Service manual

SC Series Boilers – Installation and Service Manual

3.5 AIR SWITCHES AND AIRFLOW ADJUSTMENT

The boiler is equipped with two differential air pressure switches:

1. MINIMUM AIRFLOW (Red Light). The first air pressure switch closes and the indicator is on when

there is enough combustion air supplied to the boiler for safe operation. The Minimum Airflow switch

senses positive combustion air differential pressures (∆P) between the air chamber and burner

venturi inlet. It acts as a safety device to prevent the boiler from firing when the airflow to the

combustion chamber is too low. Low airflow can result in high levels of carbon monoxide, flashback

and inefficient combustion. The minimum airflow switch is normally open and will close when the

differential pressure of the air is above the switch's set-point. The actual airflow to make the boiler

operate is slightly above the Minimum Airflow switch's set-point.

2. OPTIMUM AIRFLOW (Red Light). The second air pressure switch and indicator light allows the

installer to easily adjust the blower to the optimum combustion airflow for any installation and venting

configuration, and without the need of an inclined manometer or other such instrument. This switch

only operates the indicator light and is only for the installer's use during setup. It is not a safety

device and does not prevent the boiler from firing.

Both indicators are off when there is no call for heat. When there is a call for heat, the blower switches

on and comes up to speed. If the Minimum Airflow switch senses enough airflow, it will close (red light

ON) and allow the boiler to fire. Normally the Optimum Airflow switch will close (red light ON) but it may

be on or off depending on operating conditions.

There are two tubes connecting the air pressure switches with the air chamber and burner inlet. The clear

tube connected to the air chamber should be connected to the positive side (P1) of the switch and the red

tube connected to the burner inlet should be connected to the negative side (P2) of the switch, as

indicated on the pressure switch casing and as shown in Figure 9.

Figure 10 – Blower and air shutter

Figure 9 – Air differential pressure switch

connections