Begetube’s two phase change material units (each contain 60 litres of hydrate salt) provide the same level of thermal storage as the 600 litre thermal store next to them

A hot water cylinder once was just that — a cylinder that contains hot water. Usually copper, often with a 80 litre capacity, and sometimes with an immersion heater; a simple device that did exactly what the name suggested. But now we have buffer tanks, accumulators, thermal stores and even calorifiers. What happened?

With the increasing need for efficiency, more complex heating control systems and the potential for multiple heat sources, a simple cylinder is no longer always the answer. The problem is knowing what all these different devices do and which one to specify. The Hot Water Association provide a 78-page document that sets out exactly the definitions and uses of each. Here, we attempt to do the same thing, but in fewer pages.

Cylinder types

Vented: This is the traditional hot water system. There is a cold water tank (or header tank) in the loft, a copper cylinder in the airing cupboard and an expansion tank (smaller than the header tank) also in the loft, providing the ‘vent’. The water pressure at the tap is provided by the difference in height between the cylinder and header tank.

Unvented: This is a pressurised hot water cylinder fed directly from the mains water supply. There are header or expansion tanks and they supply hot water to the taps at cold water mains pressure — generally much better pressure than a vented system. As they are pressurised they have valves and thermostats to ensure that they do not over-pressurise and explode (generally considered, by most parties, a bad thing). They therefore need regular maintenance — which has a cost.

Unvented cylinders are becoming more popular, despite being considerably more expensive, as they ensure good pressure at all outlets in the house without the need for pumps or header tanks.

Types of Hot Water Storage Tank

A calorifier is a storage vessel that can generate heat as well as store it. A simple copper cylinder with an immersion heater could be called a calorifier. For all practical purposes the term is now redundant in a domestic situation.

A buffer tank (typically vented, and may also be called an accumulator) is a vessel containing hot water and is placed between the heat source and the heat output (such as radiators, taps, underfloor heating (UFH), etc.). A buffer tank is installed to improve the efficiency of renewable energy systems, usually heat pumps and biomass boilers.

A boiler can react to demands for heat more quickly and more efficiently than a heat pump or biomass boiler can. A 12kW boiler might deliver anything from 4kW to 12kW, depending on the demand. A 12kW non-inverter heat pump will deliver 12kW whatever the demand. If the demand can be met with 4kW, then the heat pump will only operate for a very short period, which is very inefficient and known as short-cycling.

This is less true with modern inverter-driven heat pumps as they operate in a way which is more similar to a boiler, but a buffer tank, of some form, is still a good idea for most heat pump and biomass boiler installations.


The storage vessel needs to be sized to meet both the heat source and the demand. A gas boiler heats water quite quickly so the hot water cylinder can be small — often 80 or 120 litres. A solar thermal system will produce a lot of hot water in a short period of time, then none for a long time. So the storage vessel needs to be big — 300 to 400 litres. A thermal store will typically be 200 to 300 litres, which is a happy compromise for the typical family, allowing for both traditional and renewable heat sources.

So, What do I Need?

The short answer is a thermal store. There is a need to store hot water, a thermal store does that; there is a need to buffer the heat source, a thermal store does that; there is a need to deliver water at different temperatures, a thermal store does that. But most crucially we need control.

“Control is essential,” says Brent Witherspoon of Chelmer Heating. “We need to control the output temperature and the storage temperature. But with multiple heat sources, we need to control which one to use to be able to always choose the cheapest.” And once again, a good thermal store does that.

Thermal Stores

A thermal store (which can be vented or unvented) stores hot water, as a buffer tank or hot water cylinder does, but also manages that heat until it is needed. The thermal store is filled with water heated by the heat source (boiler or heat pump) and controls the hot water as such that the top of the tank will be significantly hotter than the bottom — the top can be as hot as 85°C and the bottom could be only 30°C. In this way it acts as a buffer tank (and replaces the need for a separate buffer). The water in a buffer tank or hot water cylinder will be largely the same temperature throughout.

The thermal store will then have at least two copper coils. The top coil will be connected to the mains water supply which is heated by the hot water in the thermal store and delivered to taps, showers, etc. The bottom of the thermal store has a second coil, connected to the radiators or UFH. The thermal store will have controls that ensure water is always delivered at the required temperature – 55°C+ to radiators, 48°C to taps, etc. and 35°C to 44°C to UFH – while allowing the heat source to operate at its maximum efficiency.

three thermal storage systems from Chelmer, Worcester Bosch and Megaflo

The Ecocat thermal store from Chelmer Heating is primed for renewables integration; The ubiquitous Megaflo unvented cylinder from Heatrae Sadia; Worcester Bosch’s Greenstore range of cylinders offer fast reload times

Chelmer Heating’s Ecocat is unusual in that it has two sections within the cylinder, essentially a cylinder inside a cylinder. The top cylinder serves the hot water and the bottom is the buffer for the heating system. Both of these cylinders can be heated by three heat sources and as a consequence it is the only thermal store that can use solar thermal heat and comply with Renewable Heat Incentive (RHI) requirements.

As the Ecocat thermal store is indirectly heating water it can more simply take heat from multiple sources. Brent Witherspoon, managing director of Chelmer Heating, says: “More and more people are using renewable energy, sometimes along with a gas or oil boiler. In that case, a thermal store is the only answer.”

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