A boiler failure is usually sudden and always unwelcome, but even a planned replacement rewards careful comparison of boiler types, efficiency ratings, and installer quotes. The difference between a hasty emergency swap and a considered replacement can easily run to £1,000 or more in upfront cost, and significantly more over the boiler’s 10–15 year life in fuel bills.

For a straightforward like-for-like combi boiler swap in a three-bedroom semi, budget £1,800–£2,800 all-in from a regional firm, or £2,500–£3,500 from a national installer. Moving from a back boiler or heat-only system to a combi adds complexity: expect £2,500–£5,000 including system changes.

Boiler Types

Understanding the three main boiler types clarifies which is suitable for your property.

Combi boiler (combination)

A combi heats radiators and produces hot water on demand from the mains cold water supply, with no separate hot water cylinder or cold water tank in the loft. It is compact, fitting inside a standard kitchen unit or airing cupboard, and is the most popular choice in the UK for properties with up to three bedrooms and one bathroom.

Best suited to: smaller to medium properties; households of up to 4 people; properties where loft space is limited; conversions and flats
Limitations: hot water flow rate is limited by boiler output and mains pressure (typically 10–15 litres/minute at adequate temperature); running two showers simultaneously can cause a pressure drop; not suited to large family homes with multiple bathrooms expecting simultaneous hot water use

As an example, Worcester Bosch rates its 30 kW Greenstar 4000 combi at 12.3 litres/minute of hot water with a 35°C temperature rise.1 It needs a static mains pressure of at least 1.2 bar to deliver that flow.1

A new gas boiler, combi or not, must be at least 92% efficient (ErP seasonal efficiency) under current Building Regulations.2 Leading manufacturers include Worcester Bosch (Greenstar range), Vaillant (ecoTEC), Baxi, and Ideal. A 30–35 kW output covers most three-bedroom properties comfortably.

System boiler

A system boiler heats radiators (via an expansion vessel and pump integral to the boiler) and fills a separate hot water cylinder. With an unvented cylinder, mains cold water feeds the cylinder directly, so there is no cold water tank in the loft (a system boiler can also serve a vented cylinder fed from a loft tank). This gives higher hot water flow rates, typically 30–60 litres/minute, making it suitable for large properties and multiple bathrooms.

Best suited to: 4+ bedroom properties; homes where hot water demand is high and simultaneous use is common; properties being upgraded from a traditional gravity-fed system where the loft tanks are being removed
Limitations: requires space for a hot water cylinder (typically 150–300 litres, needing a cupboard roughly 600 mm square and 1.2–2.1 m tall); hot water temperature drops if the cylinder is exhausted before it reheats (typically 20–40 minutes)

As an example, Heatrae Sadia’s Megaflo Eco unvented cylinders range from 802 mm tall for the 70 litre model to 2,053 mm for the 300 litre model.3 The same maker quotes full reheat times of 17–39 minutes, depending on size, with an 80°C primary flow.3 It recommends a mains supply of at least 1.5 bar at 20 litres/minute.3

Heat-only (regular/conventional) boiler

A heat-only boiler requires both a hot water cylinder and cold water storage tanks in the loft. Almost all new replacements are of combi or system type, heat-only is only specified today where the existing gravity-fed system is sound, the household is large, or where an unvented system is not appropriate (very low mains pressure, for example).

Unvented cylinders

A system boiler can be paired with an unvented hot water cylinder (e.g. Megaflo, Range Tribune) that stores hot water at mains pressure, allowing power showers throughout the house without pumps. Unvented cylinders cost £600–£1,500 supply. They should be fitted by an installer registered with a competent person scheme for unvented hot water systems (often called G3-qualified), who self-certifies the work rather than notifying building control in advance.4 They need a tundish and a correctly routed discharge pipe from the safety valves.4 Manufacturers usually make an annual service a condition of the warranty.

