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What are the different types of boilers?

It can be argued – given our climate – that a boiler is the single most important appliance in the UK home, but how well do you know and understand yours? Most of us set the thermostat and then don’t give a second thought to the boiler — until it breaks down, at which point we pay someone else to look at it. But with rising energy costs, there’s never been a better time to start learning. When it comes to replacing your boiler, knowing what your options are won’t simply make your home more energy efficient and cheaper to run, it can make it more comfortable too. Read on to discover all you need to know about boilers.
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What is a boiler?

Boilers are designed to provide heat for your home (and water). They’re essentially small furnaces that heat the water passed through them. This hot water can then be distributed to your central heating system or used to supply hot water on demand or stored for later use.

Most boilers are fuelled with natural gas, but you can also obtain boilers that work with different fuels, including oil and wood pellets (biomass).

How are boilers classified? 

Boilers are classified into types according to various criteria, including their working pressure and temperature, fuel type, size and capacity and whether they condense the water vapour produced during combustion to reclaim more heat.

What are the different boiler types?

The boiler market in the UK is dominated by one particular type of boiler: the condensing boiler.

However, there are different types of condensing boiler, including combi-boilers, heat-only and system boilers. 

While most condensing boilers use mains gas, there are some alternatives for those who live off-grid, or just want a different option. Two popular alternatives are the oil boiler, which typically uses kerosene that is delivered to your home, and the biomass boiler, also known as a wood boiler.

What’s the most common type of boiler?

Condensing gas boilers are by far the most common boiler type in UK homes. According to recent UK boiler statistics, they are capable of using around 90% of their heat, making them far more energy efficient than older models. Of these, combi-boilers are the most popular.

What are the different boiler brands?

Popular brands producing combi-boilers include Baxi and Glow Worm (both Baxi heating boilers and Glow Worm boilers are British-made). Combi-boilers come in a variety of shapes and sizes to suit different requirements and households.

How does each type of boiler work?

A condensing boiler works by passing hot gas through a central chamber that heats up the water as it passes through. To increase efficiency, a second chamber uses any remaining heat to warm up water coming back into the unit from the heating system.

Of all the condensing boiler types, the combi-boiler is the most popular. A combi-boiler includes all the components required to both heat and distribute your water, so all your hot water and heating are supplied from the same unit. This makes it easy to install.

The water is heated instantly through a metallic heat exchanger within the combi boiler itself to provide you with a steady supply of hot water on demand. They require no tanks or cylinders to store hot water in – instead, water is heated only when it’s needed to increase efficiency.

Heat-only boilers, in comparison, work differently. Once heated by the boiler, water is then fed directly to the central heating system or stored within a hot water cylinder to provide hot water when it’s needed. Unlike combi and system boilers, cold water isn’t fed directly to the boiler via the mains; instead, it’s routed through a cold-water tank that’s typically situated in the loft or attic. This gravity-fed ‘open-vented’ system maintains a good flow rate even in areas with low mains water pressure.

Finally, system boilers attempt to provide a compromise between heat-only and combi-boilers. Like heat-only tanks, hot water is stored in a separate tank to provide a steady supply of hot water. Unlike heat-only systems, this is an unvented tank, so cold water is fed directly from the mains to remove the need for a separate cold-water tank. Boiler system types require fewer external components than heat-only boilers, building elements like the pump and expansion vessel into the boiler itself. System boilers are also known as ‘closed vent’ or ‘sealed system’ boilers.

A fourth type of condensing boiler attempts to combine the best features of system and combi boilers. As the name implies, combi-storage boilers offer storage to improve flow rates when multiple hot water outlets are used simultaneously while also providing hot water on demand like a regular combi boiler That said, if you’re on a tight budget, a like-for-like swap will prove the cheapest due to reduced installation requirements. Unlike system boilers, the storage is built directly into the boiler itself.

Other boiler types explained

Oil boilers provide an estimated four million homes in the UK with no access to mains gas with an alternative form of fuel. They work in a similar way to traditional boilers, with oil substituting for gas. Maintenance can be an issue because, if you run into any problems, you’ll need to get an Oftec registered engineer in to inspect it (rather than a Gas Safety Register engineer for conventional gas boilers).

A biomass boiler, or wood boiler, relies on wood pellets, chips or logs to generate heat. Different types of fuel cost different amounts – typically £400-£600/year (based on average medium usage of 12,500 kWh per year). Pellets are the most practical solution for biomass boilers even though logs are cheaper, as they can be automatically fed in to the system (you’’ll need to find a supplier for your pellets though). 

