Dual Duct Cold Deck Trouble

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I'm having trouble modeling an existing dual duct HVAC air system with eQuest. My existing dual duct system has two supply fans, one for the hot deck and one for the cold deck and they are blow-through type.

The trouble is with the cold deck and the economizer control, and I'm noticing this problem by reviewing the hourly reports of my airside system. My existing air system has all of the ventilation/economizer air located on the cold deck side. My cold deck is programmed to discharge a constant temperature of 55 F. This works fine whenever the outside air temperature is above 50 F. However, when outside air temperature is below 55 F my economizer control goes bezerk. Anytime the outside air is below 40 F, my economizer brings in 100% outside air. So, in essence my cold deck supply temperature is whatever the outside air temperature is. If the outside air temperature is between 40 F and 50 F it doesn't bring in 100% outside air, but it brings in much more outside air than is necessary to keep the cold deck at 55 F.

FYI, the problem isn't one of the following:

1.) My ventilation air requirements aren't causing the problem. If I turn off the economizer, on my energy model, the outside air percentage is fixed and the model runs correctly. Except for the fact that my cooling coil is now cooling when the economizer should be operating.

2.) My economizer is a dual-enthalpy type, but I've tried other types of economizers. They all have the same issue.

3.) My supply fan motor heat isn't causing the economizer to pull in more outside air than is necessary. In other words, my economizer isn't trying to compensate for an obscene large fan motor heat by blending down the mixed air temperature. I've checked my fan motor heat. It is only contributing 1-3 degrees of heat.

Does anyone know what is wrong? Any help is greatly appreciated.


Mark Prince

Mark Prince's picture
Joined: 2011-10-02
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