Connecting Tankless Water Heaters to Plumbing System

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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 your tankless water heater must be done in such a manner so the unit can operate properly and provide hot water on demand and in 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 unit operates.

Every tankless water heater must be installed per manufacturer's instructions and local codes, either by you or by the professional technician.

Tips when installing plumbing for water heating

When talking about the plumbing system and water heating, tankless water heater must have a 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 are either mandatory and per code, 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 the excessive temperature and pressure build-up and will prevent the pressure relief valve to open frequently due to the high pressure.

Mixing valves

Water heaters are made to produce hot water, so that is why the protection from the 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.

Recirculating pumps

Recirculating pump 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.

Water treatment devices

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

Water pressure reducer

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

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, which will restrict the pressure valve from its normal operation. Furthermore there should be no valves between the relief valve and the water heater.

The relief valve must be rated up to 150 psi and with the 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 be able to withstand high temperatures and cannot 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 the thermal expansion.

emergency plumber

Isolation valves

Many manufacturers include the 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 supposed to be installed on both cold and hot water line.

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 line, so they can be simple disconnected.

Water line installation

Keep in mind when plumbing a tankless that the water lines and connections must be approved for use in potable water heating, and that includes copper, PVC, CPVC, PEX and other types.

Union couplings or flexible pipe 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 piping 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 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, and this is why recommendation is to check models that are equipped with the recirculation pumps, so you get hot water faster when a tap is open. Good example is Rinnai and its models that operate in the Economy and Comfort modes.

All the air and debris must be purged out of the water lines, otherwise the 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 check list

  • Before plumbing the tankless water heater, flush the water line to remove the debris.
  • Ensure that water inlet and outlet are properly 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 the water lines.

  • Pressure relief valve is properly installed and tested annually.

  • Run the condensate pipe ends in the appropriate drain.

  • Plumbing lines are correctly insulated to reduce the heat loss.

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