Checking The High Limit Safety Cut-Out (Stb); Triggering The High Limit Safety Cut-Out; Terminating Or Cancelling The Test - Bosch CFB 840 Servicing Instructions

6.2

Checking the high limit safety cut-out (STB)

CAUTION: System damage due to incorrect installation
of the STB at the boiler!
The sensor must be pushed into the sensor well as far as
possible to ensure a reliable and fast shutdown in case of
excesstemperature.
B Ensure an optimum heat transfer from STB sensor to
boiler.
B The sensor must be secured in the sensor well by
means of the sensor locks (standard delivery).
B Ensure that capillaries are not damaged or kinked.
B Verify the function of the STB before commissioning.
B Regular checks must be performed in accordance
with the boiler manufacturer's specifications.
The boiler temperature must be monitored continuously at the
programmer or at the boiler display of the ZM435 or at suitable
measuring points while performing the STB check.
The burner must shut down automatically when the set cut-out
temperature (chapter 3) of the STB + 2K (example 110 °C + 2K =
112 °C) is reached. If the burner does not shut down at this point,
manually interrupt the checking procedure at once. To shut down,
release the lever of button and switch the emergency mode switch of the
burner to AUT.
After interrupting the checking procedure, check the correct routing of
the capillaries and the temperature sensor head as well as the assembly
and wiring of the boiler water temperature sensor. When in doubt,
assume the STB to be broken. In this case the broken high limit safety
cut-out must be replaced.
Fig. 13 Checking the STB using the burner emergency operation switch
[1] Thermostat selector button
[2] Burner emergency operation switch
CFB 840

6.2.1 Triggering the high limit safety cut-out

B Set burner emergency switch [2] Manual.
The burner starts.
B Pull off the thermostat selector dial [1].
B Depending on the type of controller, push the lever or button
( fig. 14) back with a screwdriver or similar tool and hold until
the high limit safety cut-out has triggered.
B Continuously monitor the boiler temperature, and interrupt the test,
if required.

6.2.2 Terminating or cancelling the test

B Set burner emergency switch (
B Depending on controller type, release the lever or button (
page 14).
B Push on the button of the temperature controller (
B Set controllers, switches and thermostats to the equipment-specific
conditions.
B Set the temperature controller to AUT.
1
DANGER: Risk to life through boiler excesstemperature!
Never perform the STB test without supervision.
B Immediately interrupt the test if the set STB
temperature is exceeded.
B Release the lever or button (depending on controller
type) of the temperature controller.
B Set burner emergency switch to AUT.
fig. 13, [2]) to AUT.
2
Commissioning | 13
fig. 14,
fig. 13, [1]).
6 720 803 704-10.1T
6 720 802 836 (2012/05)