Post on 24-Dec-2015
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HVAC 214 Electricity For Oil Heat
Fan Center Relays
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Fan Center Relay
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Fan Center Relays
• Designed to provide low voltage control of blower motors and auxiliary circuits.
• Designed to provide a good junction point for other circuits.
• Used on heating systems with air conditioning.
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Fan Center Relays
• Contain the following components:
– A low voltage transformer (step down from 120 to 24 volts)
– A pre-wired multifunction switching relay
– A Low voltage terminal board.
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Fan Centers
• Sometimes used when there are two transformers, one on the heating equipment and one on the cooling equipment.
• These transformers must be isolated from each other based on whether the thermostat is calling for cooling or heating.
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Fan Center Wiring Connections
• Connections:– R is output power from the internal
transformer– W is the junction point for W from tstat
and W from equipment– G is the connection from G from the
thermostat– Y is a junction point from for Y from the t-
stat– C is common from internal transformer
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Thermostat calls for heat
• During a call for heat, the normally closed relay contact for the low speed blower stays closed and the blower is controlled by a Fan/Limit switch mounted in the furnace plenum.
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Call for heat
Low speed blower wire goes through N/C contact onto the Fan/Limit
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The thermostat call for cooling
• During a call for cooling, the thermostat closes R to G which energizes the relay coil. When the coil is energized, the N/O contact closes and allows the blower to operate on high speed.
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Call for cooling The high speed blower wire goes through the N/O contact which closes when the coil is energized.
Thermostat closes R to G & R to Y
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Fan Center
• The W, Y, and any additional terminals are just junction points.