|MarketManager » Modeling HVAC Systems|
In general, the answer is: "Because less heating is required to counteract excessive cooling."
I will describe in more detail:
First, because you have a chilled water reset, the cold deck temperature (CDT) may rise above setpoint. As a result, when the cold and hot air are mixed in the mixing boxes, less hot air is required to attain a proper supply air temperature for the zone, thus less hot air passes over the heating coil, which therefore uses less energy.
The second part of the answer has to do with the economizers. In MarketManager, the economizers modulate so that the mixed air temperature (MAT) equals the supply air temperature (SAT) (or for a dual duct system, the CDT).
So, because you have a chilled water reset, when the CDT rises due to reduced cooling coil capacity, the economizer will reduce OA% in order to arrive at a MAT which equals the CDT. As a result, with the higher MAT, the hot deck will require less heating.
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