On the supply side, attach a 10" Tee with a damper on the side of the tee open to the crawlspace. Connect a 10" supply duct to the mainfloor. Use insulated flex duct for all of the ducts.
Partially open the supply to the crawlspace enough to keep the crawlspace 50%RH. Open the 6" fresh air enough to introduce 70-80 cfm of fresh air to the dehumidifier. The fan on the dehu can operate 24/7 to provide circulation and fresh air throughout the space.
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