10
SEQUENCE OF OPERATION
Heating, Hydronic or Steam
Both hydronic and steam heat require a one or two row
coil generally located downstream of the evaporator coil.
Controls for hydronic heat will involve a 3 way motorized
mixing or blending valve, driven by a signal from a leaving
air temperature thermostat.
Controls for low-pressure steam heat contain a motorized
throttling valve driven by a signal from a leaving air
temperature thermostat
Gas Heat
See the gas furnace instruction manual for wiring and
performance data. Additional information can be found on
the furnace access door such as the rating and serial
label, wiring and lighting instruction label.
1. Blower operation is confirmed by line voltage from the
load side of the blower contactor to the gas furnace
power supply terminals.
2. The furnace section is energized from the control
terminal across the normally closed contacts of the
combustion pressure switch, energizing pilot ignition
time delay relay heater. After delay of approximately
30-50 seconds the time delay relay’s switch closes
energizing the furnace venter motor. As the venter
operates, it causes the combustion pressure switch to
open. The ignition control energizes the pilot valve
solenoid in the combination gas valve. A high voltage
electric spark lights the pilot flame.
3. The flame sensor proves the presence of the pilot
flame generating a DC current of 0.2 microamp (or
greater) to the ignition control. The ignition control’s
internal switch action then de-energizes the spark
transformer and makes a circuit to the high fire
solenoid of the combination gas valve.
4. When there is a call for gas furnace operation the
discharge air temperature causes a change in the
resistance of a discharge air sensor thermistor. The
Electronic solid state control center measures the
sensor’s change in resistance and sends a varying DC
current to the Modulator-Regulator valve to adjust the
gas input as required.
5. The flame sensor proves the presence of the pilot
flame generating a DC current of 0.2 microamp (or
greater) to the ignition control. The ignition control’s
internal switch action then de-energizes the spark
transformer and makes a circuit to the high fire
solenoid of the combination gas valve.
Electric Heat
1. DC units that use the down flow discharge. The
installer
MUST
use an “L” shaped discharge duct
that does not have an outlet directly below the
units discharge opening, per UL1995. To guard
against the very rare occurrence of a piece of an
electric heating element breaking away. A straight
duct or outlet grill directly below the unit discharge
may allow a hot fragment to land in a conditioned
space and start a fire.
2. Blower operation is generally confined by an air
pressure differential switch as part of the heater
assembly.
3. A thermostat or thermostats in the entering outside
air energize the strip heat as required. Unit leaving
air temperature will vary depending on kW, the
number of stages, and the entering air
temperature. Calculate the temperature rise using
the following formula:
°F Rise = kW x 3413
1.08
CFM
Where °F rise is the air temperature increase, kW is the
heater capacity in kilowatts and CFM is the total airflow of
the unit.
Subtract the °F rise from the desired leaving air
temperature and set the thermostat at that value. For
multiple stages, calculate the °F rise for each stage and
set thermostats subtracting each rise from the previous
entering air temperature.
For example:
CFM 2000
kW
22.5, 3 stage, 7.5 kW each
Leaving air desired 65°F
°F Rise
=
7.5 x 3413 = 11.7°F
1.1 x 2000
Set 1
st
stage thermostat to energize at 65-11.7 = 53.3° F
Set 2
nd
stage to energize at 53.3 -11.7 = 41.6°F
Set 3
rd
stage to energize at 41.6-11.7 = 29.9°F
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