Installation and operating instruction
Gas condensing unit WTC-GB 15-B Compact and WTC-GB 25-B Compact
3 Product description
83309302 1/2018-05 La
16-196
3.3.4 Combustion control (System SCOT®)
The appliance is equipped with an electronic combustion control.
The combustion control is carried out via ionisation electrode. Depending on the
measured ionisation current, the gas volume is regulated to the amount of air avail-
able.
If excess air is reduced, the combustion temperature and therefore the ionisation
current increases. The maximum ionisation current (Io max) is achieved at excess air
of 0 % (λ=1.0).
Calibration procedures regularly determine the maximum ionization current (Io max).
Excess air is calculated from this maximum value. The setpoint of the ionisation cur-
rent (Io set) is set so that the following O
2
content is available across the entire
modulation range.
O
2
content
Natural Gas
approx. 5.0 % (λ=1.29)
Liquid Petroleum Gas
approx. 5.3 % (λ=1.31)
Example
I
o max
I
o soll
=1.0
1.28
3
2
1
1
Air number [λ]
2
Ionisation current
3
Turndown ratio
Calibration
Calibrations are carried out:
by dynamically predefined operating hours,
by dynamically predefined burner starts,
following power outage,
following the occurrence of certain faults (e. g.
F 21
,
W 22
, etc.).
Calibration can be carried out manually via the output measurement or with the
commissioning wizard.
Manual calibration is mandatory when replacing the following components:
ionisation electrode,
burner surface,
WEM-FA-G device electronics,
gas combi valve.
During calibration the CO content briefly (approx. 2 s) increases above 1000 ppm.