The S500 clearly states that a crawlspace should be dried under negative pressure. Oddly I get 2-3 a month arguing that we need to add fans.... When will the madness end?
Have you tried using a 1-hp Vortex Axial fan to create your negative draw. Then you can still use air movement to help distribute the hot dry air and the Vortex exhausts it out for you. Keep an eye on pilot lights so you don't fill the house with gas.