Is it safe to tie a water heater pressure release pipe into an HVAC condensate drain line?

Question

The current setup in my basement has a long pvc pipe going from the HVAC system to a floor drain about 8 feet away. When I had a new water heater installed the plumbers put in a drain pan underneath it but didn't set it up to actually drain anywhere.

I would like to try and hook the water heater's overpressure relief valve into the existing long pipe that's serving as my HVACs condensate drain line so that in the event it triggers and dumps the heater's contents it will flow into the floor drain instead of the drain pan. I got the idea after watching the same plumbers install my water softener and cut into the condensate line to hook up the softener's runoff hose.

Here is an overview of the pvc pipe going from the HVAC system on the left and disappearing into the wall to the floor drain on the right.

About halfway down the pvc pipe is the water heater pressure relief line that I want to tie in. Circled in red.

Here is how they hooked in the water softener.

This is what the floor drain looks like in the other room. If you look back at the first picture you can see the pipe take a 90 degree turn at the end. This goes through some drywall and ends about 2 feet later over the drain. While I'm messing around with everything I'd like to enclose this if possible. I've been having humidity issues in the room and I suspect the open drain is part of the problem.

Here is my proposed plan to tie everything in. I was originally going to go with simple straight lines, but after reading this post I think I might need to leave the p-trap in. I'm not sure of the best way to connect the water heater into this, but I'm leaning towards a flexible hose like how the water softener is tied in. I want to have as much of a slope as possible going from the HVAC system to the water heater's tie in point to reduce the chances of something flowing back into the HVAC system if the heater ever has to dump into the drain pipe. The HVAC system is a Carrier model FV4CNF003 if that helps.

So, with all of exposition out of the way my questions are:

1. Can I tie the water heater overpressure relief line into this drain line safely?
2. If so, would a flexible hose synched over the copper pipe and inserted into the pvc one be the best way to do it?
3. Can I replace the existing pvc pipe layout with one of my proposed arrangements?
4. Regardless of 1-3, is there a way to close off the floor drain in the other room?

Answer 1

No it would not be safe. I do not know the settings of your valve but I expect the water would be in excess of 180F or about 82C, to over the boiling point of water,it is under pressure. That will leave a nasty burn if you come in contact with it. If you check your building code you will probably find PVC pipe is not rated for use with hot water, your proposal would unsafe. Some of the pipe materials rated for hot water are CPVC, copper, galvanized, and PEX, check your local code. This hot water will be under pressure and could blow the PVC joints apart as well as soften them and they may collapse,and then who knows. You might consider putting a drain on the side of the overflow pan, but check with your local inspector to be sure it will be approved.