Pressure Controllers
In many steam heating systems, a line-voltage pressure controller is used to provide operating control, automatic limit protection, or a manual reset limit for protection in pressure systems of up to 300 psi. One such pressure controller is the Honeywell L404 Pressuretrol® Controller shown in Figure 15-34. It is available in a variety of different models.
Some models of the L404 are designed to provide on-off and proportional control of steam boilers fired by proportional-type burners. These can be field adjusted to operate either in unison (burner starts
at high fire) or in sequence (burner starts at some firing rate other than high fire). A common bellows assembly located in the bellows housing actuates the stop-start switch (on-off operation) and the wiper of the 185-ohm potentiometer (proportional operation).
The L404 operates strictly as a high-limit pressure safety control on steam heating boilers (see Figure 15-35). It breaks the electrical circuit on pressure rise. A variation of this controller is used for suspension-type unit heaters.
Direct control for a proportional motor operating an automatic burner can be obtained by using a pressure controller with two potentiometers operating in unison. This design makes it possible to control two motors simultaneously. It is also provided with an adjustable throttling range.
Pressure controllers are also available for vapor or vacuum systems. A pressure controller with a bellows-operated mercury switch is manufactured by Honeywell for use on vapor heating systems with pressures of up to 4 psi (see Figure 15-36). It can be used as a boiler high-limit control with cut-in and cutout settings in the vac- uum range. In such installations, the heating system must include a vacuum pump and a siphon loop.
