Here 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.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 IMPROVEMENTS. 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.
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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:
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:
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. 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 (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 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).
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.
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
In the sketch shown at left, courtesy of Carson Dunlop, 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 Hot Water Mixing Valves, Automatic Anti-Scald Tempering or Mixing Valves at a Tankless Coil - Water Pressure Balancing Type
Our photo at left, taken at a heating boiler in Portland ME, shows a popular Watts™ anti-scald valve made by the Watts Regulator Company.
You can see that the black knob permits the user to set the outgoing hot water temperature leaving the valve, and if you look closely you can see the "HOT" and "COLD" and "MIX" indicators on the three ports of the body of the valve.
Turning this Watts Reg. Co. automatic mixing valve clockwise makes the outgoing water temperature cooler. Turning the Watts mixing valve counterclockwise makes the outgoing water temperature hotter.
Hot water enters the valve's right side, cold water enters the mixing valve's bottom, and mixed or tempered hot water leaves the valve at its left side.
Using the modern Watts Regulator Co. Series LF1170 / LFL1170 Hot Water Temperature Control Valve instructions as an example, these valves control water temperature in the following ranges:
Question about tankless coil or other hot water mixing valve operation - example using the Sparco Aquamix AM 102C:
There should be a direction arrow on the anti-scald valve telling you what it is doing and which turn direction will raise or lower the outgoing water temperature.
The Sparco Aquamix AM 102C tempering valve shown (above-left) is marked to explain that if we turn this valve counterclockwise (the direction of the red arrow) the outgoing water temperature will be hotter.
Turn this anti-scald valve (and most model) clockwise (in the direction of the blue arrow) to make the outgoing water temperature cooler.
Our photo of a Sparco™ Aquamix AM102C anti-scald or tempering valve (above-left) as well as our photo of a Watts™ mixing valve shown earlier in this article both show the arrow that we describe and indicates that turning the automatic mixing valve knob clockwise makes the outgoing water cooler, and turning the automatic mixing counterclockwise makes the outgoing water hotter.
We don't describe the valve direction as "up" or "down" because these valves can be installed in more than one position, making "up" and "down" a bit confusing.
For hotter output water [on the Sparco or Watts Tempering Valve models shown]: Turn the automatic mixing valve knob counter-clockwise (on the Sparco valve above, in the direction of the red arrow) to make the output water from the system hotter.
For cooler output water: Turn the automatic mixing valve clockwise (on the Sparco valve above, in the direction of the blue arrow) to mix in more cold water and thus to make the output water from the system cooler.
Note that because these mixing valves are automatic, that is once they have been set they maintain a specified hot water temperature for water leaving the water heater, you don't have to make seasonal changes to the valve adjustment.
The automatic mixing valve, tempering valve, or anti-scald valve approach of installing this safety control at or close to the water heater is discussed further at Thermostatic Mixing Valves.
Watch out: Be sure to take a close look at the indicating arrows on your own automatic mixing valve as some tempering valve models may work differently. And for manual hot water mixing valves that we discuss below, the direction to turn the valve may not be marked, but is easy to determine, as we will explain.
If your automatic tempering or hot water mixing valve has different instructions or operates differently from those discussed here, CONTACT us and send along a photo so that we can add that information here.
By manual hot water tempering valve we mean that the plumber has installed an ordinary plumbing valve and a cold water bypass pipe to mix some cold water into the water heater's outgoing hot water supply.
This valve must be adjusted by the building occupant to obtain the desired temperature.
Our photo (left) shows a manual mixing valve (a gate valve) that is opened to add cold to outgoing hot from the tankless coils water heater. The valve in the center-right of the photo allows cold water to be added to the hot water leaving the coil. (The copper pipes were painted white - don't ask.)
A manual tempering valve does not respond to changing conditions on its own.
Watch out: a manual-only mixing valve is inherently an unsafe or unreliable means of scald burn protection. Water temperatures produced by the water heater may change for a variety of reasons surprising and even burning building occupants.
Which Way to Turn a Manual Anti-Scald Valve or non-Automatic Mixing Valve to Control Hot Water Temperature
By "manual" mixing valve we mean that an ordinary plumbing valve, usually a gate valve or ball valve is used to adjust outgoing hot water temperature, and the building occupants have to adjust the valve by hand, "manually". In most cases a manual hot water mixing or tempering valve (discussed below) will work oppositely from many models of the automatic tempering valves discussed above.
On a manual mixing valve or "tempering valve" if we "close" the manual valve by turning it clockwise we are are admitting less cold water into the mixing process, thus making the output water temperature hotter at plumbing fixtures in the building.
Here is a manual mixing valve on a hydronic heating system.
You may find a ball-valve like this controlling domestic hot water temperature, radiant floor heating temperature, or even individual heating zone water temperature.
This manually controlled valve (a human has to turn it) feeds some cold water into the hot water that is piped to building plumbing fixtures.
So, for a manually operated tempering or mixing valve,
When Should We Adjust a Manual Hot Water Tempering or Mixing Valve?
Hot Water Made by a Tankless Coil & Heating Boiler - Manual Adjustment of Tempering Valve
Watch out: if your hot water system uses a manual tempering valve and your hot water is made by a tankless coil and a hot water or steam heating boiler (as opposed to a separate hot water heater and tank), you will want to manually adjust the valve to produce a safe (below scalding temperature) hot water temperature measured at the plumbing fixture located closest to the hot water source (the heating boiler).
