Guide: Boilers
When to replace a boiler: the rule-of-half
The honest rule: when the next repair quote crosses half the cost of a new boiler and the unit is over ten years old, replacement almost always wins on a five-year total cost. Below that threshold, repair wins. The signs a heat exchanger is failing, and when a grant-backed heat pump beats a like-for-like swap, decide the rest.
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Get matchedThe rule-of-half
Take the engineer repair quote and compare it against a like-for-like new boiler installed. If the repair is more than half the cost of the new install, and the existing boiler is over ten years old, replace. Below the half threshold, repair.
The reason: a ten-year-old boiler has another major repair due within a few years on average, so paying half-cost now usually means paying again before payback.
Signs the heat exchanger is failing
- Pressure dropping below 1 bar more than once a week with no visible leak.
- Sludgy black-brown water in a radiator bleed.
- Kettling noises, where the boiler sounds like a kettle boiling.
- Frequent low-pressure error codes cleared by topping up.
- A visible white salt deposit on the casing under the flue.
Why a pre-emptive replacement can pay back
An older boiler runs at lower real-world efficiency than a current A-rated condensing combi. On a typical gas bill the efficiency upgrade saves a useful amount each year, and adding weather compensation and smart controls saves more. Over several years a planned replacement can pay for much of itself, especially against the cost of an emergency swap.
What you should not pay for
- An over-priced "powerflush only" quote on a domestic system.
- Replacing a sound older combi just because it is old; many run reliably for many years.
- A "whole-system replace" when only one component has failed, a very common upsell.
When to consider a heat pump instead
If the existing boiler is failing and the property has, or can fit, sensible insulation plus radiators that work at a lower flow temperature, the Boiler Upgrade Scheme grant of 7,500 pounds makes an air source heat pump competitive on day-one cost. Check the grant eligibility in full before committing.
How to choose a vetted trade
- Get the repair quote and a like-for-like new-boiler quote in writing, then apply the rule-of-half yourself.
- Use a Gas Safe registered engineer for the diagnosis and ask them to show the FGA and pressure readings.
- Before replacing, ask whether a grant-backed heat pump is viable for your home, since the 7,500 pound grant changes the maths.
Frequently asked questions
When is it cheaper to replace a boiler than repair it?
When the repair quote is more than half the cost of a new boiler installed and the existing unit is over ten years old. At that point a planned replacement usually wins on a five-year total cost, because an old boiler tends to need another major repair within a few years.
What are the signs my boiler is on its way out?
Pressure dropping with no visible leak, sludgy water in a radiator bleed, kettling noises, repeated low-pressure error codes, and a salt deposit under the flue. Any of these on a boiler over ten years old points towards the rule-of-half replace zone.
Should I replace my boiler with a heat pump?
It is worth checking. If your home is reasonably insulated and can run radiators at a lower flow temperature, the 7,500 pound Boiler Upgrade Scheme grant makes an air source heat pump competitive on upfront cost. Read the grant eligibility guide before committing.
Sources
Editor, Sorted Property
Oliver leads Sorted Property's editorial coverage of UK home services. He researches and writes the plain-English guides that help homeowners choose between installers and trades, drawing on the standards set by bodies such as MCS, TrustMark, the Energy Saving Trust and the Property Care Association, and is clear about what to check before any work starts.
Last reviewed: 11 June 2026