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Install a system boiler.
Sealed-system boiler with unvented or vented cylinder — for higher hot-water demand and multi-bathroom homes.
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Cylinder undersized
— runs out of hot water during peak demand.
Inadequate cold supply
— flow rate suffers if 15 mm mains.
Wrong cylinder type
— vented vs unvented matter for safety and flow.
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
System boiler only (replace existing)Existing cylinder reused
£2.2k–£3.5k
Boiler + new unvented cylinderMid-range UK install
£3.5k–£5.5k
Combi → system conversionAdding cylinder back
£3.8k–£6.5k
Premium system (35kW+ + 300L cylinder)Large family home
£5k–£8k
!
Quote spread is typically ± 18% — always get 3 quotes.
At a glance
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A system boiler works with a separate hot water cylinder (typically unvented) — heating water that's stored ready for instant high-flow use across multiple bathrooms. They suit larger UK homes with 3+ bathrooms or simultaneous hot water demand. This guide covers system boiler install costs in 2026 and when they're the right choice over a combi.
System boilers cost more to install than combis (you need a cylinder too) but deliver better flow rates for larger families.
What Does the Work Involve?
- Survey of existing system, cylinder location, gas supply.
- Power-flushing existing system.
- Boiler installation with flue, condensate drain.
- Unvented cylinder install (or upgrade existing).
- Pressure-rated pipework to cylinder.
- System balancing and commissioning.
- Building Control notification (G3 for unvented cylinder).
Typical Costs
| Configuration | Low £ | High £ | Notes |
|---|---|---|---|
| System boiler only (replace existing) | 2,200 | 3,500 | Existing cylinder reused |
| Boiler + new unvented cylinder | 3,500 | 5,500 | Mid-range UK install |
| Combi → system conversion | 3,800 | 6,500 | Adding cylinder back |
| Premium system (35kW+ + 300L cylinder) | 5,000 | 8,000 | Large family home |
How Long Does It Take?
- Like-for-like system swap: 1–2 days.
- With new cylinder: 2–3 days.
- Combi to system conversion: 3–4 days.
DIY or Professional?
Gas Safe Registered engineer for boiler. G3 qualified engineer for unvented cylinder (separate certification). Both required by UK law.
Choosing the Right Tradesperson
- Verify Gas Safe + G3 (unvented cylinder) certifications.
- Manufacturer accreditation for warranty.
- Discuss cylinder size — typically 1 person = 35L; family of 4 = 200L+.
- Confirm pipework sizing for cylinder flow rate.
UK Regulations
- Gas Safe Register: required for gas.
- G3 certification: required for unvented cylinder.
- Building Regs Part G: hot water safety.
- Boiler Plus regulations: smart controls or load compensation required.
Common Problems
- Cylinder undersized — runs out of hot water during peak demand.
- Inadequate cold supply — flow rate suffers if 15 mm mains.
- Skipped power flush — new boiler clogs.
- Wrong cylinder type — vented vs unvented matter for safety and flow.
- Bad pump or 2-port valve — system imbalance.
Frequently Asked Questions
When should I choose system over combi?
3+ bathrooms, large family, want to use multiple hot taps at once, prefer instant high-flow hot water.
What's an unvented cylinder?
Hot water cylinder under mains pressure (no header tank). Higher flow than vented but needs G3 install.
How big a cylinder do I need?
~50L/person rule of thumb. Family of 4 typically needs 200–250L. Active families with multiple showers prefer 300L.
Can I convert combi to system?
Yes — re-add cylinder. £4,000–£6,000 typical. Worth it for larger families.
How long do system boilers last?
15–25 years. Cylinder lifespan 15–20. Often cylinder fails first.
Are system boilers more efficient?
Modern systems are similar to combi in efficiency. Tank-stored water can have heat loss; combis avoid this.
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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