Is a system boiler better than a combi?

Neither is better in general; it depends on how much hot water you use at once. The Energy Saving Trust: "Large families using lots of hot water are likely to be better off with a system boiler. Smaller households using less hot water may be better off with a combi boiler." The boiler size calculator applies the same idea as rules: two showers at the same time or three bathrooms point to a system boiler, one bathroom or two used in turn to a combi, and no room for a cylinder to a combi whatever the rest.

Which is cheaper to run, a system boiler or a combi?

No source we read puts the two side by side in pounds, so this page does not. What drives the difference: a combi heats water only when a tap runs, while a system boiler keeps a cylinder hot, and the Energy Saving Trust says "heat is lost from the hot water cylinder over time"; it also advises keeping the cylinder at 60 °C to kill harmful bacteria. In England both must be at least 92% ErP as gas boilers (Boiler Plus). The calculator picks the type from how much hot water you use at once, not from running costs.

Does a system boiler cost more to install than a combi?

Usually, because of the cylinder. Worcester Bosch: "typically speaking, the cost of your average system boiler will be slightly higher than an equivalent combi boiler, largely down to the additional cost of the required cylinder", and the most accurate way to cost one is an installer who sees your current setup. The Energy Saving Trust calls combis "often the least expensive kind of boiler". If you may want a heat pump later, it adds that most heat pumps work with a cylinder, so a switch to a combi now may mean putting one back.

Is a combi or a system boiler more efficient?

The Energy Saving Trust says system boilers are "more efficient than combi boilers at producing hot water, but heat is lost from the hot water cylinder over time", and that combis' heat and hot water on demand make them "a more efficient choice for smaller households". The minimum is the same: in English homes a new gas boiler must have a product label of at least 92% ErP, whatever the type (Boiler Plus factsheet, 2018).

What is a pressurised (unvented) system, and how does it compare with a combi?

A system boiler's cylinder takes water from the mains and needs no loft tanks (Worcester Bosch). Approved Document G calls a cylinder fed from the supply pipe without a vent pipe an unvented (closed) hot water storage system, fitted with safety devices that stop the water exceeding 100 °C. Against a combi: a combi also delivers hot water at mains pressure (Worcester Bosch) but heats it as it flows, so two showers share it; the unvented cylinder stores it for several taps at once.

Who can fit an unvented cylinder?

Approved Document G (England) asks owners to choose installers with the skills for unvented systems, shown "for example, by registration with a competent person scheme for this type of work or by the holding of a current registered operative skills certification card for unvented hot water systems". Fitting one is notifiable building work: the building control body is told before work starts, unless the installer is registered with a competent person scheme, which self-certifies it. The gas work on the boiler is for a Gas Safe registered engineer (HSE).

System boiler vs combi boiler (UK): which suits your home

A combi heats water from the mains as you use it and needs no cylinder. A system boiler heats a hot water cylinder fed from the mains, so several taps and showers can run at once; a regular boiler heats a cylinder fed from tanks in the loft. Which suits your home depends on the showers you run at the same time, your bathrooms, your mains flow and whether there is room for a cylinder. The table further down is the boiler size calculator's own rules, and every other line quotes a source read on 2026-10-19.

Short answer: combi and system boiler side by side
	Combi boiler	System boiler
Hot water	Heated from the mains as you use it	Heated into a cylinder and stored
Cylinder and tanks	None	A cylinder fed from the mains, no loft tanks
Two showers at once	They share its flow	Stored water to several taps at the same time
Low mains flow	Less suitable with low water pressure (Energy Saving Trust)	Its cylinder is fed from the mains too; a regular boiler suits low water pressure (Energy Saving Trust)
Space	Needs less space	Needs room for the cylinder
Running out	Does not run out, but the flow is limited	The stored water can run out
Boiler Plus (gas, England)	One extra energy saving measure required	No extra measure in the 2018 rule
Who fits it	A Gas Safe registered engineer	Gas Safe for the boiler; an installer with unvented cylinder skills for the cylinder

Open the boiler size calculator

How each type works

Combi boiler

Worcester Bosch: "A combi (or combination) boiler provides both heating and hot water directly from the mains, all within one compact unit." The Energy Saving Trust: "Combi boilers don't use a hot water cylinder – all the heat and hot water comes directly from the boiler itself." Its kW is set by the hot water: a combi in the 24-27 kW row heats 9.8-11.1 L/min through a 35 °C rise, and only as much as the mains brings in.

