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In protecting against scalding, the manufacturer and installer must ensure a safe installation

By George Lanthier

Figure 1

A few years ago, 1998 to be exact, a good friend of mine, Bob Suffredini of Therma Flow/Everhot, and I put a book together on domestic hot water. Although it contains just about everything you would ever want to know on the subject, there’s one issue that keeps coming up all of the time, scalding. This issue is not only getting trades people sued, but also the mental and physical damage done to families and individuals can be devastating when it occurs. Here’s an excerpt from our text The Hot Water Handbook on the subject:

‘Before you can thoroughly discuss and understand an area such as ‘scalding,’ you must first learn that in the world of the product- and installation-liability attorney there is no such thing as ‘common-sense.’ In that world, the manufacturer and installer must ensure a safe installation regardless of the various uses that the domestic water system and heating system could be exposed to, and how the end-user will be affected.

‘So, although we may agree with many of you that feel the scalding issue is mostly preventable by the end-user, we will approach it from a technical standpoint as though it were not.

‘This material was specifically prepared in regards to the prevention of scalding of humans and domestic animals within the confines of the Commonwealth of Massachusetts, where the authors live and where the book is published. It should only be used as a guideline outside of the Commonwealth since local and state codes in your jurisdiction have precedent over those included here and vary from jurisdiction to jurisdiction. However, it is agreed that Massachusetts’s code is neither the toughest nor the easiest in the United States.

Figure 2

‘Our goal is to cover the heating of water by the use of any water heater, range boiler, vessel or boiler front (internal tankless), or container used for supplying hot water for domestic, culinary or sanitary purposes.

‘It should be noted at the outset that there are many known methods and techniques used by the plumbing and heating trades to reduce the output of scalding temperatures from domestic hot water heaters. It should also be noted that many of these methods are not used by many tradesmen due to high initial cost, installation costs, and operating and service maintenance costs and time, as well as customer preference.

‘However, that does not mean that these methods and devices do not work; it simply means that in most cases they are ignored for the reasons previously noted. It should also be mentioned that any of these solutions are less expensive than one hour of a liability attorney’s time. More importantly, the Gas Appliance Manufacturers Association’s Water Heater Division states that ‘more than 4,000 children are scalded by tap water every year’ and that is from electric, gas and oil-fired equipment.

‘The installation, repair and servicing of any hot water heater by persons who do not hold the appropriate licenses is not only illegal, but may also be dangerous due to those inadequately trained personnel. Those not familiar with any state’s ‘Plumbing Code’ and ‘Sanitary Code’ should not work on or about DHW heaters, and this practice is in no way condoned by the authors.”

The most effective way to prevent the scalding of humans and most domesticated animals is the use of an ‘anti-scald” valve, Figure 1, period. This valve is also now referred to generically as an ‘automatic mixing valve.” In many applications, such as hospitals and nursing homes, these valves, and other requirements, are mandated under local Plumbing Codes and State Building Codes.

Although most state codes require that a domestic hot water heater be tested, it is important to note the parameters of these tests. When referencing Massachusetts Codes we found the following:

In no case should a domestic water heater be allowed to operate with a temperature in excess of 212F.
In no case shall a tankless heater be allowed to operate with a temperature in excess of 180F.
To prevent both of these scenarios, a temperature/pressure relief valve should be installed on storage-tank-type water heaters, and a pressure relief valve on tankless heaters.In addition we also found these other requirements:

When supplying a shower of any kind, you cannot exceed a maximum temperature of 112F at the showerhead.
The Sanitary Code states that “the domestic hot water shall be a minimum of 110F and a maximum of 130F.”Another statement in the Massachusetts Plumbing Code states that the following temperatures are considered the maximum allowed for hospital applications: 125F for fixtures, 180F for kitchens and 180F for laundry. This is important to note since the Commonwealth defines a hospital as ‘any facility for the care of the sick.” This means nursing homes, rest homes and charitable homes for the aged, and rehabilitation centers. Does it also mean private homes designated specifically for use by the handicapped? We don’t know, but we wouldn’t bet against it.

Basic Considerations
The first consideration to providing the proper supply of anti-scalding hot water that will satisfy the requirements of the user is that at a higher boiler water temperature, smaller equipment may be used.

Conversely, if you are to provide adequate hot water at lower boiler water temperatures, larger equipment must be used. Essentially the basic rule for hot water production must be observed, ‘storage or recovery.”

