Boiler not heating: causes and solutions by symptom
Are your radiators cold, is there no hot water, or has the boiler locked out? First identify which service is affected, check the controls you can use safely and learn to tell a simple setting issue from a fault that needs a professional.
Why is my boiler not heating?
The most common causes involve a lack of heat demand, incorrect settings, supply problems, pressure, circulation or an internal fault. The symptoms can guide your checks but cannot, on their own, identify a faulty component.
Neither heating nor hot water works
Start by checking the display, supplies, lockout status and pressure according to your boiler's manual.
Explore this scenario →I have hot water, but the radiators are cold
Check winter mode, whether the thermostat is calling for heat and any individual heating zones.
Explore this scenario →The boiler starts but heats poorly or switches off
Distinguish normal modulation from poor circulation or inadequate heat output.
Explore this scenario →If your heating works but you have no domestic hot water, choose that scenario in the interactive diagnosis.
Interactive diagnosis: identify the symptom
Choose the situation that best describes your problem. The result suggests safe checks and compatible explanations; it cannot provide a definitive diagnosis.
What is happening to your boiler?
Choose a scenario and answer a follow-up question.
I have hot water, but the radiators are cold
First check that heating is enabled, review the timer and thermostat. If heat is being requested, note whether every radiator remains cold and consult the manual; an engineer may need to inspect the pump, diverter valve and controls.
If you smell gas or an alarm sounds, stop the diagnosis and follow the safety advice at the beginning of this article.
What to check if your boiler is not heating
Carry out these seven checks only when there are no signs of danger. None requires you to remove panels or work on gas or electrical components.
Important: the pressure reading refers to the sealed central heating circuit, not the pressure of water at the tap. Many systems run at roughly 1–1.5 bar when cold, but your boiler's manual always takes precedence.
12 reasons why your boiler may not be heating
Identify symptoms and the first appropriate check, but do not assume that an internal component is faulty without professional testing.
The heating is set to summer mode
Hot water at the taps works, but the boiler is not receiving a command to heat the radiators.
- What you can check
- Look for summer mode, a tap icon or disabled central heating on the display. Check the timer and holiday mode.
- Possible safe action
- Enable heating using the normal user controls and the manual. If the boiler does not respond, note what appears on the display.
- When to call
- You cannot change the mode, warnings appear or the boiler no longer responds to a call for heat.
The thermostat is not calling for heat
The room may already be warmer than the target temperature, the heating may be outside its programmed hours or the thermostat may have lost its connection.
- What you can check
- Compare room temperature with the thermostat setting. Check the schedule, accessible batteries in compatible models and the connection status described by the manufacturer.
- Possible safe action
- Set a reasonable temperature above the current room temperature and allow the usual response time; do not touch the wiring.
- When to call
- There is still no demand, a communication error appears or zone control fails.
No electricity or gas supply
A blank display, failure of both services or a lockout may be related to the supply, but these symptoms are not conclusive on their own.
- What you can check
- Without opening the boiler, check whether the property has electricity and look at accessible external power controls. If other gas appliances also have no supply, contact your provider from a safe place.
- Possible safe action
- Only restore household controls that you know how to operate. If you smell gas, stop all checks.
- When to call
- Circuit protection trips repeatedly, there is an unusual smell or the boiler stays off despite a working household supply.
Insufficient pressure in the heating circuit
Some boilers will not start if pressure in the radiator circuit drops below the minimum specified for that model.
- What you can check
- Read the gauge when the system is cold and compare it with the manual. Many homes operate around 1–1.5 bar when cold, but this is not universal.
- Possible safe action
- If the manufacturer allows users to top up the system, follow those instructions exactly and record the pressure before and afterwards.
- When to call
- Pressure falls again, there is a leak or you cannot identify the filling loop or valve.
Lockout or error code
A sensor may trigger a protective lockout for many reasons, including pressure, ignition, overheating, flue problems or internal components.
- What you can check
- Write down the exact code, model, symptoms and when the fault appeared. Check the official documentation for your precise model and generation.
- Possible safe action
- Reset the boiler only if the manual explicitly allows it and there are no warning signs. Do not keep resetting it if the fault returns.
- When to call
- The error is persistent, recurring, undocumented or related to safety.
Circulation pump or insufficient water flow
If water does not circulate correctly, the boiler may quickly reach its target temperature and switch off while the radiators remain cold.
