Heatec Tec-Note : Controls - HC & HCS series heaters (page 8)

Modulating actuator
The modulating actuator normally used is a Honeywell M9174C1025 (Fig. 7). It controls the firing rate of the burner in response to signals it receives from the modulating controller. It controls the firing rate by moving mechanical linkage connected to the gas butterfly metering valve, the fuel oil metering valve and the air damper to either increase or decrease the firing rate. If linkage becomes loosened from operation it will lose its adjustment and the burner will not operate properly. This is a common problem.

The actuator has two cams that are activated when the heater is started up by the Fireye flame monitor. When startup is initiated the actuator goes to high fire purge, causing one cam to close a switch inside the actuator thereby confirming that the system is in high fire purge. Then, after a preset time, the actuator automatically goes to low fire purge and the other cam closes another switch inside the actuator thereby confirming that purge is complete and that the system is ready to light in low fire.

Figure 7. Modulating actuator.

Low media level switch
This is a float switch mounted in the expansion tank (Fig. 8). It opens a limit circuit if media (thermal fluid) in the tank falls below a certain level. Moreover, when the circuit opens it also disables the circulating pump circuit. The switch automatically closes when fluid level in the tank is restored to normal. The switch is connected to the low media level relay.

Figure 8. Low medial level switch.

High fuel gas pressure switch
This is a pressure switch (Fig. 9) that opens a limit circuit if gas pressure is too high when operating on gas. When the limit circuit opens it shuts down the burner. The switch has a manual reset button.

Figure 9. High fuel gas pressure switch.