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Starting out as a new heating engineer or installer can be overwhelming given the range of technology on the market today. To provide the best service, you need a deep understanding of the different types of boilers and how they function.
At City Plumbing, we’ve created this comprehensive guide to help you navigate the pros and cons of combi, system, and heat-only boilers. Whether you’re looking for energy efficiency or ease of installation, our breakdown will give you the confidence to recommend the perfect heating solution for every client’s home.
What is a Combination Boiler?
A combination or 'combi' boiler can provide both a high efficiency of water and central heating in a single compact unit.
A gas combi boiler will heat water directly from the mains when you turn on any tap fixtures, so homeowners won't require a separate cold water storage tank in the roof space or need a hot water storage cylinder.
As the boiler can provide both heating and hot water, they are typically less expensive to install in contrast to a system boiler, as an additional hot water cylinder would be required. This coincidentally makes combi boilers suitable for smaller homes where space is limited.
That's one of the reasons why combination boilers are the most popular type of boiler in the UK, as they are used by around 80% of UK households - however, It's advised that combination boilers are not suitable for homes where the demand for hot water is higher.
When the temperature of your thermostat drops below the set point, the boiler will begin to fire up. A heat exchanger is one of the many internal components of a combi boiler that works by transferring heat from the combustion gases to the water. The hot water is then circulated around the central heating system. A valve inside the boiler will divert the heat into the domestic hot water supply, providing hot water until the tap is turned off.
There are many advantages of having a combi boiler - one is that hot water is distributed at mains pressure, which means that homeowners can get a powerful shower without needing a separate pump.
With a combi boiler, as there is no need for a water tank in the loft, homeowners will not have to worry about the potential risk of loft pipework freezing during cold winter months.
Additionally, without the extra pipework, a combination boiler installation will typically be a lot cheaper.
Overall, combi boilers are a good choice for small homes with limited space and moderate hot water demands. Most provide a high-efficiency rating that can save you money on your energy bills.
What is a System Boiler?
A system boiler is a type of boiler that's commonly used in larger properties or homes with a greater demand for hot water.
Unlike combi boilers, system boilers require a cylinder which is used for storing hot water - however, the primary heating and hot water system components are actually built into the boiler itself. This makes for a quicker and easier installation process for installers.
The boiler works by heating up water stored in a hot water cylinder, which is then distributed around the property through a network of radiators and/or underfloor heating. Unlike a regular boiler, a system boiler will not need to rely on a cold water storage tank in the loft. This makes it a more compact and efficient option and suitable for homes with little or no loft space.
Systems boilers are also compatible with solar water heating systems, which deliver many environmental benefits and lower energy bills for your clients.
With a system boiler, homeowners can take advantage of their vast array of unique features, such as:
- Suitable for homes with more than one bathroom
- Continuous supply of hot water to any number of taps at the same time
- No loft tank requirement, freeing up space and eliminating worries about leaks or frost damage.
- Economical boiler operation
- Built-in components make installation quicker and neater.
Overall, system boilers are a good choice for homes with high hot water demand as they can provide a more significant supply of hot water than combination boilers.
With any boiler, there are some drawbacks. One of the most common concerns with a system boiler is that system boilers can take longer to heat up water, which isn't ideal if your client lives in a busy household.
What is A Regular Boiler?
There's nothing run-of-the-mill about regular boilers, and it's important to not let the name fool you…
A gas regular boiler, sometimes known as a conventional, traditional or even a heat-only boiler, is designed to provide heating and hot water with the support of an additional cold water tank and hot water cylinder.
Regular boilers are best suited for properties considered larger or older because conventional boilers are an easier and faster way to replace and update an existing heating system in an older property.
This is because older heating systems won't be able to cope with the pressure that a combination or system boiler would produce, so a regular boiler would be perfect for the job.
They are also suitable for homes in a low water pressure area as the flow rate is dependent on gravity instead of being dependent on mains pressure. That's because the cold water tank is usually installed as high above the boiler as possible, typically in a loft space.
If you have a client who lives in a larger property and perhaps wants hot water available, no matter how many people are showering at 8 a.m., regular boilers are the perfect fit for their demands. Plus, they will deliver the same flow level - however, many taps and showers are running.
Additionally, regular boilers are compatible with solar hot water systems, allowing clients to heat their homes from solar energy or from the boiler.
While regular boilers have their advantages for busy homes with low water pressures. They do have a few drawbacks to keep in mind.
- Regular boilers are less efficient than other types of boilers, such as combi boilers.
- Regular boilers require more maintenance than other boilers because the hot water cylinder needs to be drained and cleaned regularly to prevent limescale.
- Regular boilers are unsuitable for homes with a high demand for hot water.
1. Combi (Combination) Boilers
The most popular choice in the UK, a combi boiler provides both heating and hot water directly from the unit without the need for external tanks.
Best For: Small to medium-sized homes, flats, and properties with limited loft space or one bathroom.
- Pros:
Space-saving: No need for a hot water cylinder or cold water tank.
Efficiency: Heats water instantly on demand rather than storing it.
