Best compact combi boilers In 2026

The best compact combi boilers to shortlist include the Worcester Bosch Greenstar CDi Compact, Ideal Logic MAX Combi2, Baxi 800 Combi 2 and Vaillant ecoFIT pure. Each offers a shallow casing, but the right choice depends on your cupboard dimensions, installation layout and hot-water needs.

For particularly limited depth, start by comparing the Ideal and Worcester models. For a location that could benefit from a rear flue, consider the Vaillant ecoFIT pure. The Baxi offers another cupboard-friendly option with a filter included and a conditional 10-year warranty.

This guide compares manufacturer specifications and practical installation considerations. It is not a laboratory test or a ranking based on independently measured failure rates. iHeat supplies and installs boilers commercially; the models available for your property will depend on your quote and installation requirements.

Best compact combi boilers at a glance

These four ranges offer shallow casings and several hot-water output options. Start by comparing their dimensions, then check the installation clearances and hot-water performance before choosing.

Output options use the manufacturers’ advertised model ratings. Central-heating output can differ.

These are casing dimensions, not the cupboard space required. Allow for the flue, pipework, filter and servicing access specified in the exact model’s installation instructions. A higher hot-water output does not increase the published casing dimensions within these ranges.

Specifications: Worcester Bosch, Ideal Heating, Baxi and Vaillant.

What makes a combi boiler compact?

A compact combi boiler combines central heating and hot-water production in a relatively small wall-mounted unit. It heats water as you need it, without requiring a separate hot-water cylinder. This makes a combi worth considering when space is limited. Energy Saving Trust explains the different boiler types.

However, "compact" does not tell you whether a particular boiler will fit inside your kitchen cupboard.

There are three separate measurements to consider:

• Casing dimensions: the boiler’s height, width and depth.

• Installation space: room for the flue, connections, pipework and accessories.

• Service access: space an engineer needs to inspect, maintain and repair it.

Depth is especially important when the boiler needs to sit behind a cupboard door. Height matters too, because the flue and connections can extend beyond the casing.

Compare the complete installation space before choosing a model. A slightly smaller casing is only useful if the finished installation remains accessible.

Which compact combi boilers are worth considering?

These four ranges offer shallow casings and different advantages for restricted spaces.

All dimensions below are height × width × depth. They describe the boiler casing, not the minimum cupboard size.

Hot-water flow rates are manufacturer figures at a 35°C temperature rise. Using the same temperature-rise condition makes the figures more useful for comparison.

Worcester Bosch Greenstar CDi Compact: a strong option for shallow cupboards

The Greenstar CDi Compact is worth shortlisting when you want a shallow boiler with several hot-water output options.

Dimensions: 690 × 390 × 280mm.

The range includes 28, 32 and 36kW models. Worcester’s installer guide lists hot-water flow rates of:

• 28CDi Compact: 11.4 litres per minute.

• 32CDi Compact: 13.1 litres per minute.

• 36CDi Compact: 15 litres per minute.

These figures are at a 35°C temperature rise. Worcester Bosch’s installer guide.

Its shallow casing is the main reason to consider it here. Choosing a higher hot-water output within this range does not increase the published casing dimensions.

What to check: the cupboard door and servicing space. Worcester specifies different front clearances for a removable cupboard door and a fixed obstruction.

Guarantee length depends on the installation package, filter and installer scheme. Confirm the cover included in your quote. Worcester Bosch product specifications and guarantee options.

Ideal Logic MAX Combi2: the shallowest casing in this list

The Ideal Logic MAX Combi2 is a useful starting point when cupboard depth is your main restriction.

Dimensions: 700 × 395 × 278mm.

That makes its casing the shallowest of these four ranges, although the small difference between models should not outweigh installation suitability. Ideal Heating’s technical brochure.

The published hot-water flow rates at a 35°C temperature rise are:

• 24 model: 9.9 litres per minute.

• 30 model: 12.4 litres per minute.

• 35 model: 14.5 litres per minute.

The range includes an Ideal System Filter. Ideal advertises a 10-year parts and labour warranty, subject to its terms, including registration within 30 days of installation. Ideal Logic MAX Combi2 specification sheet.

What to check: whether the proposed pipework needs a stand-off kit behind the boiler. The casing depth alone will not describe an installation that uses additional space behind it.

Also check the exact product name on the quote. Logic, Logic+, Logic MAX and different generations should not be treated as interchangeable specifications.

Baxi 800 Combi 2: a cupboard-friendly option with a filter package

The Baxi 800 Combi 2 offers another shallow casing, with a compact Adey MagnaClean Micro2 system filter included.

