No Heating or Hot Water? What to Check First

A sudden heating breakdown is disruptive. The underlying issue is often simple. Before booking a call-out, run through several safe external checks. They take five minutes. You need no tools.

Work down these checks in order. Test external controls before booking an engineer.

Symptom Diagnostic Triage

Use this matrix to identify what your heating symptoms mean before booking a call-out.

The 7-Point Safe Diagnostic Checklist

Work through these seven external checks. They take two minutes. Do not skip steps.

1. Power Supply and Fused Spur

Check the front of the boiler. Look at the display. An unlit display indicates a loss of incoming electrical power.

Look for the 3-amp fused spur switch on the wall near the boiler. Make sure the switch is turned on. Check the neon indicator light. Inspect your main consumer unit for tripped circuit breakers.

On site, our engineers verify incoming voltage at the spur. We use a non-contact Fluke voltage tester on the supply cable. This tool confirms whether live mains power reaches the appliance. If power is live at the wall switch but the boiler screen stays dark, an internal glass fuse on the main circuit board has blown. That fuse protects internal components from electrical surges.

2. Gas Supply and Prepay Meter

Check your gas supply at the emergency control valve by the gas meter. The handle should sit parallel with the gas pipe when open. Test a gas hob burner. Make sure gas flows normally.

If your property uses a prepay meter, inspect the digital display screen. Check the available credit balance. Depleted meter balances cut off the gas supply without warning. This causes the burner to lock out on ignition.

3. Thermostat, Programmer and Heating Controls

Inspect your room thermostat. For wireless units, replace the AA batteries. Weak batteries fade quickly. Depleted batteries often keep the screen active while losing the radio signal needed to reach the receiver.

Turn the target temperature higher. Set it at least 2°C to 3°C above the current room temperature. Listen for a click. Check your central heating programmer. Make sure it is not set to summer mode. Check that the heating timer is on.

4. Boiler Water Pressure

Inspect the pressure gauge on the front panel. Check the reading. When the heating system is cold, water pressure should sit between 1.0 and 1.5 bar.

Water pressure below the minimum operational threshold triggers a dry-fire lockout. That threshold is usually 0.5 to 0.8 bar depending on the manufacturer. Locate the external braided filling loop underneath the boiler. Open the small valves slowly. Watch the gauge rise. Stop at 1.2 to 1.5 bar. Close both taps tightly.

If pressure drops again over subsequent days, review our guide on why boilers lose pressure to isolate the leak.

5. Display Fault Codes and Manufacturer Warnings

Look at the digital display. Note any flashing fault codes. Common low water pressure fault codes include Vaillant F.22 and Baxi E119.

On Worcester Bosch boilers, low pressure shows directly on the pressure gauge or via maintenance messages on newer models. The common EA fault code represents an ignition or flame detection failure rather than low pressure. Vaillant fault code F.75 indicates that the pressure sensor did not detect a pressure jump when the pump started. That code often stems from a blocked sensor or a flat expansion vessel. Check your user manual on the manufacturer support website.

6. Frozen External Condensate Pipe

Cold weather causes frozen pipes. Modern condensing boilers discharge water through an outside plastic pipe. Sub-zero temperatures can freeze this liquid into a solid ice plug.

When condensate cannot drain, wastewater backs up inside the boiler. The appliance locks out for safety. Find the plastic pipe outside. Pour warm tap water along the exposed pipe run. This melts the ice blockage. Never pour boiling water onto cold plastic pipework. Boiling water cracks cold plastic. Reset the boiler once clear.

7. Resetting the Boiler Safely

Follow the reset procedure in your manufacturer manual. This usually means pressing or holding the reset button once. The appliance will initiate an ignition sequence.

Listen to the burner. If the boiler lights and runs smoothly, monitor the system pressure. If the boiler locks out a second time, stop there. Do not reset it repeatedly. Continued lockouts indicate an internal fault that requires a professional boiler repair by a Gas Safe registered engineer.

Combi Hot Water Priority and Diverter Valves

Combination boilers follow a strict priority rule. Domestic hot water always comes first.

When you open a hot tap or shower, an internal flow turbine detects water movement. The circuit board commands an internal motorised diverter valve to switch position. This directs primary hot water into the plate heat exchanger rather than into the radiators.

If your combi boiler produces reliable hot water while radiators remain cold, the primary burner, gas valve, and ignition sequence are working. The issue sits on the heating side. Common culprits include external thermostat controls, a failed circulation pump, an airlock, or a diverter valve cartridge seized in the hot water position. We see this often in Victorian properties across Ealing where older pipework carries suspended magnetite debris.

If radiators warm up when running hot taps, the diverter valve cartridge is passing or stuck in the heating position. In practice, mechanical valve sticking occurs even more frequently on system and heat-only boilers with external motorised zone valves than on combi diverter valves.

System and Heat-Only Boilers with Cylinders

Properties with system boilers and hot water cylinders use external motorised zone valves. These valves are usually fitted in an airing cupboard.

• S-Plan systems use two separate 2-port motorised valves. One valve opens for the radiator circuit, while the second valve controls heat to the hot water cylinder.
• Y-Plan systems use a single 3-port mid-position valve that directs flow to heating, hot water, or both together.

The Single-Demand Diagnostic Rule: If either central heating or hot water functions independently, the boiler is firing and generating heat. The fault is isolated to external system controls. These include a failed synchronous motor, a burnt-out microswitch on the valve actuator head, a seized brass valve spindle, or an uncalibrated cylinder thermostat.

If both heating and hot water fail simultaneously on a cylinder setup, check the external components. Common causes include an electrical supply failure, a seized external circulating pump, programmer failure, or a boiler lockout.

