No heating
Possible causes
- Trapped air in the system
- Faulty thermostat or timer
- Failing pump
- Low system pressure
- Stuck diverter valve
Safe to check yourself
- Bleed any radiators that are cold at the top
- Check your thermostat is calling for heat
- Check the pressure gauge
When to call a professional
If bleeding radiators and checking pressure don't resolve it, or the problem keeps returning, it needs a proper diagnosis.
No hot water
Possible causes
- Faulty diverter valve
- Blocked plate heat exchanger
- Failed immersion heater (electric/Pulsacoil boilers)
- Faulty thermistor or PCB
Safe to check yourself
- Check the boiler is actually firing when a hot tap is opened
- Check the pressure gauge
When to call a professional
This almost always needs proper diagnostic equipment to pin down — not something to keep guessing at.
Boiler losing pressure
Possible causes
- A pinhole leak in the pipework
- A leaking pressure relief valve
- Recently bled radiators
- A failing expansion vessel
Safe to check yourself
- Top the pressure back up via the filling loop if you're comfortable doing so
- Watch whether it drops again within a day or two
When to call a professional
If pressure keeps dropping after topping up, that points to a genuine leak worth finding properly.
Boiler leaking
Possible causes
- Worn pump seal
- Pressure relief valve discharging
- Corroded or damaged pipework
- Cracked heat exchanger (less common, more serious)
Safe to check yourself
- Turn off the water supply to the boiler if there's a nearby isolation valve
- Switch the boiler off safely if you can
When to call a professional
Don't remove any panels or inspect the internals yourself. If you smell gas at any point, leave the property and call 0800 111 999 first.
Boiler making a noise
Possible causes
- Limescale or sludge build-up causing 'kettling' (a rumbling, kettle-like noise)
- Trapped air in the system
- A pump starting to wear
Safe to check yourself
- Note when the noise happens — constantly, or only when heating first fires up
- Try bleeding radiators if you haven't recently
When to call a professional
Kettling in particular is worth getting checked promptly — it can mean scale building up in a way that risks damaging the heat exchanger over time.
Boiler won't fire up
Possible causes
- Ignition fault
- Low system pressure
- An interrupted gas supply
- A PCB fault
Safe to check yourself
- Check the pressure gauge
- Check whether other gas appliances in the property are working
When to call a professional
If a simple reset (per your manual) doesn't get it going, don't keep trying — repeated failed ignition attempts can trigger a fault lockout.
Boiler keeps switching off
Possible causes
- Low pressure
- A frozen or blocked condensate pipe (common in cold weather)
- The overheat safety stat tripping
- An airlock
Safe to check yourself
- Check the pressure gauge
- If you can see the external condensate pipe and it's visibly frozen, gently warming it (not boiling water) can help — only if it's easily accessible and safe to reach
When to call a professional
If it keeps happening after checking the obvious, or you're not comfortable checking the condensate pipe, that's exactly what we're for.
Error code showing
Possible causes
- Codes vary by brand and model — see our full error code guide for common Worcester, Vaillant, Baxi, Ideal and Potterton codes
Safe to check yourself
- Check your boiler's manual for that specific code
- Try a single reset per the manual's instructions
When to call a professional
If the code returns after a reset, repeated resetting isn't a fix — it needs a proper diagnosis.
Boiler pressure too high
Possible causes
- The filling loop was left open or over-filled recently
- A faulty expansion vessel
- A faulty pressure relief valve
Safe to check yourself
- Check whether your filling loop tap is fully closed
- If pressure's only slightly high, it may settle on its own
When to call a professional
If pressure stays high or keeps climbing, that's worth a proper look rather than repeatedly bleeding radiators to compensate.
Boiler pressure too low
Possible causes
- Same causes as losing pressure over time — see our full guide
Safe to check yourself
- Top up via the filling loop if you're comfortable doing so
When to call a professional
If it drops again quickly after topping up, see our full pressure guide for what that usually means.
This tool is educational, not a diagnostic guarantee — it's here to help you understand what's likely going on, not to replace a proper inspection. We never suggest DIY work on anything involving the gas or electrical side of a boiler.