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Corrosion explained

How to protect your boiler and central heating system from corrosion

Boiler corrosion

What causes corrosion?

When metal interacts with another medium such as air or water, it starts reverting to its ore form in a process known as corrosion. Rust is an everyday example. Corrosion can occur within a central heating system for a number of reasons. Quite often, it’s just a result of the system water interacting with the metals that make up the various parts of the network, picking up bits of zinc or steel or copper as it circulates. If enough air gets into the water – perhaps through a leak in the pipework – that can lead to corrosion. It can also be triggered if the alkaline level in the water is too low. This can cause all kinds of problems for your system, and could even lead to boiler breakdown.

What kind of problems can corrosion lead to?

In a poorly installed, old or badly maintained system,  corrosion can result in what is known as sludge. If left unchecked, this black, mud-like deposit can:

  • Block the secondary heat exchanger on a combi boiler.
  • Cause blockages in the pipes and radiators, inhibiting water flow.
  • Attack internal boiler components, such as sensors, seals and capillaries.
  • Corrode the metal in radiators, causing leaks.
  • Cause the pump to fail.

You can replace individual boiler parts, but widespread corrosion damage could mean you have to replace your boiler.

How do I know if corrosion is a problem?

All heating systems feature metal and water interacting at some level, so there’ll always be a small amount of corrosion happening. Only serious corrosion will lead to a sludge build-up, however. One sign that you have a problem could be if you have to regularly bleed your radiators. [link to radiator piece] This might suggest that a leak somewhere in the pipework is allowing air to enter, increasing oxidation. Or it could be that what's trapped in your radiator is not air, but hydrogen, a by-product of corrosion. If you do bleed your radiator and the water is very dark in colour, this might also suggest a serious corrosion problem.

How do I protect my system from corrosion?

Right at the point of boiler installation, the water in the central heating system should be treated with a corrosion inhibitor. Generally introduced after the system has been flushed, these chemicals help minimise the effects of corrosion by forming a protective lining on the inside of the pipes.

If you have to make a claim on your boiler cover, you might want to know the levels of inhibitor in your system. Some boiler manufacturers test the system for inhibitor as the first step when they’re called out on warranty claims. If they find that the concentration of inhibitor is not adequate, they might say the warranty is invalid.

There’s a fairly easy way to get a rough idea if you have adequate levels of inhibitor in your system. Bleed radiator water into a jar and throw in a 1p coin and a steel nail. Leave it aside for a month. If the nail turns rusty, you know that’s what’s happening to the steel in your radiators as well and you need more inhibitor.

You can also fit a magnetic filter to protect your system. Attached to the pipes which go to your boiler, these devices reduce the build-up of sludge and other magnetic particles within the boiler by capturing them before they enter. This means they are limited to sludge which is “ferrous” in nature – that is, composed of iron or steel. But the corrosion of copper, zinc or aluminium components can also result in sludge, and this variety of sludge will of course not be drawn to a magnet.

What should I do if I have a corrosion problem?

Building regulations state that all heating systems should be flushed and cleaned when a new boiler is being installed. However, sometimes the installation may not have been absolutely perfect, or maybe the pipes took a bit of a hammering during some maintenance, or maybe the whole system just hasn’t had a bit of TLC for a while.

If you suspect your system is suffering from sludge issues, you should contact a Gas Safe registered engineer. If there's a serious problem, they’ll probably suggest power flushing. This is when a powerful, low pressure flow of liquid – water, or a mix of water and cleaning agents - is pushed through the system to clear out the sludge and rust. The process will take a few hours at least, and maybe as long as a day or two if there’s a lot to be cleared out. 

Regular boiler maintenance and repair is the best way to protect your system from corrosion. We’d be happy to talk to you about a quote if you're interested in boiler cover. Our standard product includes an annual boiler service, unlimited call-outs and access to a 24/7 helpline. Should it come to it, we’d be happy to install a new boiler for you. If you want to get a good overview of boilers, have a look at our Big Boiler Handbook.


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