High-Pressure Limit Switch
The high-pressure limit switch is a safety device used to shut down the operation of the gas or oil burner when the pressure boiler pressure exceeds a preset level (commonly 5 to 8 psi). The high-pressure switch is connected to the boiler by a pigtail pipe.
temperature limit or to switch on the circulator. If the former is the case, the aquastat will close on temperature drop and open on temperature rise. The aquastat will close on temperature rise if it is used to operate the circulator.
An example of a strap-on type aquastat is the ITT General Controls L-53 Strap-On Hot Water Control illustrated in Figure 15-55. This type of aquastat responds to water temperature changes as conducted through the supply-riser pipe wall to the temperature- responsive base of the device. The enclosed switching is supplied in three basic types, depending upon their operating principles: direct- action (Figure 15-56), reverse-action (Figure 15-57), and double- action (Figure 15-58) models.
A direct-action (normally closed, N.C.) aquastat used as a high- limit control must always be located on the supply riser where it will be subjected to the maximum boiler water temperature. Its location on the riser will therefore have to be as close to the boiler as possible but ahead of any line shutoff valves.
Base of control must fit snugly against pipe.
Insert bolt through clamps and holes in strap and pull up.
Pass mounting strap through slot in base and bend to fit around pipe.
Cut off excess strap.
A double-action (single-pole double-throw, SPDT) aquastat is used to start circulator operation with a single switch actuation and to function as an operating control to maintain boiler water tem- perature. This type of aquastat should be mounted on the largest riser ahead of any valve but at a point where it will be subjected to maximum boiler water temperatures. The double-action aquastat illustrated in Figure 15-58 opens R to B contacts and closes R to W contacts on temperature rise at scale setting.
A strap-on aquastat can be mounted in any position. When mounting these aquastats, make absolutely certain that the pipe sur- face is clean and free of rust and corrosion. All rough and high spots should be filed smooth. Nothing should be allowed to interfere with the operation of the temperature responsive base of the control.