Application

Sequence of Operation

Operating Without a Supply or Exhaust

Siemens Industry, Inc.

Application Note, Application 6759

140-1335

2015-07-07

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:

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.