Boiler Not Working? Try These Checks First (Safely)

Boiler misbehaving—no heat, no hot water, or strange noises? This guide covers quick, safe checks you can do yourself, what common fault codes mean, and the point where a professional is the right call.

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Safe Quick Checks You Can Do Now Before Boiler Repair

Power & reset: Confirm mains power, consumer unit, and boiler isolation switch are on. Try a manufacturer-approved reset.
Pressure (sealed systems): Target 1.0–1.5 bar when cold. If low, top up via the filling loop per the manual.
Thermostat & timers: Check target temp, schedules, and batteries.
Pilot/ignition (older models): If you smell gas, stop and call an emergency line.
Condensate pipe (freezing weather): Thaw gently with warm (not boiling) water.
Radiators: Bleed cold tops; then re-pressurise.

Safety note: Don’t remove boiler casings or touch gas components unless you’re Gas Safe registered.

Common Boiler Symptoms & Likely Causes

No hot water, heating okay: Diverter valve sticking; plate heat exchanger blocked.
Boiler locking out: Low pressure, ignition failure, flue/condensate issues, sensor faults.
Kettling/whistling: Limescale or sludge in heat exchanger; poor circulation.
Radiators cold at top: Air in system.
Radiators cold at bottom: Sludge—powerflush or magnetic filter needed.
Pressure repeatedly drops: Leak, PRV passing, or expansion vessel failure.
Intermittent hot water: Flow sensor or thermistor issues.

What Your Boiler Error Codes Usually Mean

(Always check your exact model manual—codes vary by range.)

Worcester Bosch

EA / 227: Ignition/ flame not detected (gas/air/ignition issue)
E9: Overheat / heat exchanger temperature too high
C6: Fan speed error (flueing/airflow issue)

Worcester Bosch codes

Vaillant

F28: Ignition failed (no flame)
F29: Flame lost during operation
F22: Low water pressure
F75: Pump/pressure sensor mismatch (often vessel or sensor)

Vaillant codes

Ideal

L2: Ignition lockout / flame failure
F1: Low system pressure
F2: Flame loss

Ideal codes

Baxi

E119: Low system pressure
E133: Gas supply/ignition fault
E160: Fan/PCB communication fault

Baxi codes

Step-by-Step: Re-Pressurise a Sealed System (If Your Manual Allows)

Turn off heating, let system cool.
Locate filling loop; open valves slowly.
Watch the gauge rise to ~1.2 bar; close valves.
Reset boiler; check for leaks.
If pressure drops again, call an engineer.

Preventative Care That Actually Works

Annual Gas Safe service (combustion checks, seals, safety devices)
Magnetic filter + inhibitor to cut sludge
System flush when radiators are sludged or new boiler fitted
Limescale control in hard-water areas (scale reducer)
Keep condensate runs internal or insulated externally

When to Stop DIY and Call a Pro

Smell of gas, sooting, repeated lockouts, visible leaks, or removing case panels
Error codes returning after a reset
Pressure won’t hold; vessel/PRV likely needs work
Any electrical/PCB, gas valve, burner, or flue issue

Pricing & Transparent Fixes

Our Gas Safe engineer can come to your location, diagnose the issue, and fix the boiler on the spot if required. We operate on fair mid-market rates with no hidden extras.

Diagnosis visit — includes the issue finding of on-site assessment and basic adjustments.
Clear pricing — labour charged is clearly discussed before work proceed; parts at trade-fair rates.
Typical fixes we handle: diverter valve replacement, expansion vessel re-charge or swap, PRV replacement, sensors/thermistors, plate heat exchanger clean/replace, condensate and ignition faults.

See Boiler Repair Service | See Rates

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Need help fast?

24/7 emergencies, ~1-hour ETA. Certified plumbing, heating/boiler, drainage, gas and electrical engineers—transparent pricing, fully insured, UK-wide. Call or book online.

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FAQ

Do you really operate 24/7?

Yes. Our operators answer calls day and night, 365 days a year. Emergency jobs are prioritised.

Why does my boiler keep losing pressure?

A sealed heating system should sit around 1.0–1.5 bar when cold; if the gauge keeps dropping, the usual culprits are a tiny system leak (radiator valves, pipework, joints), a pressure-relief valve (PRV) that’s passing after a previous over-pressure event, or an expansion vessel that’s lost its air charge or has a failed diaphragm. You can safely top up via the filling loop to about 1.2 bar cold and bleed any trapped air, but if pressure falls again within days, you’ll likely need an engineer to leak-trace, recharge or replace the vessel, and check the PRV and gauge for accuracy.

Is it safe to reset a boiler?

It’s generally safe to perform a single reset if your manual advises it and you’ve checked the basics—power on, programmer/thermostat settings correct, system pressure in range, and no frozen condensate—because a one-off lockout can follow a brief supply or sensor blip. It is not safe to keep resetting a boiler that repeatedly locks out, or to reset if you smell gas, see sooting or scorch marks, or hear unusual ignition noises; repeated lockouts indicate a fault that needs a Gas Safe engineer to diagnose properly.

What bar should my boiler be at?

Most combi and system boilers are happiest at roughly 1.0–1.5 bar when cold (the sweet spot is around 1.2 bar), and it’s normal for pressure to rise to about 2.0–2.5 bar when the system is hot and circulating. If the gauge climbs to 3.0 bar and the PRV discharges water outside, or if pressure swings wildly between low cold and very high hot, that usually points to expansion-control problems—often a flat expansion vessel—or a filling loop left slightly open.

Do I need a powerflush or just a chemical clean?

A chemical clean with fresh inhibitor is usually enough for light sludge, slightly uneven radiator temperatures, or mildly discoloured water, and it’s best paired with servicing or fitting a magnetic filter to keep magnetite in check. A powerflush is warranted when you have heavy sludge symptoms—radiators cold at the bottom, kettling/whistling from the boiler, frequent pump or valve issues, very dark system water, or when connecting a new boiler to old pipework—because the higher-flow cleaning dislodges deposits that chemicals alone won’t shift.

How often should a boiler be serviced?

Annually: a yearly service maintains safety and efficiency by checking combustion quality, gas pressures, seals and flue integrity, cleaning condensate traps, verifying expansion vessel charge and safety devices, and confirming adequate inhibitor and system health; it also helps protect manufacturer warranties and, for landlords, supports compliance alongside the separate annual Gas Safety check for tenanted properties.