Cost Breakdown

Boiler unit costs

Prices below are for the boiler unit only (supply), before installation.

Boiler type and outputBudget brandMid-rangePremium
Combi 24–28 kW (2-3 bed)£600£900£1,400
Combi 30–35 kW (3-4 bed)£750£1,100£1,700
System 18–24 kW£700£1,000£1,500
System 28–35 kW£850£1,300£1,800
Heat-only 15–30 kW£650£950£1,400

Installation labour costs

Prices below are for labour only, before the cost of the boiler itself.

ScenarioLowTypicalHigh
Like-for-like combi swap (same location)£350£550£900
Like-for-like system boiler swap£450£700£1,100
Moving boiler to new location (e.g. kitchen to utility)£700£1,100£1,800
Replacing back boiler with combi (system changes required)£1,200£2,000£3,500
Full central heating installation (3-bed, 8–10 radiators)£3,500£6,000£9,500

All-in replacement costs (supply and fit)

Boiler type2-bed flat / small house3-bed semi4-bed detached
Combi (mid-range brand)£1,600–£2,400£2,000–£3,000£2,500–£4,000
System + cylinder£2,500–£4,000£3,000–£5,000£4,000–£7,000
Premium combi (Worcester, Vaillant)£2,200–£3,200£2,800–£4,000£3,500–£5,500

National comparison-site installers (British Gas, HomeServe, Boxt) typically quote at the higher end of the ranges above; local Gas Safe registered sole traders or small firms often quote less. Government analysis for the Boiler Upgrade Scheme puts the average cost of a gas boiler, including VAT, at about £3,500.5

Combi vs System Boiler: Which to Choose?

FactorCombiSystem + cylinder
Hot water flow rate10–15 l/min (dependent on boiler size)30–60 l/min (mains pressure)
Simultaneous use (2+ showers)May drop pressureHandles comfortably
Space requiredBoiler only (about 700 × 400 × 300 mm)Boiler + cylinder (typically 150–300 litres)
Mains pressure requiredAbout 1–1.5 bar for full flow (check the maker’s figure)About 1.5 bar for an unvented cylinder
Installation costLowerHigher (cylinder, zone valves)
Running costSimilar if same efficiencySimilar; system may be slightly more efficient at full load
Hot water wait timeInstant (cold start)Instant from stored cylinder
Recovery after peak useContinuous (no storage limit)About 20–40 min to reheat cylinder
Suitability for solar thermalPoor (no cylinder to store solar heat)Excellent, add solar coil cylinder
Suitability for heat pumpNot ideal, heat pumps prefer low-temperature systems with cylindersBetter, but a dedicated heat pump cylinder is preferred

What the Installation Should Include

A competent Gas Safe installation for a like-for-like swap should include:

  • Removal and safe disposal of the old boiler
  • Fitting the new boiler with all new fittings, filling loop, and pressure gauge
  • Magnetic filter (Adey MagnaClean or equivalent): manufacturers such as Worcester Bosch make a system filter a condition of their longer guarantees6
  • A scale reducer on a combi’s hot water side in a hard water area (total hardness above 200 ppm as CaCO3; check your water supplier’s data)2
  • For a combi in an existing home, at least one extra efficiency measure: weather or load compensation, flue gas heat recovery, or a smart thermostat with automation and optimisation2
  • Cleaning and flushing of the heating system, which Approved Document L expects whenever a new boiler is fitted, followed by a chemical inhibitor2
  • A power flush (£150–£400 extra) if sludge is present, or a chemical clean for cleaner systems
  • Flue extension and terminal to current safety distances (300 mm from any opening window, 600 mm from any surface facing the terminal)7
  • Gas tightness test, combustion analysis, and completion certificate
  • Building Regulations compliance certificate via Gas Safe Register8

The registered business has a legal duty to notify the work to building control, through Gas Safe Register, within 30 days and to make sure you receive a Building Regulations Compliance Certificate.8 The Gas Safe engineer self-certifies under the Competent Person Scheme.9 Keep the certificate: you may be asked for it when you sell the property.8

