Buffer Tanks For Heat Pumps: Optimizing efficiency and performance of heat pump systems

A heat pump buffer tank is generally used with an air source heat pump system or ground source heat pump, as heat pumps require a minimum system volume to operate correctly and efficiently. Buffer tanks for heat pumps act as a thermal mass to ensure minimum volume and improve system efficiency.

Installing a buffer tank / buffer vessel is logical in large properties or commercial buildings with complex heat loss patterns, significant thermal mass, low temperature hydronic heating systems, and multiple heating zones.

The buffer tank functions as a thermal reservoir or thermal storage tank, maintaining a balanced load and reducing variations in temperature and flow in the heating system.

Buffer tanks for an air source heat pump system

The key points for a heat pump buffer tank are:

A buffer tank is a thermal storage tank that stores or absorbs extra heat from a heat source, such as an air source heat pump, for later use.
Buffer tanks, or a buffer vessel are designed to reduce the cycling of a heat source and improve heat pump system performance by acting as a thermal store.
They are a hot water cylinder as such; in that they add a volume of water to the heat pump system. But they do not produce domestic hot water. If you require a hot water cylinder to produce DHW from a heat pump system, then these are listed on this webpage – hot water cylinders for heat pumps
Boilers, heat pumps, and solar thermal systems are just a few of the heat sources they can work with.
Buffer tanks are essential for reducing running costs and improving the energy efficiency of a heat pump system.

Heat pump buffer tank for an air source heat pump with optional back up heating via a boiler diagram from Flexiheat UK

A heat pump buffer tank to reduce energy consumption and improve energy efficiency

How do heat pump buffer tanks work?

A buffer tank works by storing hot water to satisfy the system’s requirements during ASHP defrosting cycles.
It reduces the short cycling of the heating system and improves heat pump performance.
The buffer tank provides a “heat sink” and “heat source” for excess heat energy.
It helps to smooth out changes in heat demand and loading on the heat pump.

Ground source heat pump systems

Ground Source Heat Pump systems (GSHPs) are effective all year round due to the consistent temperature of the ground, which remains at a steady 12 to 14 °C throughout the year. This gives GSHPs a benefit over air source heat pumps.

A ground source heat pump extracts energy by utilising an interconnected system of pipes that are installed either in trenches or in boreholes. This pipeline system contains a blend of water and anti-freeze, which effectively absorbs heat from the ground by conduction.

The heated mixture in the ground loops subsequently flows through a heat exchanger, raising the temperature of the refrigerant. By compressing the heated refrigerant, its pressure and, more importantly, the temperature is increased. The compressed and heated refrigerant is then directed through a heat exchanger, where it distributes its thermal energy into the heating circuit within the building or house, typically using a buffer tank.

Product range

To meet your needs, we offer a variety of buffer vessel options for heat pumps, categorised by the size of the tapping ports and/or the number of connecting tappings.

Our “PSS” buffer tank range for either air or ground source heat pumps

Storage capacities range from 25 to 2,000 litres.
6 bar working pressure rated
With either four connections for 25 to 300 litre units or a six-connection tapping range for our 50 to 2,000 litre models.

PSS buffer tank range for heat pump systems from 25 to 300 Litres from Flexiheat UK

25 to 300 Litre options

Technical data

Then our larger units from 500 to 2,000 Litres – these have 6 main connections as standard

500 to 2,000 Litre buffer tank models for heat pump systems from Flexiheat UK

500 to 2,000 Litre models for heat pump systems – all with 6 main connection ports

These thermal stores are made of mild steel, with the interior left untreated and the exterior coated.
This PSS range complies with all sections of the European Pressure Equipment Directive (PED). This insulated vessel range meet the ErP “C” or higher insulation grades, where applicable.
Our buffer vessels are equipped with three instrument pockets, allowing for the installation of temperature sensors or gauges.
The insulation jackets are offered for internal use in plant rooms. Additionally, there is an option to have ultra-violet resistant jackets for exterior installation. Alternatively, aluminium insulation shells are available for external mounting in coastal areas. The choice depends on the specific needs of your site.

Download the PSS buffer tank technical brochure 25 to 2,000 Litres by Flexiheat UK

Then we have another range, designated the “ACF”, these have only four main connection ports on the buffer vessel , but the 25 to 1,000 litre units have a higher rated pressure rating of 10 Bar.

