Tankless water heater plumbing: neutralized condensate and pressure relief valve
Yes down the drain is good, the condensate is slightly acidic, usually in the 3-5 pH range. The neutralizer is supposed to, well,  neutralize it back up to 7 pH but they do wear out or clog. Make sure your condensate doesn't come into contact with metals. It will eat them. Do yourself a favor and always pipe in your neutralizer like this in case it blocks up it can bypass. I always use Clear gorilla tube to allow for visual inspection.https://www.diversitech.com/en-US/item/id/11672
 
Make sure you pipe your pressure relief valve down to within 6" of the floor and not into the drain  maintaining 3/4 inch diameter all the way. In the rare event that it pops you are supposed to see the water on the floor and thus be alerted to the issue. If it's down the drain you will never know you have an issue. https://www.google.com/aclk?sa=L&amp;ai=DChcSEwjZsrDb78zgAhUOtrMKHQFmBB8YABAIGgJxbg&amp;sig=AOD64_3f3Fc_0mYzjmXXWbHAYQvAGUPvyw&amp;ctype=5&amp;q=&amp;ved=2ahUKEwiJ5abb78zgAhWCZd8KHSvbAk0Qwg96BAgLEBA&amp;adurl= this one is 50 bucks