3 Improving heating system efficiency

Over the past 60 years the UK has been transformed from a country where the majority of homes were heated with individual coal fires to one where central heating is almost universal. In 2014, an estimated 97% of British homes were centrally heated.

An open coal fire may seem very cosy and traditional, but in practice most of the heat disappears up the chimney. The thermal efficiency of a heating system = useful heat output / fuel energy input. That of a coal fire is particularly low.

Table 9 illustrates the enormous variation in thermal efficiencies of different space heating systems.

In the UK, gas is the dominant central heating fuel (see Figure 27) and it also supplies half of non-centrally heated homes. Renewable energy and energy from waste meet about 1% of domestic heating energy demand.

Figure 27 UK domestic central heating (CH) fuels, 2014 (BEIS, 2021c)

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This is a pie chart. Reading clockwise from the top the proportions are: Non central heating, 2.8%; central heating solid, 0.5%; central heating electric, 6.2%; central heating oil and other, 5.5% and central heating gas, 85.1%.

Figure 27 UK domestic central heating (CH) fuels, 2014 (BEIS, 2021c)

This growth of central heating use has been a mixed blessing for overall energy demand. The average UK dwelling today is better insulated than its 1970 counterpart, and space heating system efficiencies have increased, from an average of about 50% in 1970 to over 80% in 2011 (BEIS, 2013). It might therefore be thought that the space heating demand of the average UK home would have fallen dramatically. However, much of the potential energy saving has been taken up as increased internal temperatures. Homes are now better heated and estimated average internal temperatures have risen from about 12°C to 18°C over the same period, increasing the heat losses. It is even possible that this temperature trend will continue in the future to levels of 22°C or more found in housing in Germany and Scandinavia.

Figure 28 shows the layout of a typical domestic wet central heating system. Water is circulated through a boiler, which may be fuelled by gas, oil or solid fuel (or even an electric heat pump). The heated water flows through the radiators as well as through a heat exchanger in a hot water storage cylinder, which holds the domestic hot water used for washing, etc. The storage cylinder (if present) should be well-insulated – most are now supplied with a sprayed layer of foam insulation.

Maximise

Figure 28 A diagram of a radiator, or ‘wet’, central heating system

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This is a diagram showing the main components of a normal central heating system. At the bottom left is a labelled boiler. From this a red flow pipe is shown running horizontally to a labelled pump. An arrow indicates that the flow of water is from the boiler. The flow pipe continues through a labelled control valve and on to a set of four radiators at the right of the diagram. The flow pipe splits to enter each radiator individually. Just before it enters each radiator it passes through a labelled ‘thermostatic radiator valve or ‘T’’R’’V’’. At the opposite end of each radiator is a blue return pipe. The return pipes from each of the radiators combine and are shown returning to the boiler at the left of the diagram with an arrow showing that the water flow is towards the boiler. In the centre of the diagram a further red flow splits from that to the radiators after the pump. It rises to enter a labelled ‘hot water cylinder’ in the centre of the diagram. The flow pipe is shown running through a coiled heat exchanger inside the cylinder before emerging at the bottom as a blue return pipe. Immediately after it emerges it passes through a control valve, which is shown linked by a dotted line to a labelled ‘cylinder thermostat’ on the side of the hot water cylinder. The blue return pipe then passes from the control valve and joins the blue return pipe from the radiators, returning cold water to the boiler. A further white pipe is shown rising from the blue flow pipe immediately before it enters the boiler to a labelled ‘central heating header tank’ at the top of the diagram. This is fed from the left by a white pipe labelled ‘cold water mains supply’. Another white pipe rises from the red flow pipe immediately after leaving the boiler. This is labelled ‘expansion pipe’. It rises and curls over to feed into the top of the central heating header tank.

Figure 28 A diagram of a radiator, or ‘wet’, central heating system

There are many variants of this basic system. The most common one is the combination or ‘combi’ boiler. This supplies heat to radiators but dispenses with the hot water cylinder, acting as an instantaneous heater for domestic hot water.

Such a system should obviously be properly controlled for maximum efficiency. A good system is likely to be controlled by:

• preset time switches, or a timing programmer
• a room thermostat that senses the air temperature, usually somewhere in the middle of the house
• a hot water cylinder thermostat that senses the requirement for domestic hot water.

In addition individual radiators should be equipped with thermostatic radiator valves (TRVs) that provide extra local control in individual rooms.

Systems for larger buildings are likely to follow a similar pattern, but may have multiple zones that can be separately controlled. Large buildings such as hospitals are likely to use ‘warm air’ heating. Air is centrally heated (or cooled) and then blown through ducts to individual rooms.