When to Consider a Heat Pump Instead

The Boiler Upgrade Scheme (BUS) currently provides a £7,500 grant toward an air source heat pump installation in England and Wales, with an extra £1,500 until March 2027 for homes heated by oil or LPG that have no mains gas connection.1011 If your property has loft insulation of at least 100 mm, cavity or solid wall insulation, and windows that are at least double-glazed, a heat pump may now be cost-competitive on a 10-year total-cost-of-ownership basis.

The Energy Saving Trust puts the typical cost of installing an air source heat pump at around £12,000 (before grant; Boiler Upgrade Scheme grant: £7,500).12 Larger homes cost more: see our guide to air source heat pump costs. Running costs depend heavily on electricity tariffs, the heat pump’s seasonal performance factor (SPF: units of heat delivered per unit of electricity over a year), and whether the heating system is designed for low flow temperatures (larger radiators, wet UFH or fan coils). In UK monitoring reported to government, heat pumps in the Electrification of Heat trial averaged an SPF of 2.8, while the better-performing systems in the Heat Pump Monitor dataset averaged 3.9.13

Heat pumps are not appropriate for every property in 2026: homes with poor insulation or undersized radiators may not see the expected savings, and some homes need a larger main fuse before a heat pump can be connected. Most domestic heat pumps run on an ordinary single-phase supply. Where the extra load needs it, a network operator such as UK Power Networks can upgrade a 30 or 60 amp main fuse to 80 or 100 amps.14 A heat loss survey by an MCS-certified installer, usually carried out as part of the installer’s design and quote, is the starting point.

Boiler Warranties and Servicing

Manufacturer guarantees vary by brand and model, and most depend on the boiler being registered soon after installation. Vaillant, for example, requires registration within 30 days, or the guarantee reverts to 12 months.15 Longer guarantees usually need the maker’s accredited installer and a system filter: Worcester Bosch gives its Greenstar 8000+ range 8 years as standard, 10 years with a Greenstar system filter and up to 12 years when an accredited installer fits one.6 This is worth factoring into the total cost of ownership.

An annual service costs £70–£150 from a local Gas Safe engineer. Manufacturers generally make an annual service a condition of the guarantee, and it is also the routine safety check on the appliance.15 Service contracts from national companies run £150–£350/year including a breakdown cover element.

A boiler that is serviced annually and has a magnetic filter fitted can be expected to last 10–15 years or more; a neglected boiler tends to fail sooner.

References (15)

Figures in this note were checked against the sources below on 2 October 2026. Superscript numbers in the text point to them. Every source the site cites, by topic.