During the heating season when the boiler is hotter, you'll want to adjust the valve again to prevent the hot water temperature from being too high. In sum, you'll need to change the manual mixing valve setting twice a year.
At the end of the heating season, when the same heating boiler continues its job of making domestic hot water at a tankless coil or side-arm coil, you will need to adjust the manual tempering valve again to close off some of the mixed-in cold so that your water temperature is hot enough. That's because on most heating boilers using an aquastat or even separate individual boiler temperature controls, the boiler remains at a lower temperature when the building thermostats are never calling for heat.
Hot Water Made by a Separate Water Heater Using a Manually-Adjusted Tempering Valve
A second reason you may need to adjust the manual hot water mixing valve is that in some areas the temperature of the cold water entering the building may vary by season.
Even if your hot water is made by a separate hot water heating tank, your system might have a manual mixing valve installed and you might need to adjust that setting seasonally as incoming water temperatures change.
More information about tankless coils, how they work, what goes wrong, and their controls, is at TANKLESS COILS.
See WATER PIPE CLOG REPAIR for a discussion of loss of water pressure due to clogged piping or clogged tankless coils.
Guide to Bath Fixture Anti-Scald Valves, Automatic, Thermostatic Mixing Valves, or Tempering Valves to Avoid Hot Water Burns
Anti-scald valves that actually sense the water temperature are available built right into bathroom sink or shower fixtures, as you can see in our photo at left.
These devices permit delivery of very hot water to a building area but protect occupants from scalding by mixing in cold water right at the sink, tub, or shower.
As you can see on the control shown in our photo of a shower control in a shower-bath in Molde, Norway, fixture anti-scald valves may include a button and adjustment that lets the user demand hotter water than the control provides automatically.
On this bath shower fixture the left-hand control turns water on or off, and the right-hand control permits adjustment of the water temperature. Turning the temperature control "down" or counter-clockwise increases water temperature but the control "snaps" into a locked position at a safe hot water temperature of about 100 °F. The user can obtain hotter water by depressing a red button and turning the control further.
The point-of-use anti scald protection afforded by this type of pressure-balancing anti-scald valve is discussed further at Water Pressure-Balancing Valves.
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.
This discussion has been moved to a separate article now found at ANTI-SCALD VALVE INSPECTION
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. They 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 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 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.]
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.
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:
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]
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Frequently Asked Questions (FAQs) About hot water temperature safety control, anti-scald devices & mixing valves: choices, installation, troubleshooting
Question: Watts tempering valve problems
I had the inside of my Watts tempering valve replaced but I still have problems. In warmer weather the water is very hot but after several minutes it becomes warm. Someone said I should replace the hosing as it has a disc and spring that can malfunction. Any answers? - Ed 12/8/11
Ed, sometimes hot water rises into the hot water piping above the water heater by natural convection - it depends on piping arrangements. There are special check valves sold to prevent that trouble. I'd replace the valve rather than take a chance on an improper repair that could scald someone.
Question: how do hotels keep hot water safe and instantly available?
How do hotels insure that water stays at proper temperature and are they subject to inspections - Dawn 5/27/12
Dawn, some hotels use a system that continuously circulates hot water through a piping loop throughout the building. By that means, when you turn on hot water at the sink, hot water is right there by the sink, in the wall or floor, having only to rise up through a very short section of piping en route to the faucet. Details of using a hot water circulating loop for instant hot water are provided at
Question: hot water scalding burn report
i was recently burned in a fast food restaurant using a men's room urinal and was told this happens when the kit sink is on it forces scalding water to the urinal how is this possible - Ken 6/18/12
Ken, sorry to read that you were burned. In my OPINION there is absolutely no proper plumbing hook-up that would ever send scalding or even hot water to supply a urinal. It sounds like a plumbing mistake and most likely improper installation.
Question: hot water comes out of the cold water tap then turns cold
We have the problem that our *cold* tap water starts out hot, and only turns cold after running for a minute or so. (This is an apartment with its own 60 gallon gas water heater.) Could this be related to the anti-scald valve? - Henning Schulzrinne 9/11/12
We have a 6 unit apartment building and a tenant is experiencing very hot then cold showers after adjust it to a warm shower. It is a newer hot water heater. How do we provide consistent water temperatures for our tenants? - Joann 9/27/12
Install a tempering or mixing valve as discussed in these articles. Either at the shower or at the hot water source.
Question: I want to replace an old mixing valve on a Hoval Dulyte boiler
I am looking for a "mixing valve" for an old Hoval Dulyte boiler. It is German made and about 25 years old. The old valve was a T300, I believe, and it is now malfunctioning. I need to find a compatible part. The motor is mounted on a 3-way fitting measuring 1.25". If you have such a part, on any advice, it would be greatly appreciated. You can contact me by phone 860-267-6128 or email firstname.lastname@example.org
Maureen, you may be better off removing the old device, sealing its mounting openings, and installing a modern, readily-available anti-scald valve.
Question: Moen single lever shower control, irregular hot water, how do I avoid temperature swings
I have a shower with a Moen single lever control. I do not have a anti-scalding valve on the hot water heater. I experience significant and frequent swings in the temperature of the water from this shower if any water is being used anywhere else in the house including the basement washing machine (my shower is on the second floor). what would you recommend - John Pratapas 12/6/12
Questions & answers or comments about anti scald devices and mixing valves on hot water systems
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