Bare necessities

Bob Long, MD Eco Innovate Ltd
Bob Long, MD Eco Innovate Ltd

Heat pump expert Bob Long asks why a buffer cylinder/thermal store is essential in any system

Buffer cylinders have a large role to play in the operation of air source heat pumps, but can also be an asset in ground source systems too. Accurate selection of a buffer cylinder is essential to obtain the best result. Too large and the cylinder will waste valuable space inside the property, and too small will prove ineffective.

A buffer cylinder increases the total hydraulic volume of the heating system, storing useful amounts of thermal energy and creating a stable heating effect.  

A buffer cylinder promotes the longevity of the heat pump itself, by reducing the number of stop-starts the heat pump will endure in its lifetime.

Soft start devices and inverter driven variable-speed drives, available in high-end heat pumps, have alleviated much of this problem but even these units will benefit from a more stable operating environment.

A buffer cylinder also plays an important role in defrosting the heat exchanger of an air source heat pump (ASHP). A suitably sized buffer cylinder will ensure an adequate supply of thermal energy is available to perform essential defrost cycles, as required by all ASHPs. 

Defrost cycles use a large amount of energy over short periods of time, and rely upon the thermal storage capacity of the heating system to supply this energy.

When selecting a suitable buffer cylinder, a number of factors need to be considered.

As an example, we could imagine a 10kW heat pump operating an underfloor heating system with a circulating water temperature of approximately 35°C. To maintain this condition, the heat pump controller will probably be adjusted to cycle between low and high limits of 38°C to 41°C, and maintain the buffer cylinder within this temperature range.

The minimum size of the buffer cylinder is generally defined according to size of the heat pump, plus the rate at which the heat pump will affect the temperature of a specific volume of water.

During a defrost period, the example 10kW heat pump should be capable of removing a similar amount of energy from the thermal store, for the duration of the defrost cycle. Ideally, the energy transfer required to complete a defrost cycle should be available without reducing the temperature of water inside the thermal store to below +30°C.

During defrost periods it is advisable to stop the circulating pump feeding the emitters to avoid any negative heating effect, or impose additional load on the thermal store.

An ASHP’s defrost cycle will not generally exceed 10 minutes, during which time a buffer cylinder of 300 litres capacity might expect a temperature drop of around 8°C.

In this particular instance, a system operating an average water temperature of +40°C would finish the defrost with a residual temperature in the buffer cylinder of about 32°C.

Heat pump manufacturers employing inverter-drive technology may say a buffer tank is not required, due the power matching potential of inverter control. The absence of a buffer cylinder can be detrimental during defrost periods, however, where the energy to complete a defrost cycle must be extracted from a limited, low volume of water contained in the emitter circuit.

Under this circumstance, the heat pump is likely to complete the defrost cycle with a water temperature significantly lower than ideal.

Incorporation of additional energy sources can be easily achieved with the assistance of a buffer cylinder/thermal store. The buffer cylinder can be fitted with a number of direct and indirect input/output connections to accept energy from other renewable energy sources, such as solar thermal panels or log burners.

A suitably designed passive heat exchanger, embedded in the buffer cylinder can also provide economical pre-heat for the DHW systems.

Although a buffer cylinder represents an added cost, it provides good system stability, together with all the flexibility needed to incorporate other renewable energy sources at a later stage.