What Is an Unvented Heat Pump Cylinder?

What Is an Unvented Heat Pump Cylinder? An unvented heat pump cylinder is a mains-pressure hot water storage cylinder designed to work efficiently with an air source heat pump. Unlike a traditional vented cylinder, it does not require a cold water storage tank in the loft. It takes its cold water supply directly from the mains and stores heated domestic hot water under pressure, providing strong hot water flow to showers, baths and taps. A purpose-designed heat pump version will normally have a larger heat exchanger coil to work effectively with the lower primary water temperatures commonly produced by heat pumps.

How does an unvented heat pump cylinder work?

In a typical air-to-water heat pump system, the heat pump heats water in a primary heating circuit. This water is circulated through a heat exchanger coil inside the cylinder.

Heat passes through the coil and into the domestic hot water stored around it. The primary heating water and the potable hot water remain separate.

When a hot tap or shower is opened, mains water enters the bottom of the cylinder and pushes hot water from the cylinder towards the outlet.

Because the cylinder operates from the incoming mains supply, there is no need for a traditional cold water storage cistern.

For a smaller dwelling or light commercial application, our 100 litre compact domestic hot water heat pump may provide the required combination of compact dimensions and hot water storage.

For larger homes, explore our 200 litre heat pump domestic water heater or 300 litre heat pump domestic water heater.

For larger buildings and greater DHW demand, consider the 400 litre heat pump commercial water heater or 500 litre heat pump commercial water heater.

Why does a heat pump cylinder have a larger coil?

This is one of the main differences between a conventional indirect cylinder and a dedicated heat pump hot water cylinder.

Heat pumps generally work most efficiently at lower primary flow temperatures than gas or oil boilers.

With a lower primary temperature, there is less temperature difference between the heating water inside the coil and the domestic water stored in the cylinder. This can reduce the rate of heat transfer.

To compensate, heat pump cylinders normally use a larger coil with more heat exchanger surface area.

This allows the air source heat pump to transfer its heating output into the cylinder without having to operate at unnecessarily high temperatures, helping to maintain a better Coefficient of Performance (COP).

What is the difference between vented and unvented cylinders?

A traditional vented cylinder is normally supplied from a cold water storage cistern, often located in the loft. The height of this tank creates the pressure available at the hot water outlets.

An unvented cylinder is connected directly to the mains water supply.

This can provide much better hot water pressure and flow, provided the property has adequate incoming mains pressure and flow rate.

For households with multiple bathrooms, powerful showers or large baths, this can be a significant advantage.

It is important to remember that an unvented cylinder cannot improve a poor incoming water supply. If the mains flow rate is insufficient, fitting a larger cylinder will not automatically provide better performance.

What safety equipment does an unvented cylinder need?

Because the cylinder stores hot water under pressure, an unvented hot water system requires specific safety and expansion controls.

Depending on the cylinder design, these can include an expansion vessel or internal expansion arrangement, pressure reducing valve, check valve, expansion relief valve and temperature and pressure relief valve.

There must also be a correctly designed discharge arrangement so that water released by a safety device can be discharged safely.

In England, unvented hot water storage systems fall within the requirements of Building Regulations Approved Document G, including G3. Installation should be carried out by a person with the appropriate competence for unvented hot water systems and in accordance with the cylinder manufacturer’s instructions.

Does an unvented heat pump cylinder need an immersion heater?

Most include an electric immersion heater, or at least provision for one.

The immersion heater can provide backup hot water if the heat pump is unavailable. Depending on the heat pump and control strategy, it may also assist with rapid recovery or high-temperature hygiene cycles.

During normal operation, however, the air source heat pump should ideally provide the majority of the water heating.

Regular immersion heater operation can increase electricity consumption because direct electric resistance heating does not provide the same efficiency advantage as a heat pump.

What size unvented heat pump cylinder do I need?

Cylinder capacity should be based on the property’s domestic hot water demand, rather than simply choosing the largest cylinder that will fit.

The number of occupants, showers, baths, bathrooms and periods of simultaneous hot water use all need to be considered.

Recovery time is particularly important. Heat pumps can take longer to reheat a cylinder than a high-output boiler, so sufficient stored hot water is needed to cover peak demand.

The cylinder’s coil surface area and heat exchanger rating must also be matched to the output and operating temperatures of the heat pump.

Is an unvented heat pump cylinder the same as a heat pump water heater?

Not quite.

An unvented heat pump cylinder is normally a hot water storage vessel connected to a separate air source heat pump.

An integrated heat pump water heater combines the cylinder and heat pump technology into one appliance, typically incorporating the compressor, evaporator, fan and refrigerant circuit.

Both can provide energy-efficient domestic hot water, but they are different system arrangements.

For a typical air-to-water heat pump installation, an unvented heat pump cylinder provides mains-pressure hot water, while its larger heat exchanger allows the heat pump to heat the stored water efficiently. Correct cylinder sizing, coil selection, insulation and installation are all important to achieving good hot water performance.