System boiler with an unvented cylinder

Worcester Bosch: "Unlike a combi (combination) boiler, which heats all water on-demand, a system boiler does this with the additional support of a separate hot water storage cylinder." System boilers "take water directly from the mains", and the system "Doesn't require loft tanks". A cylinder fed from the supply pipe with no vent pipe is what Approved Document G calls an unvented (closed) system, with safety devices that stop the stored water exceeding 100 °C (rules below). Because the cylinder holds the hot water, the boiler only heats the radiators and refills it, so the installers' system rows are lower than the combi rows: 12-18 kW against 24-27 kW, 18-26 kW against 28-34 kW, 27-40 kW against 35-42 kW (Heatable, What Size Boiler Do I Need?, system / regular boiler sizing for typical UK homes).

A cylinder also needs its own controls. The Energy Saving Trust: "Two channel: for system boilers controlling heating and hot water", and with a cylinder "you should have a programmer, room thermostat and cylinder thermostat". A single-channel combi thermostat does not time the hot water. The wiring options are on the smart thermostat guide.

Regular (heat-only) boiler

The Energy Saving Trust: heat-only boilers "work in a similar way to system boilers. The main difference is that heat-only boilers have two extra header tanks to store cold water, usually installed in the loft if there is one", and they are "unlikely to be suitable for a flat, as the header tanks need to be higher than the highest hot tap or radiator". The calculator suggests one when the mains is low and more than one bathroom or shower runs at once.

Which suits your home: the calculator's rules

The boiler size calculator applies its rules in order, and the first that fits your home wins: no room for a cylinder, then a low mains, then three or more bathrooms, then two showers at once. Each line below is an example home run through the calculator's own code, with the sentences its result shows. The low mains examples use 8.0 L/min, 1.0 L/min under the bucket test's 9.0 L/min.

Table 1: which type the calculator picks, and why, for 7 example homes
Your home	Type	Why (the calculator's words)
One bathroom, one shower at a time	Combi boiler	One bathroom: a combi heats water from the mains as you use it, the type Worcester Bosch recommends for smaller homes with one bathroom.
Two bathrooms used one at a time	Combi boiler	Two bathrooms used one at a time can share a combi; the second bathroom moves it up a row, because a combi is sized for its hot water.
Two bathrooms, two showers at the same time	System boiler	Two showers at once would split one combi's flow, so a system boiler with a cylinder fits better: Worcester Bosch rates it for several outlets at the same time.
Three bathrooms	System boiler	With three or more bathrooms, Worcester Bosch expects stored hot water, and Heatable suggests a system boiler at this size.
Two bathrooms, low mains flow	Regular boiler	Your mains (8.0 L/min) is under the 9.0 L/min the bucket test expects, and Worcester Bosch says a regular boiler works on low mains pressure with the same flow at every tap. A regular boiler's cylinder is fed from the loft tank, not straight from your 8.0 L/min mains.
One bathroom, low mains flow	Combi boiler	One bathroom: a combi heats water from the mains as you use it, the type Worcester Bosch recommends for smaller homes with one bathroom. Your mains gives 8.0 L/min, which a 19.5 kW combi already heats: no combi in this row gives more hot water than your mains.
No room for a cylinder, two showers at the same time	Combi boiler	Without room for a cylinder, a combi is the type that fits: it heats water as you use it. Two showers at once share this flow between them.

Two bathrooms or two showers: is a combi enough?