What are the advantages and disadvantages of each type of boiler?

There’s no one-size-fits-all approach to boilers, which means you need to weigh up the pros and cons of each type in relation to your own circumstances. Here’s what to look out for: 

Combi-boiler pros and cons

  • Save space by removing redundant tanks and cylinders, plus the unit itself is extremely compact. Best for smaller properties with a single bathroom

  • All components for heating and hot water are incorporated within the boiler itself, which means everything is covered under a single manufacturer’s warranty

  • Popularity means it’s easier to find a Gas-safe engineer to service and maintain your boiler

  • No need to wait for hot water – it’s heated instantly on demand

  • Requires good water mains pressure

  • Water flow is reduced when serving two separate hot water outlets simultaneously (such as two separate showers, or a shower and washing up)

  • Boiler contains more moving parts, so more potential for things to go wrong than with a conventional boiler

  • High-performance boilers may be oversized for heating in properties with 15mm pipework

Heat-only boiler pros and cons

  • Better for larger properties as it can distribute hot water to multiple outlets (say shower and kitchen sink) simultaneously

  • Gravity-fed system means it’s suitable for properties with lower mains water pressure

  • System’s lower water pressure works well with older radiator systems (less chance of leaks)

  • Cheapest type of boiler to buy

  • Must wait for hot water to heat up before it can be used

  • Too much demand can result in hot water running out temporarily

  • Less efficient than combi-boilers as more heat is lost from the hot water cylinder

  • Boiler guarantee doesn’t cover external components such as the circulation pump

  • Requires more space for water tanks and cylinders

  • Potential for cold water cistern to freeze during the winter

System boiler pros and cons

  • Supports larger households through use of stored hot water

  • Like combi systems, more of the system’s components are integrated into the boiler, so comprehensive guarantee means lower maintenance costs

  • No cistern in the loft means no potential leaks or freezes to worry about. Also requires less space than a heat-only boiler

  • Requires good mains water pressure to maintain decent hot water flow rates

  • Requires more space than a combi boiler (specifically for the hot water cylinder)

  • Like heat-only boilers, are less efficient, require you to wait while water is heated, and carry the risk of running out of hot water if lots of people use it at the same time

  • High-pressure system may not be suitable for older properties

Combi-storage pros and cons

  • Built-in storage means better suited for larger properties with greater demands

  • Takes up less space than a system boiler because the storage is built into the boiler itself. However, it still requires more space than a standard combi system

  • Cheaper installation compared to switching to a heat-only or system boiler

  • Can be paired with renewable forms of energy such as solar thermal panels to heat stored water

  • Relatively small storage – typically between 40 and 200 litres, compared to system boiler storage tanks, some of which can store over 500 litres

  • There are fewer models to choose from

  • May struggle to find a suitably qualified engineer to service or repair one

Oil boilers pros and cons

  • Don’t require access to mains – oil is stored in a storage container

  • The main drawback is cost – an oil boiler usually costs hundreds of pounds a year more to run than its gas equivalent

  • More logistically challenging as you must get the oil delivered to your home. Prices vary for heating oil or kerosene, so shop around to get a good deal

Biomass boilers pros and cons

  • Typically cheaper than gas and oil – currently cost around £400-£600/year

  • Energy-efficient. The carbon dioxide given off by wood pellet boilers is similar to that absorbed by new plants, so it is a sustainable fuel. The savings made in carbon dioxide emissions is around 14.6 tonnes per year (again, according to EST)

  • Expensive to install: the Energy Saving Trust (EST) estimates installation costs of automatic-fed wood pellet boilers is between £8,000 and £15,000

  • Availability of fuel

What boiler do you need?

This depends on how you answer the questions below. Some key things to consider when looking at gas-powered combi boilers like Baxi boilers or Glow Worm boilers are:

  • How large is your property?

  • What’s your current heating system?

  • What’s the type and condition of your current pipework?

  • What’s the mains water pressure like?

  • What’s your budget?

Use the pros and cons above to determine which gas boiler best fits your needs. For example, if you’re extending your property and adding a bathroom, then you may want to switch from a combi boiler to a system boiler.

On the other hand, if you’re on a tight budget, then the cheapest option is probably a simple like-for-like swap, as it will reduce the installation costs.

If you’re looking into oil or biomass boilers, some additional considerations may have to be made, such as: 

  • Will you need planning permission for a flue or chimney?

  • Is it easy to find an oil or wood pellet supplier in your area?