Photo of an anti-scald tempering valve Mixing Valves & Anti Scald Valves on a Tankless Coil
How to install & set hot water temperature at a tankless coil

  • ANTI SCALD VALVE at TANKLESS COIL - CONTENTS: how is the hot water temperature controlled at a tankless coil on a boiler: automatic anti-scald valves or automatic compensation valves on heating boilers & at tankless coils: location, use, control, setting, safety.
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Anti-Scald protection at the tankless coil hot water system on a boiler:

This article explains how to install or locate the hot water temperature control at a tankless coil hot water system on a heating boiler. We describe what the tankless coil temperature control looks like, how it works, and how to set the output hot water temperature to avoid scalding burns. This article includes the hot water temperatures you can expect when using this type of mixing valve or anti-scald valve at a boiler.

In this article series we explain how to buy, install, adjust and inspect anti-scald equipment to prevent hot water burns. We explain what a mixing valve, tempering valve, or anti-scald valve is, where and why these valves are installed on hot water systems, and how they work.

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Guide to Installing & Setting Tankless Coil Hot Water Mixing Valves

Sparco tempering valve (C) Daniel Friedman

Above, a Sparco Aquamix AM 100C automatic tempering valve or anti-scald valve. At page top, a Watts automatic anti-scald valve installed at a home heating boiler tankless coil used to provide domestic hot water.

[Click to enlarge any image]

Article Contents

Automatic Anti-Scald Tempering or Mixing Valves at a Tankless Coil - Water Pressure Balancing Type

Photo of an anti-scald tempering valve

The Watts anti-scald valve shown above or the Sparco anti-scald valve shown earlier (or an equivalent product) is normally installed at a heating boiler tankless coil outlet or at the outlet of a stand-alone water heater.

This mixing or anti-scald valve contains an internal temperature sensor which permits the valve to add cold water to the outgoing hot water in order to limit the temperature of the hot water leaving the valve.

Our photo at left, taken at a heating boiler in Portland ME, shows a popular Watts™ anti-scald valve made by the Watts Regulator Company.

You can see that the black knob permits the user to set the outgoing hot water temperature leaving the valve, and if you look closely you can see the "HOT" and "COLD" and "MIX" indicators on the three ports of the body of the valve.

Turning this Watts Reg. Co. automatic mixing valve clockwise makes the outgoing water temperature cooler. Turning the Watts mixing valve counterclockwise makes the outgoing water temperature hotter.

Hot water enters the valve's right side, cold water enters the mixing valve's bottom, and mixed or tempered hot water leaves the valve at its left side.

Which Way do We Turn the Anti-Scald or Automatic Mixing Valve to Make Hot Water Cooler or Hotter?

Question about tankless coil or other hot water mixing valve operation - example using the Sparco Aquamix AM 102C:

I'm not sure which way to turn my hot water mixing valve. When you turn "down" on valve does it send less cold water or more. I believe it's less. Let me know. - U.S.A.


There should be a direction arrow on the anti-scald valve telling you what it is doing and which turn direction will raise or lower the outgoing water temperature.

The Sparco Aquamix AM 102C tempering valve shown (above-left) is marked to explain that if we turn this valve counterclockwise (the direction of the red arrow) the outgoing water temperature will be hotter.

Turn this anti-scald valve (and most model) clockwise (in the direction of the blue arrow) to make the outgoing water temperature cooler.

Our photo of a Sparco™ Aquamix AM102C anti-scald or tempering valve (above-left) as well as our photo of a Watts™ mixing valve shown earlier in this article both show the arrow that we describe and indicates that turning the automatic mixing valve knob clockwise makes the outgoing water cooler, and turning the automatic mixing counterclockwise makes the outgoing water hotter.

We don't describe the valve direction as "up" or "down" because these valves can be installed in more than one position, making "up" and "down" a bit confusing.

For hotter output water [on the Sparco or Watts Tempering Valve models shown]: Turn the automatic mixing valve knob counter-clockwise (on the Sparco valve above, in the direction of the red arrow) to make the output water from the system hotter.

For cooler output water: Turn the automatic mixing valve clockwise (on the Sparco valve above, in the direction of the blue arrow) to mix in more cold water and thus to make the output water from the system cooler.

