When designing a hot water return system, there are a number of things you need to be aware of if the system has a mixing valve and is recirculated. This article will cover three basic system designs for hot water systems using mixing valves and recirculation pumps. Systems that do not follow this piping arrangement have been known to cause scald injuries and deaths to unsuspecting users.

Although there are many hot water system design options, when systems have a thermostatic mixing valve, they must be piped as shown in Figure 1 to prevent scald injuries or deaths. If the tempered water return is piped only to the cold water inlet of the water heater, the circulating pump will force hot water to leak through the mixing valve’s manufacturing tolerances and allow the water leaving the mixing valve outlet to reach the full hot water temperature of the water heater. This can cause a slug of very hot water in hot water distribution piping downstream of the mixing valve. People have been scalded and killed when this piping arrangement is present.

In installations where the hot water return is piped only to the cold water inlet of the mixing valve, it will allow the system to go ambient during periods of non-use and result in the system cooling to ambient temperatures. This causes a long delay before hot water gets to the shower and poor user satisfaction.

Master mixing valve system (Located at the hot water source)

The master mixing valve system design (Figure 1) shows the proper piping arrangement for a system complying with ASSE 1017 located at the water heater or hot water source, with a recirculated return piping arrangement.   

One of the most common mistakes made by designers and installers is connecting the domestic hot water return to only the water heater in a system with a mixing valve.

If the hot water return line is connected directly to the water heater only, the circulating pump forces hot water through the mixing valve when there is no flow from a faucet in the system. When there is no flow from a faucet, cold water cannot enter the mixing valve to blend with the hot water to make tempered water. This condition causes fluctuations in temperature and pressure. To correct this problem, most mixing valve manufacturers recommend splitting the return piping after the circulation pump, as shown in the piping diagram in Figure 1. This piping arrangement allows circulated flow from both sides of the mixing valve when there is no flow from a fixture during off-peak hours.

The hot water return piping should be split after the circulating pump and routed to two locations. The first location is the cold water inlet to the mixing valve where the cooler tempered water return should be 10-20 degrees lower than the mixing valve set point. The second location is the cold water inlet to the water heater where the cooler, tempered water return is circulated back to the water heater and then through the hot water distribution piping back to the hot water inlet of the mixing valve. A careful analysis of the building’s hot water distribution system should be made. Also, a temperature gauge should be used on each branch of the tempered water return piping after the circulating pump, as close to the cold water connection as possible (about 2 to 3 feet), so that the cold water pipe will not affect the temperature gauge reading. A manual balancing valve or thermostatic balancing valve should be installed in the branch going to the water heater to close-down or reduce the flow if the hot water temperature tends to get close to the set-point temperature of the mixing valve. Some mixing valve manufacturers have many of these components included in a packaged mixing valve and circulating pump assembly.

In many buildings, during peak flow periods, the hot water can get drawn out into the hot water distribution and return piping system and the return temperature can get to within a degree or two of the mixing valve set point. When this happens, some older-style mixing valves like bi-metallic coil-type valves, need to have at least a 20-degree temperature difference between the temperature entering the hot water port and the temperature entering the cold water port for the valve to react properly. As the return water temperature approaches the set point temperature, the bi-metal technology is less efficient at responding to subtle changes. Generally, the larger a liquid-filled thermal motor is, the more reactive it is as the tempered water return temperatures get closer to the mixing valve set-point temperature. Some manufacturers of digital mixing valves can accurately control tempered water return temperatures that are within a degree or two of the mixing valve set point.         

Most mixing valve manufacturers publish a minimum temperature differential between the hot and cold water sides of the mixing valve for it to function properly. The 20-degree temperature differential in the recirculation pipe sizing methods is based on the bimetallic coil type of mixing valve, which needs a significant difference in temperature for it to sense and react to changes. Newer liquid-filled technologies with larger thermal sensing units and digital mixing valves are able to mix water with much closer hot water return and tempered water set-point temperatures. This is very important in buildings with recirculated lines where the hot water return temperature can come back to the mixing valve within a few degrees of the set-point. With the requirements for more insulation on hot water circulation piping, heat losses are minimal and hot water return temperatures are coming back very close to the temperature setting of the mixing valve.

There are new products on the market to divert the majority of the tempered water return back to the cold water side of the mixing valve when the circulated return temperature gets close to the set point of the mixing valve. This is often done with a manual balancing valve or a thermostatic diverter valve (similar to a mixing valve installed backwards), or an in-line thermostatic flow reduction valve. (Check with the valve manufacturer for control options and piping diagrams)

Digital mixing valves are quickly becoming the choice for these conditions, and manufacturers now have the ability to record and control temperatures to within a degree or two. They also can maintain the outlet temperature very close to the mixing valve set-point even when the return temperatures are coming back close to the set-point. This is only possible if the hot water return pipe is split after the circulating pump to allow flow to both sides of the mixing valve as shown in Figure 1.

