If your heating starts to go wrong, it's easy to ignore the warning signs and hope that it will mend itself. But delay might prove to be a costly affair. Heating systems don't usually break down totally—they will usually give you some sort of warning first. And sometimes ignoring those early warning signs can be dangerous.

Here's when to recognize that you should call qualified heating engineers immediately.

Strange Odours from the System

If you notice gas, fumes, or burning odours, don't wait—turn off the system and take it seriously.

A sulphur or "rotten egg" smell could signal a gas leak. A burning or pungent odour might signal overheating electrical components. Exhaust-like odours may signal an issue with the flue or ventilation.

If you notice anything unusual smell:

  • Switch off the heat at the fuse box or appliance.
  • Leave the doors and windows open to ventilate.
  • Leave the building if the smell is bad or doesn't go away.
  • Avoid using light switches, matches, or anything else that could create a spark.
  • Call the gas emergency number (01642 686 111), and then arrange for a Gas Safe registered engineer to visit.

Gas leaks and flue failure aren't something to self-diagnose them on—they're issues that need immediate professional attention.

The Flame Looks the Wrong Colour

A healthy gas flame should be blue and steady. If it's flickering or gone yellow or orange, that's normally a sign something's wrong.

It could mean:

  • Dirty burners or blocked air inlets
  • Poor combustion or lack of oxygen
  • Flue gases failing to pass through properly

On more recent boilers, the system may spit out fault codes if it senses something threatening. Don't be resetting and praying it goes away—this could be covering up for bigger issues like carbon monoxide risk.

Get an engineer to check the combustion, clean the burners, and test the ventilation system properly.

Sudden changes in sound or loud noises

All boilers make a bit of noise—but if suddenly it is louder, sharper, or more frequent, then there's a warning sign.

Common causes are:

  • Banging or knocking: Often caused by air pockets or limescale in the heat exchanger, also known as "kettling."
  • Whistling: Can be caused by faulty valve, over-pressurised system, or pump fault.
  • Grinding or screeching: Usually worn fan motor or bearing.

These issues test the system and can destroy components if not repaired. A heating engineer could bleed the system, clean out scale, or replace worn-out parts prior to their failure.

Leaks, Corrosion, or Pressure Loss

Boiler or radiator water in your home is never a good sign. If the pressure still falls even after a top-up, or you notice rust, drips, or chalky buildup on fittings, something's leaking.

Leaks, if left untreated, could:

  • Short out electrical parts
  • Cause long-term rust
  • Lead to water or mould damage
  • Reduce heating efficiency

A heating engineer can spot the cause, perhaps a faulty pressure relief valve, split pipe, or seal. Catching it early avoids further damage down the line.

No Heat—or Strange Side Effects When the Heating's On

You'll obviously know something's wrong if you have no heating or hot water at all. But subtler signs matter, too.

They are:

  • Some radiators are cold
  • The system rapidly cycles on and off (short cycling)
  • Strange spikes in your energy bills
  • Dizziness, headaches, or fatigue when the heat is operating

If you feel sick when the system is operating, open the windows, switch off the boiler, and take some fresh air. These are symptoms of carbon monoxide exposure—especially in homes without detectors.

Even slight performance faults are likely to cause faulty parts, blocked valves, or flow problems—all of which can be safely rectified by a qualified heating engineer.

While You Waiting for Support

If there is any sign of danger—e.g., gas or burning odor—switch off the system and ventilate the building.

Whilst waiting for the engineer:

  • Avoid blocking vents or taping over alarms
  • Make sure your carbon monoxide alarm is working
  • Don't forget any error codes
  • Picture leaks or drips in case the problem does alter

These facts will allow your engineer to diagnose and fix the issue quicker.

Choose the Best Heating Professional

Within the UK, make sure that your engineer is Gas Safe registered—this isn't just for boilers, but for any gas appliance. Look up if they are registered and check their ID number on the internet.

Ask if they carry generic spares for your type of boiler and if they make emergency call-outs. It being the right person to turn up quickly can be the difference between rapid repair and days of waiting—especially winter when engineers are at their busiest.

Boilers will give you clues when something is wrong—unusual smells, unusual noises, unusual fire colours, or loss of pressure. The secret is not to ignore them.

A quick phone call to a reliable heating engineer can prevent a small issue from becoming an expensive or hazardous one. Don't let it worsen. Turn off the system if necessary, remain safe, and seek assistance in as soon as possible.