- What you can check
- Note whether all radiators or only one zone are affected, and any noise or repeated stopping, without opening the casing.
- Possible safe action
- Check heat demand and accessible user controls first; pump diagnosis and replacement require a professional.
- When to call
- Radiators remain cold despite demand, the boiler stops because of overheating or the pump is suspected of failing.
A diverter valve that does not switch
In many combi boilers, a diverter valve directs heat either to the central heating circuit or to instant domestic hot water production.
- What you can check
- Note whether hot water works when central heating does not and whether the behaviour changes after turning off the hot tap.
- Possible safe action
- Do not force the actuator or open the boiler: the symptoms may also result from a thermostat, control or circulation problem.
- When to call
- Persistent signs of incorrect heat diversion between hot water and central heating.
Air trapped in radiators
Air pockets reduce local water flow and can cause gurgling noises and radiators that are cold at the top.
- What you can check
- Check whether the problem is localised and the radiator is colder at the top than at the bottom, avoiding very hot surfaces.
- Possible safe action
- Bleed radiators only if there are symptoms and you know your system's safe procedure; then check the pressure.
- When to call
- Air keeps returning, pressure drops or the radiator stays cold after correct bleeding.
Closed valves or zones not calling for heat
A closed thermostatic radiator valve, zoned programming or incorrectly adjusted lockshield valve can leave particular rooms cold.
- What you can check
- Check accessible user controls and which zones are calling for heat. Do not force seized valves.
- Possible safe action
- Change only ordinary user settings; an unbalanced installation requires a hydraulic assessment.
- When to call
- Rooms remain cold, radiator heads are jammed or zone control is faulty.
Sludge, magnetite or hydraulic blockages
Deposits can restrict water flow through filters, heat exchangers, pipework and radiators. A radiator that is cold at the bottom does not prove that sludge is present.
- What you can check
- Observe whether some radiators gradually lose performance or the system becomes noisy and needs frequent intervention.
- Possible safe action
- Do not use cleaning chemicals or dismantle internal filters. Request a professional assessment of flow and circuit condition.
- When to call
- Suspected blockage, recurring anomalies or the need for chemical cleaning.
Ignition, sensor or flue fault
The appliance may attempt to start and then lock out when it detects an abnormal condition involving gas, combustion or flue components.
- What you can check
- Read the warning and document the behaviour without touching the burner, electrodes, fan or flue ducts.
- Possible safe action
- Keep the appliance out of service when indicated by the manual or error code and request a qualified engineer.
- When to call
- Any suspected safety issue, combustion fault or persistent lockout.
Condensate, controls or heating system design
On condensing boilers, a condensate drainage fault can trigger an error; unsuitable controls or insufficient output may prevent the home from reaching its target temperature.
- What you can check
- Compare water and room temperatures, outdoor conditions, programming and visible warnings.
- Possible safe action
- Do not dismantle traps or pipes; a professional must distinguish control settings, sizing and condensate faults.
- When to call
- Recurring problems in cold weather, persistent errors or a home that fails to warm up despite sustained demand.
I have hot water, but my radiators are not heating
If domestic hot water works, the boiler can generate heat, but this does not guarantee that a central heating demand is being received or that water is circulating through the radiators.
Check the heat demand first
After summer, check that central heating is enabled. Compare the room temperature with the target setting and review the thermostat schedule. If the boiler shows a heat demand but all radiators remain cold, an engineer should assess circulation, zone controls and the diverter valve.
If just one room is affected, the cause could be a closed accessible valve, trapped air or a local imbalance. A radiator that is cold at the top and gurgles is consistent with trapped air; a cold area at the bottom can have other explanations.
The boiler heats neither the water nor the radiators
If both services fail at once on a combi boiler, first check conditions affecting the whole appliance rather than assuming that the burner is faulty.
| Indicator | What might be happening | Safe check at home |
|---|---|---|
| Blank display | No power or control fault | Check the household electricity supply without opening the boiler. |
| Warning on display | Low-pressure lockout or another condition | Record the code and consult the manual for your model. |
| Abnormal pressure | The appliance may prevent ignition | Read the gauge when cold; follow only the user instructions. |
| No general supply | External gas or electricity supply problem | Contact the supplier; if you smell gas, follow emergency procedures. |
Do not assume that both services failing means the boiler itself is broken: a schedule or lockout may produce similar symptoms. If the appliance still does not respond, seek professional diagnosis.
