I recently visited a house. The host pointed out something interesting.
The furnace AC combo is in an indoor closet and is vertical upflow and sits on top of a filter base that sits on top of the pier and beam construction that’s had part of crawl space turned into a (return air) plenum by boxing in some of the space immediately under this closet. In other words, the furnace base is elevated compared to the house’s finished flooring. The plenum height is about 10-12 inches and the length and width are about 30x30. The grates covering the plenum openings are in the living room and interior hallway. The fuel (natural) gas line (CSST) is in the house’s crawl space. In one corner of the plenum, the CSST penetrates the floor of the plenum and continues through the plenum space, exiting through the plywood floor of the closet and the CSST terminates at a black iron tee that has a connection to more black iron that goes up into the attic and the other is connecting to a gas whip that then goes into the furnace.
Is this (CSST) fuel gas line allowed in the plenum?
If it’s not allowed in the plenum, as is, can the issue be addressed by: - sleeving (with schedule 40 PVC pipe) the fuel gas line for the full length i(t’s in the plenum, sealing the top and bottom of the sleeve, or, - reducing the plenum size by completely boxing in the corner through which the fuel gas line transits, with duct board or plywood or framing lumber, sealing all new joints with caulking?
For what it’s worth, the CSST is much newer than the house itself.