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Which Wiser Kit Do I Need? The Most Common Domestic Boiler Systems Explained

Wiser smart thermostats are compatible with most boiler makes and models, but it’s crucial to know a little about the most common boiler systems in the UK, so you can choose the right kit for your home.

Our detailed guide on common domestic boiler systems explained will allow you to identify your heating system, and help you to find the right Wiser products to upgrade to smart heating.

In a hurry? Try our online product selector to find the right kit in just a few quick steps.

Overview Of Boiler Systems

Your boiler is the beating heart of your home’s central heating, providing warmth and hot water throughout the year. There are a few different types of common boiler systems in the UK, which all operate slightly differently.

No matter which of these common domestic boiler systems your home has, Wiser’s got a smart thermostat to match, so you can start enjoying better control and enhanced energy efficiency.

COMBI BOILERS – WITH ONE HEATING ZONE

A combination, or ‘combi’ boiler system, combines a water heater and central heating boiler into a single compact unit. These space-efficient systems are becoming increasingly popular among homeowners who are upgrading from older heating systems. According to the Energy Saving Trust, combi boilers are particularly effective for smaller households with low hot water demand, as unlike systems with separate hot water tanks, combi boilers heat water directly from the mains, eliminating heat loss from stored water not being used immediately.

CHARACTERISTICS OF COMBI BOILERS (one heating zone)

Central heating and hot water are supplied by a single boiler unit with no separate water tank.
The boiler is fed directly from cold mains water pressure.
Hot water is available on demand when hot water taps are used.
Requires a one-channel (a.k.a single channel) Wiser kit (Wiser Thermostat Kit 1 or Wiser Multi-zone Kit 1)

COMBI BOILERS – WITH TWO HEATING ZONES

Some combi boiler systems are equipped with two separate heating zones, each controlled by its own thermostat. This configuration allows for greater heating control over different areas of your home, which is particularly beneficial in larger homes where heating needs may vary between different rooms or floors.

CHARACTERISTICS OF COMBI BOILERS (two heating zones)

Central heating and hot water are supplied by a single boiler unit with no separate water tank.
The boiler is fed directly from cold mains water pressure.
Hot water is available on demand when hot water taps are used.
It requires a three-channel Wiser kit (Wiser Thermostat Kit 3). Note that you won’t need the third channel because the hot water control in a combi system is not separate. A two-channel kit is unsuitable for a combi system with two heating zones, as the second channel in the two-channel kit is only suitable for hot water control in a conventional boiler system (with a separate hot water tank).

ADVANTAGES OF COMBI BOILERS

Space-saving: Ideal for smaller homes, as they do not require a separate water tank, freeing up valuable space.
Cost-effective installation and maintenance: Installation and servicing are usually less expensive than other system types, thanks to fewer components and less plumbing required.
Immediate hot water supply: Provides hot water on demand, independently of the central heating, eliminating the wait time associated with heating a water cylinder.
Energy efficiency: May be more economical to run for smaller households than other boiler system types, as noted by the Energy Saving Trust, due to reduced heat loss from stored water.

DISADVANTAGES OF COMBI BOILERS

Water pressure dependency: Requires good mains water pressure to function effectively, which may not be suitable in some residential areas with low water pressure.
Limited hot water flow: Only allows for one hot water outlet (tap, bath, or shower) to be used at a time. Running multiple outlets simultaneously can significantly reduce the water pressure to each, posing a challenge for larger households with high hot water demand.
Potential for delayed response in larger homes: In larger homes or properties with multiple bathrooms, the combi boiler may struggle to meet the simultaneous hot water demands, leading to potential delays and fluctuating temperatures.

CONVENTIONAL BOILERS (with unvented hot water cylinder) – ONE HEATING ZONE

Also known as sealed or system boiler systems, a conventional boiler system with an unvented hot water cylinder uses a hot water cylinder that’s separate from the boiler but does not require a cold water tank, usually typical of vented gravity-fed systems.

CHARACTERISTICS OF CONVENTIONAL BOILERS (with unvented cylinder and one heating zone)

It consists of a central heating boiler and a separate, pressurised hot water tank fed directly by the cold water mains feed, so the tank heats and stores hot water at mains pressure.
It doesn’t require a cold water tank.
A two-channel (a.k.a dual channel) Wiser kit (Wiser Thermostat Kit 2 or Wiser Multi-zone Kit 2) is needed to control the central heating via one channel and hot water via the other.

CONVENTIONAL BOILERS (with unvented hot water cylinder) – WITH TWO HEATING ZONES

This is set up in the same way as regular conventional systems, but in this case, the system has two heating zones being controlled by the boiler, with separate programming for each, usually with two thermostats. This may be the case in larger or extended homes.

