Honeywell 6006 guide Guide to Mixing Valves & Anti Scald Valves
on Water Heating Equipment & Plumbing Fixtures

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Anti-Scald protection for water heaters:

In these hot water temperature control articles 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.

This article series answers most questions about Heating System Boiler Controls on central heating systems to aid in troubleshooting, inspection, diagnosis, and repairs. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Guide to Mixing Valves and Anti Scald Valves on Heating Equipment & Plumbing Fixtures

Sparco tempering valve (C) Daniel Friedman

Photo: a Sparco anti-scald valve or tempering valve installed at a hydronic heating boiler. Other texts refer to these safety controls as mixing valves or compensating valves or temperature compensating valves.

[Click to enlarge any image]

Anti-Scald & Hot Water Safety Articles

Definition of an anti scald valve or compensating valve - to avoid hot water burns

Anti-scald valves are used on heating and plumbing systems to avoid hot water burns at plumbing fixtures such as sinks, showers, and tubs.

An anti-scald or mixing valve mixes cold water with the outgoing hot water either automatically or manually to make sure that a person using the plumbing fixture won't be scalded. We describe and illustrate various types of automatic and manual mixing valves, anti-scald valves, or compensating valves in this article.

Definition of anti-scald valve vs. mixing or tempering valve: manufacturers recommend point-of-use anti-scald protection

Watch out: definition of mixing valve, tempering valve, anti-scald valve: most likely because of a combination of product liability concerns and because of the manufacturer of a mixing valve cannot control what other plumbing installation or operating details are followed at individual buildings, both heating boiler manufacturers (Crown Boiler Co. discussed at TANKLESS COIL INSTALLATION PROCEDURE) and mixing valve manufacturers take care to refer to products like the Sparco control shown above and the Watts mixing valve shown later on this page as a mixing valve or tempering valve.

And the manufacturers (Watts for example) typically add that point of use scald protection should be installed:

ASSE standard 1016, 1069 or 1070 listed devices should be used at fixtures to prevent possible injury.

That advice means that besides these mixing valves that are typically installed at the water heater or tankless coil, separate anti-scald protection devices are available for installation at the point of use: sink, shower, or tub etc. Those sorts of anti-scald devices are also discussed in this article.

In my experience with plumbing and heating installers, home inspectors, and consumers, these terms: automatic compensation valve, mixing valve, tempering valve, anti-scald device, are thrown about in a blizzard of usages that treat the words as synonyms.

In the course of inspecting several thousand buildings between 1976 and 2014 I have almost never found anti-scald devices at individual plumbing fixtures in private homes. The only water temperature control we typically find are mixing valves at the hot water source. Worse, sometimes there is no hot water scald protection at all.

Sources of hot water scald burn risk at buildings without adequate anti-scald device protection

Watch out: Where no anti-scald valve is installed, the risk of a person being badly burned by hot water can be significant in buildings for a variety of reasons that we will explain here, including:

Automatic compensation valves in new homes protect against severe changes in water temperature when the pressure drops; but these systems are only required to be tested at a flow rate of 2.5 gpm. It follows that when the new 1.75 gpm showerheads have no matching automatic compensation valve, anyone in the shower runs the risk of being scalded when the pressure abruptly changes. -Koeller [6]

Watch out: it can be confusing listening to plumbers, home inspectors, and building supply sales staff who toss around terms like "mixing valve", "tempering valve", and "pressure-balancing valve" a bit loosely, all referring to ways to avoid scalding burns at plumbing fixtures, but not all working the same way.

Approaches to controlling hot water temperature to avoid scalding burns

The high temperature limit control on the water heating device itself can be set to prevent scalding water from being produced. Separately
at TABLE OF SCALDING TEMPERATURES & TIMES we include a photo of a typical water heater label warning about scalding temperatures, implying that this option is one to consider.

A point of supply pressure balancing valve or automatic mixing valve (compensating valve, or a manual mixing valve) can be installed at or near the water heater, or at the tankless coil or other hot water source so that scald protection is provided even if the water heater is set to a high temperature.

