Best Practices Guide to Mixing Valves & Anti Scald Valves on Water Heating Equipment & Plumbing Fixtures
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What are mixing valves - anti-scald valves or tempering valves
Methods for avoiding scalding burns at plumbing fixtures
How to mixing valves or anti scald valves work
How to install and use anti-scald mixing valves
Questions & answers about choosing and installing anti scald devices on building plumbing and heating systems

This article series explains 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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Best Practices Guide to Scald Protection for Bathrooms, Tubs, Showers

Our home page for anti scald devices used in plumbing systems is MIXING / ANTI-SCALD VALVES. The article below, MIX VALVE SCALD PROTECTION, Best Practices, includes excerpts or adaptations from Chapter 6 of Best Practices Guide to Residential Construction, courtesy of Wiley & Sons and written by Steven Bliss.

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.

There are several 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. Below 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 (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 sometimes used to obtain more total hot water as we discuss at Mix Valve Improves Hot Water Quantity and detail at HOT WATER IMPROVEMENT.

This approach is discussed beginning at How Anti Scald Valves Work, and further reviewed by Steven Bliss at Water Pressure-Balancing Valves.
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 (to obtain more total hot water as we discuss at Mix Valve Improves Hot Water Quantity and detail at HOT WATER IMPROVEMENT).

This approach is illustrated and explained at Built-in Fixture Anti-Scald Valves and is discussed further by Mr. Bliss at Thermostatic Mixing Valves.

As noted in Chapter 6 of Best Practices Guide to Residential Construction:

There are many styles of mixer valves in tubs and showers, but all should have some sort of protection against scalding. The young and elderly are at greatest risk due to thinner skin and slower reaction times. More than 35,000 children, most under age six, are treated each year in emergency rooms for tap-water scald burns, according to the National Safe Kids Campaign.

A child exposed to 140°F water for as little as three seconds (see Table of Scalding Temperatures & Times) can sustain a third-degree burn requiring hospitalization and skin grafts. Although most scald burns occur in the kitchen, according to the Consumer Product Safety Commission, the most severe scald burns are caused by water flowing into the tub or shower. In response, many state and local codes now require antiscald protection in residential tubs and showers.

Many organizations advocate setting water heaters to no more than 120°F, which gives bathers significantly more time to move away or adjust the temperature before a burn occurs. While this strategy is helpful, it is not always reliable and can cause other problems:

Noncompliance: many homeowners turn the thermostats up to increase supply.
Water heater thermostats are often inaccurate. The ANSI standard for gas water heaters allows the temperature to vary by plus-or-minus 10°F.
Stacking effect: water at the top of a gas water heater can exceed the set point by as much as 30°F.

Use of Point of Supply Water Pressure-Balancing Valves to Control Temperature & Avoid Scalding

This is the most commonly used point-of-supply approach. These automatically adjust the water pressure to maintain the mix of hot and cold water to within 2°F to 3°F of where the user set the shower control.

If cold water is diverted to a flushing toilet or other appliance and the pressure drops, the valve automatically reduces the hot water flow proportionately to maintain the temperature.

If the cold water pressure plummets or stops altogether, the flow is reduced to a trickle.

To guard against someone accidentally turning the shower valve to scalding temperatures, these valves typically use a temperature limit stop that prevents the user from turning the shower control past a set point—typically set at or below 120°F.

Use of Point of Point of Use Thermostatic Mixing Valves to Avoid Scalding Burns

Some models of thermostatic mixing valves are more expensive than pressure-balancing valves and not widely used in the United States.

Thermostatic (temperature sensing) anti-scald valves can be installed either inline near the water heater or as part of the shower control. The inline type of valve, also called a tempering valve, adds cold water to the hot water as it leaves the water heater to maintain a constant temperature, set by the installer. These are commonly used with solar water heaters but can also be tied to a conventional water heater.

A check valve is required on the cold water side to prevent backflow, and a hot-water expansion tank is recommended to prevent excessive pressure on the hot water side.

Figure 6-1: Accessible Kitchen Design Specs: accessible sink and dishwasher work centers (C) J Wiley S Bliss

The other type of thermostatic valve is built into some high-end showers (Figure 6-54). These allow the user to set the temperature on a dial when showering.

The unit will compensate for changes in either pressure or temperature to maintain a constant delivery temperature and flow rate.

If the cold water fails or the tempered water is still too hot for any reason, the unit will shut off the flow.

As with the pressure-balancing valve, the installer sets a temperature limit stop to prevent the user from turning the shower control to scalding temperatures.

We further discuss and illustrate this approach as it is installed in a modern bathroom in Molde, Norway at Built-in Fixture Anti-Scald Valves.

[Click any image or table to see an enlarged version with additional detail, commentary & source citation.]

Retrofits to Avoid Scalding Burns at Plumbing Fixtures

In retrofits, point-of-use devices can be installed by a plumber or homeowner to limit water temperatures to 120°F. These include antiscald showerheads, as well as point-of-use devices that fit into individual plumbing fixtures, such as showerheads and bath and sink faucets.

For example, MemrySafe and ScaldShield (Antiscald Inc.) are inexpensive retrofit devices that reduce the water flow to less than 1/4 gallon per minute when the water temperature at the faucet or showerhead exceeds 120°F. These devices do not regulate temperature or pressure, but do offer protection against serious burns.

