Stack-mounted oil burner primary controls employ thermal sensors to detect ignition or flame failure. A typical stack detectorthermal sensor (see Figure 5-87) consists of a bimetal element inserted into the stack (see Figure 5-88). The thermal sensor (combustionthermostat) is usually located on the stack where the elementwill be exposed to the most rapid temperature changes. Thethermal sensor should always be mounted ahead of any draft regulator.If installed on an elbow, it should be mounted on the outsidecurve of the elbow.cycling control and a thermal detector for sensing temperature changes of the flue gases (as high as 1000°F maximum temperature). The safety switch shown on the center-left of the unit is designed to lock out if the flame is not properly established. If the flame goes out during the burner on cycle, the primary control will make one attempt to restart. If the attempt is unsuccessful, the safety switch will lock out. A manual reset is then required in order to restart the burner. The primary control shown in Figure 5-89 is used with a two-wire or three-wire primary controller.
A stack-mounted combination line voltage primary control and flame detector is shown in Figure 5-90. This type of primary control is used with constant-ignition oil burners and is designed for flange-mounting on a stack, flue pipe, or combustion chamber door of a furnace or boiler. It must be used with a line voltage thermostat or controller.