This approach is discussed beginning at ANTI SCALD VALVE OPERATION, and further reviewed by Steven Bliss at ANTI SCALD VALVE PROTECTION, Best Practices.

An temperature-sensing thermostatic mixing valve or anti-scald device can be installed at or near the water heating device so that even if the water heater is set to a high temperature the building occupants won't be scalded when hot water is run at a plumbing fixture.

To obtain more total hot water using this approach see HOT WATER QUANTITY IMPROVEMENT via ANTI-SCALD VALVE and see details at HOT WATER IMPROVEMENT.

This approach to both hot water safety and to making hot water last longer is also illustrated and explained at POINT of USE BUILT-IN FIXTURE ANTI-SCALD VALVES.

Limit switch on a steam boiler tankless coil

When a heating boiler uses a tankless coil to produce domestic hot water, a third single-function control may be installed for that purpose. In our photo a Honeywell limit control switch is being used to monitor hot water temperature at the tankless coil which is in turn mounted on a steam boiler of an older home in Portland, Maine.

You can see the black-handled mixing valve in the lower right of this photo.

Cold water from the building is entering the tankless coil via the bottom pipe (green corrosion) and hot water, heated by the coil is leaving at the upper part of the tankless coil, where it turns downwards to enter the left side of the mixing valve. Additional cold water is permitted to enter the bottom of the mixing valve, and tempered (non-scalding) hot water then leaves at the right side of the mixing valve in this photo.

In this picture of a more traditional single-function heating boiler limit control, the limit switch is being used on a tankless coil, and in the enlarged version of the photo you'll see that the temperature limit on the control is set to about 140 °F.

Below we show a sketch that explains how an anti-scald valve or tempering valve actually works.

Tables of Scalding Temperatures & Exposure Times that Cause Hot Water Burns, Laws, Regulations

We have moved this scalding temperature table.

See TABLE OF SCALDING TEMPERATURES & TIMES for a table of safe temperatures for residential hot water.

See HOT WATER ANTI-SCALD REGULATIONS for a table of Hot Water Anti-Scald Limits, Hot Water Anti-Scald Laws & Hot Water Regulations

How Automatic Anti Scald Valves, Mixing Valves, or Tempering Valves on Water Heaters Work

Sketch of a tankless coil tempering valve or anti scald valve

Anti-scald valves used with water heaters are also called tempering valves or mixing valves. An anti-scald valve mixes cold water in with the outgoing hot water to assure that hot water reaching a building fixture is at a temperature low enough to be safe.

In the sketch shown above, courtesy of Carson Dunlop Associates, an anti-scald valve (also referred to as a tempering valve , a compensating valve or an automatic mixing valve) is shown installed at the hot water piping and tankless coil on a heating boiler.

The drawing illustrates that hot water leaving the tankless coil has been heated to 180 °F. by the coil which is itself immersed in hot boiler water. The 180 deg .F. water leaving the tankless coil is mixed with 55 °F. water inside the tempering valve. This produces outgoing hot water from the valve cooled down to 140 deg F.

In the U.S. most authorities consider hot water at or below 120 deg F to be safe from scalding. Some facilities limit water temperatures to 100 deg. F.

Automatic vs. Manual Mixing Valves Work Differently and are Not Equally Safe

An automatic mixing valve or compensating valve such as those sold by Watts Regulator Co. or Sparco (and illustrated below) includes a temperature sensing mechanism that automatically adjusts the amount of cold mixed in with outgoing hot water to maintain the desired output water temperature.

A manual mixing valve such as those often installed at older tankless coil hot water systems uses a simple gate valve that is manually adjusted by the homeowner.

At a minimum the homeowner may have to adjust this valve seasonally depending on whether or not the building is also being heated by the same boiler as during the heating season the boiler will be kept hotter than during the rest of the year, making the outgoing hot water too hot.

