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Install a combi boiler.
Like-for-like swap or full conversion from system — sized, fitted and commissioned in 1–2 days.
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Hiring a boiler engineer, without the regret.
Four moves that separate a smooth job from a nightmare.
Wrong size boiler
— over-sized cycles on/off; under-sized struggles with demand.
Inadequate flue route
— restrictions limit positioning; air ventilation matters.
Skipping power flush
— system sludge wrecks new boiler's heat exchanger.
Wrong gas supply
— old 15 mm pipework may be inadequate; needs upgrading.
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
Budget combi swap (24kW)Ideal Logic / Baxi 600
£1.8k–£2.5k
Mid-range combi swap (30kW)Worcester Bosch CDi, Vaillant ecoTEC plus
£2.4k–£3.5k
Premium combi (35kW+)Top-tier brands, long warranty
£3.2k–£4.5k
System → combi conversionRemoval of tanks + cylinder
£2.8k–£5k
Boiler relocation (extra)New gas + flue routes
£500–£1.5k
!
Quote spread is typically ± 18% — always get 3 quotes.
At a glance
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A combi (combination) boiler heats hot water on demand and runs central heating without a separate cylinder. It's the UK's most-installed boiler type — small, efficient, and ideal for homes with one or two bathrooms. This guide covers what combi installation costs in 2026, the sizing decisions, and what to look for in an installer.
Always use a Gas Safe Registered engineer — gas work by anyone else is illegal and dangerous in the UK.
What Does the Work Involve?
- Survey of existing system, gas supply, and flue route.
- Power-flushing existing system (highly recommended).
- Removal of old boiler and (if applicable) hot water cylinder + cold water tank.
- New combi installation with flue and condensate drain.
- Gas connection and pressure testing.
- Magnetic filter installation (most installs).
- System balancing and commissioning.
- Building Control notification (Gas Safe Register handles).
Typical Costs
| Configuration | Low £ | High £ | Notes |
|---|---|---|---|
| Budget combi swap (24kW) | 1,800 | 2,500 | Ideal Logic / Baxi 600 |
| Mid-range combi swap (30kW) | 2,400 | 3,500 | Worcester Bosch CDi, Vaillant ecoTEC plus |
| Premium combi (35kW+) | 3,200 | 4,500 | Top-tier brands, long warranty |
| System → combi conversion | 2,800 | 5,000 | Removal of tanks + cylinder |
| Boiler relocation (extra) | 500 | 1,500 | New gas + flue routes |
| Magnetic filter (with install) | +150 | +250 | Typically included |
How Long Does It Take?
- Like-for-like combi swap: 1 day.
- System to combi conversion: 2–3 days.
- Boiler relocation: 2 days.
- Full system upgrade with relocation: 3–5 days.
DIY or Professional?
Strictly professional. Gas work without Gas Safe Registration is illegal in the UK and a serious safety hazard. Carbon monoxide leaks from poorly-installed boilers kill UK homeowners annually.
Choosing the Right Tradesperson
- Verify Gas Safe Registration at gassaferegister.co.uk — every engineer has an ID number.
- Confirm manufacturer accreditation for longest warranties (e.g., Worcester Accredited Installer for 10-year warranty).
- Get a fixed-price quote covering boiler, install, flue, filter, controls.
- Confirm Building Control notification included.
- 10-year warranty on most premium brands when installed by accredited engineers.
UK Regulations
- Gas Safe Register: legal requirement for gas work.
- Building Regulations: Part L (energy efficiency), Part J (combustion).
- ErP rating: minimum A-class boilers since 2018.
- Boiler Plus regulations: new installs must have programmer + room thermostat plus weather/load compensation or smart controls.
- Hydrogen-ready boilers: standard from 2025; mandatory for new installs from 2026 if confirmed.
Common Problems
- Wrong size boiler — over-sized cycles on/off; under-sized struggles with demand.
- Inadequate flue route — restrictions limit positioning; air ventilation matters.
- Skipping power flush — system sludge wrecks new boiler's heat exchanger.
- Wrong gas supply — old 15 mm pipework may be inadequate; needs upgrading.
- Cheap boiler choice — 5-year warranty on budget vs 10-year on premium; the maths favour mid-range.
Frequently Asked Questions
What size combi boiler do I need?
Bedrooms / bathrooms guide: 1 bath = 24-28kW; 2 baths = 30-35kW; 3+ baths often need system boiler not combi.
How long do combi boilers last?
10-15 years for budget; 15-20 for premium. Annual servicing extends life significantly.
Best combi boiler brands?
Premium: Worcester Bosch, Vaillant, Viessmann. Mid-range: Ideal, Baxi. Budget: Vokera, Main.
Should I get a 10-year warranty?
Yes if available. Pays for itself if heat exchanger fails. Requires manufacturer-accredited installer.
Combi or system boiler?
Combi: small homes, 1-2 baths, no need for stored hot water. System: 3+ baths, simultaneous demand, prefer hot water tank for backup.
How much can I save by shopping around?
Quotes can vary 30-50% for same boiler/install. Always get 3 quotes. Mid-range is usually best value.
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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