It should also be noted that if the decision is made to go with lower temperatures, the effect of those lowered temperatures on the heat-emission rate of the heating system radiation must be examined. If the combination DHW-heating system was designed for 180F water and the temperature is lowered, will the house heat? The answer, of course, is ‘no.” Keep in mind that by lowering the water temperature in any storage-tank-type system, it may lead to the formation and growth of bacterial bodies, including those linked to ‘Legionnaire’s Disease.” This will not happen with a domestic hot water system that uses a tankless only. With the tankless heater system, there is no tank, so all of the water is drawn and passed through the coil during a draw.

Proper sizing of equipment is essential. Sizing must be done to not only supply the ‘quantity” of water required, but also to ‘quantify” the amount of hot water at the desired temperature.

Stacking is common in all hot water heaters, including an internal or external tankless. Stacking occurs whenever a hot water heater develops temperature and the domestic water is not being drawn. This will normally occur during many on-and-off periods that are caused by the users through the use of many quick or short draws of hot water. Most controls and heaters in use are not designed to accommodate for this problem. It is important to identify and respect this factor since it will lead to higher temperatures during an initial draw from the heater. One of the areas we have noted is the maximum temperature allowed under the law, but does this take in the initial stacking ‘spike?” At the time we went to press, 1998, no one was sure.

Boiler-powered heaters are a major concern. These units may produce very high domestic water temperatures since the boiler may be required to operate at temperatures much higher than hot water demands. A hot water heating system designed to produce and supply 180F water to the radiation will also supply 180F water to the heat exchanger in use. Steam boilers can and may produce water/vapor temperatures on the heat exchanger in use as high as 240F during peak winter operation.

Direct-fired hot water heaters are not the perfect solution to the anti-scalding dilemma either since operating controls are also subject to stacking, and the controls can be easily changed by anyone, including the end-user. In fact, most states, including Massachusetts, dictate that a tempering or mixing valve be used on any DHW producer.

Anti-Scald Precautions
Simple and inexpensive methods can be used to prevent scalding and in the following we will offer you time-proven and industry accepted methods.

Tankless Heaters

Install a good quality mixing or anti-scald valve and trap the valve 8 inches to 12 inches below the heater.
Operate the boiler at the lowest possible temperature that will provide adequate hot water. This may require the use of a larger coil.
When an external tankless is used with a circulator, always use a flow-check valve to eliminate gravity flow through the tankless during heat cycles.
NEVER restrict the flow of cold water to a mixing valve.Tankless & Storage Tank

Operate the boiler at the lowest possible temperature that will provide adequate hot water. This may require the use of a larger booster and a larger coil.
Operate the tank aquastat at a lower temperature.
Create a “heat trap” or install a flow-check valve in the supply line from the tankless to the storage tank.
Install a “venturi” or “jet” tee to create an even temperature exiting the tank.Indirect Heaters

Operate the boiler at the lowest possible temperature that will provide adequate hot water. This may require the use of a larger indirect tank, or at least an increase in the coil size.
Operate the indirect at a lower temperature.
Create a “heat trap” or install a flow-check valve in the supply line from the boiler to the indirect.
Install a “venturi” or “jet” tee to create an even temperature exiting the tank.Direct-Fired Water Heaters
Even with direct-fired heaters, where there is a reduced risk of scalding since the operating control is not affected by the presence of a higher temperature boiler, water precautions should be taken to try and reduce scalding. Here are some recommendations:

Operate the heater at the lowest possible temperature that will provide adequate hot water. This may require the use of a larger water heater.
Lower the nozzle size on oil, and the gas flowrate on gas, if possible. If a longer burner run can be created, it will reduce the effects of stacking and be more efficient. It must be noted, however, that the burner and tank must be sized to still provide the amount of hot water required.
Install a “venturi” or “jet” tee to create an even temperature exiting the tank (see Figure 2). This piping, in tandem with an automatic mixing valve, not only will prevent stacking and scalding, but it will also provide a more abundant and consistent source of hot water and is the optimum piping arrangement for any domestic hot water heater.See ya.

George Lanthier is the owner of the Firedragon Academy, a teaching, publishing and consulting firm. He is a proctor and trainer for the industry’s certification programs and is the author of nine books on oilheating and HVAC subjects. He can be reached at 132 Lowell Street, Arlington, MA, 02474-2756. His phone is (781) 646-2584, fax at (781) 641-7099 and his Web site featuring the industry’s only ‘trade-only” discussion board can be found at

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