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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. This article series answers most questions about Heating System Boiler Controls on central heating systems to aid in troubleshooting, inspection, diagnosis, and repairs.
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.
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).
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
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
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
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.
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.
[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
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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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 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.)