Burnham CHG225 - Manual (Page 44)

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xII operation
1) The CHG boiler uses a microprocessor based control, known as a “MCBA”, to manage all boiler functions
including flame supervision and modulation. Two set point or “target” boiler supply temperatures are stored
in the MCBA’s memory; one for space heating and one for domestic water production. If an outdoor tempera-
ture sensor is connected to the boiler, the space heating supply set point will automatically adjust downwards
as the outdoor temperature increases. For more information on this feature see the discussion on boiler water
reset below.
The MCBA modulates the boiler input by varying the fan speed. As the fan speed increases, so does the
amount of gas drawn into the blower. As a result, a fairly constant air-fuel ratio is maintained across all inputs.
The MCBA determines the input needed by looking at both current and recent differences between the supply
temperature and the set point temperature. As the supply temperature approaches the set point temperature, the
fan will slow down and the input drop. Depending on the model boiler, the minimum input is between 1/4 and
1/5 of maximum input.
The MCBA also monitors boiler return and flue temperatures. In addition, all other safety controls, including
the low water cut-off and safety limit, are connected into the MCBA. The MCBA uses input from all of these
controls to either shut down the boiler when an unsafe condition exists or, in some cases, to correct the
problem.
2) The display panel has three primary modes of operation. These are:
• Standby Mode – Displays boiler’s current status. This is the default operating mode.
• Parameter Mode – Used to change control settings
• Information Mode – Displays boiler operating temperatures
Under normal conditions, the boiler is in standby mode and the display looks like that shown in Figure 11.1.
The three digits to the right of the decimal point are the boiler’s supply temperature. The digit to the left of
the decimal point is the boiler’s status code. A list of status codes, and their meanings, is shown in Table 11.3.
Figure 11.2 is a map of the menu structure for the control panel. Push the mode key to move from one mode to
the next. As you change modes, the mode you are entering is shown on the display:
a) “PArA” for Parameter Mode
b) “Info” for Information Mode
c) “Stby” for Standby Mode. Upon entering standby mode, “Stby” will briefly appear on the display
and then the display will show the boiler’s status along with the supply temperature (Figure 11.1).
The control will return to standby mode from any other mode if no key is pressed for 20 minutes.
Figure 11.1: normal Display in Standby Mode