Choosing the right boiler type can mean the difference between a household that never runs out of hot water and one that leaves the second bathroom cold after a morning rush. In the UK, three boiler types dominate: combi (combination), system, and regular (heat-only). Each suits a different set of circumstances, and fitting the wrong type is a costly mistake.

The Three Types at a Glance

FeatureCombiSystemRegular (Heat-only)
Hot water cylinder neededNoYes (usually unvented)Yes (vented or unvented)
Cold water tank in loftNoNoYes (usually)
Simultaneous hot water outletsLimitedUnlimited (cylinder capacity)Unlimited (cylinder capacity)
Best for1–2 bathrooms2–4 bathrooms, high demandOlder gravity systems, like-for-like swaps
Supply & fit cost (rough)£1,800–£3,500£2,000–£4,500£1,800–£3,500
Flue requiredYesYesYes
Typical install time1 day1–2 days1–2 days

Combi Boilers

A combi boiler heats water on demand directly from the mains, there is no storage cylinder.1 Cold water enters at mains pressure, passes through a heat exchanger, and arrives at the tap or shower at close to mains pressure.

Best for:

  • Flats and smaller homes with one bathroom and up to two people2
  • Properties with limited airing-cupboard or loft space
  • Homes with good mains pressure and flow (Worcester Bosch, for example, quotes a minimum mains pressure of 1.0–1.2 bar for full hot water flow from its Greenstar 4000 combis)3

Limitations:

  • Flow rate is finite: typically 10–15 litres per minute at a 35°C rise (Worcester Bosch’s 25 kW and 30 kW Greenstar 4000 combis give 10.2 and 12.3 l/min).3 Running two showers simultaneously will cause pressure and temperature to drop.
  • If mains pressure is poor, a combi will underperform.2
  • No hot water if the boiler develops a fault.

Installed cost (2026): about £1,800–£3,500 for a standard replacement; add £300–£600 if a new flue route is needed. The government’s own working figure for the average cost of a gas boiler, including VAT, is approximately £3,500.4

System Boilers

A system boiler heats water that is pumped to a sealed unvented hot water cylinder (typically 150–250 litres).1 The cylinder stores pressurised hot water ready for use. The boiler also feeds the central heating circuit. All components (pump, expansion vessel, pressure relief) are built into the boiler itself, no separate loft tank.

Best for:

  • Family homes with two or more bathrooms2
  • Households with peak morning demand (multiple showers at once)
  • Replacing an existing system boiler or regular boiler when the loft tank is to be removed

Advantages over combi:

  • Multiple outlets simultaneously without pressure drop
  • Hot water is available instantly from the stored cylinder (no wait for combi heat exchanger to respond)
  • Works well with solar thermal pre-heat coils in the cylinder

Limitations:

  • Cylinder takes up space (Heatrae Sadia’s Megaflo Eco indirect cylinders, for example, are 579 mm wide and about 1.1–1.7 m tall in the 125–250 litre sizes)5
  • If the cylinder runs empty you must wait for it to reheat (Heatrae Sadia quotes about 20–40 minutes from cold for its 125–300 litre Megaflo Eco cylinders on a strong primary flow; a lower-output boiler takes longer)5
  • Higher installation cost than a combi

Installed cost (2026): roughly £2,000–£4,500, with the lower end only realistic where a compatible existing cylinder in good condition is kept; a new unvented cylinder pushes the job towards the top of the range, since a 150–300 litre indirect unvented cylinder alone costs about £560–£815 before fitting. Which? puts the cost of adding an unvented cylinder, when you swap a combi for a boiler with a hot water tank, at £900–£1,400 depending on its size.6

Regular (Heat-Only) Boilers

A regular boiler, also called a heat-only or conventional boiler, works with a cold water storage tank in the loft and a separate hot water cylinder (usually vented/gravity-fed).2 It is the traditional configuration found in many older UK homes.