CHARACTERISTICS OF CONVENTIONAL BOILERS (with unvented cylinder and two heating zones)

Two heating zones on separate thermostat controls.
Pressurised hot water tank that’s separate from the boiler.
Absence of a cold water tank.
It requires a three-channel Wiser kit containing two smart thermostats (Wiser Thermostat Kit 3) to control the central heating via two channels and the hot water via the third

ADVANTAGES OF CONVENTIONAL BOILERS

No cold water tank required: Ideal for smaller homes as they do not require a cold water feed tank, which saves space.
Consistent water pressure: Delivers hot water to any number of taps at mains pressure, ensuring a steady flow regardless of how many taps are in use.
Cost efficiency: Generally more economical to operate than gravity-fed systems, which rely on the elevation of a water tank for pressure.
Hot water storage: Comes with a hot water cylinder that stores heated water, making it beneficial for larger families or households that have a high demand for hot water throughout the day.
Flexible water heating control: Allows for separate hot water control, providing the ability to heat water without running the central heating, unlike gravity-fed systems where the heating system needs to be active to heat the water.

DISADVANTAGES OF CONVENTIONAL BOILERS

Dependence on mains pressure: The performance of conventional boilers relies on adequate mains water pressure. If this supply is lost or insufficient, the system can’t deliver hot water as effectively.
Delayed hot water access: Unlike combi boilers that provide hot water on demand, conventional boilers require some time to heat water stored in the cylinder. This can lead to a delay when the stored hot water is depleted.
Requires more space than combi boilers: Although they don't need a cold water tank, the hot water cylinder still requires dedicated space, which can be a drawback in very small properties.
Potential for heat loss: Stored hot water can lose heat over time if the cylinder is not well insulated, leading to increased energy consumption to keep the water hot.

CONVENTIONAL BOILERS (with vented hot water cylinder and upgraded, fully pumped central heating)

This type of system is effectively an upgraded gravity-fed system, where the central heating has been upgraded to be fully pumped while the hot water remains gravity-fed. This type of upgrade allows separate central heating and hot water timer control, unlike standard gravity-fed systems, due to the addition of motorised valves and a cylinder thermostat.

CHARACTERISTICS OF CONVENTIONAL BOILERS (with vented hot water and fully pumped)

A system that was originally fully gravity-fed has been upgraded/retrofitted with motorised valves, so the central heating is now fully pumped — a recommended upgrade if full replacement of an older gravity-fed system is not possible.
Due to a cylinder thermostat fitted to the hot water cylinder, timer and thermostat control of the central heating is available independently of the hot water.
Four 22mm or 28mm copper pipes are connected to the boiler (sometimes three pipes if an injector T is fitted, but a fourth 22mm or 28mm copper pipe will still be nearby).
A cold water feed or expansion tank is higher than the hot water cylinder, usually in the loft or attic.
Cold taps are fed by mains pressure (so turning off mains water will only turn these off).
A two-channel Wiser kit (Wiser Thermostat Kit 2 or Wiser Multi-zone Kit 2) is required to control the central heating via one channel and hot water via the other.
It may also be referred to as BiFlo, TwinZone, Y Plan, S Plan, or S Plan Plus.

Try this to identify this system type:

On the 2-channel programmer, where you set your schedules, try setting the hot water side off and the central heating side on. If only the central heating side comes on and only your radiators get hot, you have independent control and a fully pumped system.

However, if, when turning on the central heating side, the hot water also turns on, this would indicate a gravity system.

ADVANTAGES OF CONVENTIONAL BOILERS (with vented hot water and fully pumped)

Enhanced water pressure: Particularly beneficial in areas with low mains water pressure, these systems can maintain good water pressure through the use of additional pumps. This upgrade ensures consistent hot water flow even when mains pressure is insufficient.
Ample hot water storage: The system includes a hot water cylinder that stores large volumes of hot water, making it ideal for larger households with high hot water demands. This ensures a steady supply of hot water available throughout the day.
Improved heating control: Older systems upgraded with fully pumped solutions offer better thermostatic control over the central heating. This allows for the heating to be operated independently of the hot water system, providing flexibility and improved comfort.

DISADVANTAGES OF CONVENTIONAL BOILERS (with vented hot water and fully pumped)

Lower efficiency: These systems are typically less efficient than newer, more modern heating systems. The complexity and mechanics of older fully pumped systems often result in higher running costs.
Space requirements: The need for both a water cylinder and a cold water tank makes these systems less suitable for smaller homes, where space is at a premium.
Maintenance issues: Sludge and debris can accumulate in the cold water tanks, meaning regular cleaning is needed to maintain system efficiency and prevent blockages. This adds an ongoing maintenance requirement that can be cumbersome and sometimes costly.

GRAVITY-FED SYSTEMS (conventional systems with vented hot water cylinder and non-pumped central heating)

An older and more traditional boiler system type, gravity-fed (or vented) installations feature a conventional boiler and utilise gravity and convection to heat water and distribute it to taps and radiators rather than pumping it. Cold water from tank(s) in the loft flows down by gravity to a water cylinder, where it is heated. It’s then forced to hot water taps and radiators at low pressure.

Wiser is not compatible with traditional gravity-fed systems. This type of system normally lacks water cylinder thermostats, motorised valves, and external temperature controls, resulting in very limited thermostatic control, which Wiser needs in order to work.

