Hot Water System Information

GENERAL DISCLAIMER: The information in these links are for your general information. The links above provide a comprehensive list of information from various sources that may not always be consistent. The views, opinions and design information in the manufacturer, and association links above do not necessarily reflect the views and opinions of Ron George or Ron George Design & Consulting Services or any subsidiary.

Mr. George is an expert in Designing Domestic Hot Water Systems and Investigating Hot Water Systems Failures. Below are some Hot Water Systems and Scald Burn Information.

Scald Burn Facts

1. Plumbing to Prevent Domestic Hot Water Scalds*

2. Degree of Burns*

The scald burn studies done by doctors Moritz and Henriques showed that it took approximately eight minutes of exposure to temperatures in the range of 120 Degrees Fahrenheit(F) for adults to develop a serious scald burn: Someone exposed to water at 120 F would have up to eight minutes to get out of harm's way before an injury started to develop. (It should be noted that children and infants skin is thinner and could develop burns sooner than eight minutes.) The 120 F temperature limit has become an industry standard for scald prevention in showers and combination bathtub/showers. The American Society of Plumbing Engineers (ASPE) is nearing completion of a standard on recommended temperature limits for domestic hot water fixtures. The new standard will cover temperature limits for a wide variety of plumbing fixtures. There will be a few fixtures that will have temperature limits below 120 F.

Temperature Control for Hot Water Systems

The Following is a discussion on the standards for temperature controls for domestic hot water systems.

ASSE 1016 - Automatic Compensating Valves for Individual Showers and Tub/Shower Combinations
Because showering occurs in a confined space while standing on a wet floor surface with a large portion of the body in contact with flowing water discharged from a fixed showerhead, a significant hazard is present. In addition to the risk of scalding, reacting abruptly by moving away from the flowing water can cause a serious injury from a slip or fall. ASSE 1016 valves are designed to protect against thermal shock and scalding. The standard called for a maximum temperature limit stop of 120 F and includes pressure balancing, thermostatic and combination pressure balancing/thermostatic valves. Pressure balancing devices have a pressure balancing piston or diaphragm that equalizes the pressure between the two inlet ports and maintains the same outlet temperature, as long as the incoming temperatures remain the same. Thermostatic type devices have a temperature-sensing element that senses the mixed water in the mixing chamber and adjusts the inlet ports with a bi-metallic thermal element, liquid paraffin wax piston or other type of thermal sensing element to maintain a relatively constant outlet temperature. The combination thermostatic and pressure balancing type devices have temperature-sensing and pressure-sensing elements and can compensate for both temperature and pressure changes.

During the standard update process, the development of the type TP devices in the 1990s highlighted the need for a new standard because the tests for the different types of devices were different; however manufacturers resisted creating a new standard and splitting it into three separate standards. The problem was the ASSE 1016 standard was already accepted in most of the model codes. Manufacturers would have to go back to the model plumbing code organizations and state codes and have the new standards accepted. This would have been extremely expensive and in jurisdictions where they are slow in updating to the latest edition of the code, whichever type of shower valve that kept the ASSE 1016 number would have had an advantage over the other types of valves as far as code acceptance. The compromise was to have one standard with three different types of valves and three different tests with one standard number.

ASSE 1062 - Temperature Actuated Flow Reduction Devices
Older showers with two-handled valves or single-handled mechanical mixers that do not meet the requirements of ASSE 1016 do not provide scalding or thermal shock protection. These two-handled installations can be provided with a temperature actuated flow reduction (TAFR) valve, a device that complies with ASSE 1062 and reduces the scalding risk but does not provide thermal shock protection. The TAFR valve is installed between the showerhead and the shower arm. In combination tub/shower installations, a TAFR can be screwed into the tub spout. The TAFR senses the water flowing through the device. When the water temperature exceeds the setpoint (about 115 - 117 F), the valve closes and only allows a trickle of flow. The trickle of flow is designed to allow the bather to readjust the controls to a cooler setting and to let the cooler water reset the TAFR valve. The resetting process takes about 15 - 20 seconds, but that is a small inconvenience, considering that the device can protect the bather from severe scald burns.

ASSE 1066 - Individual Pressure Balancing Valves for Individual Fixture Fittings.
To reduce the risk of thermal shock in older two-handled shower valve installations, an ASSE 1066 in-line pressure-balancing valve can be installed in the hot and cold water supply lines to the faucet. This device controls pressure imbalances that lead to thermal shock; it does not provide scald protection.