Note that because these mixing valves are automatic, that is once they have been set they maintain a specified hot water temperature for water leaving the water heater, you don't have to make seasonal changes to the valve adjustment.

The automatic mixing valve, tempering valve, or anti-scald valve approach of installing this safety control at or close to the water heater is discussed further at Thermostatic Mixing Valves.

Watch out: Be sure to take a close look at the indicating arrows on your own automatic mixing valve as some tempering valve models may work differently. And for manual hot water mixing valves that we discuss below, the direction to turn the valve may not be marked, but is easy to determine, as we will explain.

Thanks to readers Bruce Jones and Chris Martino and others for pointing out the need for clarity about about which way to turn the hot water mixing valve or tempering valve.

If your automatic tempering or hot water mixing valve has different instructions or operates differently from those discussed here, CONTACT us and send along a photo so that we can add that information here.

Tankless Coil Hot Water Mixing Valve Output Temperatures

Using the modern Watts Regulator Co. Series LF1170 / LFL1170 Hot Water Temperature Control Valve instructions as an example, these valves control water temperature in the following ranges:

Table of Hot Water Mixing Valve Output Temperatures [Watts Series LF1170 & LFL1170]

Hot Water Mixing Valve Model Cold Inlet Temperature Hot Inlet Temperature Factory Preset Outlet Temperature Output Temperature Adjustment Range Water supply pressure 
Watts LF1170-M2
60-70 °F (16-21 °C )
140-145 °F
(60-63 °C)
120 °F (49 °C)
90-160 °F (32-71 °C)
45 psi (3.15 bar)
Watts LFL 1170-M2
90 °F (31 °C)
60-120 °F (16-49 °C)


These mixing valves require a minimum supply pressure of 30 psi (207 kPa), and function with hot inlet water temperatures ranging from 120 to 200 °F (49-93 °C ), and with cold inlet temperatures from 40-85 °F(4-2 °C ).

Valve outlet temperature accuracy is +/- 3°F (1.7 °C ). There are other restrictions including a requirement that the difference between hot water and cold water pressures at the valve must not vary by more than 25%. The maximum tolerated pressure is 150 psi (10.3 bar). Lower pressure restrictions may apply when using CPVC or PEX piping & end connections.

Code approvals: CSA B125-certified and Listing: ASSE 10167 & IAPMO UPC.

Source: "Instructions Series LF1170 and LFL1170 Hot Water Temperature Control Valves", Watts Regulator Co. (2013)

Tankless Coil Anti-Scald Valve Safety Warnings

Watch out: read the installation instructions from the manufacturer of the product you are installing, both to make sure it's installed properly and thus will work as expected, and also so that you understand what to expect by way of hot water temperature control the product handles.

At TANKLESS COIL INSTALLATION PROCEDURE where we recommend the use of mixing valves we quote this warning from the Crown Boiler Company:

WARNING: Thermostatic mixing valves are intended to increase the supply of hot water available from the coil. They are not intended to prevent a scald hazard.

Similarly, the Watts Regulator Company's instructions for the installation of the Series LF1170 & LFL 1170 Hot Water Temperature Control Valves includes this warning: [Bold font is our emphasis]


This Hot Water Master Tempering Valves cannot be used for tempering water temperature at fixtures. Severe bodily injury (i.e., scalding or chilling) and/or death may result depending upon system water pressure changes and/or supply water temperature changes. ASSE standard 1016, 1069 or 1070 listed devices should be used at fixtures to prevent possible injury.

These Hot Water Tempering Valves are designed to be installed at or near the boiler or water heater. They are not designed to compensate for system pressure fluctuations and should not be used where ASSE standard 1016, 1069 or 1070 devices are required. These valves should never be used to provide “anti-scald” or “anti-chill” service.

The components of the system must be of materials with a construction capable of withstanding the high limit output temperatures of the water heating source. - Watts (2013)


Continue reading at MANUAL ANTI-SCALD TEMPERING VALVES or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.




If your hot water temperatures are too low, see TEMPERATURE of HOT WATER is TOO LOW

If you do not have enough hot water pressure or hot water quantity, see HOT WATER QUANTITY IMPROVEMENT

Or see these

Anti-Scald & Hot Water Safety Articles

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