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AQUASTAT CONTROL Functions
CHECK VALVES, WATER SUPPLY
CLOGGED SUPPLY PIPING
DEBRIS in WATER SUPPLY, Water Heater
FLOODED HEATING EQUIPMENT REPAIR
FREEZE-PROOF A BUILDING
HEAT TAPES, Heat, Insulation prevent Freeze-Up
HOT WATER SUPPLY
HOT WATER IMPROVEMENTS
MANUALS & PARTS GUIDES - HVAC
MIXING / ANTI-SCALD VALVES
NO HEAT - NO HOT WATER: HEATER DIAGNOSIS
NOISE, PLUMBING CHECKLIST
ODORS GASES SMELLS, DIAGNOSIS & CURE
PIPING IN buildings, Clogs Leaks Types
PLASTIC HEATER VENT
PLUMBING FIXTURES, KITCHEN, BATH
RELIEF VALVES - TP Valves on Boilers
SEWER GAS ODORS
SOOT on OIL FIRED HEATING EQUIPMENT
SPILL SWITCHES - Flue Gas Detection
WATER HEATER ALTERNATIVES
WATER HEATER ANODES, DIP TUBES
WATER HEATER NOISES
WATER HEATER SCALE - De-Liming Procedure
WATER HEATER SCALE PREVENTION
WATER ODORS, CAUSE CURE
WATER PIPES, Clogs Leaks Types
WATER SOFTENERS & CONDITIONERS
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 website 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. For more about kitchen and bath design and installation see BATH & KITCHEN DESIGN GUIDE and PLUMBING SYSTEM INSPECT DIAGNOSE REPAIR.
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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:
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:
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.
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.
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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