Two bathrooms used one at a time can share a combi, a row up: the installers' table puts two bathrooms in the 28-34 kW row (Heatable, What Size Boiler Do I Need?, combi boiler sizing for typical UK homes), which heats 11.5-13.9 L/min at a 35 °C rise. Two showers at the same time share that flow, so the calculator points to a system boiler. For new homes, the NHBC asks 15.0 L/min for a bathroom and one separate shower room or en-suite, which Worcester Bosch pairs with 36 kW to hot water, above the 28-34 kW row. Three bathrooms move the table to 35-42 kW for a combi, where the source adds "often consider a system boiler". By radiator count, the 8 to 14 radiators table gives both rows for each bathroom count.

Efficiency

The Government's Boiler Plus factsheet (BEIS, 2018): "The new minimum performance standard for domestic gas boilers in English homes will be set at 92% ErP", and "The boiler itself must have a product label of 92%, irrespective of the overall system label". That applies to a combi and a system boiler alike. The difference is in the hot water. The Energy Saving Trust says system boilers are "more efficient than combi boilers at producing hot water, but heat is lost from the hot water cylinder over time", and that a combi's on-demand hot water makes it "a more efficient choice for smaller households". It advises setting the cylinder's thermostat to 60 °C, "hot enough to kill harmful bacteria in the water". This page has no figure for a cylinder's heat loss: it depends on the cylinder, so ask for the model's data.

Boiler Plus adds a rule for combis only. "When a gas combination boiler is installed, an additional energy efficiency measure will be required", one of four: flue gas heat recovery systems, weather compensation, load compensation, or smart controls featuring automation and optimisation functions. For system and regular boilers the factsheet says many "require additional work to make them compatible with the energy saving devices included in Boiler Plus", with "a risk of Legionella" without it, so the rule was applied first where there is no such risk. The four measures are explained on the smart thermostat for a combi boiler page. Boiler Plus is for England only.

Running and installation costs: what drives them

This page has no pound figures: no source we read on 2026-10-19 compares the two types in pounds. What drives the cost, in the sources' words:

The cylinder. Worcester Bosch: "the cost of your average system boiler will be slightly higher than an equivalent combi boiler, largely down to the additional cost of the required cylinder". The Energy Saving Trust calls combis "often the least expensive kind of boiler".
Keeping water hot. A system boiler keeps a cylinder at temperature and loses some of that heat over time; a combi heats water only when a tap runs (Energy Saving Trust).
Your setup. Worcester Bosch: "The most accurate way of costing a system boiler would be to talk to a trusted installer who can take into account your current setup and potential needs." With a gas combi in England, the Boiler Plus measure is part of the job.
A heat pump later. The Energy Saving Trust: "Most heat pumps work with a hot water cylinder. So, if you switch to a combi boiler now, you may need to put a cylinder back in if you want a heat pump in future."

Unvented cylinders: the rules (England)

Approved Document G, requirement G3(3): "A hot water system that has a hot water storage vessel shall incorporate precautions to: (a) prevent the temperature of the water stored in the vessel at any time exceeding 100°C; and (b) ensure that any discharge from safety devices is safely conveyed to where it is visible but will not cause a danger to persons in or about the building."

Who fits it: owners "should therefore take care to choose installers who have the necessary skills to carry out this work", shown "for example, by registration with a competent person scheme for this type of work or by the holding of a current registered operative skills certification card for unvented hot water systems" (paragraph 3.40). "The installation of an unvented system is notifiable building work which must be notified to the BCB before work commences" (3.41), the BCB being the building control body, a local authority or an Approved Inspector. An installer registered with a competent person scheme for unvented systems self-certifies the work, and you are given a building regulations certificate of compliance (3.42).

The boiler itself is gas work. HSE: "Under the Gas Safety (Installation and Use) Regulations 1998 for a gas engineering business to legally undertake gas work that is within the scope of the Regulations they must be on the Gas Safe Register." Ask to see the engineer's Gas Safe identity card, which HSE says consumers are encouraged to do.

Whichever type you choose, the boiler size calculator gives the kW row for it from your bedrooms, bathrooms and radiators.

