Application

Sequence of Operation
Control Loops
9
Siemens Industry, Inc. Application Note, App 2032 140-1022
2015-08-07
Control Loops
Flow Loop – maintains the point FLOW STPT by modulating the supply air damper
point, DMPR COMD. The flow loop maintains the airflow at either OCC FLOW or
UNOCC FLOW depending on the value of OCC.UNOCC.
To enhance stable flow control, an advanced algorithm is used to calculate a
controllable setpoint as the value approaches zero cfm (lps).
FLOW is the input value for the flow loop. It is calculated as a percentage based on
where AIR VOLUME is between 0 cfm (LPS) and OCC FLOW. In the following text,
this percentage is referred to as % flow.
 If AIR VOLUME equals 0 cfm (LPS), then FLOW is 0% flow.
 If AIR VOLUME equals OCC FLOW, then FLOW is 100% flow.
The FLOW STPT percentage that corresponds to UNOCC FLOW is calculated as:
(UNOCC FLOW ÷ OCC FLOW) × 100% flow.
Example
If UNOCC FLOW = 250 cfm, and if OCC FLOW = 1000 cfm
then, in unoccupied mode the FLOW STPT
= (250 cfm ÷ 1000 cfm) × 100% flow
= 0.25 × 100% flow
= 25% flow
Since 250 cfm is 25% of 1000 cfm, the flow setpoint in unoccupied mode is 25%.
UNOCC FLOW can be set less than or equal to, but not greater than OCC FLOW.
Temperature Loop – The temperature loop will modulate HTG LOOPOUT and control
the heating elements in order to maintain the room temperature in both heating and
cooling modes.
Electric Reheat
CAUTION
Verify that the equipment is supplied with safeties by others, to ensure there is airflow
across the heating coils when they are to be energized.
The heating loop controls stages of electric reheat to warm up the room. The electric
reheat is time modulated using a duty cycle as shown in the following example.
When the controller is in cooling mode, the electric heat is OFF at all times.
Example
If the duty cycle is 10 minutes (STAGE TIME = 10 minutes), and the heating loop is
calling for 60% of heating (HTG LOOPOUT = 60%) for every 10-minute period, the
stages of electric auxiliary heat cycle as follows:
Stage 1: minutes
Stage 2: minutes
Stage 3: minutes
ON
OFF
ON
OFF
ON
OFF
With 1 stage of electronic heat 6 4 -- -- -- --
With 2 stages of electric heat 10 0 2 8 -- --
With 3 stages of electric heat 10 0 8 2 0 10