Variable Capacity Versus Fixed Capacity

March/April 2014
A version of this article appears in the March/April 2014 issue of Home Energy Magazine.
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March 01, 2014
An Experiment in the Impact of Duct Location
A new generation of fully variable-capacity HVAC units has come on the market, and they promise to deliver very high cooling efficiency. They are controlled differently than standard single-capacity systems. Instead of cycling on at full capacity and then cycling off when the thermostat is satisfied, they can vary their capacity over a wide range, thus staying on for nearly twice as many hours per day as single-capacity systems. These systems have a greater impact on conductive losses of the duct system, because cold air dwells in the ductwork (commonly located in the attic or other unconditioned space) for longer periods of time.

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