Types Of Boiler & How To Tell Them Apart
Combi Boiler
A combi boiler which is short for combination boiler, is considered the most efficient heating system. Today it is also the most popular system among the new boiler installations. It’s a compact system that provides both hot water and central heating directly through the boiler itself removing the need for bulky tanks and cylinders.
This boiler doesn’t store hot water, so it is considered very efficient as it produces hot water on demand. It’s got a fully-sealed system with a circulation pump built in, which is great news for those who look to save up space by getting rid of the old-school tanks and cylinders.
Unfortunately, nothing is perfect and a combi boiler comes with its limitations too. It won’t be able to produce a steady flow through multiple taps and also having a bath and heating the home at the same time may be a bit of struggle. If you open a hot water tap while the heating is on, priority will be given to hot water.
Good news is that there is a wide range of outputs available depending on the size of your property and your predicted hot water demand. If you choose the right power output, you shouldn’t have any problems.
Pros
- No need for the expansion tank or hot water cylinder
- Higher efficiency rate than comparable systems
- Lower new boiler installation cost
- Very compact, freeing up attic and cupboard space
Cons
- Will struggle with high hot water demand and won’t support flow through multiple taps
- More costly to service (unless still under warranty)
- Not compatible with alternative green energy systems
- Additional cost if upgrading from a different type of boiler

Conventional Boiler
Depending on professional jargon used by a plumber, these will be known under a wide array of names, all of which are used to describe the same piece of equipment. Often called regular boilers, open vent or heat only – referring to the fact that they feed the central heating system while hot water is provided through a cylinder that stores heated water.
If you have an unsightly expansion tank in your attic and a large water cylinder covered in green foam insulation, you’ve got a conventional boiler.
Although often associated with outdated technology, there’s nothing wrong with this heating system as long as the boiler itself has a high efficiency rating. The main advantage of the conventional system is that hot water is accessible from multiple taps at the same time. If you’ve used a combi boiler, you know that when you open a tap in the bathroom, there will be no hot water in the kitchen. It might turn out to be a nightmare for large families where fighting for the right to use hot water can put unnecessary stress on the morning routine. That’s where the conventional boilers step in.
They heat up the hot water cylinder that is fed by the water tank sitting in the attic. The tank also has a way of topping up the system to make up for the water that is lost due to evaporation and what’s more important – it keeps the system safe by allowing water to expand in case the system heats up too much.
An additional benefit of this configuration is that you can easily upgrade your system to multiple heat sources by replacing the cylinder to include an additional coil. You can read more about this scrolling down to our thermal solar water heating system section.
Pros
- Enjoy hot water from multiple taps simultaneously
- More reliable than other types
- Compatible with advanced renewable energy heating systems
Cons
- Will take up attic space and cupboard space to house the tank and the cylinder
- The initial boiler installation costs are relatively higher
- Less efficient due to heat loss from the cylinder

Sealed System Boiler
Or simply a system boiler is an advanced version of a conventional boiler. It works in a similar fashion by supplying the heating system directly and storing hot water in a cylinder, however, as it doesn’t require a water tank to provide pressure. It is great for homes that require instant access to hot water from multiple taps. It’s main benefit is that it frees up the space in your attic.
The expansion vessel and other bits are already integrated into the system boiler – it is a more advanced appliance. Another upside is that this type can be used with a domestic thermal solar water heating system. Scroll down for the environmental section to find out how much you can earn by joining the RHI scheme.
Nevertheless, this type of equipment also comes with its downsides, so if you can’t see yourself installing solar heat collectors on your roof, and having hot water out of multiple taps isn’t a priority, you might be better off choosing another type of new boiler.
Another downside is that both conventional and system options are limited by the volume of the cylinder. If you have a large family who want to use the bath frequently, you may find that the appliance doesn’t quite keep up with the demand. Once a cylinder is emptied, you will have to wait for the next portion of water to be heated up.
Also, producing hot water on an assumption that it is going to be needed at some point soon is not a very green thing to do. Water will need to be stored in the cylinder and no matter how well insulated it is, there will be some heat loss. If this is a concern, you might need to choose a combi boiler instead.
Pros
- Provides hot water from multiple taps instantly
- More reliable than other systems
- Compatible with thermal solar water heating systems etc.
- No need for the expansion tank
- Newer models come with a supplementary immersion heater producing hot water even when the boiler breaks down or loses gas supply
Cons
- Will take up cupboard space to house the cylinder
- The initial boiler installation costs are relatively higher
- Less efficient due to heat loss from the cylinder
Condensing Boiler
Although technically a condensing boiler can’t be considered a separate type of boiler, we feel it’s worth a mention because it sometimes creates unnecessary confusion. Besides, some properties still have non-condensing boilers in operation making their owners pay an extortionate amount of money on energy bills.
A condensing boiler will generate more hot water from the same amount of gas making it a no-brainer. In fact, it is illegal in the UK to install a domestic non-condensing boiler. The only reason why we keep mentioning this variety is that older-style properties are likely to still have 20+ years old heating equipment installed, especially when it comes to some rental properties. Especially, as some landlords do strangely prefer to keep paying high maintenance costs instead of upgrading the heating system.
Condensing boilers come with a more sophisticated heat exchanger that is capable of converting latent heat from exhaust gases. Older type heating systems simply eject the gases without any additional treatment. These gases have accumulated valuable heat that can be converted back into usable heat by condensing hot water vapour back into its liquid state and feeding it back into the system.
This method allows you to reduce your carbon footprint, save a considerable amount of money on fuel bills and even do your bit to reduce global warming by reducing the amount of excess heat released into the environment.
