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GAS HEAT SEQUENCE OF OPERATION
First Stage Heat:
1- The thermostat initiates W1 heating demand.
2- 24VAC is routed from TB1 to ignition control A3
through P2. A3 proves N.C. primary limit S10 and N.C.
rollout switch S47.
3- Combustion air inducer blower B6 is energized.
4- After the combustion air inducer B6 has reached full
speed, the combustion air proving switch S18 con
tacts close.
5- After a 30 second delay, A3 energizes the ignitor and
LO terminal (low fire) of gas valve GV1.
Second Stage Heat:
6- With first stage heat operating, an additional heating
demand from the thermostat initiates W2.
7- A second stage heating demand is received by TB1.
The second stage heat signal passes from TB1 to A3.
8- A3 energizes HI terminal (high fire) of gas valve GV1.
End of Second Stage Heat:
9- Heating demand is satisfied. Terminal W2
(high fire) is de-energized.
10- Terminal HI of GV1 is de-energized by A3
control module.
End of First Stage Heat:
11- Heating demand is satisfied. Terminal W1
(low fire) is de-energized.
12- Ignition A3 is de-energized in turn de-ener
gizing terminal LO of GV1.
Optional Low Ambient Kit:
(ETL/CSA -50° C Low Ambient Kit)
13- Line voltage is routed through the N.C. low
ambient kit thermostats S60 and S61.
K125 relay is energized closing N.O.
K125-1 contacts and energizing the low
ambient kit heater HR6.
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