Boilers are the driving force behind the central heating system. All boilers are built to do the same thing: to provide heating.
Most of the UK’s boilers are gas boilers that run off gas from the main grid.
A small percentage are LPG or oil boilers, which are popular for off-grid homes and rural properties not connected to the main gas network. Then, there are electric boilers which perform the same functions as other boilers but use electricity as an energy source.
Boilers, fundamentally, use an energy source, such as gas, to heat water. Hot water comes out of hot water taps and pumps through radiators to heat the home.
Despite all gas boilers working similarly and performing virtually the same core functions, there are three different main types:
- Combi boilers
- Standard/conventional/traditional/heat-only boilers (they go by all of those names)
- System boilers
The type of boiler installed in a home depends on the size of the property and the date the boiler was built and installed.
Let’s take a closer look at the different types of boilers:
Combi boilers are relatively modern and are most likely to be fitted in flats and small houses. They feature a single unit connected to the hot water supply and radiators. The boiler unit takes cold water directly from the mains, heats it, and pumps it to radiators, taps, showers, etc.
The main advantage of combi boilers is their ability to save space. There is just one small unit, typically mounted in a cupboard, and no water tanks. Providing direct hot water to a small number of radiators and taps makes sense for smaller properties with lower hot water demands.
When hot water demands are higher, e.g. in a 3-bed house with two bathrooms and many radiators, combi boilers might struggle - but this is changing. Modern combi boilers work well for most 3-bed houses, but it’s still wise to weigh them up against system boilers if you have two bathrooms or more (e.g. two bathrooms and an en-suite).
How do you know if you have a combi boiler?
You have a combi boiler if:
- You have one boiler unit that isn’t connected to any other cylinders.
- If you live in a flat with your own boiler, it’s almost certainly a combi boiler.
- There are five copper pipes coming from the bottom.
Pros and cons of combi boilers
- Combi boilers are compact and can be fitted into a cupboard. There’s no need for an extra water cylinder.
- Combi boilers are self-contained units that are easy to use.
- Most combi boilers are modern and highly efficient.
- New combi boilers are powerful enough to heat 3-bed homes with two bathrooms.
- Combi boilers rely on reasonable mains water pressure.
- Don’t work with power showers.
- Struggle with multiple bathrooms and high heating demands.
These boilers go by multiple names, which all amount to the same thing. They’re typically fitted in houses with a loft and are probably the most common boiler in pre-90s houses where the boiler hasn’t been recently replaced.
Conventional boilers have hot water tanks, which are typically installed in the airing cupboard. They also have a central heating feed tank and a hot water feed tank in the attic, which feeds the boiler and hot water tank by gravity. The tanks in the loft are fed from the main supply.
Conventional heating setups are typically ‘open vented’ and are open to the atmosphere in the loft, where the feed and expansion tanks are fitted. This means they’re somewhat susceptible to debris, oxidation and sludge build-up.
So, essentially, the mains supply feeds the tanks in the attic, which supplies the entire home with hot water and heating by passing it to the boiler and hot water tank. This setup ensures good pressure as the system is fed by gravity and pumps and doesn’t rely solely on the mains pressure. In addition, the hot water cylinder ensures a steady supply of hot water to multiple taps and showers too.
How do you know if you have a conventional boiler?
You have a conventional boiler if:
- You have an airing cupboard with a hot water tank and an attic with one or two additional tanks.
- The boiler is usually installed downstairs, in a cupboard, utility room or garage. It will also probably have a pilot light.
- Your hot water runs out if people in your house use lots of hot water, e.g. by taking multiple showers in quick succession.
Pros and cons of conventional boilers
- Provides a solid supply of hot water and heating at good pressure.
- Some older conventional boilers are known to be very durable.
- Work well under high hot water demands.
- Generally easy to service.
- Can be more costly to run as water is heated even when it isn’t used (providing the heating is switched on).
- Tend to be older boilers.
- More parts to go wrong, and internal components can corrode and oxidise.
System boilers are somewhat of a halfway or hybrid between a combi and a conventional boiler. Like conventional boilers, they use a hot water cylinder to keep up with higher water demands.
But, unlike conventional boilers, system boilers are installed in a closed system that isn’t fed by a tank in the loft. As a result, they’re pretty compact but deliver on multiple taps, showers and large radiator systems.
System boilers are hostage to mains water pressure like combi boilers, but that isn’t generally an issue in most parts of the UK.
System boilers rate as excellent all-rounders that suit modern radiator systems that don’t mind being placed under pressure. They’re not always suited to a straight swap with a conventional boiler unless the home’s radiators (and possibly even the plumbing) are also upgraded. That’s because older radiators don’t always like the higher pressure supplied by combi and system boilers.
Overall, system boilers are modern like combi boilers but cater to larger homes with three bathrooms (e.g. two bathrooms and an en-suite) or more.
How do you know if you have a system boiler?
You have a system boiler if:
- You have a hot water tank but don’t have a feed tank in the attic.
- Your boiler is nice and modern with digital controls.
- There are three copper pipes coming from the bottom.
Pros and cons of system boilers
- Excellent hot water provision for larger homes.
- Modern and efficient.
- High pressure if the mains is high pressure.
- Perfect for modern radiator systems.
- Higher up-front costs.
- Not ideal for older plumbing and radiators.
- Rely on mains water pressure.
How to pick a boiler type
It’s important to remember that all boilers perform the same core functions. No single boiler type is necessarily better than the others.
Combi boilers are tough to beat for smaller homes with 1 to 3 inhabitants, whereas conventional boilers are still popular for larger homes despite system boilers taking over in recent years.
What’s more important is the efficiency of the boiler and the output rating. It doesn’t matter what boiler you own if it’s not powerful enough for your home!
If you’re replacing your boiler, the replacement will likely be a combi or system boiler unless the radiators and plumbing are relatively old and wouldn’t tolerate the higher pressure of a modern boiler. In this situation, upgrading the entire heating system may be advantageous.
All boilers have to be fitted and registered by Gas Safe engineers, and they’ll advise you on what boiler is best for you.
Types of boiler FAQs
What type of boiler is best?
All modern boilers are condensing boilers. The three main types of boilers, system, combi and conventional, are definitely more similar than they are different.
Each one excels in different setups. For the most part, new boiler installations tend to involve combi or system boilers, as these make the most sense for modern radiator systems. In addition, Combi boilers have become much more powerful in recent years and can adequately heat larger homes.
Add in a hot water tank for a system boiler, and you’ve got a robust, modern heating system. Conventional systems are fine and are often worth maintaining, but there are a few scenarios where it’s worth going from a combi or system boiler setup to a conventional setup.
What is the most reliable boiler?
Top brands like Worcester Bosch, Vaillant, Ideal, Baxi and many others are there on merit and nearly always deliver reliable products. Heating engineers will have their own favourites and will advise you of the best option for your home. Overall, competition is tight, and there’s no single “most reliable” boiler brand or model.
What are the 3 types of boilers?
The three main types of gas boilers are system boilers, combi boilers and conventional boilers, which are also called traditional, standard and heat-only boilers. These boilers are fundamentally similar and provide the same core functions: distributing hot water to taps and radiators.