<div dir="ltr">I have noticed this too. The constant volume exhaust fan merely means that the fan runs constantly, but may not draw air through the space (read the help file on this, it's more eloquent than I am).<br>
<br>What I have done to remedy this issue using other system types is specify that the system fan operates as a variable volume fan, with the fan curve being linear. This will allow the fan to modulate down to the required exhaust airflow. If you look at the help files for "SUPPLY-KW/FLOW" it shows the fan curves for variable speed fans, cycling fans, discharge, etc. So this is the way that the program models cycling fans already, and you can use it to your advantage with the exhaust airflow piece.<br>
<br>Hope this works!<br><br>--<br>Karen<br><br><br><div class="gmail_quote">On Tue, Sep 23, 2008 at 12:11 PM, Yuvaraj Saravanan <span dir="ltr"><<a href="mailto:Yuvaraj.Saravanan@arup.com">Yuvaraj.Saravanan@arup.com</a>></span> wrote:<br>
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<p><font size="2" face="Courier New">Hi All,</font>
<br><font size="2" face="Courier New">I am trying to model a residential unit served by a Fan coil unit (FCU). It has a separate outside air fan and exhaust fan operating 24/7. The fan coil unit is available to operate for all hours but should operate intermittently depending on the room load.</font></p>
<p><font size="2" face="Courier New">I tried modeling this by selecting a 4-pipe FCU and specified the exhaust and outside air at the thermal zone level. I made the FCU to operate intermittently by making the "Fan on mode" to be intermittent. By doing this, what I observed in the hourly report was:</font></p>
<p><font size="2" face="Courier New">1. FCU unit cycles intermittently as intended.</font>
<br><font size="2" face="Courier New">2. Exhaust fan operates continuously (24/7) (with the specified cfm) as intended.</font>
<br><font size="2" face="Courier New">3. However, the outside air cycles per the FCU. </font>
<br><font size="2" face="Courier New">•If the FCU is off, then OA cfm is zero</font>
<br><font size="2" face="Courier New">•If FCU supply air cfm < the OSA cfm specified at the zone level, then hourly OSA cfm matches the FCU supply air cfm •If FCU supply air cfm > the OSA cfm specified at the zone level, then hourly OSA cfm matches the cfm specified at the zone level.</font></p>
<p><font size="2" face="Courier New">As mentioned before, I want my outside air to operate continuously with constant flow-rate. Can anyone suggest me a way to model this system?</font></p>
<p><font size="2" face="Courier New">Thanks in advance for your help!</font>
<br><font size="2" face="Courier New">Regards,</font>
<br><font size="2" face="Courier New">Yuvaraj</font>
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<p><font size="2" face="Arial">_</font><font size="2" color="#000000" face="Arial">_____________________________________________</font>
<br><font size="2" color="#000000" face="Arial">Yuvaraj Saravanan, PE</font>
<br><font size="2" color="#000000" face="Arial">Engineer</font>
</p>
<p><font size="2" color="#000000" face="Arial">Arup </font>
<br><font size="2" color="#000000" face="Arial">12777 West Jefferson Boulevard Suite 200</font>
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<br><font size="1" color="#000000" face="Arial"><a href="mailto:yuvaraj.saravanan@arup.com" target="_blank">yuvaraj.saravanan@arup.com</a></font><br>
<a><u></u><u><font size="2" color="#0000ff" face="Arial">www.arup.com</font></u></a>
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