Factory runs a heat duct from furnace down thru sub-floor and into area of tanks so furnace can blow warm air into that area
Then insulation is installed..........usually thinsulate type laid on the coroplast sheets that are used to enclose the underbelly.
Coroplast is like plastic cardboard. Big box hardware/sign shops etc.
Lots of self-tapping screws
Seal up any openings/seams and call it done.
Heat pads are also used for 'heated underbelly'.......either in addition to furnace duct (best method) or in lieu of the duct
Heat pads can be 120V AC or 12V DC and are t-stat controlled.
