Is it worth replacing a boiler that still works?

This is a different question from the one most articles answer. Your boiler is not broken. It fires, it heats the house, nobody is being asked to make a decision today.

So the question is not whether the appliance is bad. It is whether you would rather make this decision now, on your terms, or later on the boiler's terms. Those are the only two options, and they lead to very different jobs.

The fuel bill argument, told properly

The saving from a more efficient boiler is the reason usually given, and it is real, but the headline number is quoted on a basis most people do not meet.

Energy Saving Trust publishes two separate figures. One is for replacing a G-rated boiler with an A-rated boiler. The other, larger one, is for replacing a G-rated boiler that has no heating controls with an A-rated boiler plus a full set of controls. The bigger figure is the one that ends up in advertising, and it is buying you two things, not one. If your controls are staying as they are, the smaller figure is the one that applies to you.

Then there is the starting point. Energy Saving Trust's own test is that a gas boiler installed after 2005 is almost certainly already a condensing boiler. If yours is, you are not closing the gap between a G rating and an A rating. You are closing a much narrower one, and the fuel saving on its own is a thin reason to replace a working appliance.

Source: Energy Saving Trust, Boiler types explained

What actually ends your ability to choose

Three things quietly close the window, and none of them announce themselves.

• Parts. Once a model goes out of production the supply thins, first at the merchants and then everywhere. Manufacturers are open about this in their own guarantee terms, which reserve the right to end repair services where necessary parts are no longer available or the product has been discontinued. A repairable fault on an unsupported boiler is not a repairable fault.
• The guarantee. While it runs, a major failure is an inconvenience. The day after it expires, the same failure is a decision about money, and it arrives without warning.
• The expensive component. Most faults are small parts. A few are not, and on an older appliance the large ones account for enough of what the whole boiler is worth that repairing it stops making sense. That is the point at which the choice gets made for you.

The real argument for going early

It is not the fuel bill. It is that a planned replacement is a replacement you choose, and an emergency one is not.

Plan it and you compare installers properly, you pick the model and the position rather than accepting what is available, you book the work for a week that suits you, and you can change things while the system is open: move the boiler, convert to a combi, upgrade controls, add a filter. Energy Saving Trust's own advice is to get at least three quotes before committing, and three quotes is not something anyone gathers with no heating and a small child in the house in January.

An emergency replacement gives you none of that. You take the first installer who can come, you take the boiler they can get hold of, it goes back exactly where the old one was, and the price is whatever it is because the alternative is another cold week.

Source: Energy Saving Trust, Boiler types explained

When the honest answer is no

Plenty of people ask this question about a boiler that should be left alone. If all of the following are true, replacing it now is buying very little.

• It is under about ten years old and the manufacturer's guarantee is still live.
• It has been serviced every year and the paperwork exists.
• It is not throwing intermittent lockouts, losing pressure or making new noises.
• It still suits the house, meaning the hot water keeps up with how you actually live now.

And do not replace a boiler to fix something else

A surprising number of replacements are bought to solve a problem the boiler was not causing. Radiators cold at the top or at one end of the house is usually air or sludge in the system. Pressure that keeps dropping is usually a leak somewhere in the pipework. Hot water that runs weak at the shower is often the incoming mains flow rate, which no new combi can improve.

Each of those is worth establishing before you spend anything, because if it is not the boiler then a new boiler will not fix it, and you will have had the disruption for nothing.

The moment the maths changes

There is one situation where replacing a working boiler is straightforwardly sensible, and it has nothing to do with the boiler's condition.

If work is happening anyway, a kitchen being refitted, an extension, a bathroom coming out, then the access, the disruption and the making good are being paid for once instead of twice. Moving a boiler is a much smaller undertaking when the walls are already open. If a boiler is somewhere you have always resented, that is the window.

How to actually decide

Find out three things and the answer usually presents itself. How old the appliance is and whether the guarantee is still running. Whether the model is still in production and supported. And what an installer says about the state of the system when they look at it, not what a website says about the model in general.

If you are leaning towards doing it, do it in the summer with time to compare. That is the whole advantage of replacing a boiler that still works, and it is the only advantage that disappears completely if you wait too long.