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:
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?