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  1. #1
    Join Date
    Nov 2018
    Posts
    1
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    2 stage variable speed but truly variable speed?

    Hello,
    I'm getting a new furnace and trying to understand what exactly variable speed motor for an American standard gold 80v mean. The spec sheet lists a set of dip switch settings at stage 1 and 2 of heating to set the cfm for the blower.

    I thought a variable speed blower meant that the furnace and thermostat would decide at what speed the blower should run. So, I'm confused why there's a set of dip switches to set the speed.

    Can someone please explain? How does the variable speed come to play?

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    39,269
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    I believe the Gold 80v have variable speed motors meaning they sense duct conditions like registers shut off or filter getting dirty and adjust to try to maintain the airflow desired by the DIP switch settings. So if the DIP switch asks for 1000 CFM, the motor will try to run about the right speed to give that airflow. They also can use the Comfort R dehumidify profile which in the summer slowly ramps the blower up on a call for cooling to dehumidify better. Or use dehumidify on demand with certain thermostats so slow the blower in cooling if humidity is above setpoint. Most furnaces give you 1/2 of the cooling speed for constant fan or let you adjust it the airflow you want on constant fan.

    The other high efficiency technology is constant torque. That's the old AUD1-H single stage or the new S8X2 2 stage coming out. The CT motors work like the PSC motors everyone is used to where you have several, usually 5, speed taps to choose from. A GOOD installer would measure duct static pressure, look at the blower performance chart, and set the desired taps for heat & cool. As duct conditions change, like shutting registers or filter getting dirty, airflow backs off. Most CTs can't do any extra features like the dehumidify. A few brands set up the CT for a slow constant fan then pump up for heating & cooling.

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