I want to add a small tank-type point of use water heater under my kitchen sink. What is the best way to plumb the overflow? Obviously there is no floor drain under the sink or nearby. Is it possibly to add one somehow to the existing sink drain pipe?
You can add a “T” fitting to your sink drain there are metal versions available also. The “T” section needs to be above your P trap. This is how dishwashers are usually drained but should be fine for your overflow line.
I would also add a catch tray underneath the tank. If the house is on a crawl space I would drill through the bottom of the cabinet and through the sub floor and plumb the catch tray drain through the hole into the crawl space. You can cover the end of the drain pipe with mesh screen to prevent creepy crawlies from entering the cabinet. Also make sure you caulk around the drain pipe where it passes through the cabinet and subfloor with waterproof. Flexible caulking.