I installed my watts premier, but water keeps leaking from where the drain(black) tube was connected to the faucet base. It's definitely not leaking from the little air hole. I also cleaned the drain already, made sure it's not clogged. Any idea?

I do not understand what you are describing. Is this a tankless hot water heater? What do you mean by saying the drain tube connected to a faucet base? If a valve is leaking it would indicate a bad valve that needs replacing or repair.

Try to remove the tube from the quick connect fitting at the faucet. Cut the tube straight about 1/4”. Insert the tube to the faucet again and push it all the way in until it can move further. If that still does not fix the leak call Watts Premier technical support line at 1-800-752-5582.

I am getting leakage from the little drain hole in back of faucet. Is that due to clogged drain hose or do we need to replace the green membrane filter. The unit is about 1.5 years old.