Last updated: July 17th, 2023 at 11:26 am

The heating system has undergone a remarkable transformation, yet the conventional heat boilers still hold their relevance. These systems, also known as heat only boilers, typically incorporate two tanks located in the loft, along with a hot water cylinder and the boiler itself. The conventional or heat-only boiler is best suited for businesses and large homes that require a significant amount of water and need to use hot water across multiple taps and showers. In this comprehensive article, we will delve into the fascinating world of heat-only boilers, exploring their nature, functionality, the various types available, and any other essential insights that individuals may seek to uncover about these remarkable heating solutions.

So, What is a Heat Only Boiler?

Heat only boilers, also known as conventional vented or sealed, are a two-tank, a hot water cylinder, and the boiler unit system that efficiently heats water for central heating. They are typically found in larger properties, as they can better meet the demand for hot water across multiple taps and shower heads. Heat only boilers are not as efficient as condensing boilers, but they are a more affordable option. Of the energy used by a boiler, 94% goes to heating the home, while only 6% is lost or used to run the boiler itself.

How Do Heat Only Boilers Work?

Heat only boilers function by heating water and then circulating it throughout a residence via a system that incorporates an array of controls, including motorized valves, pumps, and tanks. The mains supply provides water to a cold-water tank situated in the loft, which then feeds it into a hot water cylinder. Subsequently, the boiler heats the hot water cylinder, allowing hot water to be supplied to taps and shower heads whenever required. Additionally, when the heating is activated, a pump circulates heated water to the radiators.

Types of Heat Only Boiler

There are two commonly used heat only boiler in the UK.

Open-Vented Heat Only Boilers

If you possess an open-vented system, the inclusion of a feed and expansion tank guarantees the maintenance of consistent water levels within the system. Its primary function is to replace any lost water resulting from evaporation or leaks. Additionally, it ensures that the expansion of water due to heating does not result in an excessive build-up of pressure.

open vented heat only boiler

Sealed Heat Only Boilers

A sealed system employs an expansion vessel as an alternative to a feed and expansion tank. This vessel, which can either be integrated within the boiler or exist as a separate containment unit, offers ample space to accommodate the expansion of water when the system undergoes heating.

Top 4 Best Heat Only Boilers in the UK

  • Grant Vortex Eco Utility 90-120 Boiler Only VTXECO26/35
  • EHC Slim Jim 7kW Single-Phase Electric Heat Only Flow Boiler
  • Worcester Bosch Greenstar 27Ri Gas Heat Only Boiler
  • Baxi 816 16kW Gas Heat Only Boiler 7749866

Grant Vortex Eco Utility 90-120 Boiler Only VTXECO26/35

The Grant Eco Utility heat-only boiler offers a cost-effective solution and incorporates a stainless-steel heat exchanger. It operates quietly and lacks any noise generation, while also providing a straightforward casing and control panel. Specifically designed for utility rooms, the Grant Vortex Eco Utility 90-120 Boiler Only VTXECO26/35 showcases an outstanding efficiency rating of 92 percent and is supported by a 5-year warranty.

grant vortex eco utility

EHC Slim Jim 7kW Single-Phase Electric Heat Only Flow Boiler

The EHC Slim Jim 7kW Single-Phase Electric Heat Only Flow Boiler is a small and efficient electric boiler, perfectly suited for smaller residences or properties with limited space. It boasts a 100% efficiency rating and operates silently, making it an excellent choice. Installing the Slim Jim Boiler is a breeze. Furthermore, it can be utilized in an ‘S’ Plan setup, allowing users to select heating only, hot water only, or both simultaneously. The boiler features a user-friendly front control panel with a temperature control dial, as well as heat and fault diagnostic indicators for the convenience of both users and installers.

EHC Slim Jim 7kW Single-Phase Electric Heat Only Flow Boiler

Worcester Bosch Greenstar 27Ri Gas Heat Only Boiler

The Worcester Bosch Greenstar gas-only heat boiler is well-suited for medium to large-sized homes and can be conveniently installed in a standard kitchen cupboard. This gas heat-only boiler is compatible with the high-efficiency Greenstar range of cylinders. It provides a maximum central heating output of 27kW and comes with a generous 7-year warranty. Additionally, it features an LED indicator for easy monitoring and operation.

