General
The limits should be checked while the heater is operating. Checking is normally done during the startup of a new heater while water is being purged.

After each limit device opens and causes the heater to shutdown you will have to depress the manual reset on the control panel. Some limit devices also have a manual reset that must also be used to reset the device for normal operation.

Checking low media level switch
Verify that the float inside the tank is properly oriented by checking the bushing at the back of switch on the outside of the tank. One of the six flat sides of the hex on the bushing should be stamped with the letter T. This flat side of the bushing should be on top and should appear to be level.

Remove cover from the low media level switch. Use the flat side of a screwdriver to depress the trip mechanism so as to open the switch contacts (Fig. 2).

Figure 2. Checking low media level switch.

Opening the switch should cause the following reactions, which indicate that the switch is operating properly:

1. Circulating pump should shut off.

2. Burner should go into alarm.

3. Low media level light on control panel should be the first light to go out.

4. Fireye flame monitor should read 3 TO P INTERLOCK OPEN.

Checking low fuel pilot pressure switch
Not all heaters have a low fuel gas pressure switch. But the panel may still have an indicator light for the switch. In that case the switch is replaced by a jumper wire. Thus, the light will react the same as it would with a switch in its normal pressure state.

On heaters with the switch, shut off the pilot gas manual shutoff valve and set HEATER CONTROL switch to ON. After pilot solenoid opens, the following reactions should occur, indicating that the switch is operating properly:

1. Low fuel pilot pressure light on control panel should be the first light to go out.

2. Fireye flame monitor should read 3 TO P INTERLOCK OPEN.

Checking high fuel gas pressure switch
Not all heaters have a high fuel pilot pressure switch. But the panel may still have an indicator light for the switch. In that case the switch is replaced by a jumper wire. Thus, the light will react the same as it would with a switch in its normal pressure state.

With the burner operating, adjust the switch to a lower setting that causes the switch to open (Fig. 3.).

Figure 3. Checking high fuel gas pressure switch

The following reactions should occur, indicating that the pressure switch is operating properly:

1. The burner should shut down.

2. High fuel gas pressure light on control panel should be the first light to go out.

3. Fireye flame monitor should read 3 TO P INTERLOCK OPEN.

Reset high pressure switch using manual reset button.