Boilers – an essential guide

OK, it’s not sexy, but it is vital. Get in the know about your boiler, and stay warm, clean and safe. You can’t say much better than that!

Gas bill
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Your boiler is an essential part of a home heating system for the vast majority of us, but it’s a feature of your home that you probably don’t stop to consider until it breaks down. 

Unless you heat your home with electricity, however, or have a heat pump, you will need to pay attention to a boiler. It needs to work efficiently to help keep bills as low as possible, and it requires regular maintenance for safe working. So, a little bit of boiler study now will pay dividends and we promise you can get back to more exciting aspects of home design after that. 

Why replace your boiler?

If your boiler is more than 10 years old, it’s probably not doing the job it should anymore, so age can be a prompt to consider an alternative. More efficient condensing boilers, which make use of the waste heat in the flue gases to pre-heat cold water entering the boiler, have been mandatory since 2005.

Equally, if you’re planning an extension or loft conversion, the extra load that’s going to be put on the boiler means replacement needs consideration. The current model may not be up the job.

Which boiler should you choose?

A combi (or combination) boiler is the cheapest option. This produces hot water on demand. A combi boiler is also the least space hungry, and you won’t need to find room for a separate hot water cylinder either, of course. However, if you need a larger supply of hot water – for example, if your home has several bathrooms – it probably won’t be suitable.

Larger homes and high hot water demand will require a system boiler or open-vent boiler to work with, respectively, a high-pressure system fed straight from the mains, which stores hot water in a cylinder, or a conventional open-vented gravity-fed system with tanks in the loft. Often like-for-like replacement is best – for example, upgrading an old system model for a new one – but some older open-vent systems may benefit from an upgrade to a system boiler. 

As well as selecting the right type of boiler for your situation, you’ll need the right size. This is determined by a heat loss calculation which takes into account the house’s size, how it’s constructed, insulation levels and airtightness, and the hot water requirements. A heating engineer should undertake this.

Get a quote for a boiler replacement

You should always use a registered installer to fit a new gas boiler. Gas and LPG boilers should be fitted by someone who is Gas Safe registered. Call on an OFTEC registered installer to put in a new oil boiler. 

It’s worth getting three quotations when you’re buying a new boiler. Make sure they cover the same elements so you’re comparing like for like. The installer will ensure the system is building regulations compliant and you should get documents to prove this. Keep these as you could be asked for them when you come to sell your home.

How much does a new boiler cost?

A straightforward gas boiler replacement plus thermostatic radiator valves will typically cost about £2,300, according to the Energy Saving Trust (EST).

Depending on the age of the boiler you’re replacing, you could see some good savings from making the swap. Getting a new A-rated condensing boiler with programmer, room thermostat and thermostatic radiators valves (TRVs) fitted in place of an old G-rated boiler (the least efficient) could save from £175 per year in a mid-terrace home to £305 in a detached if you live in England, Scotland or Wales, the EST calculates.

How to take care of a boiler

For safety, boilers are designed to cut out if something is not quite right. An error code on the display will tell the engineer the cause of any problem.

A common reason for a boiler not operating is because the pressure in the system has dropped below the minimum level needed for it to work. Pressure loss is easy to resolve by topping up your system – check the boiler’s instructions to see how to do this, or ask your installer. However, it’s essential that the cause of the pressure loss is found – often this will simply be a tiny leak in the system or water supply problems in the local area.

Many problems are caused by something within the system (that is the pipes and radiators connected to the boiler) – usually air or dirt. Air can stop the water within the system from circulating properly, while dirt can foul parts in the boiler itself. If there are gurgling noises from radiators, or the top or bottom is cool, you’ll need to bleed the radiators. Remember to re-pressurise your system if you do this.

This may be a temporary fix, though, so your registered installer should investigate the cause. For this reason, manufacturers recommend that a central heating system is flushed before a new boiler is fitted, and that a filter is then fitted to protect the boiler from any dirt that might build up in the system over time.

An issue many combi boiler owners have experienced during harsh winters is due to frozen condensate pipes. If the pipe freezes, it will cause a boiler to shut down until the condensate melts. To prevent this, condensate pipes should be installed indoors, or adequately insulated.

A boiler should be serviced annually. This should pre-empt any problems and avoid a breakdown, as well as ensure the boiler is operating efficiently. If your boiler is still under guarantee, an annual service is usually part of the terms and conditions. It is illegal for an unqualified person to attempt to repair or install a boiler.


You may be insured for emergency boiler repairs under your home policy. However, policies may only be valid if the boiler has been serviced by a qualified person within the last 12 months, so make sure you check the details.

If you’ve taken out specialist boiler insurance you should be covered for emergency repairs and an annual service. These services generally cover repairs to the boiler, and some also cover boiler replacement, dependent on the model’s age.

What about boiler safety?

When fossil fuels, such as coal, wood, oil or gas don’t burn properly colourless, odourless carbon monoxide (CO) gas is formed, which is poisonous and can cause death. If the boiler isn’t working correctly, CO can be produced and therefore your Gas Safe or OFTEC registered engineer will check your boiler’s correct functioning and for CO during the annual service. It is also important to have an audible CO alarm in your home and to test it regularly. 

If your gas boiler has a flue that can’t be checked along its whole length, you should have inspection hatches fitted, so your installer can make sure there are no breaks in the flue that could leak carbon monoxide. If you prefer not to have hatches fitted, a boiler safety shutdown device can be installed. This features a detector that is fixed against the ceiling or wall that covers the flue, and has a sensor that goes through into the void, continuously monitoring for CO. If the poisonous gas is detected, it sends a signal to the boiler to switch off, and gives out a warning signal, so you know to call your Gas Safe-registered engineer.

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