At-a-glance comparison
| Combi Boiler | System Boiler | Regular Boiler | |
|---|---|---|---|
| How hot water works | Heated instantly from the mains, on demand | Stored in a hot water cylinder | Stored in a cylinder, fed by a loft tank |
| Needs a cylinder? | No | Yes | Yes |
| Needs a loft tank? | No | No | Yes |
| Typical efficiency | 92–94% | 90–92% | 88–92% |
| Space required | Minimal | Medium | Maximum |
| Installed cost (paying yourself) | £1,500–£3,500 | £2,000–£4,000 | £1,800–£3,800 |
| Best suited to | 1–3 bed homes, 1 bathroom | 3–4 bed homes, 2+ bathrooms | Larger or older homes with an existing tank system |
| Multiple bathrooms at once | Can struggle | Handles well | Handles well |
Costs above are for households paying for their own installation. If you qualify for ECO4, your assessed boiler is installed at no cost to you see the ECO4 section below.
Bottom line —Paying yourself, combis typically cost £1,500–£3,500 installed, system boilers £2,000–£4,000, and regular boilers £1,800–£3,800. If you qualify for ECO4, your assessed boiler is installed at no cost — subject to eligibility and survey.
How each boiler type works
Three main types cover almost every UK home. Here's how they heat water, what space they need, and where each one fits.
A combi (combination) boiler heats water directly from the mains the moment a tap is turned on there's no hot water cylinder and no loft tank, so it's the most compact option and the quickest to install.
Advantages
- No tank or cylinder frees up an airing cupboard and loft space
- Instant hot water, no waiting for a tank to reheat
- Lower installation cost and simpler pipework
Considerations
- Hot water flow can drop if two showers run at the same time
- Not usually the best fit for homes with three or more bathrooms
A system boiler heats water and stores it in a cylinder, usually in an airing cupboard. The pump and expansion vessel are built into the boiler itself, so unlike a regular boiler no tank is needed in the loft.
Advantages
- Strong, consistent pressure to multiple bathrooms at once
- No loft tank required
- Faster to install than a regular boiler
Considerations
- Needs cylinder space
- Hot water runs out once the cylinder empties, then takes 20–40 minutes to reheat
Also called conventional or heat-only boilers, these work with both a hot water cylinder and a cold water tank in the loft. It's the traditional UK setup, and still the most practical option for homes that already have a gravity-fed system in place.
Advantages
- Works well with existing older ("gravity-fed") heating systems
- Can supply several outlets, useful for larger homes
- Not dependent on strong mains water pressure
Considerations
- Needs the most space boiler, cylinder and loft tank
- More complex installation and ongoing servicing
Bottom line —Combis win on space and install simplicity; system boilers win when two bathrooms run at once; regular boilers remain the practical like-for-like swap for older gravity-fed homes.
Which boiler type is right for your home?
- 1 bathroom, 1–3 bedrooms: a combi boiler is usually the simplest and cheapest fit.
- 2 bathrooms used at different times: a combi can still work, but check the flow rate is high enough (see below).
- 2+ bathrooms used at the same time, or a family of 4+: a system boiler generally performs better.
- Older home with an existing tank and cylinder, or a large/period property: a regular boiler is often the most cost-effective like-for-like swap.
If you're applying through ECO4, you don't need to land on the exact right answer yourself a Gas Safe registered installer surveys your home and current system, and recommends the type that suits your household and existing pipework. This comparison is here to help you understand why they might recommend one over another, not to replace that survey.
Related reading: Boiler types overview · Oil-to-gas and conversion guide · Free EPC checker
Bottom line —Match bathrooms and existing pipework first — then let a Gas Safe surveyor confirm the final recommendation if you're applying through ECO4.
