When your boiler packs in, the first worry is usually the bill. The good news: most repairs are a single, known part — not a mystery. Once an engineer has diagnosed the fault, the price should be predictable. Here’s what the common ones actually cost in 2026, what the genuinely expensive failures are, and how to make sure you’re paying a fair price.

What common boiler repairs cost in Southampton (2026)

These are fitted prices — part plus labour — for a standard combi. Add the diagnostic call-out (£60–£90) if it’s a first visit:

  • Thermocouple / flame sensor: £100–£180
  • Pressure sensor or relief valve: £120–£200
  • Expansion vessel (the usual "keeps losing pressure" culprit): £120–£250
  • Diverter valve (hot water but no heating, or vice versa): £150–£300
  • Pump: £200–£350
  • Fan: £200–£400
  • Gas valve: £250–£450
  • Printed circuit board (PCB): £250–£500
  • Heat exchanger: £400–£650+ — the big one

Prices vary with the make and model — parts for a premium Vaillant or Worcester cost more than a budget brand, but they’re also easier to get hold of and more reliable once fitted.

The most expensive part: the heat exchanger

If you take one thing from this article, make it this. The heat exchanger is the component the burner heats your water through, and it’s the single most expensive thing to replace — £400–£650+ once you include labour, sometimes more on a system boiler.

Because it’s so costly, a failed heat exchanger is the classic tipping point between repair and replace. On a boiler under 8 years old it’s often still under warranty and worth doing. On a 12-year-old boiler, spending £600 on a heat exchanger rarely makes sense — that money is better put towards a new unit. (I’ve written a full age-by-age repair-or-replace guide if you’re at that decision.)

The PCB (the boiler’s brain) and the gas valve are the next most expensive, at roughly £250–£500 each.

“My boiler keeps losing pressure” — what that usually costs

This is the most common call I get, so it’s worth its own section. If your pressure keeps dropping and you’re topping it up every few days, it’s almost always one of two cheap-ish fixes:

  • A tired expansion vessel that needs re-pressurising or replacing — £120–£250.
  • A weeping pressure relief valve — £120–£200.

Occasionally it’s a small leak somewhere on the system. What it is almost never is a reason to replace the whole boiler — so if someone’s opening conversation about a pressure problem is “you need a new boiler”, get a second opinion.

What about the call-out fee?

A diagnostic call-out around Southampton is typically £60–£90. A fair engineer puts that towards the repair if you go ahead. Two things to watch:

  • “Free call-out” isn’t always free — sometimes the cost is just baked into a higher repair price. Free is genuinely free only if the repair quote is still competitive.
  • Be wary of a fixed repair price quoted before diagnosis. Nobody can honestly tell you the cost before they know the fault. A proper quote comes after the diagnosis, not before.

How to avoid being overcharged

I’d rather you knew this even if you never call me. Three simple rules protect you on any boiler repair:

  1. Get the fault named first. “It’s the diverter valve” is a diagnosis you can sanity-check. “It’s playing up” is not.
  2. Ask for part and labour separately. A transparent engineer has no problem itemising it. You can look up roughly what a part should cost.
  3. Be sceptical of “replace the whole thing” for a single cheap fault. A £150 expansion vessel is not a reason to spend £2,500. If the recommendation jumps straight to replacement, a second opinion costs you one call-out and can save you thousands.

When I diagnose a fault, I tell you exactly what it is, what the part costs, and what it’ll be to fit — and if I genuinely think you’re better off replacing rather than repairing, I’ll say so and explain why. That’s the whole job.

Repair now, or wait?

If the boiler still runs but a fault is brewing — intermittent lockouts, a noise, creeping pressure — it’s cheaper to deal with it now than to wait for a total failure on the coldest night, when you’re paying emergency rates and possibly waiting for a part. The same fix is the same fix; the only thing waiting changes is your stress levels and, sometimes, the price.

If your boiler has actually stopped and you’ve no heating or hot water, that’s the point to call someone out properly rather than keep resetting it — repeated lockouts can turn a cheap fix into an expensive one.