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Operation - Electrical
Electrical Sequence
1. Supply Fan Contact (S2) Closed
• Power passes to damper which opens
• When damper is fully opened, damper relay
(D1) is energized an optional N.O. damper limit
switch (DL1) closes
• Power passes through a N.C. fault contact on
soft starter (PST(FAULT))
• Power passes to and energizes supply fan relay
(RI)
• Supply fan starts
2. Heat Contact (S4) Closed
• Power passes to N.O. fan relay which is
energized and closed
• Power passes to inlet air sensor contact (TS)
which is energized and closed if the inlet air
temperature is below the set point
• Power passes to and energizes the heat relay
(RH)
• N.O. heat relay contact (RH) closes
• Power passes to and energizes terminal 5 of the
Flame Safeguard (FSG).
• Power passes to N.C. high limit control contact
(HLC) which is closed if temperature has
remained below the high limit set point
• Power passes to terminal 6 of the Flame
Safeguard (FSG)
• Flame Safeguard (FSG) begins direct gas burner
sequence (see Direct Gas Burner sequence)
® ® ® ® ®
Direct Gas Burner Sequence
1. Supply Fan Contact Closed
• Power passes to the supply fan and heat switch
2. Heat Contact Closed
• Power passes to the heat relay then to the
Flame Safeguard
3. Flame Safeguard (FSG) Sequence
• Checks for proper airflow
• Verifies no flame present at burner
• Initiates 10 second prepurge
• Sends power to open pilot gas valve (V1) and
energizes the spark generator (SG) (clicking of
the spark generator may be heard)
• Tries for up to 10 seconds to light pilot and
confirm flame
• Powers the main gas valves open
• Shuts down spark generator
• Continuously monitors the flame and airflow
• Performs self-diagnostic check every five
seconds
13
Model VSU Make-Up Air
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