The limit circuit is designated 3-to-P or 3-P. It is a circuit that starts with terminal 3 on the Fireye flame monitor (Fig. 2) and ends with terminal P on the flame monitor. The devices listed below are in the 3-P circuit and are wired in the same sequence they are listed.

When one of the devices opens it will open limit circuit 3-P. This will cause the heater alarm indicator light to come on and the burner will shut down in a lockout state. Moreover, the display of the flame monitor will show LOCKOUT 3-P INTLK OPEN.

1. Low media level relay contacts (Fig. 5)

2. Low fuel pilot pressure switch*

3. High fuel gas pressure switch* (Fig. 9)

4. Low fuel gas pressure switch* (Fig. 10)

5. Heater fuel oil pump auxiliary contacts*)

6. Low fuel oil pressure switch*

7. High media temperature relay (Fig. 5)

8. High flue gas stack temperature switch (Fig. 11)

9. Circulating pump auxiliary contacts (Fig. 4)

10. Low media differential pressure switch (Fig. 12)

11. High media differential pressure switch (Fig. 12)

12. Combustion air auxiliary contacts (Fig. 4)

13. Low combustion air switch (Fig.13)

*On some heaters this switch (or contacts) is not used. A jumper wire is used instead. In such cases, the flame monitor will never show this switch (or contacts) as the cause of a shutdown.

In case of an abnormal shutdown, you need to identify which device initiated the shutdown (next topic).

IDENTIFYING CAUSE OF SHUTDOWN
All devices in the limit circuit sense different operating conditions. If one device opens in response to an abnormal operating condition it will open the limit circuit and shut down the burner. When the device that initiated the shutdown opens it may cause another safety device to open also.

But you need to identify the device that first responded to the condition that caused the shutdown. That condition is most likely the root of the problem.

If you are watching the clear status lights on the control panel when the shutdown occurs, the first one that goes out indicates the cause. Unfortunately, if you are not watching the lights when the shutdown ocurs you may not be able to tell which one went out first, because you may find several lights out soon after the shutdown. Even so, you should check the lights for the devices listed for the 3-P circuit in same sequence they are listed. The first one you find out likely indicates the cause.

Once you suspect a particular device in the limit circuit, you can use a voltmeter to confirm that it is actually open. A device is open when voltage is present on the terminal connection on one side of the device, but not present on the terminal connection on its other side.

Terminals for these devices are inside the heater control panel. They are marked for easy identification. Use the wiring diagram furnished with your heater to identify the appropriate terminals for the devices.

To check for voltage, connect one voltmeter lead to one terminal on the device and connect the other lead to neutral (a blue terminal). To check for voltage on its other terminal, connect one lead to the other terminal and connect the other lead to neutral (a blue terminal). Do not connect voltmeter leads across both terminals on a device because your voltmeter will not show presence of voltage when its contacts are closed.

Note that some of the limit devices automatically reset themselves after an abnormal condition has cleared.