How to Install a Tankless Water Heater


Removing an old water heater, preparing for a tankless model, and installation instructions

Installing a gas tankless water heater involves mounting the unit, connecting gas and water lines, and installing a flue.Sergeevspb / Shutterstock.com

Installing a gas tankless water heater involves mounting the unit, connecting gas and water lines, and installing a flue.

A tankless water heater, also called an on-demand water heater, saves money because it doesn’t store a large volume of heated water. What it does is provide hot water on demand so you don’t have to worry about the shower suddenly running cold.

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A tankless water heater is compact and highly efficient.

This project calls for good plumbing skills, and local codes may require that a licensed plumber do the work. If you can and choose to do the job yourself, you should know how to shut off the water to your house (see How to Shut Off the Water to Fixtures) and how to cut into and connect new fittings and pipes to your supply pipes (if you have copper pipes, see Cutting & Joining Copper Pipe).

There are a couple of basic options:

• An indoor, gas-powered tankless water heater must be vented to the outside. It also requires a gas line and a connection to a 120-volt circuit. It can be plugged into an outlet or wired directly to a service panel.

• An indoor, electric tankless water heater does not need to be vented to the outside. It does require 240-volt power. It may be plugged into a 240-volt outlet or directly wired to a service panel.

Tankless Water Heater Preparation

Before you disconnect and remove your existing water heater, be sure to do all of the necessary preparation—you don’t want to be stuck without hot water for a week or two while you’re waiting for needed plumbing, electrical, or vent work.

If you are installing a new gas unit, check to see if you need your gas meter upgraded. Because the unit uses lots of BTUs, your meter may not have a large enough capacity to supply it along with the other gas-using appliances in your home. The gas company can check this out for you and usually will upgrade the meter at little or no cost. Also have the gas company check to see that it supplies the correct amount of manifold pressure for your new unit.

If you need to install a new gas line, it is best to hire a professional plumber to do this. Black gas pipes will need to be run to the unit, and you may want to have another gas shutoff valve installed just prior to the unit being installed.

An indoor gas unit will also require venting. Because a tankless water heater produces a good deal of condensation and pushes air out with a fan, you will need to use category II stainless-steel exhaust pipe with tightly sealed silicon joints.

If you live in a cold climate, it’s a good idea to include a damper to prevent cold air from coming back in. Also install a condensate drain, which drains water from condensation through plastic tubing, which should be routed to a floor drain or a utility sink. Consult the manufacturer’s literature for the proper way to run the venting. You’ll likely need one or more elbow fittings to route the vent pipe into the unit.

A gas unit also needs 120-volt electrical power. Plan to plug it into a nearby outlet, or wire it directly to a junction box. If you are installing an electric water heater, which requires 240-volt electrical power, have an electrician route the wiring to it unless you are certain of your skills. Codes require a subpanel (which may be supplied along with the unit) to be installed next to the unit. This subpanel has its own circuit breakers so you can quickly disconnect power to the unit if needed. Run wiring into the subpanel and then from the subpanel to the unit. Be certain that the power to the water heater’s circuit is off at all times while the work is being done.

HA2016 Water-Heaters---Install

Removing the Old Water Heater

Disconnect the water lines from the old water heater.

In most cases, a new tankless unit will replace a storage water heater. To remove the old water heater, shut off the valves on the two water lines (one inlet and one outlet) leading into the existing water heater.

Use channel-type pliers or a pipe wrench to loosen and disconnect the lines from the water heater. Open the drain valve at the bottom and drain the tank.

On a gas unit, shut off the gas line valve, and disconnect the gas line. Disconnect and remove a section or two of vent pipe. If the unit is electric, you may choose to hire an electrician to disconnect the wiring as 240-volt power can present an electrocution hazard. If you choose to do it yourself, shut off the circuit at the service panel and test to make sure the power is off. Disconnect the wires from the unit and remove the old water heater. If you need help with this project, call a local water heater pro.

Installing a Tankless Water Heater

A new tankless unit can be installed flush—against the drywall or plaster—or set into the wall, between studs. If you want to set the unit into the wall, use a stud finder to locate studs and cut a hole between them. Attach the unit to the wall following the manufacturer’s instructions.

Route water supply pipes to the water heater. Be sure the pipe from the water utility leads to the inlet and the pipe leading to the house is connected to the outlet. Install shutoff valves on both lines near the unit. You also may need to add a pressure-relief valve. Use union fittings to make the final connections to the inlet and the outlet.

Open the water valves but not the electricity or gas. Open faucets in the house to run water through the unit for a minute or so. Shut off the water valves. Remove the inline filter, usually located near the inlet, and clean out any debris. Replace the filter, open the water valves, restore electrical power, and turn on the gas. Finally, test the unit to make sure it is operating properly.

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