Chantilly is located in Fairfax County.
See below
page=listings&idxID=b004&pt=1&ciID=91&city%5B0%5D=8183&ccz=city&per=25&srt=newest
Test with widget
<script charset="UTF-8" type="text/javascript" id="idxwidgetsrc-23067" src="//homes.northernva.com/idx/quicksearchjs.php?widgetid=23067"></script>
page=listings&idxID=b004&pt=1&ciID=91&city%5B0%5D=8183&ccz=city&per=25&srt=newest