Buffer vessels which are a thermal store for storing heating hot water or domestic hot water or cooling water from biomass boilers or solar hot water systems or chillers from Flexiheat UKFlexiheat UK

 Buffer Vessel /Buffer Vessels

Buffer vessels which are a thermal store for storing heating hot water or domestic hot water or cooling water from biomass boilers or solar hot water systems or chillers from Flexiheat UK

Buffer vessels are used a variety of water systems, be it domestic hot water (DHW), heating (LTHW), and or even process applications such as chilled water. The basic concept or function of buffer vessels is to heat the vessel during low demand periods and then release the heat during high demand periods, thus as such they are also called a thermal store. For chilled or cooling water applications – the reverse is applicable to the buffer vessels. A buffer vessel can be used as a thermal store for peak demand for renewable energy sources such as a biomass boiler or a heat pump heating system.

Stored heat energy / hot water storage products

Their purpose is to collect heat from one or more heat sources and store it for later use as part of a heating system. For example for a heating buffer vessels – the heat energy produced by a heat pump (air source heat or ground source heat pumps ),biomass boilers or even renewable energy sources such as solar hot water systems , might be kept in a buffer tank and then drawn out as needed to heat a wet heating system, such as an under-floor or radiator space heating system.

An open vented buffer can be used to manage an uncontrollable heat source, such as a solid fuel boiler system (pellet biomass boiler, wood burning stoves etc), without requiring the same level of safety precautions as unvented systems require as the heated water can expand into the feed and expansion tank since the buffer vessel is open to the atmosphere.

You should not use mild steel thermal storage tanks, as an open vented system is susceptible to introducing air into the heating water, and cause corrosion and damage to the mild steel buffer vessels – thus you should use glass lined buffer vessels or stainless steel buffer vessels.

Buffer tanks product construction

We can manufacture these buffer vessels in various materials – mild steel , glass lined mild steel or stainless steel buffer vessels dependant on your requirements and capacities. All vessels can be fitted with an immersion heater or heaters as backup or supplementary energy sources to the range of thermal storage buffer vessels we offer.

buffer vessels for closed heating or cooling systems as a thermal store - carbon steel or stainless steel hot water storage or chilled water energy sources from Flexiheat UK

Our Buffer vessel range – a thermal store for heating hot water or chilled water systems from 30 litres to 1,500 litres – larger units shown in our full brochure.

Technical detail

30 litres to 1500 litre buffer vessel range for thermal storage of heating or chilled water systems technical detail Flexiheat UK


Please note the “G” “I” range have larger connections – 2″ female for the 370 Litre model, and 3″ female for the 600 to 1,500 Litre sizes, but lose the dedicated immersion heater tapping, however with this range having multiple connections (4 on one side and 2 on the other of the vessel) you can utilise the spare connections for the installation of immersion heaters.

Techincal datat for our "G I" Range of larger buffer tanks 370 to 1500 litre with larger connections or ports from Flexiheat UK

Boiler buffer vessel – heating

Because a biomass boiler fired by pellet, log or wood chip – will  take longer to heat up than a gas boiler, a buffer vessel with a stored water temperature of 70 to 80 degrees C is installed to give instant heat at peak demand. Boilers, a biomass boiler in particular benefits from running longer and achieve maximum efficiency while heating the buffer vessel, whilst staying off for longer between heat cycles.

heating buffer vessels; biomass boiler; boilers

Internal picture of a buffer vessel

The heating buffer vessels are the ideal solution for storing hot water in reserve for rapid and efficient supply i.e. a thermal store, whether you’re short on boiler power or have a high peak demand.

The vessels are manufactured in mild steel, copper is not required as its heating water, and therefor a suitable corrosion inhibitor must be used on a buffer vessel for a peak demand heating solution.

We offer products from 300 to 10,000 litres in capacity , with a maximum operating temperature of 95°C , and a maximum operating pressure of either 6 or 10 Bar, in mild steel, enamelled mild steel, or even stainless if so required.


Benefits of buffer vessels in heating systems for thermal storage Flexiheat UK

Stratification –  The hydraulic circuit’s temperature can be stratified using the various temperatures available thanks to the inlet and outlet hydraulic connections positioned at varying tank heights.

Anti – condensation –  When using biomass heat generators (wood, wood chips, pellets, etc. ), it can operate as an anti-condensation circuit, ensuring that the manufacturer’s requirements are followed. These tanks allow you to comply with rules that demand the use of buffer tanks in conjunction with heat generators in some countries.

Partitioning – In addition to separating the hydraulic circuits between primary and secondary, it is possible to expand both the primary heating by connecting additional energy sources to the thermal store cylinders.

