Best Heat-Only (Conventional) Boilers in 2026

What is a heat-only boiler

A heat-only boiler is a conventional boiler designed to work with a separate hot water cylinder.

Instead of producing hot water on demand, the boiler heats water for your radiators and transfers heat to the cylinder so hot water can be stored and used later. When you run a bath, shower, or hot tap, the hot water comes from the cylinder.

This setup is commonly used in larger homes, or in properties where hot water demand overlaps. It can also be a sensible choice when you already have a cylinder and want a straightforward replacement.

How a conventional boiler works in practice

A heat-only boiler is part of a wider heating system. Most installations include the boiler, a hot water cylinder, heating controls, and pipework that links everything together.

Some homes have a vented setup with storage tanks. Others use an unvented cylinder, which does not rely on tanks. The right configuration depends on the property and the system design.

Performance is not only about the boiler. Your cylinder size, cylinder recovery, controls, and system condition can all affect hot water delivery and running costs.

Who heat-only boilers suit best

A conventional boiler can be a strong fit if:

• You want stored hot water ready for baths and showers

• Your home has one or more bathrooms and hot water use can overlap

• You already have a cylinder and want a like-for-like replacement

• You have enough space for a cylinder and associated pipework

A heat-only boiler is usually a weaker fit if you have very limited space and no cylinder in place, or if you want hot water produced instantly without stored capacity.

Fuel types for heat-only boilers

Heat-only boilers can be available for different fuels depending on the property, including natural gas, oil, and LPG.

For most UK homes on mains gas, a gas conventional boiler is typically the most straightforward option to install and service. If your home is off the gas grid, you will usually need a system designed around the available fuel and local servicing support.

This guide focuses on the heat-only boiler type and how to choose a suitable model, rather than fuel strategy for off-grid properties.

Advantages of heat-only boilers

Heat-only boilers are often chosen for practical reasons rather than headline features.

Stored hot water can support periods where demand overlaps, assuming the cylinder is sized properly. A conventional boiler can also be a sensible replacement if your home already has this layout, because a like-for-like swap is often simpler than changing the system design.

They can also work with different cylinder approaches depending on how the system is configured, which gives installers flexibility when planning upgrades.

Disadvantages to consider

A heat-only boiler needs a cylinder, and that requires space.

Hot water is limited by the cylinder. If the cylinder runs out, you wait for it to reheat. The boiler can be excellent, but a small cylinder or slow recovery can still feel disappointing at peak times.

There are also more components involved than with simpler layouts. Controls, valves, and overall water quality have a bigger impact on reliability because the boiler is only one part of the system.

Best heat-only boilers in 2026

The models below are well-known heat-only boiler options used in UK homes. The best choice is the one that suits your property’s heat demand, cylinder setup, and available space.

Worcester Bosch Greenstar Ri Heat Only (27kW)

If you want a compact heat-only (conventional) boiler from a well-known brand, the Greenstar Ri is a common choice for like-for-like replacements.

It’s often considered when you’re keeping your existing hot water cylinder and you need a boiler that fits neatly in a tighter space, such as a kitchen cupboard or utility area.

As with any conventional setup, your day-to-day hot water experience will still depend heavily on the cylinder size and how quickly it reheats.

Key features

• Type: Heat-only / conventional boiler

• Output: 27kW

• Efficiency: A-rated (around 92%)

• Mounting: Wall-mounted

• Size: 600 x 390 x 270mm

Alpha E-Tec Regular Heat Only (15kW, 20kW, 25kW)

The Alpha E-Tec Regular is typically shortlisted by homeowners who want a heat-only boiler with a few output options, so the installer can match the boiler more closely to the property.

That matters because the “best” boiler isn’t the biggest one; correct sizing helps your heating run smoothly and can avoid issues linked to oversizing.

This range makes sense if you’re keeping a conventional layout with a cylinder and you want a straightforward modern replacement.

Key features

• Type: Heat-only / conventional boiler

• Outputs available: 15kW, 20kW, 25kW

• Efficiency: A-rated (around 88%) (as per OG spec)

• Mounting: Wall-mounted

• Size: 600 x 390 x 305mm

Viessmann Vitodens 100-W Heat Only (30kW)

If you’re comparing heat-only boilers and you already know a 30kW output is suitable for your system, the Vitodens 100-W is one of the more premium-feel options that homeowners often look at.

In practice, it’s a better fit when it’s paired with the right controls and a well-set-up system, because those factors have a big impact on how consistent and efficient the heating feels day to day.

It’s still a conventional setup, so stored hot water performance will come down to the cylinder.

Key features

• Type: Heat-only / conventional boiler

• Output: 30kW

• Efficiency: A-rated (around 92%)

• Mounting: Wall-mounted

• Size: 600 x 375 x 285mm

Worcester Bosch Greenstar 8000 Life Heat Only (30kW)

The Greenstar 8000 Life heat-only model is usually considered by homeowners who want a higher-spec conventional boiler and have enough space for a larger unit.

Compared with more compact heat-only boilers, the main practical difference is the casing size and the fact it’s typically chosen as part of a more considered upgrade, not just a straight swap in a tight cupboard.

As always, the boiler is only one part of the overall system; cylinder condition, controls, and system cleanliness still do a lot of the heavy lifting for comfort and reliability.

Key features

• Type: Heat-only / conventional boiler

• Output: 30kW

• Efficiency: A-rated (around 92%)

• Mounting: Wall-mounted

• Size: 780 x 440 x 365mmSelecting the Best Heat Only Boiler

How to choose the right conventional boiler for your home

Start with your hot water routine.

If hot water demand overlaps, cylinder size and recovery can matter as much as the boiler model.

Many “not enough hot water” complaints are really cylinder issues, not boiler issues.

Next, size the boiler to your system demand. Bedrooms are a rough guide, but insulation, draughts, window area, and radiator sizing drive heat loss.

Oversizing can reduce efficiency and increase wear.

Finally, decide what you are keeping. A like-for-like swap is usually simpler.

Costs rise if you change cylinder type, relocate the boiler, rework pipework, or upgrade controls.

Typical installed cost in the UK

System complexity drives cost more than the boiler badge.

As a broad UK guide for 2026, a heat-only boiler installation is often priced in the low-thousands overall.

The main cost drivers are usually cylinder condition, required pipework changes, controls, and whether system cleaning or remedial work is needed.

Treat any range as a guide rather than a quote.

The only accurate way to price a conventional boiler install is to review the existing system and confirm what work is required.

Installation and safety notes

A heat-only boiler installed on mains gas is a gas appliance. Installation and commissioning should be completed by a Gas Safe registered engineer.

System water quality matters for performance and for many warranty conditions.

Depending on your system condition, an installer may recommend cleaning, inhibitor, and a filter.

This is often a practical reliability step, not an upsell, especially in older systems.

iHeat conventional boilers

iHeat installs modern gas boilers for UK homeowners, including like-for-like replacements where a heat-only boiler is the right fit.

If you are replacing an existing conventional boiler, the most useful next step is confirming:

• Correct output sizing for your system

• Whether your cylinder is suitable for your household

• What work is actually required for a compliant installation

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