Water Cylinders & Water Heating Systems

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, this simple device did exactly what the name suggested. But now we have buffer tanks, accumulators, thermal stores and even calorifiers. 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.

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 pressurized hot water cylinder fed directly from the mains water supply. There are headers 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 pressurized they have valves and thermostats to ensure that they do not over-pressurize and explode. 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.

Thermal Stores

A thermal store is a means of storing heat. Heat is supplied directly from the boiler (or another heat source, such as a heat pump), which eliminates efficiency losses. The thermal store typically contains one or more heat exchanges (which may be coils or external flat-plates), which are heated by the surrounding water.

A thermal store can ‘stratify’ — that is, maintain water at different temperatures, top to bottom. This makes a thermal store ideal in a home which requires higher temperature outputs for domestic hot water, and lower temperature outputs for underfloor heating, for instance.

A thermal store can also deal with multiple heat sources (for example, boiler and solar panels or heat pump and woodburning stove). In these situations, the technology comes into its own. It becomes the ‘heart’ of the system, managing the supply and distribution of heat as required.

The thermal store will 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.


Ultimately, the decision will be based on the nature of its application. Combustion and Heating Systems is a reputable boiler & burner company and was established in Johannesburg. The company’s reputation is still respected within the industry for over 27 years. We are a boiler and burner company that offers efficient, friendly and personal service. Our staff has several years of experience and knowledge to pass on to our customers.

Here at Combustion and Heating Systems we supply, install, service and maintain heating equipment such as oil and gas burners, hot water boilers, heating systems and all the associated parts. In our Johannesburg store, we have one of the biggest stock holdings of package burners and spare parts in the country. We distribute all over South Africa and neighbouring countries such as Botswana, Zimbabwe, Zambia, Mozambique, Namibia, Angola and as far north as Nigeria.

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