I'm trying to install a water softener in my garage, which apparently is pre plumbed for supply line, but not drain line.
The fact that a toilet and a sink right above the water heater at second floor and a clean out on the opposite wall outside tell me there is a drain pipe in the wall. Unfortunatelly look like the pipe is right behind my water heater.
So here is my first question: Without taking out the water heater and reinstall it back, can I open up drywall and tap water softener discharge into existing drain pipe OVER WATER HEATER? It will be 6-7 ft over the floor and 3 feet over water softener itself. Since water softener drain with pressure, I believe the height shouldn't be a problem, but I don't know if it break any plumbing code.
My second question: From what I searched so far, I know I will need a p trap to tap into the line and an air gap for health concern. Do I need to worry about overflow since the dirty water can come from second floor and the pipe won't be sealed below it?