Worcester Bosch Greenstar 27Ri Gas Heat Only Boiler

Baxi 816 16kW Gas Heat Only Boiler 7749866

The Baxi 816 heat-only boiler comes with a remarkable 10-year parts and labour warranty. It includes a compact Adey Magna clean Micro2 magnetic system filter, ensuring protection for both the boiler and heating system. This heat-only system is well-suited for all types of homes. The powerful heat-only boiler can be installed with both vented or sealed boiler systems, offering flexibility in its application.

Baxi 816 16kW Gas Heat Only Boilers

Which Homes Are Most Suited for Heat Only Boilers?

If your home is equipped with an older radiator system that may not withstand the elevated water pressure typically generated by other boiler types like system boilers, you might be advised to consider a heat only gas boiler. This recommendation is based on the potential risk of leaks that could arise from excessive water pressure.

Additionally, if you reside in an area characterized by notably low water pressure, a heat only boiler is often a suitable choice for your home. By utilizing water from a cold-water storage tank to fill the cylinder instead of directly sourcing it from the mains, this type of boiler can effectively address the challenges posed by low water pressure.

Are There Any Drawbacks to a Heat Only Boiler?

It’s important to note that heat only gas boilers have a limitation in terms of hot water supply since they draw water from a cylinder rather than directly from the mains. Once the hot water stored in the cylinder is exhausted, the boiler will need to refill it with cold water and heat it to the desired temperature for taps and showerheads.

Furthermore, it is worth considering that if you have minimal hot water consumption, a heat only boiler may not be the most efficient heating solution. This is because the cylinder may heat a larger volume of water than you actually require, whereas a combi boiler will provide only the necessary amount of water.

What Size Heat Only Boiler Do I Need?

Now that you have an understanding of how a heat boilers operates and whether you have an open vent or sealed system, the next consideration is determining the appropriate boiler size for your needs. One approach is to assess the kilowatt size of your current boiler, as a direct replacement may be sufficient.

As a general guideline, a small or medium-sized home with up to 10 radiators typically requires a 24-30 kW boiler. On the other hand, a larger home with up to 20 radiators may necessitate a larger 35-42 kW boiler. However, it’s important to note that boiler size is influenced by various other factors. Factors to consider include the number of occupants in your home, the type of building (e.g., terraced, semi-detached), the level of insulation, and the age of the property. For instance, if you reside in a terraced house, it may retain heat more effectively due to being surrounded by neighboring properties. This may indicate that a smaller boiler size would suffice.

How Often Should I Service My Heat Only Boiler?

It is strongly advised to have your boiler serviced annually. These devices play a crucial role in maintaining a warm and comfortable home, which is why it is essential to take proper care of them for various reasons.

An annual service ensures the safety and efficient operation of your heat boilers, providing the hot water and heating you require while also preserving its warranty. Booking a service for your Ideal boiler online is a quick and effortless process, granting you peace of mind.

What Are the Benefits of Heat Boilers?

Suitable For Larger Households And Businesses

The presence of a dedicated hot water tank in conjunction with a heat-only boiler enables multiple taps, showers, or baths to access hot water simultaneously. This makes regular boilers an excellent choice for accommodating larger families and commercial establishments.

Effective In Areas with Low Water Pressure

The inclusion of a cold-water cistern (header tank) allows the storage of water from the mains before supplying it to the heat-only boiler. Consequently, regular boilers prove advantageous for residences situated in regions with low water pressure.

Support For a Backup Heater

The option to install an immersion heater in the hot water tank provides a valuable contingency plan in case of boiler malfunctions or downtime.

Compatibility With Older Radiators

Traditional-style radiators can experience leaks when connected to modern boilers due to the higher pressure exerted. However, regular boilers are designed to work seamlessly with older heating systems, ensuring compatibility and preventing issues.

Integration With Solar Thermal Systems

Regular boilers can effectively harness energy from solar thermal systems like rooftop arrays. This environmentally-friendly feature empowers homeowners to reduce their carbon emissions and embrace sustainable practices.


While heat only boilers may follow conventional principles, they remain indispensable for large homes and high-demand areas where ample hot water is a necessity. With two main types – vented heat only boilers and sealed heat only boilers – these systems efficiently heat water to meet the demands of larger households and businesses. The market offers a variety of models to choose from, ensuring you find the perfect fit for your specific requirements. The best part? Heat only boilers are low-maintenance, only requiring an annual servicing to keep them operating at their best.

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