Install a Tankless Water Heater
Tankless water heaters heat water as it circulates through coils in the system, using gas or electricity on demand instead of continuously. The new water heater will need to be mounted on a wall and vented: choose its location before you start.
Turn off the water and gas. Remove old water heater. For gas water heaters, you need a tee and a valve. Install them if your existing line doesn’t have them. Run a flexible gas supply line from the tee to the water heater, but don’t attach it yet. Using copper pipe, install hot and cold water lines that run from the existing lines to the water heater. Apply flux on the joints of the pipes and solder with a blowtorch. Bell hangers hold the pipe against the wall.
Using 2"x4" lumber, create a mounting platform for the water heater 4" away from the wall. Hang the heater. Per manufacturer’s instructions, connect shut-off valve and sediment trap. Attach the gas line to the shut-off valve. Then connect to water supply lines.
Turn on the gas and bleed the line if needed. Check for leaks and turn off the gas. Next connect the vent shaft on the water heater to the vent. Using heat-resistant silicone sealant, attach a connector to the vent shaft on heater and secure with a hose clamp. Then put on the vent pipe and retaining ring. Fold over tabs to secure. Slide on an elbow, adjusting it to face toward outside vent.
Tankless water heaters require a stainless-steel flue to handle corrosive gases. Install a new one if necessary. Choose the position of your vent and drill a hole through the wall to outside. Using it as a guideline, go outside with the vent thimble and outline it on wall. Cut out a vent hole with a reciprocating saw. Attach interior and exterior vent thimble, vent pipe and hood. Turn on water and gas or plug in (for electric). Start heater as directed by manufacturer.