The Energy Saving Trust notes that heating and hot water make up more than half of a typical UK energy bill, so the appliance and the controls matter. Diagrams for each layout live on boiler types. If you already know you want a combi, go to choosing a new combi boiler. If you are unsure whether to repair, replace, or move to a heat pump, start at repair, replace or heat pump.
Empty house versus replacement. In an empty property you choose the system from a clean sheet: type, fuel, cylinder (or none), and the heating circuit. On a replacement, pipework, the flue, the gas or oil supply, and loft tanks often lock you into something close to what is already there — if that system was sized and specified properly. You can still change type (regular to combi, combi to system, boiler to heat pump), but that is a different job from a straight swap.
Choose the type first
UK domestic boilers are sold as three layouts. The fuel (gas, LPG, oil, electric) is a separate decision. The Energy Saving Trust’s boiler types explained guide is the same three: combi, system, and heat-only.
| Type | How it works | Usually a good fit when | Usually a poor fit when |
|---|---|---|---|
| Combi | One box heats the radiators and the taps from the cold main. No cylinder, no loft tanks. | One bathroom (or a careful second outlet), decent mains flow, and little cupboard space. | Two showers at once, a weak main you cannot upgrade, or you want stored hot water / heat-pump readiness. |
| System | Sealed heating circuit. Hot water from a separate cylinder (vented or unvented). Pump and expansion vessel are usually in the boiler. | More than one bathroom, high hot-water demand, or you want an immersion backup and a store. | There is nowhere for a cylinder, and a combi would actually cover the taps. |
| Regular / heat-only (conventional) | Heats radiators and a cylinder. Open-vented heating usually has a feed-and-expansion tank in the loft, plus a cold-water cistern for the cylinder. | You already have loft tanks and a cylinder, or mains pressure is too low for a combi or unvented cylinder. | A flat with no height for header tanks, or you want to drop the loft plumbing. |
Some makers sell a combi with a store — a combination boiler plus a built-in or paired tank. It can give stronger hot water than a plain combi in a smaller footprint than a full cylinder system. The trade-off is a more specific appliance and parts list. Treat it as a specialist quote, not the default.
System and regular boilers usually run on an S Plan or S Plan Plus (or a Y Plan). A combi does not. Changing type is also changing the controls and the pipework, not just the box on the wall.
Home size, bathrooms, and hot water
- Small flat or house, one bathroom. A combi is the usual UK answer if the cold main can feed it. See choosing a new combi for DHW kW and brand notes.
- Two or more bathrooms, or baths and showers used together. A system boiler and a correctly sized cylinder will supply more than one outlet at once. A large combi still shares one incoming cold main.
- Large or poorly insulated house. Heating load can be high enough that a system or regular boiler, sized from a heat-loss, is the honest heating answer. Hot water is then a cylinder problem, not a bigger combi badge.
Water pressure and flow rate
A combi and an unvented cylinder both take cold water from the mains. The water company must keep at least 0.7 bar (7 metres head) at the boundary under the guaranteed standards — that is a pressure duty, not a promise of 9 litres a minute at the kitchen tap. Measure flow at the kitchen cold tap with everything else off. Low mains water pressure explains the Ofwat figure and why a bigger combi cannot invent flow.
If the main is weak and you cannot upgrade the private supply, a regular (open-vent) system with loft storage, or a system boiler with a vented cylinder, is often the workable path. Unvented cylinders and combis need both pressure and flow.
To relate flow to kW, use kW to flow rate, LPM to kW, or hot water flow rate to kW input.
Space for a cylinder and tanks
- Combi: wall space, a flue route, a condensate drain, and a gas or oil supply. No cylinder cupboard.
- System: the boiler plus an airing-cupboard (or similar) for the cylinder. An unvented cylinder also needs a discharge pipe to a safe point.
- Regular / heat-only: boiler, cylinder, and usually two loft tanks (feed-and-expansion, and cold storage for hot water). The Energy Saving Trust notes that heat-only boilers are unlikely to suit a flat, because the header tanks need to sit above the highest tap or radiator.
If you may want a heat pump later, keeping or fitting a cylinder is the less painful route. Most UK heat pumps heat a store, not the taps directly. Switching to a combi now can mean putting a cylinder back later.
Fuel: gas, LPG, oil, electric
Type (combi / system / regular) is independent of fuel. The Energy Saving Trust’s comparison is the conservative starting point: if you have a mains gas connection, a modern condensing gas boiler is usually the cheapest of gas, oil, LPG and electricity to run. That is a running-cost observation, not a reason to ignore a heat pump.
- Natural gas. The common UK fuel where the grid reaches the property. New gas boilers are condensing. Work must be done by a Gas Safe registered engineer. In England a replacement also has to meet Boiler Plus.
