Heat-Timer® Corporation

Tankless Water Heater Installation Safety

Recently we asked the head of one of the larger Commercial HVAC companies in northern New Jersey, whether they used a standard 3-way mixing valve when installing commercial tankless water heaters.

“No,  you don’t need one…” was his answer. His opinion is not uncommon.

It is also dead wrong.

As the technology of commercial Instantaneous Water Heaters and heat exchanger design improves, the use and number of installations of Instantaneous Water Heaters in commercial applications has grown.  As a means to promote this technology many Commercial Instantaneous Water Heater manufacturers are attempting to sell this technology over tank-based water heaters by stating that an ASSE 1017 tempering valve (aka a 3-way mixing valve) is not required and that going tankless does not pose a health risk related to scalding or the growth of Legionella.

It turns out that the science and biology of Legionella growth and transmission does not support that claim at all. Installers of commercial grade tankless systems are creating a potential health risk and putting themselves and their clients in legal jeopardy, by recommending that HVAC technicians can omit the installation of an ASSE 1017 tempering valve solution.  By referencing partial facts and drawing incomplete or cherry picked scientific conclusions, the current installation practice advocated by many tankless manufacturers is in fact, creating a real public health risk.

Getting Back to the Science

As covered in the first part of this discussion, there are many reasons, both known and unknown, for the increase in Legionella cases since 2000. Contrary to conventional wisdom, it turns out that the adoption of ASHRAE 90.1 in 2001 actually increases the probability of Legionnaire bacteria as it allows for water storage at a temperature that allows the bacteria to survive.  ASHRAE 90.1 was adopted under heavy pressure from manufacturers who wanted to be able to offer cheaper equipment that ran at lower temperatures but at the end of the day, the science does not support the safety claims of recommendation and it never has.

The Problematic Manufacturer’s Claim

At a recent industry convention, a manufacturer rep for one of the leading commercial tankless water heater products made the following claim:  that if you maintain your water heater system generally in the 118*F to 120*F that Legionella does not grow and therefore that is a safe operating range for instantaneous water heaters saving both energy costs, and the need for a mixing valve.

 As justification, they presented the table below:

If one takes a closer look at the chart used in the presentation, 118*F to 122*F is a range that Legionella survives but doesn’t grow.  What happens to the Legionella that is already present in the domestic hot water system and what happens to the Legionella that is introduced by the cold water supply when there is a domestic draw?  The chart further explains a time period along with a temperature range in which only 90% of the Legionella bacteria is “killed”.  According to the chart at 122*F it can take upwards of 2 hours to kill 90% of the Legionella bacteria.  The question one should ask is what happens when new cold water is added and what happens to the active Legionella bacteria it can contain?

A more common chart that shows the relationship of temperature and Legionella is the chart based on the study completed in 1989 – “Viability of Legionella Pneumophilia in Chlorine-free Water at Elevated Temperatures” and is used as a reference by such organizations as ASSE and OSHA.

This chart shows the water temperature in which the Legionella bacteria cannot survive and is totally eliminated.  It also shows that Legionella die-off only begins above 131F and in fact still takes several hours at that temperature. For these reasons alone, it seems that continuous water heater manufacturers are rolling the dice with respect to Legionella safety and that in fact their native operating temperature should be closer to the 140 degree mark used in almost all tank based commercial domestic hot water installations.

Legionella Prevention and Scald Hazard Risk

Of course, one might think that cranking up the operating temperature of the tankless system to say 130F might solve or mitigate this problem. But since you are now delivering that water directly to the tap, you have to now worry about burning the people at the other end!  The potential scald hazard exists whenever delivering water to fixtures at temperatures above 120*F.  When storing water at the higher temperatures such as 130*F or higher to combat Legionella, one must be fully aware there is a potential scald hazard in doing so.  The following chart shows how these high water temps can quickly lead to a scald hazard:

As shown above, water temperatures that kill Legionella reliably and quickly, represent a scalding hazard when delivered directly to the user. Thus the industry standard use of a 3-way mixing valve. This problem and solution is exactly the same when using a Tankless water heating system, despite claims to the contrary by manufacturers.

Another False Argument: Tank Type Water Heaters Identified as Excellent Places for Legionella growth

In another potential selling point offered by tankless water heater manufacturers, tankless vendors attempt to make the claim that all tank based systems regardless of design, are prone to the growth of Legionella.

 This statement amounts to a broad claim that tank based water heaters, especially systems using multiple tanks, will increase the risk of Legionella.  While it is true in most commercial applications you will find multiple tanks for redundancy and capacity, this is actually completely unrelated to an increasing risk of Legionella growth.  All that matters is that the water stored is hot enough, greater than 130*F -preferably > 140*F, then the risk of Legionella growth and distribution is all but eliminated.