If you follow the hot water return pump sizing methods as outlined in hot water recirculation sizing methods published in both the ASPE Plumbing Engineering Design Handbook, or the ASHRAE Applications Handbook chapter on service water heating, you'll see both documents use a 20-degree temperature differential for determining the GPM required for the hot water return circuit. If you want a 10-degree temperature differential, you would simply calculate the required flow and adjust the final flow in GPM by multiplying by two. If you want a 5-degree differential, you multiply the final GPM by four. In the calculation method, there is a BTU loss per linear foot based on insulated or uninsulated pipe. Adjustments may be needed for the BTU loss numbers if the insulation is increased as is being required in many newer energy codes. With greater insulation thicknesses, the required GPM flow rate for hot water return systems are being reduced.

Local mixing valve

The piping diagram shown in Figure 2 is easy to pipe and maintain. Designers and installers should try to keep the system design and installation as simple and easy to maintain as possible. I have found the system to be cost-effective because you can pipe only one hot water temperature out and mix down to the various usage temperatures near the fixtures. The distribution temperature should be maintained above 124 F to keep the entire hot water distribution and recirculation temperature above the Legionella bacteria growth range of 68 F to 122 F. This minimum requirement to control bacteria growth is why it is important to have a temperature gauge after the recirculating pump, just before each hot water return branch connects to the cold water, to the water heater, and the cold water inlet of the mixing valve. Then, the temperature can be monitored for control of bacteria growth issues and the temperatures can be reduced near the fixtures to a lower the hot water temperature that will minimize scalding issues.

Dual temperature hot water systems

Dual temperature hot water systems as shown in Figure 3 offer more of a challenge to the plumbing designer and installer. They typically have a master mixing valve and two hot water return temperatures.  They also typically have two separate hot water return circulating pumps that need to be independent of each other to prevent the hot water return temperature at the cold side of the mixing valve for exceeding the set point of the valve.

For example, if the tempered water mixing valve is set at a lower temperature of about 125 F and the water heater is set to around 160 F, the high-temperature hot water return temperature would be about 140 F (with a 20-degree temperature drop across the system). The high-temperature loop hot-water-return temperature would be about 15 degrees hotter than the set point on the mixing valve. (It would be 35-degrees hotter if both return system GPM’s are designed for a 10-degree temperature difference in the hot water return loop.

Care should be taken to assure you do not create a piping system that causes the low temperature system to rise in temperature and exceed the mixing valve set point for the low temperature loop. This over-temperature danger is why the hot water return and tempered water return lines should be kept separated until after the circulating pumps and check valves. In Figure 3, notice how the tempered water return line splits after the circulating pump similar to the piping arrangement in Figure 1, and connects to the water heater and cold water side of the mixing valve.

Temperature gauge locations

Temperature gauges are not shown in these figures but should be installed at all “critical temperature control locations” in the hot water system.

These locations are:

1. Water heater inlet;
2. Water heater outlet;
3. Hot water return location before connection to the CW line to the water heater;
4. Cold water line entering the building;
5. CW inlet of the mixing valve;
6. HW inlet of the mixing valve;
7. Tempered Water outlet of a mixing valve;
8. Inlet of any heat recovery equipment; and
9. Outlet of any heat recovery equipment.

Always check with the valve manufacturer’s literature for recommended piping arrangements. Remember to include any pressure drop through the mixing valve in the head requirements for the circulating pump and verify the insulation type and thickness to determine the actual heat loss for the hot water piping.

The current methods for designing hot water return circulating pumps should probably be revisited by technical committees to address new energy code requirements for greater insulation thicknesses and to reduce the temperature drop across the system from 20 degrees to 10 degrees. Another option would be to revise the heat loss table for uninsulated and insulated pipe that shows heat loss per linear foot for a 20-degree temperature drop, 10-degree temperature drop, and a 5-degree temperature drop across the system.  There also should be information on various insulation types and thicknesses.

