Heating Fault Checks
No Heating or No Hot Water? What to Check Before You Call
If you have no heating or no hot water, first rule out gas or carbon monoxide warnings, then identify what still works and check the relevant user controls and supplies. Smell gas: leave and call the National Gas Emergency Service on 0800 111 999. For suspected carbon monoxide symptoms, get into fresh air and call NHS 111, or 999 if serious. This checklist covers London homes with domestic gas boilers and forms part of our Boiler and Heating Guides.
Call with your postcode, the boiler make and any fault code showing.
- Gas Safe registered: 648977
- Phone line open 24/7
- Same-day appointments available
Check for gas or carbon monoxide warnings before troubleshooting
Stop troubleshooting if you smell gas, suspect carbon monoxide exposure or hear a CO alarm.
Smell gas: Avoid flames and electrical switches. Open doors and windows and turn off gas at the meter only if safe, without delaying evacuation. Never enter a cellar or basement to reach the meter. Leave and call 0800 111 999 from a safe place before arranging repairs. Follow the gas emergency instructions.
Suspected carbon monoxide: Get everyone outside into fresh air. Seek medical help through NHS 111, or 999 for serious symptoms such as breathing difficulty, confusion, collapse or chest pain. Then report suspected gas-appliance CO through 0800 111 999, without delaying medical help. CO has no smell: treat an alarm as real even without symptoms. Do not re-enter until advised it is safe. See the NHS carbon monoxide advice.
DIY stops at user controls: never open the boiler casing or work on gas.
Which problem do you have: heating only, hot water only or neither?
Identify whether radiators, hot taps or both have stopped working, and whether water still flows.
Radiators heat; every hot tap runs cold
Compare outlets and normal hot-water settings. This cannot identify a failed component. Continue to “Heating but no hot water”.
Hot taps work; radiators stay cold
Check heating demand. Warm taps cannot establish that heating controls work. Continue to “No heating but hot water”.
Neither works
Observe the display and household supplies. This does not prove an internal boiler fault. Continue to “No heating or hot water”.
One tap or radiator is affected
Compare others. An isolated symptom cannot establish whole-boiler failure. Report the affected outlet or radiator when seeking assessment.
Little or no water flows
Compare the cold kitchen tap. Absent flow does not prove a boiler fault. Read the hot-tap flow FAQ.
Check your arrangement: combi boiler, boiler with stored hot water, or communal heating. Stored water can remain warm after a boiler stops, so a warm tap does not prove it currently fires. For communal heating, including a heat interface unit, use your building’s reporting arrangements.
Heating but no hot water: check which taps are affected
If heating works but there is no hot water, check whether every hot tap or just one outlet is affected.
- Compare another hot outlet.
- Note whether hot-side flow differs from cold flow.
- Record cold versus lukewarm water without putting your hand under potentially scalding water.
- On a combi, compare the normal hot-water setting with its manual; for stored water, check only the ordinary programmer schedule.
A combi diverter valve directs heating water between radiator heating and hot-water production. Understanding what a boiler diverter valve does explains one possibility, but these symptoms cannot confirm its failure. Internal causes need assessment; cylinder repairs are outside our service.
No heating but hot water: check the heating demand
If hot water works but heating does not, check that heating is enabled and the thermostat requests warmth.
Mode and timing
Check the normal heating mode, clock and schedule.
Thermostat
Set it above the displayed room temperature. Replace batteries only where the controller instructions permit.
Radiator settings
Check ordinary valve settings without dismantling or forcing anything.
Response
Note whether the boiler display changes when heating is requested.
Record whether all radiators, one zone or one radiator stays cold. No displayed demand, demand without heat and uneven heating are different observations, none of which proves a particular component has failed.
No heating or hot water: check the display and supplies
If neither service works, check the normal boiler display and whether the home has power and running cold water.
- Observe household power interruptions without opening electrical equipment.
- Compare a cold tap.
- If safely accessible and there is no gas warning, check the gas meter’s ordinary display or prepayment credit. Follow supplier instructions for supply problems.
- Record the full boiler message before changing settings.
Distinguish a blank display, a displayed fault, and an apparently operating boiler without useful heat. A blank display does not prove circuit-board failure. Never test gas with a flame, reconnect an isolated supply, replace fuses or repeatedly operate breakers.
Read the pressure gauge before considering a top-up
On a sealed heating system, compare its pressure reading with the boiler instructions before considering a top-up.
Read
Record the pressure and whether the system is cool. Heating pressure is different from tap flow.
