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The difference between a system boiler and a heat pump

Our guide shows the difference between a system boiler and a heat pump, how they use hot water cylinders and the efficiency of both.
an aroTHERM plus heat pump outside a brick house

The main differences between a system boiler and a heat pump are how they work, the fuel they use, and their environmental impact.

Both system boilers and heat pumps require a hot water cylinder, ensuring a steady flow of warmth and hot water to your home. While system boilers harness gas or oil to heat your water directly, heat pumps use electricity to intelligently draw heat from the environment to transfer warmth inside your home.

a Vaillant hot water cylinder near a door with a pair of wellies and a shovel. There is a green plant in a white pot to the right

What is a hot water cylinder?

A hot water cylinder is a well-insulated tank which stores water after it’s been heated. Water is heated through your boiler or heat pump system and is then stored, at temperature, ready for you to use whenever it’s needed.

A hot water cylinder has a thermally insulated inner tank which stores hot water heated by a boiler or heat pump. The cylinder allows multiple taps or showers to be used at the same time.

Watch the video 'What is an unvented hot water cylinder'

Is a hot water cylinder different with a system boiler vs a heat pump?

Both systems use hot water cylinders, however the role and function of a hot water cylinder differs depending on whether it’s used with a system boiler compared to a heat pump.

System boilers and heat pumps differ in their flow temperatures; system boilers can reach 65° for faster heating, whereas heat pumps generally operate at around 55°, leading to a slower heat-up process.

System boilers

  • A system boiler heats the system water, which can then be directed through the heating pipework in the cylinder, and this heats up the cold domestic water in the cylinder.
  • System boilers can use either a vented or unvented cylinder.
  • The cylinder stores hot water that is ready to be used on demand, but the system boiler is responsible for generating that hot water quickly.
  • A system boiler takes water directly from the mains and has the components for heating water built in.

HEAT PUMPS

  • With the heat pump running at a lower temperature, the thermal efficiency of the hot water cylinder plays an important role in storing hot water to the desired temperature.
  • Heat pumps require a high recovery cylinder which feature a higher KW rated coil with a larger surface area for efficient heat exchange. In comparison, a boiler cylinder typically has a smaller kW rated coil.
  • A large heat pump will require a bigger hot water cylinder to ensure there is enough stored hot water.
  • Heat pump cylinders are designed to work efficiently with the lower temperatures generated by heat pumps and usually come with large coils for efficient heat exchange.

The main difference in a hot water cylinder for a system boiler compared to a heat pump is the efficiency, sizing and heat production. A heat pump system needs a larger cylinder as the heating process is slower and operates at a lower flow temperature, whereas system boilers can produce hot water quickly, and therefore usually need a smaller hot water cylinder.

man carrying his washing with a boiler in the background

System boilers and heat pump cost

A system boiler can cost between £500 - £2,000, depending on the model, size and manufacturer. The ecoTEC plus from Vaillant is an extremely reliable option for a system boiler and is available in a range of outputs to support almost any requirement. It’s highly efficient, with precise heating controls and energy optimisation, it’s extendable with renewables systems such as solar panels and heat pumps for more efficient heating and can fit seamlessly into any home with a great modern design.

The overall cost of a heat pump will depend on the type of heat pump, overall output, and installation cost. Heat pumps can cost anywhere from £7,000 to £30,000, according to the The Eco Experts to buy and install. An air source heat pump is the most common and popular type of heat pump, transferring heat from the outside air to your house. Air source heat pumps are increasingly becoming more popular because they are highly efficient regardless of the temperature, heat pumps work well with both underfloor heating and radiators with low surface temperatures.

Efficiency in heat pumps and system boilers

Heat pumps and system boilers have a number of differences and are both efficient in different ways.

Heat pump efficiency:

  • Heat pumps are highly efficient as they transfer heat rather than generate it via combustion. The utilisation of natural energy sources such as air, ground and water mean the approach to heating and hot water production is sustainable.

  • Heat pumps operate using electricity and produce limited emissions, making them more environmentally friendly compared to traditional heating systems. To measure heat pump energy usage, you can use both Seasonal Coefficient of Performance (SCoP) and the Coefficient of Performance (CoP). These metrics illustrate the relationship between the energy consumed and the heat output, ensuring optimal performance. You can read more about energy usage in heat pumps here. The aroTHERM plus air source heat pump can achieve a potential SCoP of up to 5.03.

  • Heat pumps can operate at high efficiency levels even in cold temperatures. Vaillant’s aroTHERM plus air source heat pumps, for example, are tested to work under extreme conditions down to -20°C.

    Discover the aroTHERM plus.
  • Just 25% of the energy used by a heat pump is provided by electricity, with the remaining 75% being generated by the environment through the ground, water or air.

System boiler efficiency:

  • System boilers are fuel based, with 90-94% of the fuel’s energy converted into heat.

  • System boilers allow homeowners to store large quantities of hot water for later use, which can be efficient when multiple showers, radiators and taps are in use at the same time.

SUMMARY

Both system boilers and heat pumps use hot water cylinders to store energy, however heat pumps extract heat using an electric compressor, whereas system boilers use gas or oil. System boilers are easy to install in existing homes with high heating demands. Heat pumps on the other hand, offer a higher efficiency with lower carbon emissions.

Heat pumps also have a higher installation cost and take longer to commission compared to a system boiler but are more efficient.

Explore the features of Vaillant heat pumps and system boilers