Combi vs Heat-only vs System: Different Boiler Types Explained

The right boiler does more than keep a home warm in winter. Efficiency, cost and suitability for household needs all matter. Knowing which one fits best is essential with several different boiler types available.

Combi, heat-only and system boilers each function differently and offer unique benefits and drawbacks. This guide lays out the key details and is designed to help simplify your decision when getting a new boiler installed.

Combi boilers: compact and convenient

A combi (combination) boiler handles both heating and hot water from a single unit. No separate water tank or cylinder needed. It draws water directly from the mains and delivers hot water instantly, without limits.

Of all the different boiler types available in the UK, combi systems are one of the most popular. Their compact design and energy efficiency make them a top pick for homeowners, especially smaller properties and flats.

Pros of a combi boiler

Compact and space-saving. No need for a separate hot water tank or cylinder, making them ideal for smaller homes and flats where space is tight.

Hot water on demand. No waiting for a tank to heat up. Perfect for busy households where multiple people need hot water at different times.

Energy efficient. Combi boilers only heat water when needed, which reduces energy waste and keeps bills lower.

Simple installation. Fewer components mean less pipework and an easier installation process compared to other boiler types.

Cons of a combi boiler

Not ideal for homes with high water demand. Combi boilers struggle if multiple taps or showers run at the same time. A system or heat-only boiler might be better if you have more than two bathrooms.

Water pressure dependent. Relies on good mains water pressure. If your mains pressure is low, a combi boiler may not perform as well.

No backup immersion heater. Unlike a system or heat-only boiler with a hot water cylinder, there’s no backup if the boiler breaks down.

Who should choose a combi boiler?

A combi boiler is best for:

• Small to medium-sized homes
• Flats and apartments
• Homes with one bathroom
• Those looking for an energy-efficient, space-saving heating system

Heat-only boilers: the traditional choice

Heat-only boilers, also known as regular or conventional boilers, come with a hot water cylinder and a separate cold water storage tank (usually tucked away in the loft, gathering dust alongside the Christmas decorations.) This setup suits homes that churn through hot water, as the cylinder keeps a good supply ready to go. No queuing for the shower, no mid-wash surprises—just lots of hot water when it’s needed.

Pros of a heat-only boiler

Great for larger homes. Ideal for properties with multiple bathrooms. Stored hot water allows multiple taps and showers to run at once.

Works with older heating systems. Can be a direct replacement for an older boiler in homes with existing pipework, radiators and tanks.

Compatible with low water pressure areas. Doesn’t rely on mains pressure, making it a good choice in areas with weaker mains supply.

Backup immersion heater. If the boiler breaks down, the hot water cylinder can have an immersion heater as a backup. This makes sure hot water is still available when you need it.

Cons of a heat-only boiler

Takes up more space. Requires a hot water cylinder and a cold water tank, so it’s not ideal for smaller homes or flats.

Limited hot water. Once the cylinder runs out of hot water, you have to wait for it to heat up again.

Less energy efficient. Heating and storing hot water in advance can result in wasted energy if it’s not used.

Who should choose a heat-only boiler?

A heat-only boiler is best for:

• Larger homes with multiple bathrooms
• Households where multiple taps and showers are used at the same time
• Older properties with traditional heating systems
• Areas with low mains water pressure

System boilers: a balance between combi and heat-only

Of all the different boiler types, this one strikes a solid balance between efficiency and performance. Cold water comes straight from the mains, heats up and waits in the hot water cylinder until it’s needed. No faffing with loft tanks and no dips in pressure. Just a steady flow of hot water for the whole household.

Pros of a system boiler

Great for homes with high hot water demand. Can handle multiple taps and showers at once without losing pressure.

No loft tank required. Unlike heat-only boilers, system boilers don’t need a cold water storage tank, freeing up loft space.

More energy efficient than heat-only boilers. Stores hot water but operates with a simpler design, making it more efficient.

Works with solar heating. Compatible with solar thermal systems, making it a good option for eco-conscious homeowners.

Cons of a system boiler

Requires space for a hot water cylinder. Needs a dedicated cupboard or airing cupboard for the cylinder, which takes up more space than a combi boiler.

Hot water can run out. Once the stored water is used, you need to wait for the system to reheat the cylinder.

Installation costs can be higher. More components mean a more complex installation compared to a combi boiler.

Who should choose a system boiler?

A system boiler is best for:

• Homes with multiple bathrooms
• Families with high hot water demand
• Those who want a balance between space-saving and performance
• Properties where loft space is needed for storage or conversions

Which boiler is right for you?

Each different boiler type has its strengths and weaknesses. The best option depends on your home size, water usage and available space. Here’s a quick summary:

• A combi boiler is best for small homes needing a space-saving solution.
• A heat-only boiler works well for large households with multiple bathrooms.
• A system boiler offers a balance, delivering good hot water supply without a loft tank.

Choosing from the different boiler types gets easier with the right information. When you're ready for a new boiler, Plumbox has options to suit every household. Contact us today for expert advice, quality boilers and a team based in Cambridge. Or download our free boiler buying guide to get clued up on the best choice for your home.