Parts one and two of this series covered commissioning and troubleshooting domestic hot water (DHW) and tempered water systems. It also covered the importance of proper design, installation, and maintenance of DHW systems.
This series started by covering a checklist for inspection, troubleshooting and maintenance of DHW systems that included water heater types and sizing considerations, why you need storage tanks with copper fin-tube water heaters and with some types of tankless heaters, when to use circulating pumps, issues with aquastats and timers causing system temperature fluctuations, and why we should use temperature gauges to diagnose and document DHW system temperatures.
We continue with a checklist for temperature control valves, applications and locations for various temperature controls, DHW control valve types, temperature control valve standards, and how to protect yourself from liability if the owner refuses to install a control valve serving the hot water distribution system.
DHW System Checklist for Temperature Control Valves
A. Temperature-Actuated Mixing Valve (TMV) (source valve, near the water heater)
1. Check to see if a TMV is on the hot water distribution pipe near the water heater or in the same room as the water heater or hot water source if it is a heat exchanger.
2. For each TMV, document the following information:
3. Document the TMV flows from TMV literature:
Rated flow in gallons/minute (gpm) at 45 psi drop;
Rated flow in gpm at approximately 20 psi drop.
Most valve manufacturers list the maximum rated flow for a mixing valve at a 45 psi pressure drop, but a more realistic flow rating at a pressure drop is about 20 psi. The ASSE 1017 standard tests for maximum or rated flow at 45 psi pressure drop; many manufacturers also list flow at 20 psi drop and other pressure drops.
4. Document the TMV location/room number.
5. Document the TMV equipment ID number.
6. Document the TMV area or system served.
7. Design hot water system temperatures (if shown on drawings, schedules, details or plans):
8. Document the actual hot water system operating temperatures (as found during inspection):
9. Is insulation on the hot water distribution piping?
10. Is insulation on the hot water return piping?
11. Is a circulating pump installed downstream of the TMV recirculating hot or tempered water back through the TMV?
If yes, document the following for each circulating pump:
12. If the tempered water is circulated through a TMV, is the TMV circulated return piping routed correctly?
a. Yes; TMV and circulating pump are piped correct.
Split to cold water inlet on water heater and cold water inlet or recirculation connection on TMV.
The tempered water return pipe should split after the circulating pump and route to the cold water inlet of the water heater and the cold water inlet or return connection on the mixing valve to allow for mixed or tempered water flow through both sides of the TMV during periods of no fixture use.
c. TMV and circulating pump are piped incorrectly; return is only routed to the cold water inlet on the water heater.
This piping configuration will cause hot water to be forced through the water heater and into the hot water inlet of the TMV under a slight pressure as exerted by the circulating pump. The hot water will leak through most TMVs because manufacturing tolerances prevent getting too tight or the valve may resist movement.
Hot water leaking through the TMV allows a slug of full-temperature hot water on the outlet (mixed or tempered water) side of the mixing valve when no flow is coming from fixtures in the system.
This piping condition generally leads to complaints of no hot water or “it takes too long to get hot water” at the ends of the system and complaints of “water too hot” or “fluctuating hot water temperatures” at fixtures near the hot water source. This condition creates a thermal shock and scalding issue.
d. TMV and circulating pump are piped incorrectly; return only goes to the cold water inlet on the TMV.
This piping configuration will cause the tempered water system to circulate in a loop without being reheated; it will eventually drop to ambient temperature. When this occurs, someone can be in a shower with the valve set to a high temperature position and when the hot water arrives at the shower valve, there will be a corresponding rise in shower temperature.
This can occur when a bather is covered with soap or shampoos and can lead to a thermal shock event causing a slip-and-fall accident.
e. TMV and circulating pump are piped incorrect; document any other piping configuration that is problematic.
Check for crossover pipe between hot water systems of different system temperatures with TMVs and hot water return pipe routed to the wrong location. Sometimes crossover pipe with a valve is used to provide a backup source of hot water in larger buildings with multiple hot water systems.
Opening the valve between two different hot water systems can create a scalding condition if the systems are at different temperatures. I am aware of scald incidents that have occurred when a crossover valve from a kitchen water heater at 160 F was opened to a hotel guestroom hot water system that had some failed water heaters. A serious scald injury occurred when the valve was opened.
f. If piped incorrectly, make sure you document: who you notify about the issue and how the piping issues or error will be resolved to fix the piping condition or to allow for circulated flow from both sides of the mixing valve during periods of no fixture use.