ASSE 1017 - Temperature Actuated Mixing Valves for Hot Water Distribution Systems.
Thermostatic mixing valves complying with ASSE 1017 are designed to control temperature from +/- 3 - 7 F, depending on the size, when flowing at the required flow rate. It should be noted that ASSE 1017 has no test for compensation during pressure fluctuation, so in order to minimize pressure fluctuations between the hot and cold water lines; the mixing valve needs to be located at the hot water source. If there is a circulating pump in the system, care should be taken to make sure a mixing valve is piped properly. The tempered water return pipe should split after the tempered water circulating pump and one line should be routed to the cold water inlet of the water heater with a balancing valve to throttle this flow. The other valve should be routed to the cold water inlet of the ASSE 1017 thermostatic mixing valve. This piping arrangement allows the mixing valve to mix 100 degree return water with 140 degree hot water to get 120 tempered water deliver to the tempered water system when there is no flow from any of the fixtures. If there is no flow, no cold water can mix with the hot water to temper the hot water. If the tempered water return pipe is only routed back to the water heater, and there is no flow from a fixture, the circulating pump will force hot water to leak through or blow by the clearances in the mixing valve causing the tempered water line to rise to the full water heater outlet temperature setting. If the tempered water return line is only routed back to the cold water inlet or mixing valve return inlet, then the cooler tempered water return water will leak through or blow by the clearance in the mixing valve and the tempered water system temperature will eventually drop to ambient temperature. This would provide an opportunity for thermal shock as the hot water arrives at a shower after the system has cooled down.

ASSE 1069 - Automatic Temperature Control Mixing Valves.
The ASSE 1069 Automatic Temperature Control Mixing Valves are designed for gang shower applications such as those in a school, prison or health club with On-Off controls at each shower station. A single thermostatic mixing valve is exposed to a wide range of flow rates. This valve standard was developed to deal with a wide range of flows with tight temperature tolerances at low flows. If only one shower is flowing, the flow rate could be as low as two gallons per minute. In larger installations, it may be necessary to install a hi-low system that incorporates two or more thermostatic mixing valves. A smaller valve is used to compensate and maintain temperature during low flow conditions, and multiple valves work together as the flow rate increases.

ASSE 1070 - Water Temperature Limiting Devices.
In-line devices such as the ASSE 1070 thermostatic mixing valve limit the water temperature of the hot water supply to a shower valve, but they do not provide thermal shock protection, since additional unprotected cold water is mixed in downstream of the device. This device limits the water temperature to fixtures such as sinks, lavatories or bathtubs and reduces the risk of scalding. The device shall be either the final temperature regulation or have water further tempered downstream of the device with the addition of cold water.

Articles on Hot Water System Design and Temperature Control

Improving Bathing Safety*

Shower Safety: Special Report*

Scald Burns Can Occur in Children at Lower Temperatures*

Legionella in Domestic Hot Water Systems

What is Legionnaires' disease?

Legionnaires' disease (LEE-juh-nares) is caused by a type of bacteria called Legionella or Legionellae. The bacteria got its name in 1976, when many people who went to a Philadelphia convention of the American Legion suffered from an outbreak of this disease, a type of pneumonia (lung infection). Although this type of bacteria was around before1976, more illness from Legionnaires' disease is being detected now. This is because we are now testing for this disease whenever a patient has pneumonia.

Each year, between 8,000 and 18,000 people are hospitalized with Legionnaires' disease in the U.S. However, many infections are not diagnosed or reported because the symptoms are very similar to pneumonia and are often misdiagnosed. The number of cases is likely to be much higher. More illness is usually found in the summer and early fall, but it can happen any time of year.

Legionellae bacteria grows and thrives in warm water which is commonly found in cooling towers and water heater storage tanks. The bacteria is typically transported to humans through water vapor mists from cooling tower sprays, hot tubs and shower heads. There are several means for controlling legionellae bacteria in water supplies. Several of the links below cover methods of controlling legionellae in mechanical systems.

What are the symptoms of Legionnaires' disease?

Legionnaires' disease can have symptoms like many other forms of pneumonia, so it can be hard to diagnose at first. Signs of the disease can include: a high fever, chills, and a cough. Some people may also suffer from muscle aches and headaches. Chest X-rays are needed to find the pneumonia caused by the bacteria, and other tests can be done on sputum (phlegm), as well as blood or urine to find evidence of the bacteria in the body.

These symptoms usually begin 2 to 14 days after being exposed to the bacteria.

A milder infection caused by the same type of Legionella bacteria is called Pontiac Fever . The symptoms of Pontiac Fever usually last for 2 to 5 days and may also include fever, headaches, and muscle aches; however, there is no pneumonia. Symptoms go away on their own without treatment and without causing further problems.

Pontiac Fever and Legionnaires' disease may also be called "Legionellosis" (LEE-juh-nuh-low-sis) separately or together.

1. Cash ACME - Legionella & Scalding Resource Center*

2. CDC - Legionella Disease Information*

3. eMedicine - Legionella Infection Information*

4. Legionella.org - General Disease Information*

5. Legionnaire's Disease FAQ*

6. Legionella Outbreak News*

7. University of Texas Medical Branch - Graduate School of Biomedical Science - Legionella Infection Study*

Hot Water System Design Issues

Content Coming Soon...

Water Heater Information

Water Heater Thermostats.