Best for:

  • Replacing like-for-like in homes with an existing working gravity system where a full conversion is not justified
  • Older properties where the heating circuit was designed for low-pressure operation

Limitations:

  • Lower water pressure at taps and showers: gravity pressure comes only from the height of the loft tank above the outlet, and one bar takes about 10 m of height, so an upstairs shower often gets only 0.1–0.3 bar7
  • Loft tank and associated pipework take space and are a frost risk
  • More components mean more potential failure points

Installed cost (2026): roughly £1,800–£3,500 for a like-for-like replacement, in line with a combi swap; converting to a system boiler and unvented cylinder at the same time adds £600–£1,500 but is often worth doing.

Hard water is not a reason on its own to choose a regular boiler. In hard water areas, Approved Document L asks for measures to treat the feed water to water heaters and the hot water circuit of combination boilers to reduce limescale, whichever type you fit.8

Running Costs: Does the Boiler Type Make a Difference?

Boiler type itself has little effect on running costs, efficiency rating matters far more. A gas wet heating system going into an existing home in England must reach a minimum 92% efficiency as defined by ErP, under Approved Document L Volume 1, Table 6.2.8 An old G-rated boiler (below 70% efficient) of any type, wasting 30% or more of the gas it burns, will cost significantly more to run than a new one at that standard.

Fit a gas combi into an existing dwelling and Approved Document L paragraph 6.2 asks for one more energy efficiency measure alongside it: flue gas heat recovery, weather compensation, load compensation, or a smart thermostat with automation and optimisation. One of the four, installer’s choice, appropriate to the system.8 This is the requirement usually still called Boiler Plus.

From 1 October 2026 the price cap sets gas at 7.97p per kWh with a daily standing charge of 29.68p, so every 1,000 kWh of gas a more efficient boiler saves is worth about £80.9

Savings of a few hundred pounds a year are realistic when replacing a very old boiler: the Centre for Sustainable Energy, using Energy Saving Trust figures, puts the saving for a semi-detached home moving from a G-rated boiler to an A-rated condensing boiler with full heating controls at around £320 a year.1 Payback on a £1,800–£3,500 install is then roughly 6–11 years.

Which Should You Choose?

Choose a combi if:

  • You have one bathroom and typically no more than two people showering in the morning
  • You are short on space (no airing cupboard, no loft access)
  • Your mains water pressure and flow meet the boiler maker’s stated minimum, typically around 1 bar or more (ask your water supplier or test with a pressure gauge)

Choose a system boiler if:

  • You have two or more bathrooms with regular simultaneous use
  • You are already running an unvented cylinder and just replacing the boiler
  • You want to add solar thermal or a heat pump pre-heat coil later

Choose a regular (heat-only) boiler if:

  • You have an existing gravity system in good condition and budget is limited
  • A full conversion would require expensive pipe alterations

Gas Boilers and the Future

There is no date in current UK policy for ending gas boiler installations in existing homes. The Warm Homes Plan, published in January 2026, sets no phase-out deadline and instead backs grants, loans and market growth, on the view that heat pumps will be the best and cheapest form of electrified heating for most homes.10 New-build is the exception: under the Future Homes Standard, new homes get low-carbon heating, high energy efficiency and solar panels by default.10 Its regulations were made in March 2026 and come into force in England on 24 March 2027.11

A well-chosen high-efficiency gas boiler installed today is a sound investment with a 10–15 year expected lifespan. Future-proofing the pipework for a later heat pump conversion is worth doing, and the regulations already push that way: where a wet heating system is newly installed or fully replaced (appliance, emitters and pipework together), Approved Document L paragraph 5.10 requires it to be sized to work at a maximum flow temperature of 55°C or lower, not the 70–80°C older systems were designed around.8 A boiler-only swap does not trigger that, but sizing radiators for 55°C anyway is the cheap version of the same insurance.

Getting a Quote

When asking for quotes, provide: number of bathrooms and bedrooms, current boiler type and location, flue route options, and whether you want a new cylinder or to retain the existing one. The legal basis for using a registered engineer is regulation 3 of the Gas Safety (Installation and Use) Regulations 1998: nobody may work on a gas fitting unless competent to do so, and the business doing the work must be a member of a class of persons approved by the Health and Safety Executive, which is the Gas Safe Register.12 Check the Gas Safe Register before hiring.

