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Repair, replace or upgrade?
Cold rads, no hot water, F22 fault code? Describe what's happening — the AI tells you if it's a £150 repair or a £3k replacement.
Decode boiler fault codes.
Worcester, Vaillant, Baxi, Ideal — common lockout codes and what the engineer should be doing about each.
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kW sizing, modulation ratios, Benchmark cert, manufacturer warranty terms. Know what to ask before they recommend the most expensive box on the van.
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Top tips
Hiring a boiler engineer, without the regret.
Four moves that separate a smooth job from a nightmare.
Get a Gas Safe ID before letting them in.
Boilers are gas appliances. Anyone working on yours must be Gas Safe registered — and on the right scheme for your fuel (LPG, oil, natural gas are separate).
Demand the Benchmark certificate.
Every new boiler comes with a Benchmark commissioning checklist. Without it, the manufacturer warranty is void — and the certificate goes in your seller's pack later.
Size the boiler to the radiators, not the house.
Oversized combis short-cycle, wear out fast and cost more to run. A heat-loss calc — not a 'same as before' swap — is the sign of a careful installer.
Power-flush before fitting.
On a 10-year-old system, sludge will write off your new boiler's heat exchanger in 18 months. A power-flush should be quoted; if it's not, ask why.
Costs & timeline
Know what it costs. Know when it ends.
Indicative UK ranges and what affects price.
Cost range
By job type
Inc. VAT · 2026
Source: NMT quotes
Source: NMT quotes
Boiler service (annual)
£80–£150
Combi boiler swap (like-for-like)
£1.8k–£3k
Combi boiler upgrade (premium)Vaillant/Viessmann tier
£2.8k–£4.5k
System boiler + cylinder
£3k–£5.5k
Power flush
£450–£900
Conversion from back-boilerInc. removal + replumb
£3.5k–£6k
!
Quote spread is typically ± 18% — always get 3 quotes.
At a glance
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Boiler Engineer explained
"Boiler lockout" is when the boiler stops itself and shows a fault code instead of running normally. Usually the boiler can be reset; sometimes the issue is serious. This guide covers what UK boiler fault codes mean, the reset procedure, and when to call Gas Safe.
Reset Procedure (Most Brands)
- Note the fault code (will help engineer).
- Find reset button (R symbol or "Reset" usually).
- Press and hold 2-5 seconds.
- Wait 30 seconds for boiler to fire.
- If it re-locks immediately — call Gas Safe.
- If it runs — issue may be intermittent; service due.
Common Fault Code Meanings
| Brand | Code | Likely cause |
|---|---|---|
| Worcester Bosch | EA | Ignition failure |
| Worcester Bosch | E9 | Overheat |
| Vaillant | F22 | Low pressure |
| Vaillant | F75 | Pump or sensor |
| Ideal | F1 | Low pressure |
| Ideal | L2 | Flame failure |
| Baxi | E125 | Pressure |
| Baxi | E133 | Flame failure |
Typical Diagnosis & Repair Costs
| Issue | Low £ | High £ |
|---|---|---|
| Diagnosis visit | 80 | 180 |
| Ignition repair (electrodes) | 150 | 350 |
| Fan replacement | 200 | 500 |
| PCB replacement | 250 | 600 |
| Major component (heat exchanger) | 800 | 2,000 |
UK Regulations
- Gas Safe required for boiler repair.
- Reset button safe to use; component replacement isn't DIY.
Common Problems
- Repeated lockout — call engineer.
- No fault code displayed — power off then on; may reset.
- Frost lockout in winter — condensate pipe frozen; thaw with warm water.
Frequently Asked Questions
Can I just keep resetting?
Once or twice for intermittent issues. Repeated lockout means there's a real problem.
Why does boiler keep locking out?
Common: low pressure, ignition issues, dirty filter, frozen condensate pipe.
Should I replace or repair?
Repair if cost < 50% of new install. Replace if over.
What if no fault code shows?
Power-cycle the boiler. If still won't fire, call engineer.
Are fault codes universal?
No — each brand has its own. Manual or app lookup needed.
Is lockout dangerous?
No — it's the safety mechanism preventing operation when something's wrong.
This guide was written with AI assistance and is intended for general information only. Prices are estimates based on UK averages and may vary by region. Always get at least three quotes and consult a qualified professional before starting any work.
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