13. Disconnect switch. Is a power disconnect switch or power switch installed on the circulating pump?
Yes; electric panel number, circuit breaker number or disconnect switch number.
No; power disconnect switch is required by the electrical code.
14. Circulating pump flow and pressure:
a. Determine the total gpm required to maintain the system temperature.
Use the heat loss calculations of ASHRAE, ASPE or other method to determine heat loss in piping based on the system temperature, pipe size, insulation type and insulation thickness.
b. Determine the friction loss at the required flow for the hot water distribution system (supply and return). This includes friction loss at balancing valves to determine if the pump head and flow in gpm will be adequate for the hot water distribution system.
The pump head requirements are based upon the design length and size of pipe and anticipated friction loss in the hot water supply and return circulated piping system.
c. Determine if the selected circulating pump is sized properly based upon the required flow and friction loss for the circulated distribution system.
Obtain the circulating pump flow rate in gallons per minute at the published pressure in feet of head from the manufacturer’s pump curve data.
15. Velocity. Check the hot water return flow velocity to see if it is within the maximum velocity limits of the pipe material manufacturer and equipment manufacturer(s).
a. If velocity limits are exceeded, consider increasing the recirculation pipe diameter to lower the velocity or lowering the flow in gpm, which will cause a greater temperature difference across the system. With greater temperature differences, it may be possible for return temperatures to always be in the ideal temperature for Legionella bacteria growth.
If the lower flow option is done, consider a form of secondary disinfection that is appropriate for the recirculated hot water system to control Legionella bacteria growth.
Pipe size changes should be done in the design phase. If the pipe is already installed, document the condition in writing to all parties (the owner/property manager/construction manager/contractor, etc.) of the velocity/pipe sizing issue and potential risks of erosion or pipe failure/floods. Then discuss lower flow rates and disinfection options with the building’s water management team.
b. Pipe size near the circulating pump inlet in inches; velocity in feet/second (fps).
c. What is the DHW return piping material?
1. If the hot water return piping is copper or plastic: Is it sized to keep hot water return flow velocity under the manufacturer’s maximum recommended flow velocity?
Maximum velocity of 2 fps for some plastic pipe manufacturers. Maximum 5 fps for copper at hot water temperatures up to 140 F, and a maximum velocity of 2 to 3 fps for copper over 140 F.
Yes, hot water/hot water return flow velocity is within limits of the piping/equipment manufacturer.
No, hot water/hot water return piping manufacturer flow velocity exceeds limits.
2. Is the TMV/digital mixing valve (DMV) sized properly for the hot water or tempered water system and inlet/outlet temperatures?
Verify the flow rate by determining the water supply fixture units, convert to gpm and then determine if the gpm is within the mixing valve manufacturer’s published flow range. If the flow is too high, the temperature could drop off. If the flow is too low, there could be temperature fluctuations in some types of mixing valves.
At low flows, mixed water may not flow over the thermal element uniformly, causing temperature control issues and potential thermal shock and scalding issues.
Most mixing valve manufacturers publish minimum flow rates; some require a circulated flow through the mixing valve for the valve to mix properly.
3. Document if the TMV meets the standards listed in the applicable plumbing code.
See standards listed below in section 3. The TMV should meet the application listed in the standard. For example: TMVs for hot water distribution systems at or near the water heater or hot water source should comply with either ASSE 1017 or CSA B-125.3.
If there is another standard listed on valve, document here.
2. TMV valve types.
a. Bi-metal coil. This is old technology; they rely on a thermal motor made of two different metals that expand at different rates, bonded together and coiled up so when they are exposed to different water temperatures, the coil will unwind or wind up to react to changes in temperature.
This type of thermal motor has very little torque or driving force and is often susceptible to seizure or sticking in hard water conditions. It is very important to inspect and clean this type of valve at regular intervals to ensure the valve’s thermal motor or coil is still moving freely.
Most mixing valves and water heaters require water softening in hard water conditions and regular maintenance to descale moving parts and heating surfaces. Bi-metal coil-type valves generally have poor accuracy at low flows and react relatively slowly compared to other valve control technologies. Bi-metal coil designs generally provide minimal force acting on the temperature actuation moving parts of a valve.
