can be expensive.

Check the hot-water setting

Make sure the domestic-hot-water control has not been turned down or disabled. Baxi explains that a tap icon normally represents hot water while a radiator icon represents heating, although the exact interface varies.

Do not confuse the central-heating flow temperature with the hot-water temperature. Baxi’s guidance states that changing the heating flow temperature on a combi does not change water at the taps. Use the exact user guide and make only ordinary control changes.

Check water flow and supply

Confirm that cold water flows normally from several taps. If the whole property has poor or no cold-water supply, the combi cannot provide normal hot water. Check for known water-supply work and ask whether neighbours are affected.

If cold flow is strong but one hot outlet is weak, clean only an easily removable aerator or shower head where its instructions permit. Do not dismantle a thermostatic shower or fixed plumbing simply to test the boiler.

Check pressure and the display

Read the pressure only against the model’s user guide. Baxi’s general no-hot-water guidance notes that many boilers operate around 1 bar, but the correct cold range varies. If the gauge is already normal, do not add water.

Record the complete fault code exactly. Do not use a code meaning from another Baxi model, and do not repeatedly reset. One restart may be appropriate only when the matching user guide tells the user to do so.

If there is a hot-water cylinder

With a system or heat-only boiler, working radiators prove only that the central-heating circuit can run. The hot-water channel may have its own programmer schedule, cylinder thermostat, motorised valve and circulation route.

Check whether the programmer’s hot-water channel is on and whether the cylinder thermostat is set normally. If the timer has separate heating and hot-water lights, note which one appears. Do not remove the programmer, cylinder thermostat or valve cover.

Was the cylinder given enough time?

A stored-water system needs time to reheat after baths, showers or heavy use. Run one normal hot-water period and wait for the time described in the system instructions. If water remains cold while radiators heat, record whether the boiler ever responds to the hot-water channel.

Do not leave the boiler firing indefinitely or raise stored-water controls to extreme temperatures. Stored hot water must be managed carefully because low storage temperatures can increase hygiene risk while excessively hot water can scald.

Could a motorised valve be involved?

Many cylinder systems use one or more external motorised valves to direct heated water towards radiators, the cylinder coil or both. A valve or its control can fail in a heating position, leaving radiators hot while the cylinder receives no heat.

You can report whether a nearby valve makes a noise or whether its ordinary external indicator changes, but do not remove its electrical head, force its lever or work on the wiring. A heating engineer must test the control sequence safely.

What if the cylinder has an immersion heater?

An immersion heater is a separate electrical water heater. If fitted and known to be safe, it may provide temporary hot water when used exactly as its instructions describe. It does not repair the boiler circuit.

Do not use it if the switch is hot, damaged, wet or repeatedly trips the electrics. Do not remove the immersion cover. Electrical diagnosis belongs to a competent professional.

What the working radiators do—and do not—prove

Hot radiators show that the system has power, has accepted a heating demand and can produce and circulate heat in at least one mode. On a combi, that makes a total loss of gas or ignition less likely at that moment. On a cylinder system, it says little about the separate hot-water controls.

It does not prove that every sensor, valve, pump, heat exchanger or control is working. The symptom narrows the investigation but does not make internal diagnosis safe for a homeowner.

What not to do

• Do not remove the boiler casing.
• Do not force a diverter or motorised valve.
• Do not bridge thermostat or programmer wiring.
• Do not keep resetting the boiler after the fault returns.
• Do not add pressure when the gauge is already in its normal range.
• Do not lower a cylinder temperature without system-specific advice.

When to arrange a repair

Arrange professional diagnosis if every hot outlet is affected, the display recognises demand but water stays cold, a fault code returns, the boiler repeatedly locks out, or the cylinder does not heat despite a correct schedule. A sudden change after recent work should also be reported to the installer.

Any work inside a gas boiler must be performed by an appropriately qualified Gas Safe registered engineer. For a cylinder system, the engineer may also need to test external controls and valves.

Gas and carbon monoxide safety

If you smell gas, open doors and windows, avoid flames and electrical switches, turn off the supply at the meter only if you know how, leave if necessary and call the National Gas Emergency Service on 0800 111 999. Do not continue testing the boiler.

If a carbon monoxide alarm sounds or anyone develops symptoms such as headache, dizziness, nausea, weakness or confusion, go outside and seek urgent advice. A boiler that heats radiators is not automatically safe.

What to tell the engineer

• the complete Baxi model name;
• whether it is a combi or has a cylinder;
• whether every hot outlet is cold;
• whether the display shows a hot-water demand or flame;
• the exact code and pressure reading;
• whether the problem followed recent work; and
• which programmer and thermostat lights are on.

Frequently asked questions

Can a diverter valve cause heating but no hot water?

It is one possible professional diagnosis on relevant systems, but the symptom alone does not prove it. Flow detection, sensing, heat transfer and controls can produce similar results.

Why is one tap cold but others are hot?

That usually points towards the affected outlet, mixer or local pipework rather than the boiler. Compare another tap before booking the right trade.

Should I reset the Baxi boiler?

Only when the exact model guide instructs a restart for the displayed condition. If the fault returns, stop resetting and arrange diagnosis.

Can low pressure stop hot water?

Some sealed boilers will stop or show a fault when system pressure is too low. Use the model’s stated range; do not assume that every no-hot-water problem needs filling.

Why does heating work but the cylinder stay cold?

The hot-water schedule, cylinder thermostat, valve or cylinder-heating circuit may not be completing its separate demand. Safe testing requires the system layout and suitable instruments.

Related boiler help

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• Baxi hot-water temperature fluctuating
• Boiler diverter-valve symptoms