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JOHNSON CONTROLS
135
SECTION 5 – SEQUENCE OF OPERATION
FORM 145.05-NOM7
ISSUE DATE: 10/31/2019
5
Hot Water/Steam Heat
Programmed Data
The following parameters are programmed into the
Unit Controller through the User Interface:
• PROGRAM key, HEATING subsection:
• “HEATING SYSTEM” must be set to USER
ENABLE
• “HW VALVE ACTION” must be set to either
DIRECT or REVERSE
• OPTIONS key, HEATING subsection: “HEAT-
ING SYSTEM TYPE” must be set to HOT WA-
TER/STEAM
• SETPOINTS key, HEATING subsection:
• For CV units: “1ST STAGE HEATING
SETPOINT” (default is 85.0°F) and “2ND
STAGE HEATING SETPOINT” (default
is 100.0°F) must be programmed for CON
-
STANT VOLUME
• For VAV and FlexSys
units: “HEATING
SAT” (default is 100.0°F) must be pro
-
grammed for VARIABLE AIR VOLUME or
FLEXSYS
Active SP
See
to determine what the
“SUPPLY AIR TEMP ACTIVE SP” value is for Con-
stant Volume units.
On VAV and FlexSys units, the “SUPPLY AIR TEMP
ACTIVE SP”
is the “HEATING SAT.”
The STEAM or
HOT WATER heat becomes active as soon as the unit
transitions into an OCCUPIED HEATING or UNOC-
CUPIED HEATING mode.
Sequence of Operation
The Unit Controller uses the SUPPLY AIR TEMP as
described below to determine when to increase or de-
crease the heating capacity.
• When the SUPPLY AIR TEMP is less than the
“SUPPLY AIR TEMP ACTIVE SP,” the Unit
Controller increases the amount of heat
• When the SUPPLY AIR TEMP is greater than
the “SUPPLY AIR TEMP ACTIVE SP,” the Unit
Controller decreases the amount of heat
• When “UNDER FLR TEMP OVRD” is active,
the valve is controlled to “UNDERFLOOR TEMP
CONTROL SP”
The Unit Controller sends a 0 to 10 VDC signal to the
hot water or steam valve as described below:
•
If the unit is configured for
DIRECT,
an increase
in heating demand results in an increase in output
voltage to the valve
•
If the unit is configured for
REVERSE, an in-
crease in heating demand results in a decrease in
output voltage to the valve
Freezestat Operation
If the control is not in an active HEATING mode but
the supply fan air proving switch is closed, the Unit
Controller controls the modulating valve to prevent the
Supply Air Temperature from dropping below 38.0°F.
If the supply fan air proving switch is open (unit OFF)
and the Outdoor Air Temperature is less than 40.0°F, a
voltage signal is sent to the modulating valve to open
to 100%, or 0 % if the valve Action is programmed for
“REVERSE.”
Freeze Fault
The Unit Controller monitors the status of the Freez-
estat.
• Freezestat Open = Normal
•
Freezestat Closed = Fault (Closes at 35.0°F)
• Freezestat closed for 10 seconds, the Unit Con-
troller fully opens HW/Steam valve:
• 100% if DIRECT
• 0% if REVERSE
The Unit Controller starts a 5-minute Freeze Trip tim-
er. If the Freezestat opens during this period, the unit
resumes normal operation.
If the Freezestat remains closed after the 5-minute
Freeze Trip timer expires, the Unit Controller shuts
down the unit and displays a “LOCKOUT–HOT WA-
TER FREEZE” fault.
Units with a WATERSIDE ECONOMIZER have a
freezestat installed from the factory that acts the same
way as mentioned above. The unit might not have a hot
water or steam coil to actuate.