When you need a new boiler, not a flush

Signs the boiler is the problem, not the water

• It is over fifteen years old and has needed more than one repair in the last two seasons.
• Parts are getting hard to source. Once an engineer is hunting for a discontinued PCB, the economics have turned.
• A single repair is quoted at a substantial fraction of a replacement. A £700 heat exchanger on a fifteen-year-old boiler rarely makes sense.
• It is a back boiler or a floor-standing unit of considerable age. Serviceable, but nearing the end.
• Efficiency is poor by design. A pre-condensing boiler is losing a meaningful share of your gas up the flue regardless of how clean the system is.
• It keeps locking out and nobody has been able to find a persistent cause.

Where the flush still comes in

If you are replacing the boiler, the system still has to be cleaned — and now it genuinely is not optional. BS 7593:2019 sets out that the system is cleaned before the new boiler goes in, a permanent filter capturing magnetic and non-magnetic debris is fitted, and the correct inhibitor dose is added.

The reason is warranty. Manufacturers decline claims on heat exchangers damaged by debris from an uncleaned system, and they ask for evidence of the clean when assessing. A new boiler installed onto an uncleaned sludged system is a boiler you may effectively have no warranty on.

The useful consequence: if you are getting a boiler replaced, the flush should be quoted as part of that job rather than bought separately. It is cheaper bundled and it is done at the right point in the sequence.

What a boiler quote should itemise

You cannot compare two quotes that are single numbers. A quote you can actually assess separates out:

• Boiler make, model and output in kW — and the reasoning for that output, ideally a heat loss calculation rather than "same as the old one".
• Manufacturer warranty length, and what conditions keep it valid.
• System clean: chemical flush or power flush, stated explicitly.
• Magnetic filter: make and model, supplied and fitted.
• Inhibitor: product and dose.
• Any pipework alterations, gas supply upgrade, flue work, or condensate run.
• Controls — thermostat, TRVs, weather compensation — and whether they meet current Building Regulations.
• Gas Safe registration number, Building Regulations notification, and commissioning documentation.
• Making good, disposal of the old unit, and what is not included.

Rough costs

A straight combi swap in the same position, typically £1,800 to £3,000 depending on boiler and location. A system or heat-only boiler with a cylinder, or any change of boiler position or type, runs higher — commonly £3,000 to £5,000 once pipework, flue and making good are accounted for. London sits at the upper end. A system clean and filter, if quoted separately, adds roughly £400 to £1,000 on top.

These are broad ranges because the job varies enormously. Get three quotes, itemised as above, and compare like with like.

Worth considering before you commit

If your boiler is near the end anyway, it is worth at least pricing a heat pump alongside it. Grant funding and the economics have both moved, and the system cleanse you would need for a boiler is much the same work you would need for a heat pump. Replacing a boiler now is a decision you live with for another decade or more.

Tell us what the boiler is doing and we will put you in front of a Gas Safe engineer who can price a repair, a replacement and the system clean, and tell you which of the three is actually worth doing.