Dimensions: 700 × 395 × 285mm.

Baxi publishes the following hot-water flow rates at a 35°C temperature rise:

• 824 Combi 2: 10.2 litres per minute.

• 830 Combi 2: 12.2 litres per minute.

• 836 Combi 2: 15 litres per minute.

Its published front clearance is 5mm during operation, increasing to 450mm for servicing. That distinction matters when planning a cupboard door or nearby obstruction. Baxi’s technical specifications.

What to check: space for the filter and access to the connections, as well as the boiler itself.

Baxi advertises a 10-year parts and labour warranty, subject to its conditions. The supplied filter must be installed and registered at the same time as the boiler for that cover to apply. Baxi 800 Combi 2 warranty information.

Vaillant ecoFIT pure: worth considering for a rear-flue installation

The Vaillant ecoFIT pure is worth considering where its top or rear flue options could help the installation layout.

Dimensions: 700 × 390 × 295mm.

It is the deepest of these four shortlisted ranges, so check available depth carefully before assuming it will fit behind a cupboard door. Vaillant ecoFIT pure product specifications.

Vaillant’s technical sheet lists hot-water flow rates at a 35°C temperature rise of:

• ecoFIT pure 825: 10.4 litres per minute.

• ecoFIT pure 830: 12.2 litres per minute.

• ecoFIT pure 835: 14.1 litres per minute.

The same sheet also publishes figures at a 42°C rise, so check which condition a comparison is using. Vaillant ecoFIT pure technical sheet.

What to check: whether a rear flue is suitable for your property. It is an installation option, not a guarantee that the existing flue position can be reused.

Vaillant advertises extended cover through its Advance scheme, with qualifying requirements including a boiler protection kit. Confirm your actual guarantee in writing.

Will a compact boiler fit inside your cupboard?

A compact boiler can be installed inside a suitable cupboard, provided the completed installation meets the requirements for the exact model.

Do not order a boiler or build an enclosure from the casing dimensions alone.

Measure the space that is actually available

Record the internal height, width and depth. For depth, measure from the proposed mounting surface to the inside of the closed door.

Check whether any of the following reduce that space:

• Hinges, rails and shelves.

• The cupboard back panel.

• Existing pipes or electrical fittings.

• Uneven walls.

• Door hardware that projects inwards.

An external cupboard measurement does not tell you how much usable room is inside.

Allow for connections and accessories

The installation also needs space for the flue, water and gas connections, condensate pipe and any specified filter or mounting accessories.

Some components may sit outside the cupboard. Others may need space below or behind the boiler.

Ask your installer to confirm the proposed layout before changing the cabinetry.

Check access with the door open

A boiler may need relatively little clearance from a cupboard door during normal operation but much more room in front for servicing.

The door must open sufficiently or be removable where required. Nearby walls, worktops and permanent fittings must not prevent access.

An engineer should also be able to reach the controls, isolation points and filter without dismantling unrelated kitchen units.

A worked depth example

Worcester publishes a 280mm casing depth for the Greenstar CDi Compact and a 20mm clearance to a removable cupboard door.

For an illustrative depth check:

280mm boiler + 20mm door clearance = 300mm.

This only checks casing depth and the specified door gap. It does not confirm that a nominally 300mm cupboard is suitable, or account for the mounting arrangement and obstructions.

Worcester also specifies servicing space in front of a fixed surface. Its guide gives 600mm, with a reduced allowance only under particular conditions. Worcester Bosch clearance requirements.

The practical lesson is simple: “fits inside the cupboard” and “can be installed and serviced there” are separate checks.

What size compact combi boiler do you need?

Choose the physical size around your available space, then select the output around your home’s heating and hot-water requirements.

A small casing does not necessarily mean a low output. Equally, a high-output combi is not automatically the best choice for a small home.

Separate heating output from hot-water output

A combi boiler has specifications for both central heating and domestic hot water. The number in its model name does not necessarily describe the maximum output available to your radiators.

For example, Ideal lists the Logic MAX Combi2 30 with a maximum hot-water output of 30.3kW, while its central-heating output is 6.1–24.2kW at a mean water temperature of 70°C. Ideal Heating’s product data.

This is why matching a boiler to a fixed number of bedrooms or radiators can be misleading.

Your installer should consider heat loss, insulation, emitters and heating controls, alongside hot-water demand.

Understand what the flow rate means

A hot-water flow rate describes how much water the boiler can heat each minute under stated conditions.

A 35°C temperature rise means raising the incoming water temperature by 35°C. For example, heating water from 10°C to 45°C is a 35°C rise.