They can utilize renewable energy sources such as solar, via an indirect coil or coils to help heat the system or other renewable energy sources.

Our buffer vessels are the ideal solution, by storing hot water which is thermal energy in reserve for quick, effective and efficient supply, whether you’re short on boiler power or dealing with a high peak demand.

buffer tanks for heat pumps; cylinders buffer vessels; heating; heat pump buffer


With options of vertical low, vertical extra-low or even customized dimensions to suit site installation requirements.

Low height buffer vessels from Flexiheat UK ; to handle peak demand for heat pumps and biomass boilers in heating sytems

Low height buffer vessel options from Flexiheat UK

We offer ErP  energy rated buffer vessels – From 50 litres to 500 Litres capacities in accordance with the European Directive 2009/125/EC. You can download the full brochure here –


Or the range of heating hot water storage buffer vessels from 800 to 5,000 litres in capacities, in plain mild steel, galvanized mild steel or even stainless steel.


Domestic hot water buffer vessels / domestic hot water storage tanks

Our domestic hot water buffer vessel product range can also be called domestic hot water storage tanks. Historically these used to be manufactured from copper , but due to the fact that copper vessels can’t take as much pressure as stainless steel manufactured buffer vessels, stainless steel units have taken the market in the stainless steel copper argument

Also copper is not as durable as stainless steel, as copper is more prone to stress and crevice corrosion. Copper solutions for a thermal store / thermal energy vessel are also subject to the copper commodity prices which can fluctuate wildly these days. Stainless steel prices used to be a lot more than copper , but these days the price of stainless steel products is a lot closer to copper products , these factors, mean copper buffer vessel’s are less competitive in terms of long-term product installed life cycle costs as opposed to stainless steel units.

stainless steel domestic hot water buffer vessels from Flexiheat UK ; buffer vessel for storing hot water

Storing hot water – stainless steel hot water buffer vessel option

We do a full range of stainless steel or glass lined mild steel domestic hot water buffer tanks from 200 to 5000 litres in capacity in a vertical or horizontal configuration.

Domestic hot water storage Bessel or tank from Flexiheat UK; hot water buffer tank; hot water buffer vessel

Domestic hot water storage buffer vessel- can be fitted with an immersion heater or heaters as back up if so required


domestic hot water storage tank dimensions 200 to 1000 litres for domestic hot water storage

200 to 1000 Litre Buffer Vessels for Domestic Hot Water Storage

With options of vertical low, vertical extra-low or even customized dimensions to suit site requirements, and optional accessories such as a magnesium anode or an electronic anode for the enamelled versions – stainless versions don’t require any protection.

We can also supply larger volume units and bespoke units in enamelled mild steel, stainless steel (recommended for an uncontrollable heat source on an open vented system) at special request.

hydraulic equipment – tapping’s and dimensions for the 1,500 to 6,000 Litres buffer vessel models

domestic hot water storage tank dimensions 1000 to 6000 litres Flexiheat UK ; storing hot water

1500 to 6000 Litre DHW Storage tanks

Both types can be fitted with immersion heaters connected, for either direct top up or emergency back up of the hot water systems, we can manufacture or supply the immersion heaters in various heat / energy outputs to suit the system requirements for the thermal store. Or supply units that can utilize renewable energy , via an indirect coil to heat the hot water systems capacities.

Energy saving insulation

We supply energy efficiency class leading B or C ErP insulation rated buffer products to comply with the building regulations and European Directive 2009/125/EC

Chilled water buffer vessels – Steel

Chilled or cooling water services cylinders buffer vessels are used to assist chillers in air conditioning systems to avoid excessive chiller cycling and temperature control issues, and low water volume systems may require additional buffer capacity installed as a solution.

These products are available with the options of either a perforated central divider plate or multiple flow dividers -which helps reduce fluid mixing and convective turbulence due to temperature differences in the chilled water. They also ensure the reduction of compressor start-ups of the chillers.


With standard products from 30 to 6,000 litres in capacity ,with rigid or anti-condensation polyurethane insulation.

Dimensions of our chilled water buffer vessels or storage tanks from Flexiheat UK

hydraulic equipment – tapping’s and dimensions

Technical detail of our chilled water buffer vessels or tanks from Flexiheat UK

We can also supply a range of buffer vessel for chilled water products above 5000 litres in volume, and bespoke units at special request.

Download the Buffer Vessels Brochure from Flexiheat UK

Should you require more product information, availability, technical support or prices on our complete range of buffer vessels – for heating, domestic hot water or chilled water then please don’t hesitate to get in touch – contact our sales service team on 01202 822221 or use our email contact form via the contact tab above to message us.