Central AC - How does 2 speed air handler work?

[goshenHeat]

I will be replacing my 20 year old split central AC system with a new outdoor condenser & indoor air handler. I notice that the air handlers I am looking have two speeds. How does the AC control system determine the air handler speed setting? How does the control system determine when to switch the air handler to the higher speed?


Thank you.

[HayZee518]

motor speeds

the thermostat controls the fan speed according to demand.

[dfitz3390]

yes the thermostat controls speed but speed is probably different in heat v. cool if unit controls heating side . sometimes duct length is a factor too