Frequently asked questions

Is a system boiler better than a combi?: Neither is better in general; it depends on how much hot water you use at once. The Energy Saving Trust: "Large families using lots of hot water are likely to be better off with a system boiler. Smaller households using less hot water may be better off with a combi boiler." The boiler size calculator applies the same idea as rules: two showers at the same time or three bathrooms point to a system boiler, one bathroom or two used in turn to a combi, and no room for a cylinder to a combi whatever the rest.
Which is cheaper to run, a system boiler or a combi?: No source we read puts the two side by side in pounds, so this page does not. What drives the difference: a combi heats water only when a tap runs, while a system boiler keeps a cylinder hot, and the Energy Saving Trust says "heat is lost from the hot water cylinder over time"; it also advises keeping the cylinder at 60 °C to kill harmful bacteria. In England both must be at least 92% ErP as gas boilers (Boiler Plus). The calculator picks the type from how much hot water you use at once, not from running costs.
Does a system boiler cost more to install than a combi?: Usually, because of the cylinder. Worcester Bosch: "typically speaking, the cost of your average system boiler will be slightly higher than an equivalent combi boiler, largely down to the additional cost of the required cylinder", and the most accurate way to cost one is an installer who sees your current setup. The Energy Saving Trust calls combis "often the least expensive kind of boiler". If you may want a heat pump later, it adds that most heat pumps work with a cylinder, so a switch to a combi now may mean putting one back.
Is a combi or a system boiler more efficient?: The Energy Saving Trust says system boilers are "more efficient than combi boilers at producing hot water, but heat is lost from the hot water cylinder over time", and that combis' heat and hot water on demand make them "a more efficient choice for smaller households". The minimum is the same: in English homes a new gas boiler must have a product label of at least 92% ErP, whatever the type (Boiler Plus factsheet, 2018).
What is a pressurised (unvented) system, and how does it compare with a combi?: A system boiler's cylinder takes water from the mains and needs no loft tanks (Worcester Bosch). Approved Document G calls a cylinder fed from the supply pipe without a vent pipe an unvented (closed) hot water storage system, fitted with safety devices that stop the water exceeding 100 °C. Against a combi: a combi also delivers hot water at mains pressure (Worcester Bosch) but heats it as it flows, so two showers share it; the unvented cylinder stores it for several taps at once.
Who can fit an unvented cylinder?: Approved Document G (England) asks owners to choose installers with the skills for unvented systems, shown "for example, by registration with a competent person scheme for this type of work or by the holding of a current registered operative skills certification card for unvented hot water systems". Fitting one is notifiable building work: the building control body is told before work starts, unless the installer is registered with a competent person scheme, which self-certifies it. The gas work on the boiler is for a Gas Safe registered engineer (HSE).

Sources

Energy Saving Trust, Boiler types explained (last updated 20 May 2026), read 2026-10-19.
Worcester Bosch, What is a system boiler? (undated), read 2026-10-19.
Worcester Bosch, Boiler types (undated), read 2026-10-19.
Energy Saving Trust, Thermostats and heating controls (last updated 28 September 2026), read 2026-10-19.
GOV.UK (BEIS), Boiler Plus: New standards for domestic boiler installations from April 2018, frequently asked questions (03/01/2018, answers dated to 12/10/2018), read 2026-10-19.
GOV.UK, Approved Document G: Sanitation, hot water safety and water efficiency (2015 edition incorporating 2016 and 2024 amendments) (page last updated 1 October 2024), read 2026-10-19.
Health and Safety Executive, Gas Safe Register (domestic gas) (page dated 2024-11-06), read 2026-10-19.
Heatable, What Size Boiler Do I Need? (Boiler kW Calculator) (undated), read 2026-09-20.
Worcester Bosch, Technical Bulletin TB 116 B: NHBC policy for combination boilers (03/2019), read 2026-10-12.
Worcester Bosch, Gas boilers (undated), read 2026-10-12.
Worcester Bosch, Regular boilers (undated), read 2026-10-12.
Baxi, Baxi 800 Combi 2, technical data (undated), read 2026-10-12.
Affinity Water, Water pressure: your responsibilities (bucket test) (undated), read 2026-10-12.
Wikipedia, Specific heat capacity (undated), read 2026-10-12.

