[Equest-users] Winter compressor usage with Economizer

[Keith Swartz]

Justin,

When I have experienced winter cooling in my models it turned out to be from a problem with the economizer low limit. Although you can enter only the high limit in the wizard (Screen 6 of 7 for an HVAC system) the program sometimes enters a low limit of 45F, which will cause a chiller or DX cooling system to run whenever cooling is needed when the outside temperature is below 45F.

To check the economizer low limit, and presuming you are working in a wizard, (If you are still working in a wizard, you can still look at things like you are working in the detailed interface, but you can't make any changes here unless you click on "Mode" in the menu bar and select "Detail Data Edit.") click on "Component Tree" near the lower left corner. Then click the "Air-Side HVAC" module button along the top. In the component list on the left side double-click on one of the systems. Go to the "Outdoor Air" tab, and in the "Outside Air and Economizer" subtab there is a field in the lower right called "Economizer Low Limit." I often see a red "45.0" there. It's interesting that it's red because red text indicates an entry that was entered by the user instead of a default value, which is green. (There are other colors with different meanings too.) But in the wizard, there is no place to enter an economizer low limit!

I add the economizer low limit to my list of things to change when I get to the Detailed Interface. You can enter a number for the low limit, or you can select no lower limit by right-clicking on the field and selecting "Restore Default." See the help screen for more details.

________________________________
Keith Swartz, P.E., LEED AP

Energy Engineer / Senior Project Manager
Energy Center of Wisconsin
455 Science Drive, Suite 200, Madison, WI 53711
Phone: 608.238.8276 x123
Fax: 608.238.0523

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From: Justin Chin [mailto:jchin.bu at gmail.com]
Sent: Thursday, June 16, 2011 2:57 PM
To: equest-users at lists.onebuilding.org
Subject: [Equest-users] Winter compressor usage with Economizer

Hello all,

  I am currently having a problem where the compressors on my DX units are kicking on during the Winter months even though I have an OA discharge economizer on.  The economizer has a high limit of 58F and a maximum of 1 OA fraction.  The discharge temperature is 55F, so I am not sure why the compressors would turn on in the winter.  Any help is appreciated.

Regards,
Justin
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