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).
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.
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.
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.
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.
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.
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).
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:
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
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
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
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
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
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
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?