I am planning to run the PRV discharge line down into the crawl space and would like to run it into what I've been told is a flood drain to the sewer... essentially a 4 inch flex line like you'd see attached to the downspouts.

When the new condensing furnace was installed the contractor ran the condensate line into this "sewer line' since it's right below the furnace area where I want to mount the water heater.

What's a good legal way to do this? Would I have to mate up some type of floor drain to the sewer drain and then plumb the PRV dishcharge so it discharges into the free air above the drain, or can the discharge line go right into the sewer line?

What I'm calling the sewer drain line only extends about 4-6 inches beyound the base of the foundation footing.

I could post a picture if that would help.

The termination point of a PRV discharge line must point downward. When extending a pipe through a footer wall we must install a 90deg elbow on the end of the line to turn it downwards. (This is to minimize the risk of scalding water spraying out of the line and hitting someone standing near the point of discharge)

Under the IRC when terminating outdoors the line must terminate within 6" of finished grade. Under the UPC it must terminate between 6" and 24" above finished grade.

When terminating into the house DWV system or a storm drain system such as you have the PRV line must terminate a minimum of 2" above the flood level rim of the drain receptor, whether that is a floor drain, utility sink or a sump vessel. (The intended purpose of the 2" air gap is to prevent bacteria from entering the line and potentially going up the line and into the heater vessel.)

In your case, you could install a tee on the storm drain line and stub a section of the storm drain up slightly above grade, then mount your PRV discharge line pointing down into the open standpipe. Just make sure that the bottom of the PRV line is 2" above the highest level of the riser from the tee.

(Note, your 4" line is a storm drain therefore you are not required to install a P-trap below the riser.)

I suppose another option would be to install some type of drip pan with a drain under the small water heater (the heater will be mounted about 2 feet off the floor on the wall behind the furnace), and then run that drain down into the storm drain, with the PRV and condensate lines discharging above the drain in the drip pan.

If I went with option 2, could the drain from the drip pan just run down and into the storm drain without any mechanical connection. That is, just penetrate xx inches into the drain?

As always, thanks for the superb and complete advice :-)