Product facts in the Ad block come from the merchant's own page, read on the day shown beside the price. This page quotes documents; it has not tested any boiler or cylinder. Boiler Plus and Approved Document G are described for England only.

Every quote with its read date

Boiler types, rules and costs

Combi boilers: Energy Saving Trust, Boiler types explained (last updated 20 May 2026): "Combi boilers don't use a hot water cylinder – all the heat and hot water comes directly from the boiler itself. This makes them a good option if you don't have much space, and they're often the least expensive kind of boiler." "Combi boilers may be less suitable for: Houses with multiple bathrooms or higher hot water demands. The 'on demand' feature of a combi boiler works less well when you need hot water in multiple places at once. Houses with low water pressure." Worcester Bosch, Boiler types (undated): "A combi (or combination) boiler provides both heating and hot water directly from the mains, all within one compact unit." — https://energysavingtrust.org.uk/advice/boilers/ and https://www.worcester-bosch.co.uk/boiler-types, read on 2026-10-19
System boilers: Energy Saving Trust, Boiler types explained (last updated 20 May 2026): "System boilers have a separate water cylinder to store hot water. They're more efficient than combi boilers at producing hot water, but heat is lost from the hot water cylinder over time." "you can draw hot water from the storage tank to multiple taps at the same time." Worcester Bosch, What is a system boiler? (undated): "Unlike a combi (combination) boiler, which heats all water on-demand, a system boiler does this with the additional support of a separate hot water storage cylinder"; "Doesn't require loft tanks, freeing up space for future conversion"; "the water is already heated up and stored ready in your hot water cylinder". Worcester Bosch, Boiler types (undated): system boilers "take water directly from the mains"; "they require space for a separate hot water store" — https://energysavingtrust.org.uk/advice/boilers/ and https://www.worcester-bosch.co.uk/products/boilers/system-boilers and https://www.worcester-bosch.co.uk/boiler-types, read on 2026-10-19
Regular (heat-only) boilers: Energy Saving Trust, Boiler types explained (last updated 20 May 2026): "Heat-only boilers, also called regular or conventional boilers, work in a similar way to system boilers. The main difference is that heat-only boilers have two extra header tanks to store cold water, usually installed in the loft if there is one." "Heat-only boilers are unlikely to be suitable for a flat, as the header tanks need to be higher than the highest hot tap or radiator." — https://energysavingtrust.org.uk/advice/boilers/, read on 2026-10-19
Combi hot water at mains pressure: Worcester Bosch, What is a system boiler? (undated), on combination boilers: "Hot water is delivered at mains pressure, for powerful showers without the need for a pump"; "There is no risk of 'running out' of hot water like in a system boiler, as combi boiler create hot water on demand" — https://www.worcester-bosch.co.uk/products/boilers/system-boilers, read on 2026-10-19
Controls for a cylinder: Energy Saving Trust, Thermostats and heating controls (last updated 28 September 2026): "Single channel: for combi boilers. This only controls central heating as hot water is provided on demand." "Two channel: for system boilers controlling heating and hot water, or combi boilers controlling two heating zones (eg upstairs and downstairs)." "If you have a system or regular boiler with hot water cylinder, you should have a programmer, room thermostat and cylinder thermostat." "You should set the temperature of the hot water thermostat to 60°C. This is hot enough to kill harmful bacteria in the water"; with a system or regular boiler the boiler's temperature "should be no lower than 65°C so that it can heat your cylinder to 60°C" — https://energysavingtrust.org.uk/advice/thermostats-and-heating-controls/, read on 2026-10-19
What a system boiler costs against a combi: Worcester Bosch, What is a system boiler? (undated), "How much do system boilers typically cost?": "The most accurate way of costing a system boiler would be to talk to a trusted installer who can take into account your current setup and potential needs. But typically speaking, the cost of your average system boiler will be slightly higher than an equivalent combi boiler, largely down to the additional cost of the required cylinder." Energy Saving Trust, Boiler types explained (last updated 20 May 2026): combi boilers are "often the least expensive kind of boiler" — https://www.worcester-bosch.co.uk/products/boilers/system-boilers and https://energysavingtrust.org.uk/advice/boilers/, read on 2026-10-19
A cylinder and heat pumps: Energy Saving Trust, Boiler types explained (last updated 20 May 2026): "Most heat pumps work with a hot water cylinder. So, if you switch to a combi boiler now, you may need to put a cylinder back in if you want a heat pump in future." — https://energysavingtrust.org.uk/advice/boilers/, read on 2026-10-19