The boiler runs but heats poorly or switches off
A radiator's surface temperature is not the same as the room temperature. Modulating controls deliberately lower the water temperature when less energy is needed.
Starts and stops
This may be normal control cycling or indicate insufficient flow, a sensor problem or lockout. Note whether the system reaches its target and whether errors appear.
Lukewarm radiators
This can be normal with modulating boilers and low-temperature systems. Assess actual comfort rather than judging only by how hot the radiator feels.
The house does not warm up
Check the schedule, target temperature, heating zones and insulation. Controls, heat demand and system sizing need to be assessed together.
How to interpret the behaviour without mistaking it for a fault
The boiler cycles its burner on and off. Many appliances adjust their output to avoid delivering more heat than necessary. When the thermostat is close to its target, the boiler may reduce the flow temperature or briefly switch off the burner. This is different from very frequent cycling accompanied by persistently cold radiators or overheating warnings. Record how often it stops and whether the room temperature continues to rise: that information will help the engineer, although it cannot replace measurements.
The radiators stay lukewarm. With modulating controls or weather compensation, radiators do not have to remain very hot. If the home stays at the desired temperature, lukewarm radiators may mean the controls are delivering the right amount of heat. But if a room fails to reach its target for a prolonged period, check the schedule and report the problem. Do not automatically increase the water temperature without considering the emitters, insulation and manufacturer's instructions.
Some rooms take much longer to warm up. Heat distribution depends on flow to each circuit, radiator settings, room orientation, heat loss through windows and each room's heating needs. If one room remains cold while others reach their target, first check the accessible controls for that zone. Hydraulic balancing and reviewing system sizing are technical tasks, not adjustments to improvise.
Underfloor heating behaves differently. Because it heats a large area at a lower temperature, underfloor heating may take longer to change room temperature. Alternating extreme setpoints or expecting an instant response after every adjustment is not a reliable way to assess its performance. If there is a genuine fault, an installer should compare flow rates, flow and return temperatures, mixing valve controls and circuit configuration against the system design.
Quick-reference table: symptom, possible cause and what to do
Use this table for initial guidance, then consult the relevant section.
| Observable symptom | Possible causes | First safe check | When to call an engineer |
|---|---|---|---|
| Neither service works | Supply, lockout or pressure | Read the display and pressure gauge | The boiler stays unpowered despite supply or an error persists |
| Hot water works, heating does not | Summer mode, thermostat, diverter valve, circulation | Check mode and thermostat setting | There is a heat demand but all radiators stay cold |
| Heating works, no hot water | DHW controls, flow detection or heat exchanger | Check DHW mode and other taps | Every hot-water outlet is affected |
| Only one radiator is cold | Valve, trapped air or local flow | Check accessible controls and signs of air | The problem persists or a valve is seized |
| Pressure keeps falling | Water loss or another issue | Record the pressure when cold | Pressure falls again |
| Repeated lockouts | Model-specific error codes | Record the code and consult the manual | A permitted user action does not resolve the fault |
| The entire house heats poorly | Controls, heat loss, flow rate | Review timers, setpoints and zones | The home fails to reach its target without explanation |
| Blank display | Power supply or electronics | Observe the household electrical supply | Power is available but the display remains off |
How central heating works and where it can fail
In a combi boiler, the sealed central heating circuit is separate from the domestic water used at taps. This simplified diagram is not a repair guide for any particular model.
Domestic hot water and central heating are not the same
The gauge normally displays the pressure in the sealed central heating circuit. Domestic hot water comes from the mains and, in an instantaneous combi boiler, is heated through a heat exchanger. Depending on the appliance, a diverter valve can direct heat between the two services.
If a radiator is cold but hot water works, that alone does not prove the pump or diverter valve is faulty: first check the user controls and the actual hydraulic layout.
Common mistakes to avoid
Resetting again and again
Repeated lockouts may indicate an unresolved fault. Record the code and stop resetting.
Constantly topping up
If pressure repeatedly drops, find the cause rather than topping up indefinitely.
Changing technical settings
Installer menus and sealed components are not household controls.
Confusing two pressures
The heating gauge does not directly measure the pressure or flow at your taps.
Bleeding without symptoms
Unnecessary bleeding can reduce pressure and distract from the real cause.