  1. 1technicalsupplier figureWorcester Bosch, guidance.RN-Q9RHFQManufacturer's specification for the Greenstar 4000 range. It gives the minimum static mains pressure for maximum hot water flow on the 25 kW and 30 kW combis.
  2. 2technicalvery strongHM Government (GOV.UK).RN-IHEXOXThe full Approved Document L Volume 1 for dwellings, 2021 edition with the 2023 amendments, for use in England. This is the statutory guidance on conservation of fuel and power: the U-value tables for new and replacement elements, the rules on when an existing element must be upgraded, controls and commissioning for heating systems, and the notification requirements. If a note quotes a target U-value for a wall, roof, floor, window or door, this is where the number comes from.
  3. 3technicalsupplier figureHeatrae Sadia, a data sheet.RN-YHTQW5Manufacturer data for the Megaflo Eco indirect unvented cylinder. Nominal capacities are 70, 125, 145, 170, 210, 250 and 300 litres. Insulation is 60mm thick on every size. The immersion heater is 1 x 3 kW on all sizes. Empty weights are 25, 31, 35, 39, 43, 50 and 58 kg, but full weights are 125, 180, 204, 233, 253, 300 and 358 kg, which is the figure that decides whether a floor or an airing cupboard base can take it. Standing heat loss runs 332.15 kWh/year at 70 litres up to 616.85 kWh/year at 300 litres, so an oversized cylinder is a permanent standing cost. Maximum flow at 3 bar is 76 l/min across the range. The cylinder has an insulated temperature and pressure relief valve.
  4. 4technicalvery strongMinistry of Housing, Communities and Local Government.RN-YAW64MRequirement G3(4) states the hot water supply to any fixed bath must be designed so the water delivered to that bath cannot exceed 48 degrees C; paragraph 3.65 says this is achieved with an in-line blending valve or other temperature control device with a maximum temperature stop, and 3.66 accepts valves to BS EN 1111:1999 or BS EN 1287:1999 that fail safe. G3(3) requires unvented storage vessels to prevent stored water exceeding 100 degrees C and to convey safety discharge somewhere visible and safe. Regulation 36 and Requirement G2 cap potential consumption in a new dwelling at 125 litres per person per day, or the optional 110 litres per person per day where a planning condition specifies it; G2 applies only to dwellings erected or formed by material change of use. Sections G4 sanitary conveniences, G5 bathrooms and G6 food preparation areas set scale of provision.
  5. 5costvery strongDepartment for Energy Security and Net Zero.RN-AC5XPIA price, true at 2026-09. The summary business case for the Boiler Upgrade Scheme 2026 to 2030. Sets out the grant amounts, the budget, expected installation volumes and the department's reasoning for continuing the scheme. This is where the scheme's own numbers come from, as opposed to the application page, which tells a householder what to do but not what it is expected to cost the state or how long the money lasts.
  6. 6technicalsupplier figureWorcester Bosch, guidance.RN-TVF1W9Worcester Bosch's page on guarantee lengths for its boiler ranges. It gives standard and extended terms and the filter and accredited installer conditions.
  7. 7technicalvery strongMinistry of Housing, Communities and Local Government.RN-IU0X2MThe 2022 amendments took effect 1 October 2022 and extended the carbon monoxide alarm provisions. Table 2 sets the minimum flue sizes: 200mm diameter for a fireplace with an opening up to 500mm by 550mm, 125mm diameter for a closed appliance up to 20kW burning smokeless or Clean Air Act compliant fuel, 150mm for other closed appliances up to 30kW and 175mm for those above 30kW and up to 50kW. Diagram 17 gives the flue outlet clearances: at or within 600mm of the ridge the outlet must be at least 600mm above the ridge, elsewhere on a roof it must be at least 2300mm horizontally from the nearest weather surface and either 1000mm above the intersection or as high as the ridge, and it must be 1000mm above the top of any openable rooflight or dormer within 2300mm. Bends must be no more than 45 degrees to the vertical and paragraph 1.49 limits a sweepable flue to four changes of direction.
  8. 8technicalstrongGas Safe Register, guidance.RN-3GZ0JEGas Safe Register's consumer factsheet on notification of gas work in England and Wales, revised April 2016. Explains which gas work must be notified to building control, that a registered engineer does this through the scheme, and what certificate the householder should receive afterwards. The page blocks automated fetching, so it is described from its title and publisher.
  9. 9technicalvery strongGOV.UK, a government page.RN-FM4G39The official list of competent person schemes and what each one covers, plus what self-certification means: a registered installer certifies their own work as compliant and notifies building control, instead of the council inspecting it. This is the source for which trades can self-certify what, which is why a FENSA or Gas Safe registered installer does not need a separate building notice.
  10. 10technicalvery strongGOV.UK.RN-9EFNVZThe grant values: £7,500 towards an air source heat pump, £7,500 towards a ground source heat pump including water source heat pumps and those on shared ground loops, and £5,000 towards a biomass boiler. A further £1,500 is available until March 2027 where the property is heated by oil or LPG and has no mains gas connection. Only one grant is available per property. Capacity limits are 45kWth for an individual system, 70kWth for multiple heat pumps and 300kWth for shared ground loops. Hybrid systems pairing a gas boiler with a heat pump are not eligible, and an existing low carbon heating system cannot be replaced under the scheme.
  11. 11technicalvery strongGOV.UK (Department for Energy Security and Net Zero), a government page.RN-DJPZSBThe Boiler Upgrade Scheme application page: the grant amounts for an air source heat pump, ground source heat pump and biomass boiler, who is eligible, and the fact that the installer applies on the householder's behalf rather than the householder claiming it back. This is the official statement of the grant, so it is the figure to use rather than an installer's summary of it.
  12. 12costfairEnergy Saving Trust.RN-X9NXFYA price, true at 2026-09. Energy Saving Trust advice page on air source heat pumps, giving the typical installed cost for a UK home.
  13. 13technicalvery strongDepartment for Energy Security and Net Zero, guidance.RN-NQD61GA May 2026 research report published by DESNZ for the Committee on Fuel Poverty. It compares measured seasonal performance of UK heat pumps across monitoring datasets.
  14. 14technicalfairUK Power Networks, guidance.RN-MKMAIMUK Power Networks' page on domestic main fuse upgrades. It explains upgrading a 30 or 60 amp fuse to 80 or 100 amps for heat pumps and other new loads.
  15. 15technicalsupplier figureVaillant, guidance.RN-5QDLYPVaillant's terms for its domestic boiler guarantees. It sets the 30-day registration rule, the annual service condition and what the guarantee covers.

