While performing ac maintenance, I discovered system was low on refrigerant. Recommended to customer they should replace system, or allow us to add a dye kit/sealer to perform a leak investigation. Customer asked that we add refrigerant (4lbs R410A) and dye kit, but perform the leak investigation later on in the summer. Explained to customer we cannot guarantee the system will hold the refrigerant, and if it loses charge the system will most likely need to be replaced. Also replaced expansion tank on customers water heater as the old one had failed. Added st5 expansion tank, bled air from lines. Cycled and tested, water heater working properly at this time. As for the air conditioner, the system is running but chud can not guarantee how long it will be before refrigerant leaks out again. Replaced customers filter. 16x25x1.