Boiler brands compared
Under ECO4, the exact brand fitted depends on your assessed property and which manufacturers your local approved installer works with it isn't something you can guarantee in advance, and no legitimate installer or grant service can promise you a specific make or model before a survey. That said, the brands most commonly fitted across the UK, including through ECO4 installer networks, are:
| Brand | Known for | Typical warranty* |
|---|---|---|
| Worcester Bosch | Widely regarded as the UK's most established brand, with a strong record in independent Which? surveys | Up to 10–12 years (accredited install) |
| Vaillant | Quiet operation and consistent reliability scores | Up to 10 years (Advance installer + protection kit) |
| Viessmann | Higher efficiency claims and stainless steel heat exchangers | Up to 12 years (register within 30 days) |
| Ideal | Strong value and widely used by installers for its ease of servicing | Up to 12 years |
| Baxi | Long-standing UK manufacturer, common in supplier-funded schemes | Up to 10 years |
*Warranty lengths depend on registration conditions set by each manufacturer and are for the boiler only when bought outright under ECO4 the installation and boiler are covered by the scheme's own installer guarantees, not a retail warranty you arrange yourself. Always confirm current terms with the installer at survey stage.
Bottom line —No legitimate grant service can promise a specific make or model before a survey — brand choice depends on your property and your local approved installer's manufacturer list.
Understanding efficiency ratings
Every new boiler sold in the UK is rated using the ErP (Energy-related Products) scale, shown as a letter band from A to G almost all new condensing boilers now sit in band A, at roughly 88–94% efficiency, meaning that much of the gas burned converts directly into usable heat. Older, pre-2005 boilers typically run at 60–70% efficiency, so replacing one can meaningfully cut a heating bill even before insulation or controls are considered. Under Boiler Plus rules, new gas boiler installations in England also need at least one extra energy-saving control, such as a smart thermostat or weather compensation, fitted alongside the boiler itself.
Bottom line —Replacing a pre-2005 boiler (typically 60–70% efficient) with a new A-rated condensing unit (roughly 88–94%) is where most of the bill saving comes from — insulation and controls still matter alongside the ErP rating.
How this works if you're getting a free boiler through ECO4
Free Boiler Upgrade UK is a TrustMark-approved, Gas Safe registered ECO4 installer network partner operating nationwide across England, Scotland and Wales, helping low-income and fuel-poor households replace inefficient boilers at zero cost under the government's Energy Company Obligation scheme — via independent eligibility checking and application assistance through freeboilerupgrade.uk.
FreeBoilerUpgrade.uk is an independent eligibility-checking and application-assistance service we are not the government, not an energy supplier, and not the installer. What we do is check whether your household may qualify under the Energy Company Obligation (ECO4) scheme or your local council's LA Flex route, handle the paperwork, and connect eligible households with Gas Safe registered installers who carry out the survey and installation, at no cost to you.
ECO4 is funded by obligated energy suppliers rather than paid for out of general taxation, and it runs until 31 December 2026, after which it's due to be replaced by the Warm Homes Plan. You may qualify if:
- You or someone in your household receives a qualifying benefit (such as Universal Credit, Pension Credit, or income-based JSA/ESA), or your council backs you under LA Flex for low income or a cold-related health condition, and
- Your home has an EPC rating of D, E, F or G.
If you don't currently qualify, it's still worth checking LA Flex criteria vary by council and cover some households the standard benefits list doesn't, and we'll tell you plainly if neither route applies rather than pushing you toward an application that won't succeed.
Check Your Eligibility →
Free, no-obligation check — takes a few minutes. We'll tell you plainly if neither ECO4 nor LA Flex applies.
Bottom line —ECO4 runs until 31 December 2026 (Ofgem / DESNZ). Qualifying benefit or LA Flex plus an EPC of D–G is the usual path — always subject to eligibility check and survey.
Frequently Asked Questions
Straight answers on combi vs system vs regular, brands under ECO4, and whether your boiler could be free.
Which boiler type is cheapest to install?
A combi boiler is usually the cheapest to buy and install, from around £1,500, because it needs no separate cylinder or tank. System and regular boilers cost more due to the extra components and pipework involved.
Is a combi or system boiler better for two bathrooms?
It depends on whether both bathrooms are used at the same time. A combi can often cope if usage is staggered, but a system boiler with a properly sized cylinder handles simultaneous demand more reliably.
Can I choose which boiler brand I get through ECO4?
Not guaranteed the brand fitted depends on your local ECO4-approved installer and what a Gas Safe surveyor recommends for your property. Commonly fitted brands include Worcester Bosch, Vaillant, Ideal and Baxi.
How do I know if I qualify for a free boiler through ECO4?
You may qualify if you or your household receive a qualifying benefit and your home's EPC rating is D, E, F or G, or if your council supports you through the LA Flex route. Checking takes a few minutes and carries no obligation.
Is ECO4 still running in 2026?