Temperature controls for hot water systems

As a member of the ASSE International, Scald Awareness Task Group, I strive to educate and give guidance to the public and plumbing industry on potential scalding hazards associated with domestic hot water systems. In addition, I serve on the ASSE main product standards committee and many of the ASSE product standards dealing with hot water temperature controls. Domestic hot water from showers, bathtubs and sinks accounts for a significant percentage of all scald burns in children, the elderly and disabled persons. As water flows have been reduced over the years, the risk of thermal shock and scalding has increased. The efforts to prevent thermal shock and scald injuries have been the driving force for many new product designs, innovations and inventions related to temperature control devices to address pressure imbalances and temperature fluctuations that are the causes of the incidents in showers and tub/showers.

ASSE temperature control standard application chart

The ASSE Scald Task Group developed an application chart to address each hot water temperature control product standard’s application. This was done to help guide people in choosing the appropriate product for each specific application as to where the temperature control devices are intended to be used in a hot water system. Over the years, I have received many calls and e-mails from inspectors, installers, engineers and others asking if certain types of devices can be used in certain installations. In many cases, there are misapplications, and someone would argue that the device they chose controls the temperature, but the product standard and associated tests did not cover the given application and somehow, they felt a need to appeal to me to explain the scenario.

For example, one person asked if an ASSE 1017 temperature actuated device (a master thermostatic mixing valve) would be appropriate for controlling the temperature of emergency showers and eyewashes. I told this person, “No, absolutely not according to the scope of the ASSE standards.” I explained that an ASSE 1017 mixing valve can fail and allow the shuttle to move over toward the hot position, which will allow all hot water to flow or be pushed by the circulating pump and come out of the mixing valve. Water temperatures over 100 F can cause damage to the eyes. Emergency fixture mixing valves and master mixing valves should be designed to fail to “closed” or fail to “cold.” The thermostatic master mixing valve and thermostatic emergency mixing valves use a thermal motor (either a bi-metal coil or a paraffin wax or alcohol liquid-filled thermal sensing element) to sense the outlet water temperature and adjust the shuttle position depending on the temperature.

In either case, a breach of the thermal sensor wall, releasing the thermal hydraulic fluid or a broken or seized-up/scaled-up bi-metal coil can enable a mixing valve shuttle to allow scalding hot water to flow from the valve. Some fail-safe mixing valve designs have a spring that pushes the valve’s shuttle to a fail-safe or fail-cold position if the thermal motor fails. It is a product design issue. This, along with the fact that there is no cold water by-pass included in the 1017 valve, is why an ASSE 1017 valve should not be used for an emergency fixture application.

The ASSE 1071 emergency fixture mixing valve standard titled: “ASSE 1071-2012 — Performance Requirements for Temperature Actuated Mixing Valves for Plumbed Emergency Equipment,” requires a full by-pass to cold water if there is a problem. An ASSE 1017 valve cannot perform the same safety functions as the temperature limiting and cold water by-pass functions designed into a valve complying with ASSE 1071. Anyone using an ASSE 1017 valve for emergency fixtures should immediately remove the device from service and provide a temporary means of protection and replace it with the correct valve. ASSE 1071 devices should not be installed in recirculated systems because the water sits stagnant in these systems for very long periods of time, which makes it conducive to bacteria growth.  The Authority Having Jurisdiction (AHJ) is OSHA, and in most cases, it relies on a combination of plumbing code requirements, OSHA requirements and industry standards for emergency fixtures for the final approval of most installations. Owners, engineers and installers need to be aware of the manufacturer’s installation requirements, which may exceed the requirements of the codes, standards and manufacturer’s installation instructions.

It is also important for devices listed to multiple standards that are installed, still comply with the scope of the standard for the given application. There are many other valve types installed in locations that do not match the scope for the device described in the product standard. For these reasons, ASSE decided to have the Scald Awareness Task Group prepare an application chart to serve as a reference tool for adding clarity to locations where the devices should and should not be installed.

The task group’s assignment was to develop an application chart that was published as, a design guideline for temperature control devices, that clearly explains where each temperature control device, within ASSE’s product performance standards, should be used, and create an understandable reference tool that everyone in the plumbing industry can use.

The ASSE product performance standards for temperature control devices are as follows:

1. ASSE 1016-2011/ASME A112.1016-2011/CSA B125.16-11, Performance Requirements for Automatic Compensating Valves for Individual Showers and Tub/Shower Combinations.

2. ASSE 1017-2009, Performance Requirements for Temperature Actuated Mixing Valves for Hot Water Distribution Systems. (Although this device provides significantly more control to hot water system temperatures for purposes of maintaining temperatures for control of bacteria and microorganisms that are common in low temperature distribution systems, than systems with no master distribution system temperature control, it is not an anti-scald device or thermal shock control device without further controls downstream)

3. ASSE 1062-2006, Performance Requirements for Temperature Actuated Flow Reduction (TAFR) Valves for Individual Fixture Fittings

4. ASSE 1066-1997, Performance Requirements for Individual Pressure Balancing In-Line Valves for Individual Fixture Fittings.

