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How dangerous can a faulty boiler be?

How dangerous can a faulty boiler be?

Let’s start by putting your mind at rest. Modern, good quality boilers are incredibly safe. If yours is made by one of the big brands (Ideal, Worcester, Glow-Worm, Viessman, Vaillant etc.), then it will come with a host of safety features – pressure gauges, sensors, trips, relief valves and so on – that are designed never to let the boiler’s temperatures or pressures reach critical proportions.

You, the homeowner, will also play a vital part in that safety. You should familiarise yourself with the instructions and find out what all the messages, lights and gauges mean on the front of the boiler.

It’s also useful to get to know what sounds the boiler makes under normal operation, as unusual noises often come before technical warnings when something’s amiss. Your main responsibility is to ensure you have a professional boiler engineer give it its annual service. That way, any wear and tear can be discovered and fixed, and the engineer can make sure everything’s running normally and safely.

Now, we’ll cover some of the risks that can occur with a damaged or neglected boiler.

Carbon monoxide escape

Carbon monoxide (CO) is a very dangerous gas because it’s a silent killer. It doesn’t have a smell, so it can poison a family without their knowing. Tiny amounts of CO are produced when gas is burnt in a well functioning boiler, but it, along with the main exhaust gas (carbon dioxide, CO₂) should be vented to outside where they pose hardly any risk to life.

The risk here is twofold, though. First, if the boiler isn’t burning properly, i.e. if the fuel and oxygen levels aren’t mixing efficiently, then the proportion of CO will rise. Second, if the vent is blocked, this toxic gas can end up inside the home rather than outside.

Your annual inspection and service should cover both risk factors. The engineer will test the fuel/air mix and fine tune or fix the components that influence it. They’ll also check the flue is free of leaves, cobwebs, birds’ nests and so on to ensure safe exhaust (you can also do this yourself from time to time).


Boilers can overheat if they are boiling water much more slowly than they are intended to. When cool water from your radiators passes over the flame, or cold mains water goes through the heat exchanger, if it’s going too slow, it will heat up more than intended, just like when you leave your soup over the hob for too long.

The usual causes of this are partially blocked heat exchangers or faulty pumps, both of which should be picked up at your annual service.

Gas leak

Badly maintained boilers could develop leaks, which can be dangerous inside or outside the building. Unlike CO, mains gas does have a smell, so you should hopefully be able to detect this if it’s happening. It’s possible that the leak might be nothing to do with the boiler itself, but if you smell gas:

  • Do not turn on or off any electrical devices or switches
  • Open as many doors and windows as you can
  • Evacuate the building
  • Turn off the mains gas valve (if you can do it quickly)
  • Call 1800 20 50 50

Water leak

Just like gas, water enters and circulates via pipework, and any pipework can leak. The most common place is at joints or sharp bends, but damaged or corroded pipework can also spring a leak anywhere. A cold water leak comes with its own risks, such as electrical contact, damage to property or growth of mould, and should be dealt with as soon as you find it.

Hot water has all those risks plus the obvious risk of scalding, so any water leak from within the boiler or from around the central heating system should be dealt with immediately. Switch off the boiler and, if possible, turn off the mains water, and phone a boiler engineer, central heating fitter, plumber or water provider straight away (depending on where the leak is).

Stick with the experts

As we said at the start, boilers are generally safe and these risks are very rare. However, the risk is increased when you or your boiler engineer cuts corners with installing or servicing. That’s why you should stick with the experts when you’re having anything one with your boiler.

Published: 1 July 2022