Guide to Installing & Setting Tankless Coil Hot Water Mixing Valves

This discussion has moved to ANTI SCALD VALVE at TANKLESS COIL

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


Tankless Coil Hot Water Mixing Valve Output Temperatures


Guide to Using a Manual Hot Water Tempering Valve on the Domestic Hot Water Supply

This discussion has moved to MANUAL ANTI-SCALD TEMPERING VALVES

Point of Use Anti-Scald Valves, Guide to Bath Fixture Anti-Scald Valves, Automatic, Thermostatic Mixing Valves, or Tempering Valves to Avoid Hot Water Burns


How Can an Anti-Scald Valve Improve the Hot Water Volume & Quantity?

Using an anti-scald valve or "mixing valve" or "tempering valve" on a hot water supply system can increase both the actual and the apparent total quantity of hot water available. That is, building occupants get a longer time in the shower, provided the hot water is used with some sense, as we explain here.

We will have more "heat" stored in the hot water tank (or more heat stored inside the heating boiler that in turn is going to heat a tankless coil and thus heat water passing through the coil) if we can keep the hot water tank (or boiler) at a higher temperature.

Watch out: when we heat the hot water tank or boiler to a higher temperature we also increase the chances of someone being scalded at the tap.

Therefore, to avoid scalding, we mix cold in with the outgoing hot water (the right way to do this).

Where a mixing valve is not installed (watch out for scalding!) we can be smarter at the plumbing fixture itself by turning on less hot water flow and more cold water flow so that the water is not scalding.

This combination of keeping water in the hot water tank (or boiler) hotter, but causing the volume of hot water to flow more slowly at the individual plumbing fixture, mixing in enough cold to get adequate flow quantity and safe temperature, is a risky (scald risk) alternative to using a mixing valve.

We don't recommend this alternative, (see MIX VALVE SCALD PROTECTION, Best Practices) but we explain it here as you may encounter this approach, especially in an older building and one where there are no small children nor elderly occupants (who are at greater risk of being scalded).

In either case, by starting with a hotter water heater or boiler and by mixing in cold in the outgoing water (or by regulating water pressures at the fixture), we consume hot water in the water heater tank more slowly. That means longer time in the shower.

In sum, with careful installation of appropriate safety controls such as an automatic mixing valve or automatic tempering valve (or by careful use of a manual mixing valve), we can heat water in the water heater storage tank or tankless coil to a higher temperature, mixing or tempering it as it leaves the water heater or at the point of use in the building to avoid scalding.

The result is more total hot water available from the same water heating equipment.

Details about various ways to get more hot water or better hot water pressure and flow are at HOT WATER IMPROVEMENT where we provide a detailed guide to improving hot water quantity and flow when a tankless coil is installed as well as using other methods of hot water heating.

Watch out: setting water heater temperature higher to attempt to obtain more total hot water at the plumbing fixtures without also installing and properly setting anti-scald devices can result in dangerous, even fatal hot water scalding burns.

Inspection of Mixing Valves or Anti-Scald or Tempering Valves

This discussion has been moved to a separate article now found at ANTI-SCALD VALVE INSPECTION

Mixing 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)

Question: repair parts for Danfoss Mixing Valve

I am looking for repair kit for this Danfoss TMV mixing value or O-rings for use in my building in Nova Scotia, Canada - Anonymous by private email, 2017/04/03

Danfoss anti-scald TMV mixing valve repair (C) GS Danfoss anti-scald TMV mixing valve repair (C) GS


I can see why you asked, Anon, as it's difficult to find repair parts for Danfoss TMVs in Nova Scotia.

The first document below lists the components and parts of the mixing valve you describe - or a model close to it. The second provides instructions for a newer Thermostatic Mixing Valve or TMV from Danfoss, the Series 30 HR/HV.

Danfoss does not sell directly to the public. To buy Danfoss equipment or repair parts, check with your local plumbing and heating supplier or with one of the online Danfoss TMV repair part kit suppliers we'll list below.


Continue reading at ANTI SCALD VALVE PROTECTION, Best Practices 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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