The efficient and safe operation of a hot-water heating system requires the use of a variety of different types of controls, which can be roughly divided into either system-actuating controls (e. g., the room thermostat, burner controls, and circulating pump controls) or safety controls (e. g., high-limit controls, pressure-relief valves, and pressure-reducing valves).
The safety controls prevent damage to the system by shutting it down when pressure and temperature levels become excessive. The high-limit control, or aquastat, is an example of such a control. It is
Figure 7-25 Heat exchanger used in connection with a steam boiler to provide hot water to baseboard heat-emitting units. (Courtesy National
Better Heating-Cooling Council)
A device designed to operate in conjunction with the circulating pumps and is located on the hot-water boiler. If the pressure or temperature of the hot water exceeds the design limits of the system, the system is shut down until conditions return to an acceptable level. Some aquastat relays provide multizone control when used with a separate circulator and a relay for each zone.
Another very important safety control for hot-water heating systems are pressure-relief valves. These valves are designed to open when pressure in the boiler reaches a certain level and close when the pressure returns to a safe level again. Pressure-relief valves must be installed in accordance with current ASME or local codes.
These and other controls (e. g., main shutoff valves, pressure-reducing valves, and drain cocks) are described in considerable detail elsewhere in Chapter 15, “Steam and Hot-Water Space Heating Boilers” of this volume, and in Chapters 4, “Thermostats and Humidistats,” and 9, “Valves and Valve Installation” of Volume 2.