Core Cylinder Types

• Unvented Cylinders: Fed directly from the cold water mains, providing high-pressure hot water without needing a loft tank. They must be installed by G3-qualified professionals due to high-pressure safety requirements.

• Vented (Gravity) Cylinders: Supplied by a cold water tank in the loft. Pressure is determined by the height of the tank. They are simpler and cheaper to install but often provide lower pressure.

• Direct vs. Indirect:

  • Direct: Water is heated by internal electric immersion heaters.

  • Indirect: Water is heated via a coil connected to an external boiler or heat pump. 

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Advanced & Specialized Systems

• Smart Cylinders (e.g., Mixergy): These use "top-down" heating to only heat the amount of water you actually need, reducing energy waste by up to 20-30%. They learn your usage patterns via machine learning and can be controlled via smartphone apps.

• Multi+ Systems (e.g., Daikin Multi+): An all-in-one solution that connects an outdoor heat pump to up to three indoor air-conditioning units and a domestic hot water tank (usually 90L or 120L). It provides heating, cooling, and hot water from a single compact system.

• Thermal Stores: Unlike standard cylinders, the water in the tank stays there and acts as a "heat bank". Mains water is heated instantly as it passes through a heat exchanger inside the store. They are excellent for integrating multiple heat sources like wood burners and solar thermal.

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Core Cylinder Types

• Unvented Cylinders: Fed directly from the cold water mains, providing high-pressure hot water without needing a loft tank. They must be installed by G3-qualified professionals due to high-pressure safety requirements.

• Vented (Gravity) Cylinders: Supplied by a cold water tank in the loft. Pressure is determined by the height of the tank. They are simpler and cheaper to install but often provide lower pressure.

• Direct vs. Indirect:

  • Direct: Water is heated by internal electric immersion heaters.

  • Indirect: Water is heated via a coil connected to an external boiler or heat pump.

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Indoor Unit

In modern heating systems, particularly those using heat pumps, a hot water cylinder is often formally referred to as an Indoor Unit. 

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Why the name change?

In traditional gas or electric systems, the "cylinder" was just a storage tank. In a heat pump setup, the cylinder often becomes a more active component of a two-part system: 

• Outdoor Unit: The heat pump itself, which extracts energy from the air or ground.

• Indoor Unit: The hot water cylinder (often integrated with a control system) that receives that energy to heat and store your water. 

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Common Variations

Depending on the manufacturer and setup, these indoor units come in two main forms:

• Integrated Units: An all-in-one floor-standing cabinet that houses the hot water cylinder, expansion vessel, and control electronics. It looks like a tall fridge and is designed to be aesthetically suitable for kitchens or utility rooms.

• Split/Compact Units: A smaller wall-mounted unit, containing the controls and pumps, paired with a separate standalone hot water cylinder. This is used when space is tight or if the cylinder needs to be in a different room from the main controls. 

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Key Differences from Standard Cylinders

While they look similar, an "indoor unit" cylinder designed for a heat pump has a crucial internal difference: Surface Area.
Because heat pumps operate at lower temperatures than gas boilers, the internal heating coil must be much larger (often double the size) to transfer heat effectively. You generally cannot use a standard boiler cylinder with a heat pump for this reason. 

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