Forcing valves and controls
Seized or deteriorated components should be assessed before being operated.
Following another brand's manual
Error codes and procedures vary by manufacturer, model and generation.
Working on combustion components
Do not interfere with gas, the burner, flue system, internal electrical components or safety devices.
When to stop and call a professional
Safe checks at home
Only user controls, external supplies, target temperature, operating modes, display and pressure gauge, in accordance with the manual.
Technical assistance
Recurring errors, unstable pressure, poor circulation, internal valves or sensors require a qualified engineer.
Emergency
If you smell gas, a CO alarm sounds, someone has symptoms consistent with poisoning or there is any immediate danger, leave the area and call 112 from a safe place.
How to reduce the risk of losing your heating
Before cold weather arrives, test the heating mode and make sure the thermostat and timer reflect how the home is used. Observe the radiators and note pressure drops; do not ignore leaks or repeated errors.
Before winter
Turn on the heating in advance, check the heat demand and confirm all zones respond. Review your model's manual.
During the heating season
Keep track of unusual pressure or performance changes. Request an assessment if air, noise or lockouts keep returning.
Regulated maintenance
Consult the current Spanish RITE regulations and the operating and maintenance manual. The general RITE table specifies maintenance at least every two years for wall-mounted gas boilers up to 70 kW in homes, unless stricter requirements apply. Do not confuse this with the gas installation inspection.
Frequently asked questions about a boiler that is not heating
Why is my boiler not heating even though the pressure is correct?
Correct pressure rules out only one possible cause: the thermostat, selected mode, circulation or internal components may still be at fault. Check the heat demand and record any display messages.
Why do I have hot water but cold radiators?
On a combi boiler, central heating may be disabled, the thermostat may not be calling for heat, or there may be a circulation or diverter valve issue. Start with the user controls.
What should I do if the boiler will not start when I turn on the heating?
Check heating mode, the timer and setpoint. Read the display and pressure gauge and consult the manual. Seek help if lockouts persist.
Why does my boiler keep switching on and off?
It may be modulating normally near the desired temperature, or there may be a heat-demand, flow or control issue. Record the frequency, codes and temperatures for the engineer.
Why are my radiators not heating after the summer?
Check summer/winter mode, timer and accessible valves. Cold areas or gurgling can indicate air in the system.
Could the thermostat be the problem?
Yes. A setpoint below room temperature, an unsuitable schedule or loss of communication can prevent a call for heat. Carry out external checks before blaming the thermostat.
How can I tell if the circulation pump might be faulty?
Cold radiators during an active call for heat and temperature-related shutdowns can indicate poor circulation, but do not prove a pump fault. Internal checks are for professionals.
Why does the boiler heat for a few minutes and then stop?
This may be normal cycling or a stop caused by reduced flow, a sensor, overheating or an error. If it keeps happening without adequate comfort, record the pattern and contact an engineer.
Is it normal for the house to take a long time to heat up?
It depends on heat loss, radiators, outdoor temperature and thermal inertia. Underfloor heating usually responds more slowly than lightweight radiators. What matters is reaching the intended setpoint.
What does an error code mean?
Only the manual for the correct brand, model and generation can give a reliable meaning. Copy the exact code and avoid repeated resets.
Should I bleed my radiators if the boiler is not heating?
Only when symptoms suggest air, such as gurgling or a radiator that is cold at the top. Bleeding unnecessarily may lower pressure without resolving a general fault.
What should I do if the pressure keeps dropping?
Do not keep topping up as a permanent solution. There may be a leak or another issue that requires investigation.
Can I reset a locked-out boiler?
Only if the manual permits it and there is no smell of gas, alarm or other danger. If the lockout returns, stop trying and seek help.
Why do some radiators heat up while others stay cold?
Check accessible controls in the affected rooms. Trapped air, insufficient flow, sludge or hydraulic imbalance may also be involved.
When should I request a technical inspection?
For recurring errors, unstable pressure, circulation problems, leaks or no heat after safe checks, as well as scheduled maintenance.
What information does the engineer need?
Brand and model, exact error code, cold pressure, whether hot water or heating works, when the problem started and which user settings you have checked.
Heating solutions and materials for installers
Browse boilers, thermostats, valves, components and heating accessories in the Industrial Ginés catalogue. If you need to repair a domestic boiler fault, contact the relevant service engineer or a qualified heating company.
