AO Smith Tankless Water Heater Installation

You’ve probably been there before. You get a call to replace a water heater, you show up to the job site, and the customer’s interested in switching to tankless. The catch? You’ve never installed one before. Don’t worry, SUPPLY.com and AO Smith (the world’s largest manufacturer of water heating equipment) are here to help trade professionals like you become well versed in how to install, replace, and repair them.

At a high level, AO Smith provides an excellent training course for anyone interested in learning about tankless water heaters. You can sign up for hands-on training in Ashland City, Tennessee, home of the largest water heater factory in the world, or you can take the free two-hour online course with a certification test at the end of the training (we highly recommend both).

To give you a head start, we took their online course and have consolidated what we learned below. Use this guide as a quick reference when you’re working on an AO Smith tankless water heater.

First thing’s first – you need to know what type of water heater you are installing, and how it works. Refer to the model types graphic below to see the difference between condensing and non-condensing water heaters.

AO Smith tankless water heater installation

What You Need

Now that you know what you’ll be working with, you can get started on the install. Almost everything you need is included with the water heater. You’ll find:

  • Installation manual
  • Maintenance sheet (customized version for each model)
  • Coupling for condensing water heaters (allows transition from metal to PVC)
  • Termination screens (keeps anything from getting outside of the heater)
  • Easy-Link communication cable (for models with 199,000 BTU and greater)

You’ll also need an isolation valve and a pressure relief valve  – we’ll get to that later. Indoor heaters include a power cord, but outdoor heaters must be field wired. Use the installation manual in tandem with the tips in this guide to make sure everything is properly installed.


Before you get into the install, keep in mind that the heater needs to be properly turned off. The enable /disable switch on the heater turns off heat but does not kill power to the water heater. You need to turn off the switch that is inside the heater to avoid shock, or just unplug it or go to the circuit breaker.

Condensate Connection

If you are working with a condensing water heater, look first to the condensate connection. Flue gases are cooled down to the point that they build up, resulting in acidic condensate. Due to its acidic nature, the condensate connection will need to be neutralized to about a 7 pH. The neutralizer will need to be replaced if the pH ever falls to 6 or below.  Make sure you use PVC instead of metal fittings here – the acid will eat away at the material.

Water Connections

Next, look at your water connections. In order to isolate the water heater from the plumbing system, isolation valves should be installed with every heater, with the union facing the water heater. A pressure relief valve is required as well to release excess pressure in the tank. The pressure relief valve will need to be field supplied. Finally, make sure the hot and cold water heaters are connected properly (labeled blue for cold and red for hot) to ensure proper water flow.

Recirculation Systems

Recirculation systems circulate water to keep it warm, allowing hot water to be readily available. It is very important that the recirculation system is sized correctly, and that you maintain at least 2 gallons of flow when the recirculation pump is flowing.  For a single heater, you’ll really want to keep it at 2-4 gallons per minute. Anything below 2 gallons will cause a flash fire and shut down the system. In multi-heater setups, size the pump accordingly to ensure appropriate flow. This way the hot water flow will not alert the second heater to come on unnecessarily.

“P” models have a built-in pump. To keep the pump from running constantly, use an Aquastat or a timer that will help regulate when the pump needs to be on or off.

A.O. Smith Tankless Water Heater

Thermal Expansion

Thermal expansion tanks are recommended for all water heaters and required for heaters with recirculation systems. An expansion tank will help keep the pressure off of your plumbing system by absorbing any excess water created by the heating process. Thermal expansion tanks need to be pressurized before install with incoming water pressure.

Water Quality

Water quality needs to be checked and treated regularly. Hard water minerals build up and solidify in the water heater tubes, blocking flow and creating leaks. A water softener, which replaces the hard minerals with sodium, is one preferred method of treatment.

We recommend using the AO Smith Product Preserver, which uses a template assisted crystallization process. This process keeps minerals from solidifying and will protect the water heater. The cartridge in the preserver is replaceable and lasts up to two years. It supports flow rates up to 7 gallons per minute. Allow installations with copper pipes to age 3 weeks before installing a product preserver. As the copper ages it can give off elements that damage the media.

Gas Connections

Gas connections release gas equivalent to the amount of water to heat on demand. There are a few things to keep in mind when working with gas.

  • Most AO Smith gas fittings are ¾” in size, but double check to see what you are working with.
  • Don’t twist the gas connections, which are made from a frail metal. Use a backup wrench to ensure they stay straight.
  • Look at the pressure of the gas coming in. In most cases, it will come at a low pressure, but in some cases, you will need a regulator.
  • Regulators may also use a vent limiter (check valve).


The water heater should not be installed near any contaminated air. If the area is confined, like a laundry room, it is more likely to be contaminated. Check the installation manual to see the specific venting requirements for your water heater, like diameter and length. The longer the vent, the harder the fan will have to work. Remember to factor in the length of the fittings to the total required length.

With non-condensing water heaters, use a stainless steel vent material. You’ll also need a condensate collector at the bottom of the vent for non-condensing heaters, as they have no means to get rid of that excess water. With a condensing unit, you can also use a stainless steel material – when there’s a recirculation system the heat from the glue gases makes it unsafe to use PVC.

The “C” Model (Concentric Vent) uses Centrotherm venting, a plastic pipe with a metal interior which releases exhaust gas and simultaneously brings air in. Any commercially available pipe with plastic on the outside and metal on the inside will work, just keep in mind that if you are replacing a current system, you cannot mix the Concentrotherm pipe with what is already there. “C” Models need the condensate collector at the bottom.

Up to 8 condensing water heaters of the same model can be common-vented. Each common-vented heater needs a non-return valve, which will close off and keep flue gases from escaping if the heater system is not running. The common-vent can extend anywhere from 4-10 inches, though the DIP switches must be set to maximum length no matter the size of the vent. Check your heater manual to make sure you have the correct DIP switch settings for your unit.


With the AO Smith Easy-Link, up to 4 water heaters with 199,000 BTU can be linked together with the materials in the heater package. These are linked the same way as a typical plumbing system, with the parent male plug on the first heater connecting to the second heater with the female plug – this is repeated across the four heaters.

AO Smith Tankless Hot Water Heater

Multi-Unit Controller

You can hook up more than 4 tankless water heaters together (up to 20) using the multi-unit controller. The male-female cable process is the same as the Easy-Link system. The female end of the connection will go into the heater to the male parent connector. On the second cable, connect the female end to the third connection. Remember that it’s important that the multi-unit control has power, but that it’s on standby.

These are the basic sections that the training covers. A big part of working with water heaters, however, is fixing issues in units that are already installed. AO Smith provides more insight into the different error codes you’ll run across and how to troubleshoot – stay tuned for a SUPPLY.com guide to use whenever you’re on the job and need a quick reminder of what error you’re dealing with.

Interested in a water heater or have any questions? Give our product experts a call at 877-781-3114!