Stop and Call a Gas Safe Engineer If

Safety always comes first. Stop your checks and arrange a professional inspection under any of the following conditions:

• You smell gas anywhere in the property. Immediately open windows. Do not use electrical switches. Turn off the meter emergency valve if safe. Leave the premises. Call the National Gas Emergency Service on 0800 111 999.
• Your carbon monoxide alarm sounds, or occupants experience unexplained headaches, nausea, or dizziness.
• The boiler locks out repeatedly after a single safe reset attempt.
• Water is actively leaking from the boiler casing or discharging through the external copper relief pipe.
• You notice scorched electrical components, burning smells, or continuous fuse board tripping.
• Any check requires removing the boiler casing. Under the Gas Safety (Installation and Use) Regulations 1998, opening room-sealed casings or working on gas assemblies is restricted to qualified Gas Safe engineers.

What Our Engineer Does on the Visit

When attending a heating breakdown, our engineers follow a methodical diagnostic procedure:

1. Electrical supply verification: Testing incoming voltage at the fused spur and internal terminals using a non-contact Fluke tester and digital multimeter to isolate power supply and fuse failures.
2. Control and fault log interrogation: Reading digital fault history codes and checking live temperature and pressure sensor parameters through the boiler diagnostics menu.
3. Hydraulic and circulation testing: Testing hot water delivery and clipping temperature probes to flow and return pipework to verify circulating pump operation and heat transfer.
4. External control testing: Checking 230V switching signals across the room thermostat, programmer, and motorised zone or diverter valve microswitches.
5. Flue gas and combustion analysis: Using an electronic flue gas analyser to verify correct combustion ratios, burner pressure, and flue integrity.
6. Fixed written quote: Explaining the confirmed fault clearly and providing a fixed price for required replacement parts before starting repair work.

Related Problems It Can Be Mistaken For

Several minor heating issues can mimic a complete boiler breakdown:

• Single cold radiator: If one radiator stays cold while others heat up, the boiler is operating normally. The issue is usually a seized thermostatic radiator valve pin or trapped air requiring bleeding.
• Cold shower with hot taps: If hot water runs hot from kitchen taps but runs lukewarm in the shower, the problem is a scaled or worn thermostatic mixer shower cartridge rather than a boiler failure.
• Tripped circuit breaker: A tripped domestic socket circuit cutting power to the boiler fused spur can be mistaken for a mechanical appliance fault.

Fixed Diagnostic Pricing

We believe in complete pricing transparency. Our rates are fixed. The boiler repair cost guide lists typical part prices, and the emergency boiler repair cost guide covers out-of-hours visits.

Our diagnostic visit includes complete electrical testing, hydraulic checks, and fault diagnosis. We always provide a written fixed quote before fitting any replacement parts. We agree prices upfront.

For long-term protection, our Homeowner Plan at £9.99 a month includes the standard annual service, priority booking if the boiler breaks down and 10% off labour. If your boiler is over 12 years old and suffering repeated component failures, an energy-efficient boiler installation or modern heating upgrade may prove more cost-effective than continuous repair bills.

Frequently Asked Questions

How much does a boiler breakdown diagnostic cost?

Our diagnostic fee is £120 for weekday daytime visits between 08:00 and 18:00 across West London. Saturday daytime visits are £160, evening visits between 18:00 and 21:00 are £180, and emergency overnight, Sunday, or bank holiday dispatch is £220. The visit includes full electrical, hydraulic, and control testing. Any required replacement parts and subsequent repair labour are quoted as a clear fixed price before work begins.

Can I fix no heating or hot water myself?

You can safely check external issues such as testing thermostat batteries, checking the fused spur switch and consumer unit breakers, repressurising the filling loop to 1 to 1.5 bar, and gently thawing an external condensate pipe. However, you must never remove the boiler casing or tamper with gas valves, internal wiring, or flue components. Work inside the appliance legally requires a Gas Safe registered engineer.

What should I do if I smell gas when checking my boiler?

If you smell gas, act immediately to keep everyone safe. Do not operate light switches, plug sockets, or open flames. Open doors and windows to ventilate the property. If safe to reach, turn off the emergency control valve beside your gas meter. Leave the building straight away and call the National Gas Emergency Service on 0800 111 999 from outside. Do not re-enter until declared safe.

Why is my boiler display completely blank with no power?

A blank display screen indicates that incoming mains electricity is not reaching the boiler controls. First check that the 3-amp fused spur switch next to the appliance is switched on and inspect your home consumer unit for a tripped breaker. If external power is confirmed live at the wall spur, an internal glass fuse on the boiler circuit board has likely blown to protect against electrical surges.

Why do I have hot water from the taps but no central heating?

When domestic hot water heats normally, your boiler burner and primary heating circuits are functioning. The fault is isolated to the heating side of your system. Common culprits include a room thermostat with depleted batteries, a programmer set to summer mode, a failing central heating circulation pump, or a motorised diverter valve stuck in the hot water position. An engineer can test switching signals to locate the fault.

How many times can I press the reset button if my boiler locks out?

You should only press the reset button once after correcting a clear external issue, such as topping up low water pressure or thawing a frozen condensate pipe. Pressing reset repeatedly can overload electrical components and will not clear an internal mechanical defect. If the boiler locks out a second time, leave it off and book a Gas Safe registered engineer to diagnose the cause.

How quickly can an engineer attend a boiler breakdown in West London?

Our telephone line is answered 24/7 by an AI receptionist that takes caller details at any time. Genuine emergencies, such as uncontrollable leaks or hazardous faults, can be dispatched at any time at our emergency rate. Routine daytime breakdown visits run within scheduled morning (08:00 to 12:00) or afternoon (12:00 to 18:00) arrival windows across Kensal Rise, Queen’s Park, Acton, and surrounding West London areas.