- LPG. Off-grid alternative to natural gas. Same three boiler types exist. You need tank or bottle space and a delivery contract. Gas Safe still applies. The Energy Saving Trust describes LPG as the most expensive of gas, oil and LPG to run, with lower CO2 than oil.
- Oil (typically kerosene). Common off-grid, especially in parts of Northern Ireland. Needs a storage tank, bunding where required, and an OFTEC-trained installer rather than Gas Safe. Oil boilers have their own efficiency minimums under Approved Document L (see Boiler Plus). Bio-kerosene blends exist; treat them as a specification question for the installer and the boiler maker.
- Electric boilers. Almost all of the electricity they draw becomes heat at the appliance, but a unit of electricity usually costs more than a unit of gas, so running cost is often high for whole-house heating. They can make sense for a small, well-insulated property with no gas, or as a backup. They do not need a flue. An electric boiler is not a heat pump: a heat pump moves heat and can deliver more heat than the electricity it uses.
Solid fuel (wood, pellets, coal) is a different appliance class and a different set of regulations. It is not a drop-in gas-boiler replacement.
Heat pumps and hybrids
A new fossil boiler is not the only option. The Energy Saving Trust describes air source heat pumps as more than three times as efficient as a gas boiler because they move heat rather than burning fuel. That efficiency depends on a proper heat-loss, emitters that can run at a lower flow temperature, and a cylinder. Use the heat pump checker as a first filter, then air source heat pumps and installation.
A heat pump is worth a serious look when the house can be made to work at a low flow temperature (often with larger radiators or underfloor heating), there is space for an outdoor unit and a cylinder, and you are in England or Wales where the Boiler Upgrade Scheme may contribute to a standalone heat pump. Grant amounts and eligibility change — check gov.uk and the figures on our heat pumps page rather than a quote from memory. Hybrids (heat pump plus a fossil boiler) are not eligible for that scheme. Scotland and Northern Ireland have separate arrangements.
A hybrid can be a stepping stone where a full heat-pump retrofit is awkward this year, but the controls have to let the heat pump do most of the year’s work or you simply own two appliances. If the job is more advanced than a standard type-and-size choice, get a design survey rather than picking a badge kW from a table.
Size it in kW
Kilowatts on the badge are not one number. On a combi, domestic hot water usually sets the size (typically the mid-20s to high-30s kW). Heating load for a typical insulated house is often much lower, and the boiler should be range-rated down for heating so it can condense. On a system or regular boiler, kW is mainly the heating load; the cylinder is sized in litres and recovery time, not by inflating the boiler to combi DHW levels.
The only size that really counts is a heat-loss calculation by the installer (room by room, to the current Part L method). Bedroom counts and “one kW per radiator” are starting bands, not a design. Since June 2022, Approved Document L in England expects that calculation when a boiler is replaced, so the boiler is not much larger than the home needs.
Use what size boiler for a first band from radiator count, bathrooms, and type. Then, on site, a heat-loss plus a measured flow. Related tools:
- Heating system output / heat-source design
- All radiators max power output and radiator new output
- Hot water recovery time (cylinder systems)
- Toolbox — the heating-design calculators in one list
Rough starting bands only (not a heat-loss):
| Typical combi DHW rating | Typical system / heat-only heating rating | |
|---|---|---|
| Small flat / house, ~1 bath, ~up to 10 radiators | Around 24–28 kW | Around 12–18 kW |
| Average house, 1–2 baths, ~10–15 radiators | Around 28–34 kW | Around 18–24 kW |
| Larger house, several outlets, many radiators | Around 32–42 kW — still limited by the cold main | Around 24–30 kW, or more if the heat-loss says so |
Oversizing heating makes the boiler cycle and spend less time condensing. Undersizing means the house never catches up on a design-day. Both are why the survey beats the table. Flow temperature versus output is explained in flow temperature and power output.
Efficiency: condensing and ErP
A condensing boiler reuses heat from the flue gases that older boilers throw away. The Energy Saving Trust’s rule of thumb: a gas or LPG boiler fitted after 2005, or an oil boiler after 2007, is normally a condensing boiler (plastic flue, a condensate pipe to a drain). New replacements are condensing.
ErP (Energy-related Products) is the labelling regime for seasonal space-heating efficiency. In England, a gas boiler fitted in an existing home must have an ErP seasonal space-heating efficiency of at least 92% — that is a Boiler Plus / Part L figure, read from the boiler’s product data, not an “up to” brochure line. Oil has its own minimums (91%, or 86% for an oil combi) under the same Approved Document. More on running the system well is on boiler energy efficiency. Turning the flow temperature down, or the equivalent on a system or regular boiler, is usually worth more than chasing a one-point ErP difference between two current A-rated boxes.
Boiler Plus (England)
From 6 April 2018, gas boiler work in existing homes in England has had extra minimums, now sitting in Approved Document L. The full page is Boiler Plus. In short:
- Gas boiler ErP seasonal efficiency at least 92%.
