Real 2026 London prices for the common repairs, what moves the number, and how to avoid overpaying.
The short answer
The rule engineers use: if a repair costs more than half the price of a new boiler and your boiler is over 12 years old, replacement usually wins on total cost. Under 10 years old, repair almost always makes sense. Between the two, the specific fault decides.
What the law requires
The half-cost rule, and why age changes everything
Every repair-or-replace decision is the same bet: is this repair the last one for years, or the first of a series? Age is the best predictor we have. Under ten years old, a boiler is mid-life; parts are plentiful, the platform has years left, and even a £400 repair is buying a long runway. Past twelve years, three things shift at once: components begin failing in clusters rather than singly, efficiency has drifted well below the boiler’s day-one figures, and parts availability starts to thin, with some items reconditioned-only. That is why the same £450 PCB is a sensible repair on a six-year-old boiler and a poor one on a fifteen-year-old. The half-cost rule packages this into one honest sentence: a repair above half the price of a new boiler, on a unit past twelve years, rarely pays off. A typical London combi replacement runs £1,800 to £2,800 installed, so the practical threshold sits around £900 to £1,400 of repair on an old boiler, and well-quoted repairs almost never reach it unless the fault is major.
The faults that tip the decision
Some repairs are verdicts in themselves. A cracked or leaking main heat exchanger on an older boiler is the classic terminal diagnosis: the part alone can approach half a new boiler’s price before labour. A PCB failure on a unit past twelve years is the textbook borderline case. Repeated failures are their own signal regardless of size: two engineer visits in eighteen months means you are paying a new boiler’s deposit in instalments while keeping the old boiler’s bills. On the other side, plenty of dramatic-feeling faults are cheap: ignition electrodes, sensors, diverter valves and pressure faults are routine fixes at any age, and no honest engineer condemns a boiler over any of them.
The running-cost side of the maths
Repair-or-replace is not only about the repair bill. An old G-rated boiler converts roughly 70 per cent of its gas into useful heat; a modern A-rated condensing boiler exceeds 90 per cent. On a typical London gas bill, that efficiency gap is worth a meaningful sum every single year, which quietly subsidises the replacement long after the install is paid for. Add the value of a fresh warranty of up to ten or even twelve years (a decade of repair bills you will not have) and the honest comparison is never “£450 repair versus £2,300 boiler”; it is five-year total cost against five-year total cost. Sometimes the old boiler still wins that comparison, and when it does, we say so.
Sitting on a quote and not sure which way it points? Read us the boiler’s age and the fault, and we will give you both sets of numbers straight. Call 020 3000 0000.
How to protect yourself from a bad recommendation
The market fails people in both directions: firms that upsell replacements the boiler did not need, and firms that string out repairs on a boiler that died two winters ago. Four defences work. Get the fault named specifically, in writing, with a fixed repair price; “it is not worth fixing” without a diagnosis is a sales line, not an assessment. Apply the half-cost rule yourself against real installation prices, not inflated ones quoted to make the repair look good. On any condemnation, a second opinion costs little and regularly saves four figures. And check who benefits: an assessor who only sells boilers has one answer; ask for the repair quote anyway and watch how the conversation goes.
Repair or replace, quick answers
How long should a boiler last?
Twelve to fifteen years well serviced is normal; annual servicing is the biggest single factor in reaching the top of that range.
Is a 20-year-old boiler worth repairing?
Only for the cheapest faults, and even then as a bridge while you plan the replacement on your own schedule rather than a January breakdown’s.
Do new boilers really cut bills?
The efficiency gap is real and measurable, typically 10 to 20 per cent off gas use when replacing a unit over a decade old; savings scale with how much heating you actually use.
Should I wait for the old boiler to die completely?
No. A planned installation is cheaper and calmer than a distress purchase in a cold snap, and you keep your choice of boiler instead of taking whatever is in stock. Our boiler installation guide covers costs and types in full.