Figure 10. Low fuel gas pressure switch.

High flue gas stack temperature switch
This is a temperature sensing switch (Fig. 11) that opens a limit circuit if flue gas temperature exceeds its factory setting of 1000 degrees F. When the limit circuit opens, it shuts down the burner. The switch must be manually reset, which requires removing its cover and pressing a reset button. (The manufacturer of this switch calls it a differential expansion temperature control.)

Figure 11. High flue gas temperature switch.

Differential pressure switch
The differential pressure switch currently used on all Heatec helical coil heaters is shown in Fig. 12. It is Barksdale Dialmatic pressure switch CDPD2H-A8OSS. It shuts down the burner if it senses abnormal differential pressures at either of its two switches. Abnormal differential pressure is a symptom of inadequate flow of thermal fluid through the coil.

This protects the coil from overheating and prevents breakdown of thermal fluid. The burner cannot be restarted until adequate pressure/flow is restored. There is no manual reset for this switch. It automatically resets when adequate pressure/flow is restored.

HCS heaters have a time delay relay in the pressure switch circuit to prevent unintended shutdown of the heater when flow is briefly interrupted when temperature control valves in the system open and close.

Note: Excessive buildup of trash in the strainer will reduce flow of thermal fluid through the helical coil and may cause the pressure switch to shutdown the burner.

A separate Tec-Note provides information on setting Barksdale pressure switches. Another Tec-Note covers pressure switches used prior to the Barksdale switch.

Figure 12. Center: Barksdale pressure differential switch.
Left: Outlet pressure gauge. Right: Inlet pressure gauge.

Low combustion air switch
This is a switch (Fig. 13) that opens a limit circuit if combustion air pressure differential is too low. When the limit circuit opens it shuts down the burner in a lockout state. The display of the Fireye flame monitor (Fig. 2) will indicate LOCKOUT 3-P INTLK OPEN.

The burner cannot be restarted until the flame monitor is manually reset. The air pressure switch automatically closes when adequate air pressure is restored.

Figure 13. Low combustion air switch and pilot gas solenoid.

As previously explained, there is no indicator light labeled LOW COMBUSTION AIR. However, if the low combustion air switch opens due to low combustion air, the green LIMITS MADE light will go out, but none of the clear lights will go out. Thus, the limits light in effect functions as a low combustion air light in addition to a limits made light.