What is a buffer vessel and what does it do?
A buffer vessel is a pressurised storage vessel installed within a heating system to increase the overall water volume of the circuit. This additional thermal mass helps to prevent short-cycling — where a heat source switches on and off rapidly because the system volume is too small to absorb its output — and improves overall system stability. Buffer vessels are used with boilers, heat pumps, and heat network plant to ensure the heat source operates efficiently within its intended modulation range.
Do heat pumps require a buffer vessel?
Many heat pump installations benefit from a buffer vessel, though it is not always a mandatory requirement. Heat pumps typically operate most efficiently when they can run for sustained periods at a stable output rather than cycling frequently. Where the heating distribution system has a low water volume — as is common with underfloor heating manifolds or small radiator circuits — adding a buffer vessel increases the system volume and allows the heat pump to run longer cycles at higher efficiency. Modutherm's Heat Pump Buffer Vessels are available from 500–5,000 litres and are specifically configured for heat pump primary circuits.
What is the purpose of an expansion vessel in a heating system?
An expansion vessel accommodates the increase in water volume that occurs as water in a sealed heating system is heated. Without an expansion vessel, the pressure in the system would rise to unsafe levels as the water expands. The vessel contains a pre-charged air or nitrogen cushion separated from the system water by a flexible membrane; as the water expands, it compresses the gas, absorbing the pressure rise and keeping the system within its operating limits. Modutherm's expansion vessels are available in wall-mounted and floor-standing configurations from 24–500 litres.
What size buffer vessel do I need?
Buffer vessel sizing depends on the type and output of the heat source, the water volume of the connected heating distribution system, and the minimum run time required for the heat source to operate efficiently. For heat pump applications, guidance from CIBSE and heat pump manufacturers typically specifies a minimum system water volume based on the heat pump's output in kilowatts. A heating or mechanical engineer will calculate the required vessel capacity at the project design stage. Modutherm's range spans 200–20,000 litres, covering installations from small commercial premises through to large-scale multi-building sites.