The rules in this note are those that apply in England. Scotland, Wales and Northern Ireland have their own building standards and permitted development rights.

References (12)

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. 1technicalfairCentre for Sustainable Energy, guidance.RN-5926D3Consumer energy advice from the Centre for Sustainable Energy charity. It describes a combi as heating on demand and a system boiler as heating water held in a storage cylinder.
  2. 2technicalfairEnergy Saving Trust, guidance.RN-IRAQA2How to identify and choose a domestic boiler. Heating and hot water account for over half of annual energy spend. A boiler is a condensing model if it is a gas or LPG boiler installed after 2005, or an oil boiler installed after 2007. On cylinders, upgrading hot water cylinder insulation to 80mm saves around £45 a year in Great Britain and £35 in Northern Ireland, and when replacing a cylinder it should be sized no larger than needed. Replacement costs quoted on the page are around £3,500 and £3,200 by system type. Notes solar water heating can supplement but never fully replace a boiler or immersion heater because sunlight varies through the year. Last updated 20 May 2026.
  3. 3technicalsupplier 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.
  4. 4costvery 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.
  5. 5technicalsupplier 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.
  6. 6costfairWhich?.RN-4XIXAGA price, true at 2026-09. Which? guide to boiler and associated installation costs; it gives the cost range and average for a power flush of a five-radiator heating system.
  7. 7technicalsupplier figureMira Showers, guidance.RN-QGKAH6Shower manufacturer's guide to domestic water pressure. It states that one bar equals about 10 metres of head, which sets the pressure a gravity system can deliver.
  8. 8technicalvery strongMinistry of Housing, Communities and Local Government.RN-L0FCJXThe in-force U-value tables a DIY insulation or door note must quote, with the exact numbers. Table 4.2, limiting U-values for new fabric elements in existing dwellings, sets roof 0.15, wall 0.18, floor 0.18, swimming pool basin 0.25, window 1.4 or Window Energy Rating Band B minimum, rooflight 2.2, doors with more than 60 per cent of the internal face glazed 1.4 or Doorset Energy Rating Band C minimum, and other doors 1.4 or Doorset Energy Rating Band B minimum. Table 4.3, for existing elements, gives a roof threshold of 0.35 and an improved value of 0.16, which is the figure a loft insulation note needs: paragraph 4.12 says a retained element whose U-value is higher than the column (a) threshold, for example through a loft or garage conversion, should be upgraded to the column (b) value. Paragraph 4.13 allows a lesser upgrade only where the improved value is not technically or functionally feasible or would not achieve a simple payback of 15 years or less. Paragraph 4.8 requires a replacement element to be no worse than the one it replaces as well as meeting Table 4.2. Paragraph 4.10 gives the heritage exception: where character must be maintained, a centre pane U-value of no more than 1.2 or low-emissivity secondary glazing. Note 9 records that the timber window concession of 1.6 or Band C ended on 14 June 2023. The 2021 edition came into force on 15 June 2022; a 2026 edition exists at https://assets.publishing.service.gov.uk/media/69c122a6cfa346b9d4704a55/ADL1_2026.pdf.
  9. 9costvery strongOfgem.RN-YYE8U2A price, true at 2026-09. The live source for any running-cost sum in a heating note. For 1 October to 31 December 2026 the direct debit cap is 26.32p per kWh for electricity with a 54.83p daily standing charge, and 7.97p per kWh for gas with a 29.68p daily standing charge, giving a typical annual dual fuel bill of £1,723, a 4% rise on the previous period. Ofgem states these are the average unit rate and standing charge for direct debit customers in England, Scotland and Wales; gas prices include 5% VAT while electricity carried no VAT in this period. The cap is reset every three months, so any note quoting a pence-per-kWh figure should be dated against this page.
  10. 10technicalvery strongDepartment for Energy Security and Net Zero, guidance.RN-AS0G0PPublished 21 January 2026 and last updated 18 March 2026. It commits £15 billion of public investment over this Parliament, 2025/26 to 2029/30, to upgrade up to 5 million homes by 2030, targeting over 450,000 heat pump installations a year by 2030 and solar panels on up to 3 million additional homes. The Boiler Upgrade Scheme gets £2.7 billion to 2030, keeping the £7,500 grant for hydronic heat pumps, adding new £2,500 grants for air-to-air heat pumps and heat batteries used for central heating, and £5,000 for biomass boilers in rural areas. A Warm Homes Fund provides a £5 billion financial transactions facility including £1.7 billion for low and zero-interest consumer loans, rolling out in phases from 2026, and the Warm Homes Social Housing Fund and Local Grant merge into a single low-income scheme from 2027/28. Private rented sector properties must meet EPC Band C across two metrics by October 2030, subject to a £10,000 spending cap, and an EPC C standard is introduced for the social rented sector for the first time. Future Homes Standard regulations were to be laid in Q1 2026.
  11. 11technicalvery strongMinistry of Housing, Communities and Local Government, guidance.RN-WMRZGFThe government's circular letter to building control bodies announcing the Future Homes and Buildings Standards: the Building Regulations etc. (Amendment) (England) Regulations 2026 and the 2026 editions of Approved Documents L and F. It gives the purpose (new buildings built with low carbon heating and high energy efficiency), the in-force date of 24 March 2027 (24 September 2027 for higher-risk building work), and the transitional arrangements, under which work notified before 24 March 2027 and started before 24 March 2028 stays under the previous rules.
  12. 12technicalvery strongThe National Archives (legislation.gov.uk), legislation.RN-UFY1XPRegulation 3(1) makes it unlawful for any person to carry out work in relation to a gas fitting or gas storage vessel unless he is competent to do so, and 3(2) places the duty to ensure that on the employer, on every other employer or self-employed person with any control over the work, and on anyone who required the work to be done at a place of work under their control. Regulation 3(3) goes further: no employer may let an employee, and no self-employed person may themselves, carry out work on a gas fitting or service pipework unless that employer or self-employed person is a member of a class of persons approved by the Health and Safety Executive, which in practice is registration with the Gas Safe Register. Regulation 3(4) exempts only replacing a hose or regulator on a portable or mobile space heater and replacing the hose connecting a re-fillable cylinder to installation pipework. Regulation 3(7) makes it an offence to falsely pretend to be a member of an approved class. This is the legal basis for saying a boiler must be fitted by a Gas Safe registered engineer.

