How to Install a Tankless Water Heater

Installing a tankless water heater can improve your home’s energy efficiency and provide an endless hot water supply. In the video above, This Old House plumbing and heating expert Richard Trethewey demonstrates the installation of an on-demand water heater.

While it’s possible for DIYers with significant plumbing and electrical experience to tackle the job, this is an advanced project that will likely require a professional plumber – especially for gas installations. You can see our picks for the best tankless water heaters here.

How Tankless Water Heaters Work

Tankless water heaters use high-powered burners to rapidly heat water as it flows through a heat exchanger. When a hot water tap is turned on, cold water travels through a pipe into the unit, where it’s heated by either a gas burner or electric elements. This process ensures a constant supply of hot water without the limitations of a storage tank. Some of the benefits include:

• Energy efficiency: According to the U.S. Department of Energy, tankless units can be 24%–34% more efficient than conventional storage tank water heaters.
• Space savings: These compact units take up significantly less space than traditional tank-style heaters.
• Longevity: Tankless water heaters typically last longer than conventional units, with a lifespan of 20 years or more. Tank water heaters generally have a lifespan of around 12 years.

Preparing for Tankless Water Heater Installation

Evaluate your household’s hot water needs to determine the appropriate size and capacity of the tankless unit. Consider factors such as the number of bathrooms, appliances that use hot water, and peak usage times.

Select an appropriate location for your tankless water heater. Tankless units can be installed indoors or outdoors, depending on the model and local climate. The chosen spot should allow for proper ventilation and easy access for maintenance.

Step-by-Step Tankless Water Heater Installation Process

While you’re likely going to need a professional contractor to handle the installation, the steps below provide an idea of what is involved.

First, you’ll need to remove the old water heater.

1. Shut off the water and gas supply to the existing water heater.
2. Drain the tank completely.
3. Disconnect the water and gas lines.
4. Remove the old unit from its location.

Then, you’re ready to install the new tankless unit.

1. Using the hole saw, drill a hole on the outside of the house near the location of the water heater. Run PVC pipes for the fresh air intake and vent through that hole.
2. Connect the cold water supply line to the water heater and the hot lines out of the heater using the ¾” copper pipe.
3. An on-demand water heater requires an electrical outlet. An electrician may need to assist in adding one near the installation location.
4. A crossover valve can be installed at the furthest plumbing fixture to allow the built-in recirculation pump to keep hot water at the ready.
5. Turn on the cold water supply and gas supply, then power the water heater.

Setting Up the Tankless Water Heater

Proper electrical and gas connections are crucial for safely operating your tankless water heater. For electric models, ensure the unit is connected to an appropriately sized circuit breaker and wiring. Gas models require gas line sizing and pressure that meets the unit’s demands.

Proper venting is also essential for gas-powered tankless water heaters to ensure safe operation and efficient performance. The manufacturer should provide instructions on the venting requirements of the model you choose.

Once the tankless water heater is installed and connected, adjust it for optimal performance. Most tankless water heaters allow you to set the desired water temperature. The recommended setting is typically between 120°F and 140°F, balancing comfort, safety, and energy efficiency.

Modern tankless units often feature digital control panels allowing precise temperature control and programming. Familiarize yourself with the control panel and set it according to your household’s needs.

Tankless Water Heater Testing and Maintenance

Once your new tankless water heater is installed, you can test the system. Turn on the water supply and check for leaks. For gas models, turn on the gas supply and check for gas leaks using a soapy water solution. Finally, turn on the power to the unit and test hot water at various fixtures.

Your new system will last longer and work more effectively if you also perform the following regular maintenance tasks:

• Flush the system annually to remove mineral buildup.
• Clean or replace air intake filters as needed.
• Inspect venting systems for obstructions or damage.
• Schedule professional maintenance checks as recommended by the manufacturer.