Easier Install: Less pipework and fewer components mean lower installation costs.
- Cons:
Water Pressure: Struggles to supply hot water to multiple taps simultaneously.
No Backup: If the boiler breaks, there is no immersion heater backup for hot water.
2. System Boilers
System boilers require a separate cylinder to store hot water but incorporate the pump and expansion vessel within the boiler itself.
Best For: Large family homes with multiple bathrooms and a high demand for hot water.
- Pros:
High Demand: Can deliver hot water to multiple showers and taps at the same time without a drop in pressure.
Tidy Installation: Most of the major components are built-in, making it cleaner than a regular boiler setup.
Solar Compatible: Many modern cylinders used with system boilers can be integrated with solar thermal panels.
- Cons:
Space Required: You must have room for a hot water storage cylinder (usually in an airing cupboard).
Heat Loss: Even well-insulated cylinders lose some heat over time.
3. Regular (Conventional) Boilers
Also known as "heat-only" boilers, these are the traditional setup. They require both a hot water cylinder and a cold water storage tank (usually in the loft).
Best For: Older properties with traditional plumbing and homes where the existing pipework cannot handle the high pressure of a combi or system boiler.
- Pros:
Low Pressure Friendly: Ideal for older radiators that might leak under the high pressure of a closed system.
Massive Volume: Great for homes where multiple people need hot water at once.
Backup: Hot water cylinders often have an immersion heater for use if the boiler fails.
- Cons:
Bulky: Requires significant space in both the loft (tanks) and a cupboard (cylinder).
Wait Times: Once the hot water is gone, you have to wait for the boiler to heat the entire tank again.
Complexity: The most complex to install due to the amount of external pipework and tanks.
A Quick Summary of the Three Main Boiler Types
Combi (Combination) Boilers
What is a Combi boiler? A Combi boiler is a compact, "all-in-one" unit that provides both central heating and hot water. It is a self-contained system that doesn’t require separate water tanks or cylinders.
How does it work? It connects directly to the mains water supply. When a hot tap is turned on, a heat exchanger warms the water instantly as it flows through the boiler. It prioritizes hot water, meaning it temporarily pauses the central heating while the tap is running.
Why are they so popular? They are the UK’s most popular choice because they are incredibly space-efficient and cost-effective. By heating water only when needed, they eliminate the energy waste associated with storing hot water.
Who is a Combi boiler for? Small to medium homes (1–3 bedrooms).
- Properties with one bathroom.
- Homes with limited space (no loft or airing cupboard).
Pros and Cons:
- Pros: Instant hot water; compact size; cheaper installation; high energy efficiency.
- Cons: Struggles with multiple showers at once; depends on strong mains water pressure; no backup hot water if the boiler fails.
System Boilers
What is a System boiler? A System boiler provides heating and hot water via a separate storage cylinder. While it needs a cylinder to hold hot water, most of the major pumping and expansion components are built into the boiler unit itself.
How does it work? Like a Combi, it takes water from the mains, but instead of heating it only when a tap opens, it pumps hot water into a storage cylinder. This reservoir of hot water is then ready to be sent to multiple outlets simultaneously.
Why are they so popular? They offer a "middle ground" between Combi and Regular boilers. They provide the high-volume hot water performance of a traditional system but with a neater, easier installation since they don't require tanks in the loft.
Who is a System boiler for? Large families with high hot water demand.
- Homes with multiple bathrooms used at the same time.
- Eco-conscious homeowners looking to integrate solar thermal energy.
Pros and Cons:
- Pros: Constant hot water to multiple taps; solar-compatible; no loft tanks needed.
- Cons: Requires an airing cupboard for the cylinder; hot water can run out and need time to reheat; more heat loss than a Combi.
Regular (Conventional) Boilers
What is a Regular boiler? Also known as "heat-only" or "traditional" boilers, these systems consist of the boiler unit, a hot water cylinder, and a cold water storage tank (usually located in the loft).
How does it work? It is a gravity-fed system. Cold water from a loft tank fills the hot water cylinder. The boiler heats the water in the cylinder, which is then drawn down to the taps. A second small "feed and expansion" tank in the loft maintains the water level in the heating circuit.
Why are they so popular? They are the go-to choice for replacing boilers in older properties. Because they operate at lower pressure (gravity-fed), they are less likely to cause leaks in aging pipework or radiators that aren't built for modern high-pressure systems.
Who is a Regular boiler for? Older properties with traditional plumbing.
- Areas with low mains water pressure.
- Very large homes with multiple bathrooms and high demand.
Pros and Cons:
- Pros: Ideal for low-pressure areas; can handle high demand; immersion heaters in the cylinder provide backup hot water.
- Cons: Takes up significant space (loft and cupboard); complex/expensive to install from scratch; takes time to reheat once the tank is empty.
What is an Electric Boiler?
Similar to gas boilers, an electric boiler can come in three different types;
- Storage boilers are a type of boiler that has a hot water cylinder built in that stores hot water for use when needed. The water is heated up during off-peak hours when electricity is cheaper.
