Operating Instructions

Application 2522: VAV with Electric Reheat or Baseboard Radiation
Siemens Building Technologies, Inc. 3-31
Electric Reheat
CAUTION:
Verify that the equipment is supplied with safeties by others to ensure that there is
airflow across the heating coils when they are to be energized.
CAUTION:
Do not set HTG FLOW MIN (Point 33) to 0 cfm (0 lps). A minimum airflow should
be provided across the heating coils, for ventilation and for dispersing heat.
The heating loop controls up to two 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
Assume the duty cycle (STAGE TIME, Point 89) is 10 minutes, REHEAT START (Point 22) is
0%, and REHEAT END (Point 23) is 100%. The following table shows the intervals during
which heat is on and off for various levels of demand from the heating loopout (HTG
LOOPOUT, Point 80).
One stage of heat: Two stages of heat
minutes Stage 1: minutes Stage 2: minutes
HTG LOOPOUT ON OFF ON OFF ON OFF
40% 4 6 8 2 0 10
60% 6 4 10 0 2 8
100% 10 0 10 0 10 0
Baseboard Radiation
The baseboard radiation can be either a two-position valve or electrical resistance heating.
If the controller is in cooling mode, the heating valve is closed.
When in heating mode, the controller operates the heating valve to maintain the heating
setpoint as if it were a single stage of reheat.
Sequencing Logic (Optional)
The settings of FLOW START (Point 16), FLOW END (Point 17), REHEAT START (Point 22),
and REHEAT END (Point 23) determine how the damper and reheat modulation are
sequenced while in heating mode. These points represent the values of HTG LOOPOUT
(Point 80) at which modulation of the damper and reheat begin and end.