5. ASSE 1069-2005, Performance Requirements for Automatic Temperature Control Mixing Valves.

6. ASSE 1070-2015/ASME A112.1070-2015/CSA B125.70-15, Performance Requirements for Water Temperature Limiting Devices.

7. ASSE 1071-2012, Performance Requirements for Temperature Actuated Mixing Valves for Plumbed Emergency Equipment

Click here for the ASSE Guideline for Temperature Control Devices.

A Scald death related to an improperly piped mixing valve

About 20-something years ago, I wrote about problems associated with oversized thermostatic mixing valves and how they may not be able to accurately control outlet hot water temperatures when a large valve has only a single low-flow fixture flowing at off-peak hours. That article led to my involvement in a scald investigation where an elderly man had died from scald burns he received in a shower. There were several experts in that case that produced the old article I had written that warned of the inability of oversized mixing valves to adequately control the mixed water outlet temperature when only one shower was flowing in a large building. Upon my investigation, I found that oversizing was not the cause in that installation.

The building was three-stories tall with three wings, and it had about 120 assisted living apartment units with a kitchen, dining room, hair salon, laundry room, community room, offices and a mechanical room on the first floor. The hot water system consisted of three, 120-gallon water heaters. There was a 2-inch hot water pipe routed from the water heaters to a large thermostatic mixing valve with an ASSE 1017 standard listing. The mixing valve had 2-inch hot and cold water inlets and a 2-inch tempered water outlet with a relatively small thermal element. There was a 2-inch hot water pipe leaving the mechanical room and it split into three branch mains routing to each wing of the building in 1-1/2-inch hot water branch mains.

During several flow tests, the measured temperature stayed relatively the same. The mixing valve was working properly during my inspection. Mixing valves should be sized based on the flow, not the pipe size to which they are connected. This generally results in valve sizes that are one or two pipe sizes smaller than the mixed water outlet pipe, but in this case, the valve was the same size as the pipe. After some investigation, it did not appear to be an oversized thermostatic mixing valve because there was water circulated through the mixing valve that exceeded the minimum flow requirement of the valve. There had been a change in the piping between the scald incident and the time I inspected the system. The old piping arrangement was documented in photos and showed the original piping had the tempered water return piping returning only to the cold water inlet pipe of the water heater. That confirmed the problem, but still nobody seemed to realize it was a problem upon inspection, because during the day, when there was flow from fixtures in the building it was working as intended.

I reviewed my site visit notes and examined the photos of the hot water system in the mechanical room. The flow and temperature measurements indicated it took more than 4 1/2 minutes for the water temperature to stabilize during the flow test. This was how long it took for the hot water to get from the mechanical room to the deceased man’s apartment near the end of the hallway. I reviewed the chronology of what happened from the reports of the scald incident. Before the victim died, he gave a statement of what happened in the scald incident to some witnesses and medical professionals. He entered the shower and set the temperature to the normal setting that he was comfortable with. Several minutes into his shower he was shampooing his hair and suddenly the water temperature spiked to a steaming hot temperature. His eyes were filled with shampoo and he could not see the shower controls, so he grabbed the shower curtain to pull it between him and the scalding hot water.  When he was done showering, he got dressed had some small blisters on his face, chest, arms and legs. He went to lunch in the dining room where other residents commented on his bright red skin and some of the blisters starting to appear on his face, forehead and chest. He later went back to his room to lay down and take a nap. When he awoke, his skin was peeling and sloughing off in big chunks, and much of it had stuck to the bed sheets. He called for help and was taken to the hospital where he died several days later from sepsis infection related to his burn injuries and loss of skin.   

I reviewed the maintenance records and they revealed a lot of information. A few months before the scald incident, there were three old water heaters and one of them developed a leak. A plumbing contractor was called to make repairs to the leaking water heater. The plumber noticed the water heater had rusted through, so he closed the isolation valves and shut off the gas and drained it. The temperature was raised to adjust for the loss of one water heater. The plumbing contractor also gave them a price to replace everything in the mechanical room with new piping, new water heaters, new circulating pumps and a new thermostatic mixing valve. The owner, fearing the other two heaters were near the end of their life expectancy, agreed to all the proposed changes in the mechanical room.

