Question: How to use an old sink drain for new tankless overflow

[Tidybuoy]

I'm installing an indoor tankless water heater in an old (no longer used) wash room. I've replaced all of the plumbing and installed gas however, I still have the original sink drain that needs to be capped off. It is currently flush with the wall but open.

The tankless water heater has both a pressure relief valve drain line and a small condensate drain line that need to be routed to the outside or to a suitable drain. I'm wondering if there is a fitting out there that I could adapt my unused sink drain for this. It would be the cleanest setup.

I'm also wondering if this drain would require any type of drain trap as that is no longer there (was originally under the sink). The current drain is exposed to the sewer so I'm thinking this needs to be some kind of sealed configuration.

Any advice would be appreciated.

[1990C4S]

I have drilled and tapped my ABS drain pipes to accept pipe fitting (drains only).

You could do 1/4NPT for condensate and 3/8NPT for pressure relief. Then screw in a hose barb. You also add a Tee and add the appropriate fittings.

Might not be to code...mine is above my trap in a unused sink, so the condensate prevents the basement trap water from evaporating away.

[Tidybuoy]

Thanks. I'll figure something out. I need to take a photo and measure the actual drain, I believe it is 2" or whatever is standard for a sink sewer connection. The plumbing is cast iron female and there was a galvanized pipe screwed into that. Right now it's just a big hole in the wall with cast iron threads.

I was envisioning some type of abs or pvc that screws into the 2" that then has much smaller fittings for the two overflows.

[Tidybuoy]

I think that this is what I need - Fernco reducing donut. However, I am uncertain if I need to have a trap to prevent sewer gases getting into the house or will it matter since it will be connected to a pvc output from the pressure relief valve and no air can escape.

[dad911]

Originally Posted by Tidybuoy

Thanks. I'll figure something out. I need to take a photo and measure the actual drain, I believe it is 2" or whatever is standard for a sink sewer connection. The plumbing is cast iron female and there was a galvanized pipe screwed into that. Right now it's just a big hole in the wall with cast iron threads.

I was envisioning some type of abs or pvc that screws into the 2" that then has much smaller fittings for the two overflows.

The tankless drain will drip and keep the trap full. You don't need or want to make a tight connection. you want a 2" or 1-1/2" trap with a vertical leg, and drop the smaller condensate line into it.

Just like the flex line from a laundry line drains into a larger pipe.

[dad911]

For an example this is my house. The 3/4" condensate line drops into the larger pipe at my laundry box.

If you removed a sink, you need the trap, it's not in the wall.

[Rusty Heap]

"while you're in there" If you have the option, I recently added a hot and cold water outside faucet for warm water to wash the car.

https://www.lowes.com/pd/Woodford-3-4-in-PEX-x-12-in-L-Model-22-Freezeless-Anti-Rupture-Hot-and-Cold-Vertical-Sillcock/1000549343

[looneybin]

You need to have a trap , but the condensate line and pressure relief lines have to have an air gap, they can’t be directly attached to the drain with fittings (unless the fittings provide the air gap)

[wdfifteen]

Originally Posted by looneybin

You need to have a trap , but the condensate line and pressure relief lines have to have an air gap, they can’t be directly attached to the drain with fittings (unless the fittings provide the air gap)

+1
You don’t want a sealed connection between the heater and the sewer pipe. Any pressure changes in the sewer line would be transmitted to the heater. I would install a basic P trap and route the hoses from the heater into it.

[Tidybuoy]

Thanks all!

I will add a small trap for this. I took a look this morning and it looks to be a 2.5" diameter opening. Although there is an existing trap pipe, I'm going to replace with new pvc. Behind the wall is a vent to the roof for this drain.