It does not mean the hot tap runs at 35°C.

When incoming water is colder, the boiler needs to raise its temperature further to reach the same outlet temperature. This can reduce the available hot-water flow.

Compare specifications at the same temperature rise, then discuss how that performance relates to your taps and showers.

Check your incoming water supply

A higher-rated boiler cannot compensate for an inadequate incoming water supply.

Before selecting a model for stronger hot-water performance, have the available flow and pressure assessed under realistic conditions, including when other outlets are being used.

This is particularly important if you already experience a substantial drop in shower performance when another tap opens.

Think about simultaneous use

The important question is how much hot water you need at once.

Consider whether your household regularly:

• Runs two showers together.

• Fills a bath while someone showers.

• Uses a kitchen hot tap during a shower.

• Plans to add another bathroom.

A combi may work well when outlets are used separately but be less suitable for frequent simultaneous demand. Energy Saving Trust identifies high hot-water demand and low water pressure as reasons a combi may be less suitable. Boiler selection guidance.

For a fuller explanation, read our guide to what size combi boiler you need.

How much does a compact combi boiler cost to install?

For a budgeting reference, iHeat’s average boiler installation order value was around £2,800 including VAT for fitting dates from September 2025 to August 2026.

This rounded figure covers a mix of boiler installations, rather than compact combi boilers alone. It includes recorded package extras and price adjustments, so it should not be treated as a fixed price for any model in this guide. The data was reviewed on 14 September 2026. See our new boiler costs guide for the calculation and scope.

Your quotation depends on both the boiler and the work required. Replacing an existing combi in the same position generally involves less work than relocating it or converting a system with a hot-water cylinder.

When comparing compact boiler quotes, check whether the total includes:

• VAT, the boiler and installation.

• The required flue and pipework alterations.

• Heating controls, system cleaning and a magnetic filter.

• Removal of the old boiler and any redundant equipment.

• Any cupboard alterations needed for installation and servicing access.

Compare complete installed prices. A smaller casing does not automatically mean a cheaper installation, particularly if fitting it requires changes to the cupboard, flue or pipework.

Are compact boilers efficient and reliable?

A compact casing alone does not tell you how efficiently a boiler will run or how long it will last.

For efficiency, compare the exact model’s published rating and consider how the heating system will be controlled and commissioned.

Compatible controls can adjust heating to demand. Suitable flow-temperature settings can also help a condensing boiler operate efficiently while maintaining comfort. Energy Saving Trust’s heating controls guidance.

Look beyond the maximum output

Ask about the boiler’s minimum heating output as well as its maximum.

A home may need relatively little heat during milder weather. How far the boiler can reduce its output is therefore relevant, particularly in a small or well-insulated property.

The installer should assess this alongside the heating system and compatible controls.

Compare warranty conditions

Warranty length is useful, but it is not a direct measurement of reliability.

Before choosing, confirm:

• The cover included with your specific installation.

• Registration requirements and deadlines.

• Required filters or accessories.

• Servicing obligations.

• Any exclusions and how to arrange a repair.

Include ongoing servicing in your ownership budget.

Keep maintenance access practical

A compact installation should remain accessible after the kitchen is finished.

Avoid placing permanent shelves or fittings across service access. Keep the controls and required clearances available, and follow the manufacturer’s instructions for the enclosure.

A boiler that fits neatly today should still be straightforward to maintain later.

When should you consider a different boiler type?

A compact combi is a useful space-saving option, but fitting the smallest appliance should not be the only objective.

If several people regularly need hot water at once, ask your installer to compare a combi with a suitable system boiler and cylinder arrangement.

If you already have a cylinder that serves your household well, discuss the practical consequences before removing it. You may gain storage space, but your hot-water delivery will change.

Energy Saving Trust also notes that most heat pumps use a hot-water cylinder. If you plan to move to a heat pump in future, retaining suitable cylinder space may be worth considering. Energy Saving Trust boiler guidance.

If cupboard depth is not a restriction, you can widen your shortlist using our best combi boilers guide.

Find a compact boiler that fits your home

Start with your usable space, then check hot-water demand and the installation layout.

The Ideal Logic MAX Combi2 and Worcester Bosch Greenstar CDi Compact are useful starting points for restricted depth. The Baxi 800 Combi 2 offers another shallow option, while the Vaillant ecoFIT pure is worth considering where its flue choices suit the property.

Before choosing, have the cupboard measurements, water supply and required access checked. Then compare complete installation quotes for suitable models.

Find a new boiler with iHeat and flag any cupboard restrictions when discussing your installation.