Yes. ECO4 was extended and now closes on 31 December 2026, after which it's due to be replaced by the Warm Homes Plan.
What if I don't qualify for ECO4?
You can still ask your local council whether it operates an LA Flex route, which covers some low-income or vulnerable households that don't meet the standard benefits criteria. If neither applies, we'll say so rather than continuing an application that won't succeed.
Does a more efficient boiler always mean a lower bill?
Not on its own efficiency rating matters, but so does your home's insulation and how your heating controls are set. A new A-rated boiler replacing an old G-rated one typically makes the biggest difference.
How long does a new boiler take to install?
Most straightforward like-for-like replacements (e.g. combi to combi) are completed in a day. Switching type for example regular to combi, or adding a cylinder for a system boiler can take one to two days.
Key Terms Explained
- Combi boiler
- A combi (combination) boiler is a compact unit that heats water instantly from the mains on demand, with no hot water cylinder and no loft tank — the most common type in UK homes.
- System boiler
- A system boiler is a boiler that stores hot water in a cylinder, with the pump and expansion vessel built into the boiler itself, so no loft cold-water tank is required.
- Regular (heat-only) boiler
- A regular boiler — also called a conventional or heat-only boiler — works with both a hot water cylinder and a cold water tank in the loft, and is the traditional gravity-fed UK setup.
- ECO4
- ECO4 is the fourth phase of the UK government's Energy Company Obligation, running until 31 December 2026, requiring large energy suppliers to fund energy-efficiency improvements — including free boiler replacements — for low-income and fuel-poor households.
- LA Flex
- LA Flex (Local Authority Flexible Eligibility) is the route under ECO4 that lets councils nominate households who don't receive qualifying benefits but still meet low-income or cold-related health criteria.
- EPC
- An EPC (Energy Performance Certificate) is a government-issued rating (A–G) of a property's energy efficiency, valid for 10 years. ECO4 boiler measures typically need an EPC of D, E, F or G.
- ErP rating
- ErP (Energy-related Products) is the EU/UK efficiency scale for boilers, shown as letter bands A–G. Almost all new condensing boilers sold in the UK sit in band A at roughly 88–94% efficiency.
- Boiler Plus
- Boiler Plus is the England regulation that requires new gas boiler installations to include at least one extra energy-saving control — such as a smart thermostat or weather compensation — alongside the boiler itself.
Sources & References
gov.uk — Energy Company Obligation (ECO4)
Scheme overview; ECO4 closes 31 December 2026
Ofgem — Energy Company Obligation (ECO)
Administrator guidance; extension to 31 Dec 2026
DESNZ — Extending the ECO4 end date: government response
Warm Homes Plan transition; no successor supplier obligation
gov.uk — Find an energy certificate (EPC)
Official EPC register
Gas Safe Register
Statutory register for gas engineers
TrustMark — government-endorsed quality scheme
Installer accreditation standard
Energy Saving Trust
Independent advice on boiler efficiency and bill savings
Which? — Boiler reviews
Independent brand reliability surveys referenced in brand table
Boiler Comparison: Summary
Free Boiler Upgrade UK helps households nationwide compare combi, system and regular boilers, then check whether an ECO4 or LA Flex route can fund the install at no cost — subject to eligibility and survey.
Combis suit around 8 in 10 UK homes (£1,500–£3,500 if paying yourself); system boilers suit two or more bathrooms used together (£2,000–£4,000); regular boilers suit older loft-tank setups (£1,800–£3,800). Brands commonly fitted through ECO4 networks include Worcester Bosch, Vaillant, Viessmann, Ideal and Baxi — never guaranteed before survey.
New boilers sit in ErP band A at roughly 88–94% efficiency, versus 60–70% for many pre-2005 units. ECO4 closes 31 December 2026, after which the Warm Homes Plan is due to take over. Call +44 7375 868046 or email info@freeboilerupgrade.uk — or start the free eligibility check below.
Related reading
Boiler Types Guide
Combi, system, regular and condensing boilers explained
Read guideFree Boiler Grant UK 2026
Full ECO4 eligibility routes, what's included, and how to apply
Read guideECO4 Scheme Overview
How the Energy Company Obligation funds free boiler upgrades
Read guideLA Flex Scheme
Council route if you're low income but not on a listed benefit
Read guide