MUA-SVX004B-EN
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Air Distribution
Two basic air control systems are used to deliver conditioned air to the occupied space:
“constant” fan operation and “intermittent” fan operation.
Intermittent Fan Operation
This air control system is available on duct furnaces when the fan time delay relay is ordered
(optional). The thermostat turns the gas on whenever the temperature drops below the
thermostat setting. The fan time delay relay allows the unit to fire for a period of 60 seconds
before fan operation. It also allows the fan to operate for 120 seconds after burner shutdown.
Constant Fan Operation
Duct Furnaces: This air control system is standard on duct furnaces without a fan time delay
relay. The blower operates constantly, circulating air through the unit, through the discharge,
continuously into the occupied zone, and back to the return. The thermostat turns the gas valve
on or off, raising the temperature of the air to maintain comfort conditions in the occupied area.