AAON RK-60 - Manual (Page 13)

13
SEQUENCE OF OPERATIONS
I. GENERAL INFORMATION
A. HEATING
1. Natural Gas
When the thermostat calls for heating, W1 makes R to
the heat relay (HR) all N.O. (Normally open) contacts
close and all N.C. (normally closed) contacts open. The
combustion motor starts and as the pressure decreases in
the flue outlet box the ignition control is energized. The
control sends 24 VAC to the main gas valve and high
voltage to the ignitor. If a burner flame has been detected
after 10 seconds, the spark is extinguished and the flame
continues. If a flame has not been detected after 10
seconds, the gas valve closes, the spark ceases and the
induced draft blower continues to purge the heat ex-
changer. After 45 seconds of purge, the ignition system
will attempt to light the burners again. Should no flame
be detected after 3 tries, the ignition control locks out the
system.
On a fault the gas train is shut down by a main limit
located in the heat exchanger area or by an auxiliary
limit mounted in the supply air fan housing.
2. LP (Propane) Gas
The sequence for LP Gas is the same as above but upon
non-proof of burner the gas train will enter a 100%
lockout condition.
3. Electric
When the thermostat calls for heat 'W1' makes 'R' to the
heat relay 'HR'. All N.O. contacts close, and all N.C.
contacts open. The heat relay makes 'R' to the first stage
of electric heat.
On a fault condition the main limit located in the supply
air or the auxiliary limit located in the supply air fan
housing will remove power from all contactors.
If additional heating is required a second set of elements
can be turned on by 'W2'.
OPTIONAL - When available the electric heat can be
sequenced to provide a constant discharge air tempera-
ture.
4. Steam or Hot Water
This option adds a steam coil down stream of the cooling
coil (if supplied). Connections and controls are provided
by others.
B. COOLING
1. Packaged Units
When the thermostat calls for cooling from the space, 'Y1'
makes 'R' to 'CC1' through the LPS (low pressure switch),
HPS (high pressure switch) and optional GOT (guaran-
tee off timer).
On units 26 through 60 tons 'CC3' is also made. If
additional cooling is required 'CC2' and 'CC4' (on 26-60
ton) are made through their respective pressure switches
and timers.
2. DX Only - Coil Units
When the thermostat calls for cooling from the space, the
condensing unit is energized (refer to manufacturers
instructions for sequence of operation). The evaporator
blower contactor is energized simultaneously with the
condensing section.
3. Chilled Water Coil Units
The blower contactor is energized to provide supply air on
a signal from the space thermostat. All other controls are
by others.
C. OPTIONAL ECONOMIZER
When cooling is called for and the unit has the econo-
mizer option installed, temperature switch ECS (or En-
thalpy) allows the economizer operation when the out-
side air reaches the required setpoint. (Some options use
dry bulb sensing and some options use enthalpy sensing
to determine the outside air (O.A.) condition).
When the economizer is in operation 'Y1' controls the
opening and closing of the dampers, 'Y2' is then able to
control the compressors which 'Y1' normally controls. A
modulating economizer is also available. The operation
is the same as the standard economizer except that the
motor modulates the damper position to maintain a
preset mixed air temperature.