The maintenance records showed that shortly after the replacement project, there were numerous complaints of water temperatures that were too hot in rooms near the mechanical room and water temperatures that were not hot enough from rooms further away from the mechanical room during off peak hours. The facility made numerous service calls to the plumbing contractor to come to the facility to correct this problem and every time he came out it was mid-day and there was water use in the system and the temperature at the outlet of the mixing valve appeared to be within normal tolerances. The maintenance records also showed a couple of circulator pump failures and replacements in the weeks after the mechanical room renovations and before the scald incident. To me, these were signs of the circulating pumps being dead headed.

I asked myself how they could have been dead headed. The old photos revealed the answer to these questions. The hot water return piping was installed wrong because it was routed back only to the cold water inlet of the water heater. This piping arrangement would have caused the circulating pumps to try to force hot water through the mixing valve when the thermostatic element was sliding the shuttle over to close-off the hot water port. The piping arrangement would have dead-headed the circulating pumps. That would explain the circulating pumps being replaced a couple of times during that short period. The dead-headed circulating pumps would also have caused very little or no flow in the hot water distribution piping during periods of non-use or at night time hours. This explained the complaints of not enough hot water at the ends of the hallways (over a 4-1/2-minute wait for the first draw in the mornings because the mixing valve was restricting circulation, and the hot water mains were reaching ambient temperature at middle and the ends of the system). It also explains the complaints of water that was too hot in rooms near the mechanical room (where a slug of really hot water leaked past the mixing valve).

The thermostatic mixing valves are manufactured with tolerances that allow the temperature adjustment shuttle to move (like a piston), so there is always a manufacturing tolerance that allows circulated hot water to leak or flow around the mixing valve shuttle even when the mixing valve was trying to shut-down the hot water port.

In this case, the water heaters were normally kept at 150 F and the mixing valve reduced the distribution temperature, but during the period when one of the heaters was down, the storage temperatures were raised to a higher temperature to make-up for the loss of one water heater. When the water heaters were replaced, the temperature of all three heaters were set to the higher temperature that the contractor found on the old heaters. This allowed a slug of very hot water to develop downstream of the mixing valve during periods of non-use after he work was completed and the piping was not installed correctly. This is also what led to the complaints about water that was way too hot in the rooms near the mechanical room and complaints about lack of hot water or very long waits for hot water at rooms near the end of each hallway.

Several weeks after the scald incident, the plumber realized the piping was wrong when during one of the service calls about temperature issues, he proposed to replace the mixing valve and the mixing valve representative asked if he had installed it in accordance with the piping diagram in the installation manual. The contractor realized the piping mistake, the piping was changed, and the system began working properly. This piping mistake highlighted the deadly serious importance of splitting the tempered water return piping after the hot water return circulating pump and routing the tempered water return to both the cold water inlet of the water heater and to the cold water inlet or hot water return piping connection of the mixing valve. This allows flow from both sides of the mixing valve during periods of non-fixture-use. Some mixing valve manufacturers recommend including a balancing valve or an in-line thermostatic element on the hot water return branch running to the water heater to allow for balancing or adjustment to reduce the flow to the hot water side of the valve if the hot water return temperature climbs because of hot water usage in the system.

The resulting code change

Shortly after this case was settled, I assisted the local hospital burn center with scald burn prevention information for its website in memory of Emil Melaniak. Next, I submitted a code change to the International Plumbing Code to require hot or tempered water systems that are circulated through a mixing valve to be piped with a tee after the circulating pump to split the tempered water return and route to both the cold water inlet of the water heater and to the cold water inlet or tempered water return connection of the mixing valve to prevent this from occurring in the future.

The code change addressed recirculating pumps used with thermostatic mixing valves and first appeared in the 2003 edition of the International Plumbing Code. The code language was as follows:     

The 2003 IPC language was as Follows: 607.2.3 Recirculating pump. Where a thermostatic mixing valve is used in a system with a hot water recirculating pump, the hot water or tempered water return line shall be routed to the cold water inlet pipe of the water heater and the cold water inlet pipe or the hot water return connection of the thermostatic mixing valve.

This code language was updated and currently the 2018 edition of the IPC has the following language:

The 2018 IPC Code language is as follows: 607.2.2 Piping for recirculation systems having master thermostatic valves. Where a thermostatic mixing valve is used in a system with a hot water recirculating pump, the hot water or tempered water return line shall be routed to the cold water inlet pipe of the water heater and the cold water inlet pipe or the hot water return connection of the thermostatic mixing valve.

During several recent investigations of hot water systems where the tenants are complaining about temperature fluctuations, I checked the mixing valve piping arrangement and surprisingly I have found many systems where this same piping issue is causing temperature fluctuation issues and potentially a dangerous scalding water condition. I hope this article will help raise awareness and prevent this tragedy from occurring again.