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Many people use hot tap water from boilers daily for showering, bathing, washing clothes and dishwashing. A boiler in its environment is shown in Figure 3.1.
When a tap is opened, hot water is supplied within a few seconds, usually at a temperature of 40-65°C. However, hot water is often required by several users at the same time of day (the bathing period). There is a therefore a problem of hot water availability.
An indirect boiler heats water in a primary circuit, as shown in Figure 3.2. Heat from this hot water is transferred to tap water or space heating water before the water returns to the boiler and is reheated. (In this chapter, only indirect boilers are discussed.)
Hot tap water can be produced in two ways:
If there is a risk of the water temperature getting too high in a tap water system, which is the case for all systems in this chapter, a thermostatic mixer should be installed before the taps. The thermostatic mixer mixes cold tap water with hot tap water from the accumulator or the heat exchanger, which ensures that hazardous temperatures are not reached (see Figure 3.3).
For instantaneous tapwater solutions, wall-hung gas boilers are the dominant concept used in Europe. There are three solutions for preparing instantaneous hot tap water in wall-hung gas boilers:
Secondary heat exchangers and bithermic heat exchangers are the dominant solutions.
Secondary Heat Exchanger System
The most common solution is the system using a secondary heat exchanger. In this system, the burner heats water in the primary heat exchanger. This water is used as the hot medium in the radiator system or, when there is a demand, for the tap water system through the secondary heat exchanger. A system using a SWEP E-model as secondary heat exchanger is shown in Figure 3.4.
Figure 3.5 shows the layout of a secondary heat exchanger system with the principal components. It is important to point out that there are alternative designs. The functions of the various components are described below.
If you would like to know more about boiler applications we recommend that you to download the free online digital handbook "A Technical Handbook for Heating Applications". Take me to download.
