User Manual Eccotemp FVI12-NG Tankless Natural Gas Water Heater

The recommended operating PSI is 14 minimum. More exactly, it is 14 PSI + pipe pressure loss + pressure loss + 5 PSI safety margin. This is for pressure at the heater. To go from lower to upper floors, add 0.44 PSI per foot.

This unit heats water by 77F rise at a flow rate of 2.3 GPM (and a proportionately lower temperature rise with a higher flow rate, and higher temp. rise with lower flow rate). All tankless water heaters operate on the same principle.

A low-flow showerhead uses 1.5 GPM. A sink uses about 0.75 GPM. A clothes washer uses about 2 GPM. A dryer doesn't use any hot water. Most recently manufactured dishwashers heat their own water.

You should add all of those flow rates that would be running at the same time. Only you know this. Then use math to convert the 77F rise at a flow rate of 2.3 GPM to what you'll be using. Then add it to your NYC incoming water temperature, which might be 35F in winter.

A 50 gallon tank is a beast that takes up about 10 times more square footage in your closet/basement. The tank is less energy efficient by 5% to 20%, depending on monthly usage. A tankless can be put near to the point of use, to reduce the time to get hot water. A tank is less flexible in location.