A number of different factors must be considered when selecting circulators for both large and small heating systems. The selection factors include:
• Amount of water to be handled
• Temperature of the water to be handled
• Head against which the pump must operate
• Working head of the system
• Pump suction head
Data necessary for selecting a suitable water-circulating pump are supplied by pump manufacturers. Always check the manufacturer’ s specifications for the maximum working pressure and the maximum operating temperature of the pump before deciding on which model to use. These operational limits can be found in the manufacturer’ s specification sheet, in the installation manual, or on the pump nameplate. The maximum working pressure and maximum operating temperature of the system must not exceed those of the pump. If they do, it can lead to possible property damage, serious injury, or even death.
The temperature of the water handled by the pump will determine the type of pump packing selected. The working head of the system is the sum of the static head and the friction. The pump will have either a positive or negative suction head.
The circulators used in small residential hot-water heating systems are installed directly in the return line. These are commonly single-suction pumps in which both the motor and the pump share a common shaft. The major objection to installing a pump directly in the return line is that the pipe connections may have to be broken to remove the pump for service and repair. However, some pumps such as the Bell & Gossett Series 90 model shown in Figures 10-15 and 10-16 can be serviced without removing the entire unit. If the circulator must be completely removed from the return line, a good idea is to install shutoff (check) valves above and below the unit so that the entire system does not have to be drained.
The operating head of circulators used in smaller hot-water heating systems is limited, and it is often general practice to size the pipelines of the system after selecting a pump capable of meeting the requirements of the system.
Pump manufacturers recommend that cast-iron pumps be selected for circulating water in a hydronic space heating system. Bronze pumps are recommended for pumping domestic (potable) hot water.