- Time and temperature control (timer or programmer, plus a room thermostat — or one programmable thermostat that does both).
- On a gas combi, one extra measure: flue-gas heat recovery, weather compensation, load compensation, or a smart thermostat with automation and optimisation.
From 15 June 2022 the same document also expects room-by-room thermostatic control (usually TRVs except in the room with the main thermostat), boiler interlock, a heat-loss for sizing, commissioning (including inhibitor to BS 7593), and, for a fully new wet system, design for 55 °C flow where practical. Wales, Scotland and Northern Ireland have their own Building Regulations — the installer works to the country the home is in.
Controls
Minimum controls are a legal floor, not a good system. A modulating control (OpenTherm or the maker’s own bus) lets the boiler match the load instead of banging on and off at full temperature. See modulating boilers and smart controls. Weather or load compensation is one of the Boiler Plus extra options on a gas combi and is worth having on system and regular boilers too.
Pick the control the boiler actually speaks. A “smart” on/off thermostat is not the same as a modulating one. Brand-specific notes for Vaillant and Worcester Bosch live with those makers.
Warranty
UK law gives a statutory minimum. Manufacturer packages are longer if you register the boiler, fit the specified filter, and have it serviced every year on their terms. Lengths differ by brand and by the pack the installer is on — Worcester Bosch guarantee lengths is an example of how much the small print moves. A long warranty from a brand with no local service is a paper comfort. Parts supply and an engineer who already knows the range usually matter more than the headline years.
Installation
Gas and LPG work in the UK must be carried out by a Gas Safe registered engineer. Oil is OFTEC (or equivalent competence). In England the gas job is notified through the competent person scheme; keep the Building Regulations Compliance Certificate. The Energy Saving Trust also recommends comparing quotes and looking for TrustMark registered businesses.
A proper install is more than hanging the boiler: flue and plume, condensate, gas or oil pipe sizing, water treatment, inhibitor, a magnetic filter where required, balancing, Benchmark (or equivalent), and a handover of the controls. For combis that sequence is on combi boiler installation and new combi boiler. If the existing boiler still has life in it, boiler repair may be the better spend this winter.
What drives the cost
We are not publishing a single “average UK boiler price” here. Fitted cost depends on the job, not the badge. Typical factors:
- Like-for-like versus a system change. Same place, same type is the cheaper shape of job. Moving the boiler, converting from regular to combi (stripping tanks and the cylinder), or adding a cylinder to a combi house is extra labour and materials.
- Type and fuel. A combi avoids a cylinder. A system or regular job includes the store and often more controls. Oil and LPG add tank and fuel-line work.
- Flue, condensate, and gas pipe. A new flue route, an internal condensate pump, or an undersized gas supply can dominate the quote.
- System condition. Flushing, inhibitor, a filter, and radiator or TRV upgrades are part of doing the job to Part L, not optional extras on a responsible quote.
- Controls and Boiler Plus extras. Weather compensation, OpenTherm, or a smart thermostat should appear as specified items, not a vague “controls included”.
- Access and making good. Cupboard size, scaffolding, and decorating after a move all sit outside the appliance price.
For fitted combi bands see combi boiler cost. For a site-specific figure, use quote. The Energy Saving Trust suggests getting more than one installer quote.
A practical order of decisions
- Is this a repair, a boiler replacement, or a heat-pump / hybrid conversation? (Decide.)
- What fuel do you actually have — or could you have?
- Combi, system, or regular, from bathrooms, mains flow, and space for a cylinder or tanks.
- A heat-loss (and a flow measurement if it is a combi), not a bedroom count.
- A boiler and control your local Gas Safe (or OFTEC) engineer will support, that meets Boiler Plus where that applies.
- Written quote: ErP, range-rating, extras, flushing, filter, guarantee registration, and the compliance certificate.
Related
- Boiler types
- Choosing a new combi boiler
- System boiler
- What size boiler
- Boiler Plus
- Combi boiler installation
- New combi boiler
- Heat pumps and hybrid heat pumps
- Boiler repair
- Quote
Sources
- Energy Saving Trust — boiler types explained
- Energy Saving Trust — air source heat pumps
- Approved Document L (conservation of fuel and power) — includes the Boiler Plus rules for England
- BEIS Boiler Plus factsheet (92% ErP, controls, combi extra measures)
- Boiler Upgrade Scheme (England and Wales)
- Gas Safe Register — find an engineer
Guidance only, expanded from the old hub.myboiler.com stub (WordPress page 750). Gas work in the UK needs a Gas Safe registered engineer. Building Regulations differ by nation. Efficiency labels, grant rules, and guarantee packs change — check the current Approved Document, scheme pages, and the installer pack for the job.