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Demand Controlled Ventilation (DCV)
Demand Controlled Ventilation (DCV) function requires a space
air CO
sensor be connected to the W7220 controller. The CO
2
sensor provides a 2 to 10 vdc signal proportional to the space CO
level. This sensor is available as a factory-installed option (located
in the unit's return air plenum) or as a field-installed accessory.
See Fig. 1 for model number nomenclature; check Position #9 for
codes E, F, G or H indicating a factory-installed CO
W7220 automatically recognizes the connection of this sensor and
self-enables the DCV function after the Configuration period.
DCV with Single-Speed Fan System
During DCV, the outside air damper modulates between two user
configurations depending upon the signal level of the space or re-
turn air CO
sensor representing the space occupancy level. The
2
lower of these two positions is referred to as the Minimum IAQ
Damper Position (designated VENTMIN) while the higher is re-
ferred to as Economizer Minimum Position (designated MINI-
MUM POSITION or VENTMAX). The VENTMIN position
should be set to an economizer position that brings in enough fresh
air to remove contaminants and CO
than people; this airflow rate is designated Va. The VENTMAX
should be set to an economizer position that brings in enough fresh
air to remove contaminants and CO
cluding people at the design condition for maximum space occu-
pancy; this airflow rate is designated Vbz. See Fig. 73.
VENTMAX
VENTILATION FOR PEOPLE
VENTMIN
VENTILATION FOR SOURCES
100
INSIDE OUTSIDE CO
Fig. 73 — DCV Single-Speed System Setpoints
DCV with Two-Speed Fan System
Ventilation codes require that the same ventilation rates (Vbz and
Va, expressed as CFM) be provided regardless of supply fan
speed. When the supply fan speed is reduced, the internal static
pressure in the unit's return plenum also decreases. If the same
outside air damper position is retained, the airflow rate through the
OA damper decreases below the Va and Vbz levels. To restore
ventilation rates to design levels, the damper positions VENTMIN
and VENTMAX must be automatically adjusted when the fan
speed changes. The W7220 provides this function when it is con-
figured for 2-speed fan operation through a second set of damper
position setpoints.
During operation at High fan speed, the damper setpoint limits are
designated VENTMIN H and VENTMAX H. Damper operation
is same as described in the DCV with Single-Speed Fan System
section, above.
sensor. The
2
generated by sources other
2
generated by all sources in-
2
700
DIFFERENTIAL
2
During operation at Low fan speed, the damper setpoint limits
change to VENTMIN L and VENTMAX L. These settings are
higher than the comparable High speed settings and cause the out-
2
side air damper to open more to allow the same Va and Vbz air-
2
flow rates to be admitted to the space.
Adjust the DCV setpoints VENTMAX H and VENTMAX L with
supply fan speed in High speed and Low speed respectively to
provide the design load ventilation airflow rate Vbz by measuring
outside air temperature, return air temperature and supply air tem-
perature. Make damper position adjustments with at least 10°F
temperature difference between the outdoor and return-air tem-
peratures. See Fig. 74.
To determine the damper setpoint position, perform the following
procedure for each condition setpoint, with mechanical cooling
OFF:
Calculate the appropriate supply air temperature using the follow-
ing formula:
TS = (TO x Vbz/CFM) + TR x (CFM – Vbz)/CFM
TS = Supply Air Temperature
TO = Outdoor Air Temperature
Vbz = Design Maximum Ventilation CFM
CFM= Unit Supply Airflow Rate
TR = Return Air Temperature
As an example:
Unit Airflow Rate at High Speed is 4000 CFM
Ventilation CFM at design occupancy Vbz is 1200 CFM
TO = 60°F
TR = 75°F
Required TS = 60 x (1200/4000) + 75 x (4000 – 1200)/4000
=60 x 0.30 + 75 x 0.70 = 18.0 + 52.5
= 70.5
At the W7220 keypad, enter the parameter SETUP –> VENT-
MAX H and adjust the setpoint value until the observed Supply
Air Temperature (MA TEMP) reaches 70.5. Press the
button to save this setpoint to controller memory.
When determining VENTMIN setpoints, substitute the value for
Va in place of Vbz in the formula.
DCV Setpoint
The SETPOINTS parameter DCV SET defines the space CO
level above which the DCV mode begins to open the outside air
damper beyond its VENTMIN ventilation lower limit. This
setpoint should be a minimum of 100 ppm greater than the out-
door ambient CO
level to ensure the outside air will be capable of
2
diluting the space CO
400 ppm; adjust the setpoint DCV SET to 500 ppm if outdoor
CO
level is not known. The factory default value for DCV SET is
2
1100 ppm.
Economizer Occupancy Control
The 24-v signal that terminates at the W7220's OCC input to place
the economizer control in Occupied mode when the supply fan
starts is routed through the rooftop unit's Integrated Staging
Control (ISC) Board at the board's OCCUPANCY jumper. To im-
plement an occupancy control for the economizer operation, con-
nect a contact set at ISC OCCUPANCY quick-connect terminals
and cut jumper JMP1. To allow automatic occupancy mode, close
the control contacts. To place the economizer in Unoccupied
mode, open the control contacts. See Fig. 75.
55
level. A typical value for outdoor CO
2
(Enter)
2
is
2
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