  1. Upgrading and replacing your boiler, Centre for Sustainable Energy, accessed 29 September 2026. cse.org.uk

  2. Boilers, Energy Saving Trust, accessed 2 October 2026. energysavingtrust.org.uk

  3. Greenstar 4000 Specifier’s Guide, Worcester Bosch, accessed 29 September 2026. worcester-bosch.co.uk

  4. Boiler Upgrade Scheme 2026 to 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

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

  6. Boiler prices: how much does a new boiler cost?, Which?, accessed 2 October 2026. Priced at 2026-09 and left there. which.co.uk

  7. The ultimate guide to water pressure, Mira Showers, accessed 29 September 2026. mirashowers.co.uk

  8. Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments, Ministry of Housing, Communities and Local Government, accessed 9 September 2026. assets.publishing.service.gov.uk

  9. Energy price cap unit rates and standing charges, Ofgem, accessed 30 September 2026. Priced at 2026-09 and left there. ofgem.gov.uk

  10. Warm Homes Plan, Department for Energy Security and Net Zero, accessed 9 September 2026. gov.uk

  11. The Future Homes and Buildings Standards: Building Circular 01/2026, Ministry of Housing, Communities and Local Government, accessed 29 September 2026. gov.uk

  12. Gas Safety (Installation and Use) Regulations 1998, regulation 3: qualification and supervision, legislation.gov.uk, accessed 9 September 2026. legislation.gov.uk