Consider an indoor gas water heater installed in a suitable closet on the ground floor of a single-story residence. The TPRV drain pipe leads to the outdoors, so it begins upward from the valve for a few inches into the attic crawl space, then runs horizontally to penetrate an exterior wall, then bends downward outside and terminates at a height of 6 to 24 inches above the ground/flood level. The path has no places that would trap water.
Is this acceptable? Does the initial upward path of the drain violate the “downward” requirement for TPRV drains?
If this is unacceptable, do you have any suggestions for how to run an acceptable TPRV drain, or do I need to relocate the water heater outside?
It is a plumbing code violation to run any temperature and pressure relief valve uphill.
You cannot terminate the relief valve piping inside the closet if drainage will damage the floor.
One solution would be to install a floor drain under the relief piping, with the drain tied into your plumbing system.
A simpler method may be to run the relief valve piping through a hole in the floor to the basement or crawlspace where it could then be directed to a drain or sump or left to drain into the soil.
If your climate is subject to freezing, you cannot drain the relief valve piping to the exterior of the house or to unconditioned space like a vented crawlspace.
Water heaters also require a overflow pan under them when they are installed in the living space as is this one. The overflow pan must also be drained to a floor drain or by other means to an area where it will not cause damage or physical harm.