- Combination boilers are the most efficient type of electric boiler that heats water for both central heating and domestic hot water for taps, baths, or showers on demand.
- Flow boilers can heat water as it passes through the boiler. They are the least efficient type of electric boiler, but they are also the simplest and cheapest to install.
Each serves the same function as a gas boiler, but instead is fuelled by electricity instead of gas. Electric boilers will convert electricity into heat to warm up the water that provides heating to your customer's home and tap fixtures rather than burning gas fuel.
Electric boilers are usually less common than gas boilers, however - they are a good solution if a customer's property is not connected to a gas network.
Electric boilers can provide customers with a better efficiency rating than other gas boilers as they use less energy to heat homes, saving customers hundreds on heating bills. As they don't produce carbon emissions, they are considered better for the environment to meet net-zero goals by 2050.
Are Oil Boilers Still Necessary?
Oil boilers are considerably less common than gas boilers, however - there are over four million homes in the UK that aren't connected to the gas main that needs to be heated in other ways - such as oil.
Having an oil boiler is often the best way to do this. The oil tank can be fitted on-site and filled with oil as and when it's needed. The tank should be located on your customer's property, and there are specific installation regulations that need to be followed by installers.
- First, the base of the oil tank must be durable enough to support the weight of the tank and its contents.
- Secondly, the oil tank must be situated on solid ground, such as paving slabs or concrete and ensure they are at least 42mm thick and extend out at least 300mm beyond the tank's perimeter.
- Lastly, suppose the oil tank is located quite close to your customer's home. In that case, a firewall must be positioned between the tank and your home.
An oil boiler operates in a very similar fashion to gas boilers. In this case, oil is used, which is ignited in the combustion chamber, and in turn, the heat exchanger will warm up cold water, either from a cold water tank in a conventional system or from the mains in a combi system.
The heated water can then be used in radiators, taps and showers to provide customers with heating and hot water.
What is an LPG Boiler?
LPG boilers stand for Liquid Petroleum Gas, which works the same as traditional gas boilers. The only key difference is that the petroleum gas is stored in a tank.
The tank is located separately outside the customer's property, usually in the garden, and has to be topped up by an LPG supplier. It is possible to have a tank underground if you do not wish to lose any garden space.
LPG boilers work similarly to natural gas boilers as they can continue to produce energy to heat radiators or underfloor heating and provide hot water for your customers.
It's proven that LPG is known to be a safe and clean gas, making it a very efficient method of home heating for off-grid properties.
There are some advantages of using an LPG boiler, such as;
- LPG is considered a cleaner fuel in comparison to oil as it burns less carbon, preventing soot and ash build-up.
- LPG is more dense than oil, meaning you can get more heat from burning the same quantity.
- LPG boilers can be installed at a range of properties, from park homes, cabins and huts, as they tend to be smaller meaning they do not take up as much space.
- Less noise is produced thanks to their compact size.
There are, however, some drawbacks for installers to take into consideration as well, like;
- The cost of LPG can be more expensive than mains gas.
- LPG has to be delivered via a lorry or truck, so it's important they can access the property quickly and safely.
- Before installing an LPG external tank, customers may need planning permission from our local authority.
- LPG is not subject to a price cap, meaning the supplier can charge whatever they want. This has been a significant issue during the current cost of living crisis.
- LPG is a cleaner fuel, however - still derived from fossil fuels, meaning it is not a renewable choice.
What are Hydrogen Ready Boilers?
Installers need to understand what the term hydrogen-ready or hydrogen blend means, as more newer boilers will have the ability to burn either natural gas or pure 100% hydrogen.
Hydrogen-ready boilers such as the Ideal Logic Max range provide the key to converting the existing gas distribution networks from natural gas, predominantly methane, to hydrogen.
Unlike most carbon-heavy fossil fuels such as coal, oil and gas - hydrogen does not produce carbon dioxide when combusted.
This means that by using hydrogen blend-ready boilers, carbon emissions could be significantly reduced in homes, thus helping to combat global warming and improve environmental sustainability, allowing us to decarbonise all sectors of the UK economy to meet our net zero targets by 2050.
Are You a Homeowner Choosing a New Boiler?
While this guide is designed for trade professionals, we know that choosing the right heating system is a big decision for any homeowner. Picking between a combi, system, or regular boiler depends on your household’s water usage, the number of bathrooms you have, and your available space.
To help you make the right investment for your property, we’ve created a dedicated guide to simplify the process.
Boiler Pricing at a Glance
Combi Boilers – Starting from £650
- What’s Included: This price covers the boiler unit only. While budget-friendly models start at this price point, premium brands typically average between £1,000 – £1,500.
System Boilers – Starting from £1,150
- What’s Included: This covers the boiler unit itself. When quoting, remember to factor in the additional cost of a separate hot water storage cylinder.
Regular Boilers – Starting from £730
What’s Included: This is for the "heat-only" unit. While these units are often the most affordable, they require the most external components, such as cold water tanks and cylinders.
Help your customers make the switch to the best boiler today. Take a look at our City Plumbing website for more information.
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