Boiler Plus: 92% ErP: GOV.UK (BEIS), Boiler Plus: New standards for domestic boiler installations from April 2018, frequently asked questions (dated 03/01/2018, answers to 12/10/2018): "The new minimum performance standard for domestic gas boilers in English homes will be set at 92% ErP. The standard will also make timers and room thermostats an explicit requirement for all gas and oil systems"; "The boiler itself must have a product label of 92%, irrespective of the overall system label" — https://assets.publishing.service.gov.uk/media/5b2cc1e2ed915d586e2d8fe9/Boiler_Plus_Factsheet_v3.pdf, read on 2026-10-19
Boiler Plus: the extra measure with a gas combi: GOV.UK (BEIS), Boiler Plus: New standards for domestic boiler installations from April 2018, frequently asked questions (dated 03/01/2018, answers to 12/10/2018): "When a gas combination boiler is installed, an additional energy efficiency measure will be required." "The energy saving technologies that can be used to comply are: Flue gas heat recovery systems; Weather Compensation; Load Compensation; Smart controls featuring automation and optimisation functions" — https://assets.publishing.service.gov.uk/media/5b2cc1e2ed915d586e2d8fe9/Boiler_Plus_Factsheet_v3.pdf, read on 2026-10-19
Boiler Plus: system and regular boilers: GOV.UK (BEIS), Boiler Plus: New standards for domestic boiler installations from April 2018, frequently asked questions (dated 03/01/2018, answers to 12/10/2018), question 9 "What about heat only and system boilers?": "Many system boilers and most regular boilers on the market today require additional work to make them compatible with the energy saving devices included in Boiler Plus. This may involve additional costs and additional knowledge for the installer, without which there may be a risk of Legionella." "It makes sense to first apply the new standards in systems where there is no risk, without any additional work, and consider extending the rules in the future" — https://assets.publishing.service.gov.uk/media/5b2cc1e2ed915d586e2d8fe9/Boiler_Plus_Factsheet_v3.pdf, read on 2026-10-19
Unvented cylinder: GOV.UK, Approved Document G: Sanitation, hot water safety and water efficiency (2015 edition incorporating 2016 and 2024 amendments, for use in England; page last updated 1 October 2024), key terms: "Unvented (closed) hot water storage system means a vessel fed with cold water from a supply pipe or dedicated storage cistern (without a vent pipe) and in which water is heated directly or indirectly", "fitted with safety devices to prevent water temperatures exceeding 100°C"; "Vented (open) hot water storage system means a vessel fed with cold water from a dedicated storage cistern" — https://assets.publishing.service.gov.uk/media/66f6c6ce3b919067bb4828cc/ADG_with_2024_amendments.pdf, read on 2026-10-19
Requirement G3(3): GOV.UK, Approved Document G: Sanitation, hot water safety and water efficiency (2015 edition incorporating 2016 and 2024 amendments, for use in England; page last updated 1 October 2024), Requirement G3(3): "A hot water system that has a hot water storage vessel shall incorporate precautions to: (a) prevent the temperature of the water stored in the vessel at any time exceeding 100°C; and (b) ensure that any discharge from safety devices is safely conveyed to where it is visible but will not cause a danger to persons in or about the building." — https://assets.publishing.service.gov.uk/media/66f6c6ce3b919067bb4828cc/ADG_with_2024_amendments.pdf, read on 2026-10-19
Who installs an unvented cylinder: GOV.UK, Approved Document G: Sanitation, hot water safety and water efficiency (2015 edition incorporating 2016 and 2024 amendments, for use in England; page last updated 1 October 2024), paragraph 3.40: "Building owners and occupiers should therefore take care to choose installers who have the necessary skills to carry out this work. These skills can be demonstrated for example, by registration with a competent person scheme for this type of work or by the holding of a current registered operative skills certification card for unvented hot water systems." Paragraph 3.41: "The installation of an unvented system is notifiable building work which must be notified to the BCB before work commences." Paragraph 3.42: if the installer is registered with a competent person scheme for unvented hot water systems "it will not be necessary for the work to be notified in advance to the BCB"; "the building owner/occupier will be given a building regulations certificate of compliance". BCB "means Building Control Body: a local authority or an Approved Inspector" — https://assets.publishing.service.gov.uk/media/66f6c6ce3b919067bb4828cc/ADG_with_2024_amendments.pdf, read on 2026-10-19
Gas Safe Register: Health and Safety Executive, Gas Safe Register (page dated 2024-11-06): "Under the Gas Safety (Installation and Use) Regulations 1998 for a gas engineering business to legally undertake gas work that is within the scope of the Regulations they must be on the Gas Safe Register." "Gas Safe registered engineers carry an identity card which consumers are encouraged to ask an engineer for before they have gas work carried out in their home." The register covers "Great Britain, Northern Ireland, the Isle of Man and Guernsey" — https://www.hse.gov.uk/gas/domestic/newschemecontract.htm, read on 2026-10-19

