Five-minute Guide to… Hot Water Storage
We may think that hot water storage is simple, but there’s more to the subject than meets the eye, says Tim Pullen
Do you need a thermal store or unvented cylinder in your home? There’s more to storing hot water than meets the eye, as energy-efficiency expert Tim Pullen explains
Appearances can be deceptive. The advent of more efficient boilers, multiple heat sources and the need to lose as little heat as possible from the cylinder means that a hot water storage system is no longer just a copper cylinder in the airing cupboard.
Herein lies the problem for the homeowner: the terminology and functionality of different options. We have simple cylinders, direct or indirect, combination cylinders, buffer tanks, vented and unvented tanks, thermal stores, calorifiers, accumulators, and most of the above in horizontal versions, too. In some cases, where the boiler is a long way from the bathroom, a buffer tank will be used to prevent long delays in getting hot water to the tap. These are simply a relatively small tank with no heating capability. When huge volumes of hot water are needed, an accumulator (the same as a calorifier) will be used. These are like a buffer tank, but big, usually many thousands of litres.
In reality there are only two types of cylinder in common use: the indirect, vented cylinder or the thermal store (both of which are the subject of this article).
Indirect, Vented Cylinder or Thermal Store?
The traditional hot water storage is an indirect, vented cylinder — a copper or stainless steel cylinder with a coil in it that is connected to the boiler in a closed loop. Cold water is supplied to the cylinder from a tank, usually in the loft. The boiler heats the water in the coil and that, in turn, heats the water in the cylinder. This indirect heating prevents contamination of the water from the taps. The cylinder will have a pipe leading to the tank in the loft, so that if the water in the cylinder overheats it can overflow safely to the tank. The water pressure at the tap is determined by the difference in height between the tap and the tank in the loft. There are many unvented options for indirect cylinders available. These are typically used where water pressure is an issue, either because mains pressure is poor or a header tank cannot be installed. These ensure good pressure at the tap by pressurising the system. They work well, but come with a maintenance overhead as they need annual maintenance to ensure they are safe.
A thermal store, meanwhile, is a means of storing heat. Heat is supplied direct 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.
There are two further main differences between a traditional indirect cylinder and a thermal store. The traditional indirect cylinder will maintain the water in it at broadly the same temperature throughout. 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. And, while an indirect cylinder will deal with a single heat source (plus an immersion heater), a thermal store can deal with multiple heat sources (for example, boiler and solar panels or heat pump and woodburning stove).
In these situations, where there are multiple heat sources, a thermal store comes into its own. A good thermal store will select the cheapest heat source to call on when heat is needed. It becomes the ‘ heart’ of the system, managing the supply and distribution of heat as required.
Size and Costs
Indirect cylinders tend to be smaller than thermal stores as they are not, primarily, storing and managing heat. A thermal store will generally be in excess of 200litres and indirect cylinder less than 150litres. The actual size will vary with the house size and the number of occupants. It needs to be calculated properly by an expert, to ensure that it is big enough to meet demands, but not so big that the heat source cannot get it up to temperature.
An indirect cylinder will be around £150 to £300, depending on size and quality. A good thermal store, like the Chelmer Ecocat or the Gledhill Torrent, could be over £2,000, again depending on size — reflecting the fact that a thermal store is an energy management system, and without it your renewable energy system will just not work properly.
Which is Best for Your Home?
Indirect cylinders have been around for many years doing a perfectly good job. Plenty of simple indirect and mains-pressure systems are still sold, and where there is a single heat source and broadly similar heat output demands (ie radiators and domestic hot water), that is the right option. But they are probably not best where there is underfloor heating and some form of renewable energy.
The advent of condensing boilers, the movement away from combi boilers and the increasing use of low temperature heat sources like heat pumps means the thermal store is becoming increasingly necessary. Add to that potentially high temperature inputs like solar thermal systems and woodburning stoves and the low temperature output needed by underfloor heating, and precise heating management becomes a requirement. H
THERMAL Store Cylinder The Ecocat thermal store ( below) is designed to work with heat pumps and boilers and can also be optimised to work with solar thermal systems, ranges and woodburning stoves. Here’s how it works…