Worcester Bosch boiler hot water works but heating does not

If a Worcester Bosch combi provides normal hot water but the radiators remain cold, the boiler can produce heat in at least one mode. The fault is more likely to involve the heating demand, controls, pressure, circulation or the way heat is directed than a complete loss of gas or power.

That observation narrows the search, but it does not identify an internal part. Start with the thermostat, programmer, heating mode, pressure and display. Keep the casing closed and use the exact user guide for the model.

Confirm that hot water really is normal

Run one hot tap and allow it to settle. Note whether the water becomes properly hot, the flame symbol appears and the temperature remains steady. Briefly warm or fluctuating water is a different symptom from fully normal hot water.

If the boiler has a separate cylinder, this guide’s combi logic does not apply in the same way. Stored hot water can remain available even when the boiler itself is not responding, so identify the system before drawing conclusions.

Check that heating mode is enabled

Many Worcester controls have a summer or hot-water-only setting that intentionally leaves central heating off. Look for a radiator symbol, heating temperature setting or seasonal mode described in the user guide.

A power interruption or clock change can also alter schedules. Confirm the programmer has the correct day and time, then use a temporary heating-on period rather than rewriting the whole schedule during diagnosis.

Turn the room thermostat above room temperature

Worcester’s broken-boiler guidance recommends checking that the thermostat is above the current room temperature. Raise it enough to create a clear demand and wait a few minutes. Watch for a heating or flame symbol on the boiler and listen for ordinary operation.

If the thermostat is wireless, check its display and batteries as the control instructions describe. Do not open the receiver, bridge terminals or assume that a lit thermostat has successfully communicated with the boiler.

Does the boiler receive the heating demand?

The most useful distinction is whether the boiler display changes when heating is requested. No radiator or demand symbol can point towards mode, schedule, thermostat, receiver or external wiring. A recognised demand followed by no circulation, lockout or a fault code shifts the professional investigation elsewhere.

Photograph the display before pressing restart. Record every letter, number and symbol exactly. Worcester ranges use different interfaces and codes, so use only the matching model literature.

Check the pressure on a sealed system

Worcester’s Greenstar i guidance says a sealed system is typically around 1 bar when heating is off and rises while operating. Other models can differ. If the reading is below the model’s permitted range, the boiler may not run central heating normally.

Only repressurise when the exact user guide identifies the filling method and you understand it. Do not add water when the pressure is already normal. Frequent pressure loss needs investigation rather than repeated topping up.

Check ordinary radiator controls

Make sure at least one radiator valve is open and that a main room thermostat is not in a room where every radiator valve is shut. If every thermostatic radiator valve is closed, the system may have little or no route for normal circulation.

Do not force seized valves or dismantle them. If only one or two radiators are cold while others heat, the problem is more likely local to those radiators, their valves, trapped air or system balance than a boiler that has lost heating completely.

If every radiator stays cold

Check whether accessible heating pipes near the boiler begin to warm after a confirmed demand, without touching anything that may be hot. Note whether the boiler fires briefly, runs continuously, stops with a code or never acknowledges the request.

Possible professional diagnostic areas include external controls, circulation, changeover components, sensors and wiring. The symptom does not justify removing the casing or replacing a pump or valve without testing.

Could the boiler be in anti-cycle or pump-overrun mode?

Modern boilers do not necessarily fire continuously whenever the thermostat is calling. They may pause between burns or run a pump after the burner stops. A short delay can be normal if rooms and radiators are already warm.

It is not normal for all radiators to remain cold through a sustained demand while the room is below target. Record how long the delay lasts and what the display says rather than resetting immediately.

What if the thermostat says it is heating?

A smart thermostat can show “heating” because its app has requested heat, but that does not prove the receiver has switched or that the boiler has accepted the signal. Check the receiver’s ordinary status lights and connection information from the control maker.

If controls were recently replaced, wiring or configuration may need checking by the installer. Do not unlink and rewire equipment as a diagnostic shortcut.

One restart, only when the guide says so

If the display shows a condition for which the Worcester user guide tells the user to reset, perform one controlled restart. Give the boiler time to complete its sequence and record what happens.

If the code returns or heating still does not start, stop resetting. A recurring safety shutdown needs diagnosis; repeated restarts can hide the sequence without repairing the cause.

If hot water works but one heating zone does not

A multi-zone home may have separate thermostats and motorised valves. If one floor or zone heats while another stays cold, compare the relevant schedules and thermostat demands. The boiler itself may be responding normally to the working zone.

Do not remove valve covers or work on mains wiring. Record which zone creates a boiler demand and which does not so the engineer can test the correct control path.

If there is a hot-water cylinder

Normal hot water may simply mean the cylinder is still holding stored heat. Check whether the boiler responds when the cylinder later calls for heat. A system or heat-only installation can include programmers, zone valves and pumps separate from the boiler.

Tell the engineer whether the cylinder was heated recently by the boiler or by an immersion heater. That detail prevents stored heat from being mistaken for proof that the boiler’s hot-water mode is currently working.

What the symptom suggests to an engineer

Worcester professional fault-finding material separates a boiler that fails during central-heating demand while hot water remains available from a total no-heat fault. It directs competent engineers to check the external demand, timer, heating setting and then model-specific internal components.

That is useful context, not a homeowner repair list. Electrical measurements, casing removal, combustion checks and internal valve or control work must be left to qualified people.

What not to do

• Do not remove the boiler casing.
• Do not bridge thermostat or receiver wires.
• Do not force an internal or external motorised valve.
• Do not keep resetting a returning fault.
• Do not refill when pressure is normal.
• Do not assume working hot water makes the appliance safe.

When to arrange a repair

Book professional diagnosis if the controls create a clear demand but every radiator remains cold, a fault code returns, pressure repeatedly falls, the boiler fires only briefly, or the problem began after control or plumbing work. Also call if the display is blank despite a confirmed supply.

A Gas Safe registered engineer can test the control signal, circulation and model-specific boiler sequence safely. Provide photographs and your notes so the intermittent pattern can be reproduced.

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.

If a carbon monoxide alarm sounds or someone develops possible exposure symptoms, go outside and seek urgent medical advice. HSE says an appliance suspected to be unsafe must be turned off and checked by a Gas Safe registered engineer.

What to tell the engineer

• the full Worcester Bosch model name;
• whether it is a combi or cylinder system;
• whether hot water is fully normal;
• whether the boiler shows a heating demand;
• the exact fault code and pressure reading;
• whether any radiator or zone heats; and
• recent power, control or plumbing changes.

Frequently asked questions

Why does the tap work but the radiator symbol never appears?

The boiler may not be receiving a central-heating request. Check mode, schedule and thermostat first, then have the control path tested if the demand still does not appear.

Can low pressure stop heating while hot water still works?

It can on some models and conditions. Compare the cold reading with the exact user guide, but do not top up a normal system.

Is the diverter valve definitely faulty?

No. It is one possible area on relevant combis, but controls, circulation, sensing and other faults can create the same symptom. Testing is required.

Why are only upstairs radiators cold?

That points more towards radiator valves, air, balance or circulation than a complete loss of boiler heating. Treat it as a different pattern and record which radiators work.

Will resetting restore central heating?

A reset may clear a temporary lockout when the model guide instructs it. If heating still fails or the code returns, arrange diagnosis rather than repeating it.

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