The calculator's rules and figures

Combi boiler kW: Heatable, What Size Boiler Do I Need? (Boiler kW Calculator) (read 2026), combi boiler sizing for typical UK homes: 1 bathroom, 0-10 radiators, 1-2 bedrooms -> "24-27kW"; 2 bathrooms, 10+ radiators, 3-4 bedrooms -> "28-34kW"; 3+ bathrooms, 20+ radiators, 4+ bedrooms -> "35-42kW (often consider a system boiler)". Ideal Heating, Combi Boiler Size Guide (read 2026): 1 bedroom "24-27kW", 2-3 bedrooms "28-34kW"; 24kW "Up to 10" radiators, 28kW "Between 10 and 15", 32kW "Between 15 and 20" — https://heatable.co.uk/boiler-advice/what-size-boiler-for-my-home and https://idealheating.com/products/boilers/ideal-combi-boilers/combi-boiler-size-guide, read on 2026-09-20
System boiler kW: Heatable, What Size Boiler Do I Need? (Boiler kW Calculator) (read 2026), system / regular boiler sizing for typical UK homes: 1 bathroom, 0-10 radiators, 1-2 bedrooms -> "12-18kW"; 2-3 bathrooms, 10+ radiators, 3-4 bedrooms -> "18-26kW"; 3+ bathrooms, 20+ radiators, 4+ bedrooms -> "27-40kW" — https://heatable.co.uk/boiler-advice/what-size-boiler-for-my-home, read on 2026-09-20
Specific heat of water: Wikipedia, Specific heat capacity (read 2026): water's specific heat capacity is "about 4184 J⋅kg−1⋅K−1 at 20 °C", i.e. 4.184 kJ to warm 1 kg of water by 1 °C — https://en.wikipedia.org/wiki/Specific_heat_capacity, read on 2026-10-12
Temperature rise for combi flow: Baxi, Baxi 800 Combi 2 technical data (read 2026): "Maximum DHW flow rate (Delta T @ 35'C rise) (l/min)". Worcester Bosch, Technical Bulletin TB 116 B (03/2019): the flow rate at the outlet "is achieved with a 35°C temperature rise from the incoming mains cold water temperature" — https://baxi.co.uk/professional/products/boilers/combi-boilers/baxi-800-combi-2 and https://www.worcester-bosch.co.uk/support/literature/download/B, read on 2026-10-12
Hot water demand by bathrooms: Worcester Bosch, Technical Bulletin TB 116 B (03/2019), "NHBC policy for combination boilers": a combi "should be selected with the following minimum rated output to hot water": "0.15 L/sec (9 L/min) = 25 kW", "0.20 L/sec (12 L/min) = 30 kW", "0.25 L/sec (15 L/min) = 36 kW". "Example 1 – Dwelling has single bathroom with shower over the bath. Minimum NHBC requirement is 0.2 L/sec (12 L/min)"; "Example 2 – Dwelling has Bathroom and one separate Shower room/en-suite. Minimum NHBC requirement is 0.25 L/sec (15 L/min)"; "Houses with a greater number of bathrooms or shower rooms / en-suites are likely to require a stored water solution." The NHBC standard is for new homes — https://www.worcester-bosch.co.uk/support/literature/download/B, read on 2026-10-12
Mains flow bucket test: Affinity Water, Water pressure: your responsibilities (read 2026): use the first tap off the supply pipe, usually the kitchen tap, with "all other taps and appliances using water" off, and "fill a 4.5 litre (1 gallon) bucket"; "It should take 30 seconds to fill the bucket with water - If it takes longer to fill then it's likely you have a water pressure or flow issue." 4.5 litres in 30 seconds is 9 litres a minute — https://metering.affinitywater.co.uk/water-pressure/your-responsibilities, read on 2026-10-12