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MIXING / ANTI-SCALD VALVES
  Define Mixing Valves, Anti Scald Valves
  Table of Scalding Temperatures & Times
  Hot Water Anti-Scald Regulations
  How Anti Scald Valves Work
  Installing & Setting Anti-Scald Valves
  Which Way To Turn the Mixing Valve
  Using a Manual Hot Water Tempering Valve
  Built-in Fixture Anti-Scald Valves
  Mix Valve Improves Hot Water Quantity
  Inspect Anti-Scald Valves
MIX VALVE SCALD PROTECTION, Best Practices
  Water Pressure-Balancing Valves
  Thermostatic Mixing Valves
  Retrofits to Avoid Scalding Burns

Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. Thanks to Alan Carson and Bob Dunlop, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education including the ASHI-adopted Home Inspection Training Program (home study course), publications such as the Home Reference Book, report writing materials including the Horizon report writer, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
A.O. Smith water heater safety sticker: 182734-000 Rev.00 observed on the gas fired water heater shown at High Efficiency Gas Water Heaters
Thanks to reader J.S.A. for discussing hot water mixing valves and which way to turn the valve - July 2010
Thanks to reader Athena for discussing hot water mixing valves and which way to turn the valve to make water hotter or cooler - October 2010.
"Domestic Hot Water Scald Burn Lawsuits, The Who, What, When, Why, Where, How", Dr. D. Bynum Jr., Vernon L. Petri, Esq., John T. Myers, paralegal, Seminar and Technical Paper for the 25-28 Oct 98 Annual ASPE Meeting at the Indianapolis Convention Center, Indianapolis IN, websearch 09/21/2010,original source: http://media.wattswater.com/F-H20LS-Reprint.pdf [Large PDF]

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
Fuel Oil & Oil Heating Magazine, 3621 Hill Rd., Parsippany, NJ 07054, 973-331-9545
Goodman Furnace High Temperature Plastic Vent HTPV safety recall US CPSC notice
Home Heating System Should Be Checked [for proper venting and for CO Carbon Monoxide Hazards - DJF]
Inspection Procedures for Oil-Fired Heating Systems Detailed step by step approaches for inspecting complex systems]
Lennox Pulse Furnace Safety Inspection/Warranty Program: Carbon Monoxide Warning
Oil Tanks - The Oil Storage Tank Information Website: Buried or Above Ground Oil Tank Inspection, Testing, Cleanup, Abandonment of Oil Tanks
Plastic Heating Vent Pipe & Other Heating Safety Recall Notices
Weil McLain Model GV Gas Boiler/gas valve CPSC recall/repair
Domestic and Commercial Oil Burners, Charles H. Burkhardt, McGraw Hill Book Company, New York 3rd Ed 1969.
National Fuel Gas Code (Z223.1) $16.00 and National Fuel Gas Code Handbook (Z223.2) $47.00 American Gas Association (A.G.A.), 1515 Wilson Boulevard, Arlington, VA 22209 also available from National Fire Protection Association, Batterymarch Park, Quincy, MA 02269. Fundamentals of Gas Appliance Venting and Ventilation, 1985, American Gas Association Laboratories, Engineering Services Department. American Gas Association, 1515 Wilson Boulevard, Arlington, VA 22209. Catalog #XHO585. Reprinted 1989.
The Steam Book, 1984, Training and Education Department, Fluid Handling Division, ITT [probably out of print, possibly available from several home inspection supply companies] Fuel Oil and Oil Heat Magazine, October 1990, offers an update,
Principles of Steam Heating, $13.25 includes postage. Fuel oil & Oil Heat Magazine, 389 Passaic Ave., Fairfield, NJ 07004.
The Lost Art of Steam Heating, Dan Holohan, 516-579-3046 FAX
Principles of Steam Heating, Dan Holohan, technical editor of Fuel Oil and Oil Heat magazine, 389 Passaic Ave., Fairfield, NJ 07004 ($12.+1.25 postage/handling).
"Residential Steam Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
"Residential Hydronic (circulating hot water) Heating Systems", Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
"Warm Air Heating Systems". Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
Heating, Ventilating, and Air Conditioning Volume I, Heating Fundamentals,
Boilers, Boiler Conversions, James E. Brumbaugh, ISBN 0-672-23389-4 (v. 1) Volume II, Oil, Gas, and Coal Burners, Controls, Ducts, Piping, Valves, James E. Brumbaugh, ISBN 0-672-23390-7 (v. 2) Volume III, Radiant Heating, Water Heaters, Ventilation, Air Conditioning, Heat Pumps, Air Cleaners, James E. Brumbaugh, ISBN 0-672-23383-5 (v. 3) or ISBN 0-672-23380-0 (set) Special Sales Director, Macmillan Publishing Co., 866 Third Ave., New York, NY 10022. Macmillan Publishing Co., NY
Installation Guide for Residential Hydronic Heating Systems
Installation Guide #200, The Hydronics Institute, 35 Russo Place, Berkeley Heights, NJ 07922
The ABC's of Retention Head Oil Burners, National Association of Oil Heat Service Managers, TM 115, National Old Timers' Association of the Energy Industry, PO Box 168, Mineola, NY 11501. (Excellent tips on spotting problems on oil-fired heating equipment. Booklet.)
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Best Practices Guide to Scald Protection for Bathrooms, Tubs, Showers

Use of Point of Supply Water Pressure-Balancing Valves to Control Temperature & Avoid Scalding

Use of Point of Point of Use Thermostatic Mixing Valves to Avoid Scalding Burns

Retrofits to Avoid Scalding Burns at Plumbing Fixtures

Questions & Answers regarding this article

Ask a Question or Search InspectAPedia

Technical Reviewers & References

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair