Norwegian bath anti-scald valve Best Practices Guide to Mixing Valves & Anti Scald Valves on Water Heating Equipment & Plumbing Fixtures

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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.

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Best Practices Guide to Scald Protection for Bathrooms, Tubs, Showers

Sparco tempering valve (C) Daniel FriedmanOur 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.

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There are several approaches to controlling hot water temperature to avoid scalding burns:

  1. 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.
  2. 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.
  3. 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

Sparco tempering valve (C) Daniel FriedmanThis 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.

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

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.

For more about kitchen and bath design and installation see BATH & KITCHEN DESIGN GUIDE and PLUMBING SYSTEM INSPECT DIAGNOSE REPAIR.

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