Code requires that both the water heater PRV discharge line and the furnace condensate line must either terminate to free air outside the structure or into an approved "indirect waste receptor"

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.)

OK, I think I follow... by installing a tee there will still be the open end of the storm drain and the PRV and condensate lines will terminate at least 2 inches above the vertical 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 :-)

If the water heater is mounted over a floor surface that can be damaged by water you are required to install a drip pan, however, code prohibits discharging the PRV into the drip pan.

LOL... I haven't built anything to code yet...

You need to be able to tell if the PRV is allowing water out since it indicates a problem (like a failed thermostat in an electric heater).

Not sure why this is coming up again after a year and a half... but I did just as Lazypup suggested and ran the PRV discharge down into the crawl space and terminated it in the free air above the storm drain, to which I had affixed a 90 degree adapter with plastic grate. I also terminated the condensate line for the A/C in a similar fashion.

When I passed my plumbing inspection the inspector did not want to see anything in the crawl space (which is about 4 ft tall), all he cared about was that the PRV discharge was routed somewhere.. and the crawl space was fine with him.

I can understand Brickeye's concern that by running the line into the crawl space you may not see if it has been discharging... but I think that would be pretty obvious as we walk by this little water heater in the laundry room all the time and if the thermostat was stuck you'd definitely hear the water flowing out the PRV discharge.