Application

Sequence of Operation
Heating Safety
27
Siemens Industry, Inc.
Application Note, App 6750
140-1328
2015-07-07
The interaction between the table statement and the general exhaust air velocity
feedback loop can be summarized as follows:
 When the general exhaust air volume is near setpoint, the table statement controls
it.
 When the general exhaust air volume is not near its setpoint (either too high or too
low), both the table statement and the general exhaust air velocity feedback loop
work together to bring the general exhaust air velocity close to its setpoint as
quickly as possible.
 When the general exhaust air velocity is so low that it cannot be accurately read by
the general exhaust AVS, the table statement controls it.
Heating Safety
NOTE:
As a safety feature, these applications include MODHTG FLO to ensure that
adequate airflow is present before heating coils are energized. When the supply
airflow (in fpm as derived from the supply air velocity sensor) is greater than
MODHTG FLO, then the internal point “ok_to_mod” is set to Yes and the modulating
heating device is allowed to modulate.
The default value is 300, which means that the airflow over the heating coil must be at
least 300 fpm.
Since CFM = FPM x Duct Area x Flow Coefficient, the default value of 300 fpm
equates to the following cfms:
In a 12-inch diameter duct and a typical flow coefficient of .7, 300 fpm equates to 158
cfm.
12 inch diameter = .75 sq ft
.75 sq ft * 300 fpm * .7 = 158 cfm
In an 8-inch diameter duct and a typical flow coefficient of .7, 300 fpm equates to 74
cfm.
8 inch diameter = .35 sq ft
.35 sq ft * 300 fpm * .7 = 74 cfm
If the application uses hot water heat rather than electric heat, then MODHTG FLO
may be set lower than the default value of 300. This allows reheating to occur even if
the box is operating below its designated minimum flow setting.
In the open loop mode, the measured airflow is always 0. To re-enable heating, the
MODHTG FLO value must be set to 0. When used with an electric reheat, the 0 should
be set only after a thorough safety review that shows the electric heating mechanism
has sufficient internal safeguards (that is, resettable shutoffs) that would operate if the
system actually did have insufficient flow.
Using fpm flow rather than cfm makes the feature less dependent on duct size.
There is hysteresis (deadband) around the flow threshold. The heating turns off below
a flow of MODHTG FLO, and does not turn back on until the measured flow rises to a
level of 50 fpm more than MODHTG FLO. Between MODHTG FLO and MODHTG FLO
+ 50, the internal point “ok_to_mod” will not change value.