Water heater thermostats do not control temperature. I cringe every time I hear a radio talk show host or some one tell people to turn their water heater thermostats down to prevent scalding. Turning the thermostat down will not prevent scalding. I see the same mis-guided advice dispensed in newspaper handyman columns that talk about water heaters or home safety. Water heater thermostats cannot be relied upon to control the hot water temperature to a hot water system. Although water heater manufacturers recommend that installers set thermostats at 120 - 125 F, and although most of them ship the water heaters at lower temperature settings, it is impossible to accurately control the water heater temperature with a water heater thermostat. The plumbing engineering community continues to recommend storing hot water at 140 degrees Fahrenheit and mixing the hot water with an ASSE 1017 thermostatic valve to deliver hot water to the plumbing system at about 120 degrees Fahrenheit. This reduces the threat of Legionella bacteria growth in the tank, and allows a smaller water heater than one with a lower stored temperature.

Hot water systems are designed and water heaters are generally sized to store hot water at 140 F. When the thermostat is set at a lower temperature, the water heater has a reduced capacity to deliver hot water. As a result, when users run out of hot water, and the water heater thermostat is re-adjusted to a temperature above 120 F, this increases the temperature to a scalding temperature and in some cases it could be adjusted even higher than the original setting. The water heater will continue to store and deliver water at scalding hot temperatures. The solution is using a thermostatic mixing valve not tinkering with the water heater burner controls.

Water heater thermostats were never intended to provide precise temperature controls for hot water systems. For example: the thermostat dial calibration test of ANSI Z21.10.1-1998, which is the applicable standard for gas-fired water heaters, allows the temperature to vary 10 degrees above or below the thermostat setting. I have talked to water heater manufacturers that have indicated that the controls can vary as much as 15 to 18 degrees Fahrenheit above or below the set point of he thermostat. The thermostat is inserted into the lower portion of a water heater tank and turns the fuel supply to the heater on and off. Most newer water heater thermostat dials have no way to know what the temperature in the tank is. There are letter designations, numbers, or words like, warm, hot and very hot. There is generally never a fixed temperature indicated on the dial.

Theoretically, if the water heater thermostatic element is set at 120 degrees Fahrenheit, the burner would come on when the temperature at the thermostat reaches about 105 degrees Fahrenheit. The burner stays on until the water around the thermostat element near the bottom of the heater reaches about 135 degrees Fahrenheit (about 30 degrees higher than when the burner came on and 15 degrees above the theoretical set point of the thermostat).
Most people don't realize that the maximum temperature limit test of the ANSI Z21.10.1 Gas Water Heater Standard allows the outlet water temperature of the water heater to rise 30 F above the thermostat setting. This provision in the standard accounts for the phenomenon known as "stacking" or "thermal layering". The hot water is less dense and rises to the top of the hot water tank. The cooler water drops to the bottom of the tank. Stacking or layering occurs when hot water rises to the top of the heater due to recurring short duration heating cycles caused by a frequent number of small quantity hot water uses. This phenomenon can occur in any type of storage water heater and generally is more significant in vertical heaters.
At the top of a water heater that is theoretically set for 120 F to prevent scalding, the temperatures can easily reach 150 to 165 degrees Fahrenheit. These extremely high temperatures will cause third degrees burns and severe scald injuries in an instant upon contact with the skin. This is why I highly recommend installing the proper type of an ASSE 1017 thermostatic mixing valve on the outlet piping of a water heater to limit the hot water distribution temperatures to a maximum safe delivery temperature of 120 F. If high temperature hot water uses are required in a building, I recommend installing an ASSE 1070 thermostatic mixing valve on the local branch piping serving a fixture or group of fixtures. The mixing valve can then reduce the hot water temperature to a safe temperature.

Water Heater Rescue*

Water Heater Manufacturers

Storage Type Water Heaters:

A.O. Smith Water Heaters

American Standard

American Water Heater Company

Bradford White

Hubble Water Heaters

John Wood Water Heaters

Laars Water Heaters

Lochinvar

Marathon Water Heaters

Patterson-Kelley

PVI Industries

Rheem Manufacturing Company

State Industries

Instantaneous Water Heaters:

Gas Fired Copper Fin Tube Heaters:

A.O. Smith Water Heaters

HW Circulation System Information

Hot Water Re-Circulation Systems

Circulating Pump Manufacturers

Circulating pumps in domestic hot water systems should not be cast iron. Pump manufacturers make circulating pumps out of stainless steel or all bronze construction specifically for domestic hot water systems.

Thermostatic Mixing Valve Information

Design Considerations for Domestic Hot Water Return Systems with Mixing Valves*

Hot Water Temperature Maintenance and the Codes

Master Mixing Valve Sizing

Thermostatic Mixing Valve Manufacturers

ASSE 1017 Listed Valves

Apollo Valves (Conbranco Industries)

Armstrong - RADA