Buffer vessel range for heat pumps 25 to 5000 Litre - ACF range from Flexiheat UK

ACF range of thermal stores / buffer vessel from 25 to 5,000 Litres to add minimum volume to heat pump systems

10 Bar working pressure rated from 25 to 1,000 Litres
The 500 Litre unit comes with 3″ connections – which is very large for this size of buffer vessel
6 Bar rated from 1,500 to 5,000 Litres
Four connecting tappings – marked the “DN1’s “on the following diagram

Download the Buffer vessel range for heat pumps 25 to 5000 Litre brochure – ACF range from Flexiheat UK

Types of buffer tanks

There are different types of buffer tanks available, either mild steel or stainless steel.
Buffer tanks can be used for both heating and cooling systems.
They are available in various sizes, ranging from 50 to 6,000 litres.
A buffer tank doesn’t usually contain a heat exchanger or coils; we manufacture a buffer tank with an intermediate coil or coils to separate the heat pump system’s heated water (the primary circuit) from the secondary circuit, e.g., radiators or underfloor heating. This is hydraulic separation.
Mild steel buffer tanks are suitable for sealed heating or chilled water systems only.

Benefits of using a buffer tank with a heat pump

Using a buffer tank as a thermal store with a heat pump can improve heat pump performance and reduce cycling.
It can help reduce running costs and improve energy efficiency.
A buffer tank can provide a bypass mechanism to keep the heat pump running at a minimum flow rate.
It can also help to reduce the risk of overheating and improve system reliability.

When should I use a buffer tank in a heating system?

A buffer tank is typically used in heat pump installations to prevent short cycling and maintain a minimum flow rate through the heat pump.
It is recommended to use a buffer tank with air source heat pumps, especially in systems with underfloor heating and multiple thermostats.
A buffer tank can also be used with ground source heat pumps or a solar thermal system (normally a solar PV system).
It is not always necessary to use a buffer tank, and the decision to use one should be based on the specific needs and system design of the HVAC system.

How do you size a buffer tank for a heat pump?

The size of the buffer tank should be determined by the heat pump manufacturers guidance and the specific needs of the heating system.
A buffer tank should hold approximately 15 litres per 1 kW of heat pump capacity as a rule of thumb.

Do all heat pumps need a buffer tank?

No, not every heat pump needs a buffer tank. Inverter heat pumps can reliably adjust their output to match demand. If you turn on the heat in another part of your property or raise the temperature, the heat pump will boost its heat output to suit the additional demand.

Keep in mind that the design of each heating system differs depending on the size, age, and intended usage of your property.

For example, an underfloor heating system with a thermostat in each room for independent heating control, even with an inverter heat pump, requires the installation of a buffer tank due to the smaller water volume of the heating system in a single room or smaller heating zones, which cannot support the heat pump’s increased output.

Also, it is not easy to size a heat pump system so that it operates in a broad range of ambient temperatures, even with inverter heat pumps. For the more moderate outside air temperatures that we see in the UK, a heat pump capacity is probably too big if you want it to run at its quietest for most of the time. These more moderate temperatures necessitate a buffer tank.

Installation and connection of buffer tanks

A buffer tank can be connected using only two connections, which prevents most of the flow from passing through the buffer under most operating conditions.

This design minimises mixing and only allows the buffer to come into play when necessary.

You can use the buffer to inject additional heat from other sources, like solar heat or excess heat from a backup boiler heating distribution system.

To convert excess energy into useful heat, add a coil or electric immersion heater to the buffer.

Maintenance of buffer tanks

Regular maintenance is necessary to ensure the optimal performance of the buffer tank.

Regularly inspect the tank for signs of corrosion or damage.

To prevent debris accumulation, the buffer tank must be cleaned and drained on a regular basis.

A qualified heat pump installer should perform any maintenance or servicing.

Common applications of buffer tanks

Buffer tanks are commonly used in heat pump systems, including air-source heat pumps and ground-source heat pumps.
They are also used in solar thermal systems and biomass boiler systems.
Buffer tanks can be used in both residential and commercial heating systems.
They are suitable for use with underfloor heating and radiator systems.

Design Considerations

The design of the buffer tank should take into account the specific needs of the heating system.

Correct tank sizing is necessary for optimal performance.

The material used for the tank should be durable and resistant to corrosion.

The tank should be well-insulated to minimise heat loss and comply with the ErP regualtions.

Get in Touch

Buffer tanks are an essential component of heat pump systems, providing a range of benefits including improved heat pump performance and reduced running costs. They are suitable for use with a range of heat sources, including air source heat pumps, ground source heat pumps, and solar thermal systems. By understanding the different types of buffer tanks, how they work, and when to use them, heat pump installers can design and install efficient and effective heating systems. Contact Flexiheat UK on 01202 822221 or use our email contact form to get the latest price and delivery times on our buffer tanks for heat pumps.