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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.

  • So this house is a pier-and-beam foundation with a crawl space, right? And the CSST in the crawl space is a replacement line from the outside buried gas line? Was the original line black iron running from the buried line inside an exterior wall up into and across the attic and down into the heater closet? Was the buried line also replaced? (I have this exact same furnace design including plenum except I'm on a slab and the black iron line comes down into the heater closet containing the furnace and water heater.) – Jim Stewart Jun 27 at 22:45
  • Yes, pier and beam with crawl space. The CSST was replacement for a corroded/pinhole-leaking black iron pipe (it must have been touching the soil to start the corroding) that was also in the crawl space. The original iron pipe penetrated the plenum in the same way the CSST does now. The new black iron that goes into the attic is the replacement feed line for a water heater that's further away from the gas meter but the crawl space narrowed and I told plumber he could abandon the original route and go into the attic. – coderjohn Jun 28 at 0:19

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