  1. Greenstar 4000 specifier’s guide, Worcester Bosch, accessed 29 September 2026. worcester-bosch.co.uk

  2. Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments, HM Government (GOV.UK), accessed 7 September 2026. assets.publishing.service.gov.uk

  3. Megaflo Eco Indirect unvented cylinder: technical data sheet, Heatrae Sadia, accessed 29 September 2026. heatraesadia.com

  4. Approved Document G: Sanitation, hot water safety and water efficiency (2015 edition incorporating 2016 and 2024 amendments), HM Government (GOV.UK), accessed 7 September 2026. assets.publishing.service.gov.uk

  5. Boiler Upgrade Scheme 2026–2030: Summary Business Case, Department for Energy Security and Net Zero, accessed 2 October 2026. Priced at 2026-09 and left there. assets.publishing.service.gov.uk

  6. Guarantee length, Worcester Bosch, accessed 29 September 2026. worcester-bosch.co.uk

  7. Approved Document J: Combustion appliances and fuel storage systems (2010 edition incorporating 2010, 2013 and 2022 amendments), HM Government (GOV.UK), accessed 7 September 2026. assets.publishing.service.gov.uk

  8. Notification of gas work in England & Wales (consumer factsheet, revised April 2016), Gas Safe Register, accessed 7 September 2026. gassaferegister.co.uk

  9. Competent person schemes, GOV.UK, accessed 7 September 2026. gov.uk

  10. Apply for the Boiler Upgrade Scheme: What you can get, GOV.UK, accessed 7 September 2026. gov.uk

  11. Apply for the Boiler Upgrade Scheme: Overview, GOV.UK, accessed 7 September 2026. gov.uk

  12. Air source heat pumps, Energy Saving Trust, accessed 30 September 2026. Priced at 2026-09 and left there. energysavingtrust.org.uk

  13. How to ensure a successful transition to heat pumps for households at risk of fuel poverty (report for the Committee on Fuel Poverty), Department for Energy Security and Net Zero, accessed 29 September 2026. assets.publishing.service.gov.uk

  14. Fuse upgrade, UK Power Networks, accessed 29 September 2026. ukpowernetworks.co.uk

  15. Domestic boiler guarantee terms and conditions, Vaillant, accessed 29 September 2026. vaillant.co.uk