Application

Sequence of Operation
Operating Without a Supply or Exhaust
28
Siemens Industry, Inc.
Application Note, App 2928
140-1302
2015-07-07
8
340
3.2
9
390
3.5
10
521
3.8
11
531
4.1
12
598
4.9
13
691
5.8
14
798
7.0
15
0
0
16
1200
10
Example 5 - Multi-point Table
The point values above are for the exhaust table. Open loop supply operation is
similar, but the points used are 1 through 16; not 30 through 46.
Since the actual airflow being measured is 0, the heating safety requirement for
minimum airflow is not met and heating will not occur. To re-enable heating, the FLO
value must be set to 0, but only after a thorough safety review that the electric heating
mechanism has sufficient internal safeguards (that is, resettable shutoffs) that would
operate in the event that the system at some time actually did have insufficient flow.
NOTE:
Since
PID Only Mode uses PID without a Venturi table and Open Loop Mode uses
Venturi table without PID, these two modes cannot be used at the same time.
Operating Without a Supply or Exhaust
It is possible to run this application without the supply or exhaust if the corresponding
flow coefficient is set to zero. When the flow coefficient is zero and the offboard air
module is not connected the air velocity sensor will not display a FAIL status and the
flow loop will be allowed to run with a flow value of zero. See the
Application Notes
section for more information.
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: