Connecting Tankless Water Heaters to a Plumbing System

What you should know when connecting a tankless water heater and home plumbing. Tips for DIY plumbing to prevent hot water problems, scalding, leaking, noise...

Plumbing installation of the tankless water heater must be done in such a manner so the unit can operate properly and provide hot water on demand and in a continuous flow, with leak-free connections and protected from freezing. The water heater life and performance depend on the properly designed plumbing system, water quality, water pressure, and the environment where water is coming from and the unit operates.

Every tankless water heater must be installed per the manufacturer's instructions and local codes correctly.

Tips when installing plumbing for water heating

When talking about the plumbing system and water heating, a tankless water heater must have the minimum water flow for its normal operation and use the pressure relief valve as the safety element. Installation of the following elements in the plumbing system or water heater is either mandatory and per codes or recommended for better performance and safety.

Expansion tanks

In the closed water supply system, the thermal expansion tank must be part of the system, which will protect the unit from excessive temperature and pressure build-up and will prevent the pressure relief valve from opening frequently due to the high pressure. Buy expansion tanks on

Mixing valves

Water heaters are made to produce hot water, so protection from scalding temperatures is important. The solution is to lower the temperature by mixing hot and cold water, or by installing the mixing valves, especially those with the low water resistance.

Check out reviews and prices of mixing valves.

Recirculating pumps

Recirculating pumps can be used to provide instantaneous hot water delivery at the tap or shower. Also, install a pressure meter on the inlet to prevent the boiling inside the water heater, which causes abnormal sounds and is decreasing the heat exchanger life.

Check out reviews and prices of recirculating pumps.

Water treatment devices

If you live in an area with hard water (high mineral deposits in plumbing), you might want to consider installing sediment filters and water treatment devices.

Tip: Buy a water softener to prevent water heater damages due to hard water and sediment buildup.

Water pressure reducer

In the case of the high water pressure from the municipality water pipeline, often cut-offs or water hammer, you might want to consider the installation of the depressurizing valves, backflow preventers, check valves, etc.

See how much do water pressure reducers cost online.

Installing a pressure relief valve

Each tankless unit in North America must have the pressure relief valve installed per ANSI for all water-heating systems. When installing this valve, you must not plug it or install any reducing fittings, restricting the pressure valve from its normal operation. Furthermore, there should be no valves between the relief valve and a water heater.

The relief valve must be rated up to 150 psi and with a rating that exceeds the BTU input of the tankless water heater. It should be installed in the outlet line and close to the water heater. You must also install a discharge line from the relief valve to the appropriate drain to prevent any injuries from the scalding water. The discharge line must withstand high temperatures and not exceed the maximum length (code regulation).

To ensure the proper work of the relief valve, it is recommended to test the valve annually. Keep in mind that periodic discharges might happen due to thermal expansion.

Pressure relief valves are not expensive and are relatively easy to replace.

emergency plumber

Isolation valves

Many manufacturers include isolation valves with the water heater. By installing the valves, you will be able to isolate the heater and perform any service and maintenance. They are supposed to be installed on both cold and hot water line.

If isolation valves are not included with your water heater, you can review and buy on

Manual control valve

A manual control valve must be installed in the water inlet of the water heater and before it is connected to the plumbing. To make servicing easier, it is recommended to use unions on both hot and cold water lines so that they can be simply disconnected.

Water line installation

When plumbing a tankless, the water lines and connections must be approved for use in potable water heating, including copper, PVC, CPVC, PEX, and other types.

Union couplings or flexible pipes for connecting pipes should be installed to reduce the force applied to the piping. Avoid too many joints and keep the piping simple and as short as possible as the longer tubing means greater heat loss. The size of the pipes and fittings needs to be the same.

Make sure that there is no crossover and the unit is leak-free. Ensure that there is the minimum water pressure in the plumbing system (usually 50 psi). All tankless water heaters require a minimum water flow so the unit can activate, and that water flow is 0.4-0.5 gallons per minute.

I have seen many reviews and complaints about the slow hot water delivery, so the recommendation is to check models that are equipped with the recirculation pumps to get hot water faster when a tap is open. A good example is Rinnai and its models from RUR series.

All the air and debris must be purged out of the water lines; otherwise, damage to the unit might occur. Water filter cleaning should also be performed regularly.

When plumbing a gas-powered tankless water heater, make sure that non-metal pipes are kept away from the heater and its flue.

Plumbing checklist

  • Before plumbing the tankless water heater, flush the water line to remove debris.
  • Ensure that water inlet and outlet are adequately installed: hot to hot and cold to cold, and there is no cross-connection.
  • Ensure there are no leaks in water lines and between pipes and fittings.
  • Ensure that the water pressure and water flow are per requirements, and the air is purged out of the water lines.
  • A pressure relief valve is installed correctly and tested annually.
  • Run the condensate pipe ends in the appropriate drain.
  • Plumbing lines are correctly insulated to reduce heat loss.
Find Trusted Plumbers & Get Free Quotes