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Old 07-27-2013, 08:54 PM   #1
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Central AC - How does 2 speed air handler work?


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.

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Old 07-27-2013, 09:11 PM   #2
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Central AC - How does 2 speed air handler work?


Ask your installer. He sold you on it.

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Old 07-27-2013, 09:22 PM   #3
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Central AC - How does 2 speed air handler work?


Ramping profiles determine when an ECM speeds up or slows down. They are controlled by one or several sets of dip switches on the printed circuit board, and the literature from the unit will tell you what modes do what...
But I wouldn't mess with them unless you know what you're doing. Those settings are hard for a guy w/o any time working with them to wrap his head around.
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Old 07-27-2013, 09:30 PM   #4
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Central AC - How does 2 speed air handler work?


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Originally Posted by goshenplumber View Post
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.
2 speed air handler, or variable speed air handler?
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Old 08-02-2013, 12:05 AM   #5
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Central AC - How does 2 speed air handler work?


Word of caution, 2 speeds generally mean half of what your duct is sized for in first stage. Saves money on utility but only "comforts" the interior of your house as there is generally not enough air flow to fill up your duct in first stage. 2 stage equipment is better for smaller stacked houses or houses with 1 or 2 occupants. Depending on many factors, duct size, length of runs, and horizontal length of your house, 2 stage isn't always the best. Make sure to discuss all these with your contractor.
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Old 08-02-2013, 05:35 AM   #6
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Central AC - How does 2 speed air handler work?


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Word of caution, 2 speeds generally mean half of what your duct is sized for in first stage. Saves money on utility but only "comforts" the interior of your house as there is generally not enough air flow to fill up your duct in first stage. 2 stage equipment is better for smaller stacked houses or houses with 1 or 2 occupants. Depending on many factors, duct size, length of runs, and horizontal length of your house,

That would be caused by undersized duct work. Duct work that was probably sized to a .1" friction rate.
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Old 08-05-2013, 12:24 PM   #7
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Central AC - How does 2 speed air handler work?


When I think of the term 2 speed air handler I think non variable speed single stage cooling unit. If that is the case the control board or relay in the inside unit would energize one speed for cooling (typically high) and another speed for heating and possibly a third speed for constant fan.


In a system with a variable speed indoor unit and a 2 stage cooling outside unit typically your thermostat would energize a lower capacity at the outside unit and lower fan speed in the indoor unit during the 1st call for A/C or heat for a heat pump. The second stage would energize by the thermostat if the first did not satisfy the demand. If a heat pump and you live in an area where supplemental heat is needed next would come a third stage. These indoor units are controlled by variable speed motors. These motor will ramp up or down to deliver the designed CFM (if possible) for each stage.

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