Instead tankless manufacturers use uncommon examples of boiler configurations and boiler designs that are not in the mainstream of applications. The examples used are extreme cases of stratification of water temperatures within a tank, claiming that the lower portion of a common water tank where the cold water feeds in, is extremely cold, below 100*F.

In fact, for most boilers and water heaters designed in this century, this is not a valid operating scenario. The “stacking” or stratification effect occurs when short or intermediate domestic draws are made on the water heater in which the draws are large enough to initiate a call for heat, but not large enough to dissipate the stored energy within the tank.  In a boiler and indirect water heater application, the stored water temperature within the tank can easily approach the boiler supply water temperature of 180F.  In fact, stratification of water temperature within the water heater is a good thing in the prevention of Legionella bacteria.  The only negative is that it presents a potential scald hazard if an ASSE 1017 mixing valve is not utilized.

Second, the manufacturers of tankless systems fail to explain the dynamics of water blending that occur as the cold water supply and building recirculation water enters into the tank.  There is the initial blending of these water temperatures as they enter the bottom of the tank.  This blended water temperature can be calculated based on the flow volume and water temperature using the following equation:

C gal x C temp = (A gal x A temp) + (B gal x B temp)

C gal = the total of cold and recirculation flow
C temp = the blended temperature of the cold and recirculation water
A gal = the flow rate of the cold water supply (the flow rate of the domestic demand)
A temp = the temperature of the cold water supply
B gal = the flow rate of the building recirculation
B temp = the temperature of the building recirculation

Once you have calculated the blended temperature of the cold and building recirculation entering the tank, you can use the same equation to calculate the effect of the cold and building recirculation water entering the tank to its overall storage temperature.  In this calculation the variables are as followed:

C gallons = the total storage volume of the tank
C temp = the blended temperature of the tank
A gallons = the flow volume of the cold and recirculation entering the tank
A temp = the temperature of the cold and recirculation water (as calculated in the above equation)
B gallons = the storage volume of the tank minus the amount of cold and recirculation flow volume.
B temp = the temperature of the tank prior to the domestic draw.

Example:  A commercial 100 gallon water heater tank (direct or indirect fired type) currently stored at 140*F with 15 gal of building recirculation returning to the tank at 120*F.  There is a 10 gal domestic draw occurring with 60*F cold water entering in from the street.  What is the overall effect on the stored water within the tank assuming the tank does not initiate a call for heat during this domestic draw?

Using the first equation we calculate the blended temperature of the cold water supply with the building recirculation water and find the blended temperature is 96*F.

25 gal x C temp = (10 gal x 60 temp) + (15 gal x 120 temp)

C temp = 96*F

Then using the second equation we can calculate the overall effect on the stored water temperature with the water heater tank:

100 gal x C temp = (25 gal x 96 temp) + (75 gal x 140 temp)

C temp = 129*F

Again, the above calculations represent the absolute worst case as it assumes the water heater never initiated a call for heat to recover.  A typical thermostat on a water heater has a differential of 7 degrees, so there is a good possibility that the water heater is responding to this domestic draw and is adding heat.

In applications with a copper finned water heater and a storage tank, the tank water temperature is even more uniform. In these designs, the cold water and building recirculation enters the water heater and is heated prior to entering the tank.  This is actually the best method as the cooler water is heated directly and the storage tank can circulate back to the water heater to maintain a high storage temperature.

On a direct fired water heater or indirect fired water heater tank the bottom layer of water temperature does not remain cold and is not a breeding ground for Legionella bacteria as stated by tankless manufacturers.  By design a water heater tank has the heating element or coil at the bottom portion of the tank.  On electric type water heaters, there are typically 2 or more heating elements, one near the bottom portion of the tank and one near the upper portion of the tank.  This is to ensure the water within the tank is heated thoroughly and the desired storage temperature is maintained.


As engineers, designers or installers of domestic hot water systems, it is our responsibility to deliver hot water to the end users in the most efficient, but safe means as possible. Tankless water heaters have many selling points for use in smaller domestic hot water delivery systems.

But the idea that these systems don’t need a mixing valve, shouldn’t be one of them.

There are 2 main areas of safety that you need to consider in that design:

  1. Minimize the potential for a scalding hazard.
  2. Ensure no growth of Legionella in the domestic hot water system.

The simplest way to meet both of these safety concerns is to store the domestic water at a high temperature and deliver it at a safe water temperature using an ASSE 1017 mixing valve.

It does not matter what type of water heating system you have or are considering, an ASSE 1017 mixing valve will work on all types of water heaters – tank styles or tankless.

The thing to remember as an engineer, designer or installer is that you will bear the responsibility of liability in the eyes of lawyers in the event an individual or individuals become sicken or scalded.

Overall the industry desperately needs to change its mindset with regard to the recommendations of ASHRAE 90.1 and the use and installation of a mixing valve in these settings.