Wiser thermostat and multi-zone kits 2 and 3 may be compatible with gravity-fed systems that have been retrofitted or upgraded with pumps and cylinder thermostats. Please speak to our technical support helpline on 0333 6000 622 for further advice on your system’s compatibility.

Gravity-fed systems are no longer fitted new and do not comply with current building regulations. Therefore, homeowners are increasingly opting to replace older gravity-fed systems with combi or fully pumped conventional boiler systems.

CHARACTERISTICS OF GRAVITY-FED SYSTEMS (non-pumped)

A distinctive feature of a gravity-fed system is four 22mm or 28mm copper pipes connected to the boiler (sometimes three pipes if an injector T is fitted, but a fourth 22mm or 28mm copper pipe will still be nearby).
A hot water cylinder is situated higher than the boiler (normally in an airing cupboard) and lacks a cylinder thermostat.
A cold water feed or expansion tank is situated higher than the hot water cylinder, usually in the loft or attic.
Cold taps are fed by mains pressure (so turning off mains water will only turn these off).
The central heating is not available independently of the hot water.
Typically, there is only ON/OFF boiler control, and there is no time control.
In residential areas with low water pressure, a gravity or gravity/pumped system is usually the best option. Some systems incorporate a pump to help provide greater pressure at the taps

Try this to identify this system type:

See if your system has a time controller or if it is just on and off at the boiler. Controlling your system by turning the boiler on and off could indicate a gravity system.

On the 2-channel programmer where you set your schedules, try setting the hot water side off and the central heating side on. If doing this automatically turns on the hot water side as well, this would indicate a gravity system.

ADVANTAGES OF GRAVITY-FED SYSTEMS (non-pumped)

Adequate pressure in low-pressure areas: These systems can be advantageous in residential areas with low mains water pressure, as they do not rely solely on the pressure from the main water supply.
Large volume hot water storage: Equipped with a hot water cylinder, gravity-fed systems can store a significant volume of hot water, making them ideal for larger households. This ensures that there is always a ready supply of hot water for various uses throughout the home.

DISADVANTAGES OF GRAVITY-FED SYSTEMS (non-pumped)

Reduced efficiency: Compared to more modern systems, gravity-fed systems are generally less efficient. Their reliance on gravity rather than pumps often results in higher energy consumption and increased operating costs.
Dependent heating and hot water supply: In gravity-fed systems, the central heating must be activated to heat the water, meaning that hot water cannot be heated independently of the heating system. This can lead to unnecessary heating and wasted energy.
Space-consuming: The requirement for both a water cylinder and a separate cold water tank means these systems occupy more space, making them less suitable for smaller homes.
Limited control options: Typically, these systems lack sophisticated controls such as water cylinder thermostats and motorised valves, which restricts precise temperature regulation and efficient energy use.
Incompatibility with modern smart controls: Gravity-fed systems are not compatible with modern smart heating controls like Wiser, as they cannot control the hot water and heating elements separately.
Maintenance requirements: Sludge and debris tend to accumulate in the cold water tanks over time, requiring periodic maintenance to ensure the system operates efficiently and to prevent blockages.

WISER IS ONLY COMPATIBLE WITH GRAVITY-FED SYSTEMS THAT HAVE BEEN UPGRADED TO A FULLY-PUMPED SYSTEM WITH MOTORISED VALVES AND A CYLINDER THERMOSTAT.

FAQs: Common Boiler Systems Explained

What Are the Most Common Boiler Systems in UK Homes?

The most common types in the UK include combi boilers, conventional boilers with vented or unvented cylinders, and system boilers. Each common boiler system offers different advantages depending on household size, water usage, and energy needs.

Which Boiler System is Best for Integrating with Smart Home Technologies Like Wiser?

Modern boiler systems, like combi and system boilers, work well with smart technologies. Wiser kits are especially suited to upgrading these common boiler systems, providing efficient heating control and helping manage energy use effectively.

How Can Modern Boilers and Smart Controls Reduce My Home's Carbon Footprint?

Energy-efficient boilers such as modern combi and system boilers can significantly reduce home carbon emissions compared to older heating systems. By upgrading your combi and system boilers with a smart heating kit from Wiser, you can further optimise your home’s energy use with features like tailored schedules, multi-zone heating and IFTTT.

Understanding common domestic boiler systems is essential for any homeowner looking to upgrade their heating system to smart heating. By integrating a Wiser smart heating kit, you can enjoy enhanced control, efficiency, and proven savings of up to 30%* on annual energy bills.

If you're not sure whether your system is fully pumped, would like to know what changes may need to be made to your gravity-fed system to be compatible with Wiser, or would like to purchase Wiser for a fully pumped system, we encourage you to contact our support team who'll help you find the kit you'll need, call 0333 6000 622 today.

Use our product selector to easily find the right Wiser kit and nearby stockists in just a few simple steps.

^{*Smart thermostat system percentage savings based on BEAMA and Salford University research statistics 2021. Savings are a guideline and vary based on your home’s thermal performance.}