Compare
A top-up is appropriate only if the manual permits it and you can identify the correct filling arrangement. Read our.
Stop
Seek advice for recurring pressure loss, visible leaks, excessive readings or uncertainty.
Do not repeatedly top up or apply sealed-system filling instructions to an open-vented system.
Consider one reset only if your boiler instructions allow it
Attempt one reset only when the exact model’s instructions permit it and there are no gas or CO warnings.
Record the complete message first, follow the specified user procedure and observe the result. Our guide explains how to reset a boiler safely.
Stop if the manual requires it, the fault persists or the boiler locks out again. A successful restart does not explain recurring faults. Never repeatedly reset, cycle the power or restart an appliance labelled unsafe.
When to stop checking and call an engineer
Stop when permitted checks do not restore operation, a fault returns or further investigation requires opening equipment. Report observations rather than choosing a replacement part. Gas and CO emergency instructions take priority over every repair route.
An urgent loss of heating or hot water
Seek urgent repair advice if someone cannot keep warm or meet essential care needs. Explain which services have failed, current conditions and any vulnerable occupants. After excluding gas or CO danger, seek.
A persistent boiler fault after the safe checks
An unresolved boiler fault needs an engineer to establish its cause. Report persistent lockout, recurring symptoms or every hot tap running cold. Internal gas-appliance investigation requires an appropriately qualified Gas Safe registered engineer. Arrange.
Heating is requested but radiators remain cold
A heating request without radiator heat needs assessment of the circuit and boiler response. Report whether one radiator, one zone or everything is affected. Pumps, valves and circulation are possible investigation areas, not confirmed causes. Arrange.
Heating controls still do not produce a response
Arrange assessment if permitted settings and battery checks produce no response. Record the controller model, displayed status and recent power cuts or setting changes. Communication, wiring, controls or the boiler may be involved. Stop before receiver or wiring access and arrange.
Boiler repair, servicing and gas work
Call with your boiler symptoms and the checks you have made
When you call, describe what stopped working and what the boiler displays so we can discuss the next step.
Have these details ready:
- London postcode and accessible boiler make/model.
- Heating or hot-water symptoms and affected outlets.
- Full displayed message and applicable pressure reading.
- When the problem began and checks attempted.
- Vulnerable occupants or essential care needs.
Contact All Service 4 U for Gas Safe registered engineers. Call us 24 hours a day, 7 days a week.
Call about a heating or hot-water fault: 020 3740 6785
020 3740 6785
Frequently Asked Questions
Why is no water coming out of the hot taps?
Absent hot-tap flow points to a water-supply or plumbing-path problem that needs distinguishing from water flowing cold. Compare another hot outlet and the cold kitchen tap. Record whether all water, only hot-side flow or one outlet is affected. If cold supply is also absent, check your water supplier’s interruption information. Otherwise seek appropriate plumbing or building-management advice; do not disconnect pipes or force valves.
No hot water from a Worcester boiler: what should I record?
Record the exact Worcester model, complete displayed message, whether heating works and which hot outlets are affected.
- Find the model in user documents or an accessible external label.
- Copy the whole message.
- Note whether radiators heat.
- Distinguish absent flow from cold or lukewarm water.
The matching manufacturer instructions determine code meanings and permitted user actions. These observations do not establish which part needs repair.
Could freezing weather have stopped my boiler?
A frozen external condensate pipe can stop a condensing boiler, but cold weather alone does not confirm it. Only consider warm, never boiling, water where manufacturer instructions permit and the pipe is correctly identified and safely accessible at ground level. Watch for ice and slipping. Do not use flames, ladders, cut pipes or access internal traps. Inaccessible pipework or a persistent fault needs professional help; any reset remains subject to the earlier one-attempt rule.
Who should a tenant contact about no heating or hot water?
Report the loss promptly to your landlord or managing agent, following gas or CO emergency instructions first where relevant. Include when it began, affected services, checks made and household needs, and keep a record of the report. For communal heating, follow the building’s reporting arrangements. Do not assume the problem requires a private boiler repair or commission work without the appropriate authorisation.
Can I restart a boiler that an engineer has labelled unsafe?
Do not restart or reconnect a boiler switched off or disconnected as unsafe. Keep the warning notice, follow its instructions and arrange the specified remedial assessment with an appropriately qualified Gas Safe registered engineer. In rented accommodation, notify the landlord or managing agent. An unsafe finding does not, by itself, establish that replacement is the only remedy; assessment determines what is needed.
Still have a question or need urgent assistance? Call 020 3740 6785, tell us what has happened and we will tell you what happens next.