Combi boilers: Worcester Bosch, Gas boilers (read 2026): "Combi boilers are perfect for smaller homes with one bathroom and provide instant hot water" — https://www.worcester-bosch.co.uk/products/boilers/gas-boilers, read on 2026-10-12
System boilers: Worcester Bosch, Gas boilers (read 2026): "System boilers are more suited for larger homes with more than one bathroom and require a cylinder". Worcester Bosch, Understanding various boiler types (read 2026): "System boilers are well-suited to homes that need hot water for several outlets at the same time. They take water directly from the mains, offering strong flow rates and steady performance even when more than one tap or shower is running" — https://www.worcester-bosch.co.uk/products/boilers/gas-boilers and https://www.worcester-bosch.co.uk/boiler-types, read on 2026-10-12
Regular boilers: Worcester Bosch, Regular boilers (read 2026): "Traditional boilers can operate effectively even if your mains water pressure is low"; they store "the heated water in bulk and allows for multiple showers, taps, and baths to run simultaneously"; "they will deliver the same level of flow however many taps and showers are running"; "installed with two tanks", the cold water tank "usually installed as high above the boiler as possible, like in your attic or loft" — https://www.worcester-bosch.co.uk/products/boilers/regular-boilers, read on 2026-10-12
Three or more bathrooms: Worcester Bosch, Technical Bulletin TB 116 B (03/2019): "Houses with a greater number of bathrooms or shower rooms / en-suites are likely to require a stored water solution." Heatable, What Size Boiler Do I Need? (read 2026): the 3+ bathroom row of the combi table reads "35-42kW (often consider a system boiler)" — https://www.worcester-bosch.co.uk/support/literature/download/B and https://heatable.co.uk/boiler-advice/what-size-boiler-for-my-home, read on 2026-10-12

What size boiler do I need? UK combi boiler size calculator
Combi boiler size calculator (UK): 24-27 kW for 1-2 bedrooms, 28-34 kW for 3-4 bedrooms or 2 bathrooms, 35-42 kW for 5+ bedrooms. Hot water L/min too.
What size radiator do I need? (BTU and watts by room size and type, UK)
BTU/h and watts for UK room sizes by type from the merchants' ft³ x factor rule, each adjustment, and the flat x4 rule. A 4 x 4 m living room: 7,500 BTU/h.
What size boiler for a 3-bedroom house? (UK: 28-34 kW combi, 18-26 kW system)
A 3-bedroom house with one or two bathrooms needs 28-34 kW from a combi or 18-26 kW from a system boiler on the UK tables; heat loss by floor area.
What size boiler for a 4-bedroom house? (UK: 28-34 kW combi, 35-42 kW with 3 bathrooms)
A 4-bedroom house is 28-34 kW combi or 18-26 kW system with one or two bathrooms, 35-42 kW combi with three. Combi or system, heat loss, radiators.

Last updated 2026-10-19. Sizing estimates from published charts and rules of thumb (ENERGY STAR, manufacturer guides), each with its source and date: a Manual J load calculation by an installer is the final word.