It is very important to remove the cover over the thermal motor coil and inspect and clean the thermal motor regularly. New valves should be inspected quarterly for the first year; if limited signs of scale and sediment, then inspections can be extended to longer intervals until a safe approximation of an inspection and cleaning regimen can be established.
Lack of preventative maintenance of this type of valve can allow the valve to get stuck and fail to control outlet temperatures, which can lead to scald injuries and death.
b. Paraffin wax. Liquid paraffin wax is a fluid that expands and contracts greatly with changes in temperature; it is often used in thermal elements to create the driving force for a TMV thermal motor.
Thermal elements or thermal motors come in various sizes and styles; larger sensing elements provide greater driving force, movement and temperature control accuracy. TMVs can be used in moderately hard water and control the temperature, but there is still a need for regular inspection for scale buildup and deliming/descaling maintenance on the valve’s moving parts.
Water softening is recommended in hard water conditions with regular maintenance and deliming to descale moving components. Deliming is often done with a mild acid solution were components can be bathed in the acid-water solution. In some cases, the water heater and mixing valve can be delimed by using isolation valves and deliming ports with valves to pump a mild acid solution through the water heater and mixing valve. These deliming ports must be built into the system to allow this to occur.
c. Digital. Digital mixing valves are electronic mixing valves relying on digital technology, measuring differences in resistance through different temperatures of water that have different density and resistance to achieve accurate temperature control.
These valves incorporate electronic components, including temperature sensors, and can be programmed to deliver specific temperature outputs, resulting in a high degree of precision (within 1 to 2 degrees). They allow continuous monitoring or data logging of hot water distribution temperature and DMV inlet and outlet temperatures. Water softening is recommended in hard water conditions with regular maintenance to descale.
3. Point-of-use temperature control valves.
a. ASSE 1016/ASME A112.1016/CSA B125.16, Performance Requirements for Automatic Compensating Valves for Individual Showers and Tub/Shower Combinations.
ASSE 1016 is a product performance standard for individual showers or tub-shower combinations. The devices are intended to control the water temperature to wall-mounted showerheads, either in individual shower or tub/shower combination fixtures, to reduce the risk of scalding and thermal shock. They are intended to be installed at the point-of-use, where the bather or bather’s attendant has access to flow and final temperature control mechanisms and where no further mixing occurs downstream of the device.
The three types of individual control valves covered are:
• Pressure-balancing valves (Type P) include a mechanism that senses incoming hot and cold water pressures and compensate for fluctuations to stabilize outlet temperature;
• Thermostatic valves (Type T) have a mechanism that senses outlet temperature and compensate for fluctuations in either incoming hot and cold water temperature or pressure to stabilize outlet temperature;
• Combination thermostatic/pressure-balancing valves (Type T/P) sense outlet temperature and incoming hot and cold water pressures and compensate for fluctuations in incoming hot and cold water temperatures or pressures to stabilize the outlet temperature.
Document the following:
b. ASSE 1062, Performance Requirements for Temperature-Actuated, Flow-Reduction (TAFR) Valves for Individual Supply Fittings.
This standard applies to TAFR valves for individual supply fittings, which use a thermal element to react to high-temperature hot water and shut down the flow to a trickle. These valves are intended for use in line with or are integrated into individual plumbing supply fittings such as showerheads, and bath, utility, sink and lavatory faucets.
They shall automatically reduce flow within 5 seconds in response to outlet temperatures greater than a preset actuation temperature, not to exceed 120 F, to limit exposure to high-temperature water discharged from an individual supply fitting.
For each shower or tub-shower valve, document the following:
c. ASSE 1066, Performance Requirements for Individual Pressure-Balancing Inline Valves for Individual Fixture Fittings.
The ASSE 1666 standard applies to automatic pressure-balancing inline valves for individual fixture fittings used to equalize incoming hot and cold water line pressures. The purpose is to minimize mixed water temperature variations due to pressure fluctuations when used in conjunction with a mixing valve or two-handle valve set.
They are not designed to limit the maximum outlet temperature at the point of use. These devices are intended for use with individual plumbing fixture fittings used with showers, bathtubs, utility sinks and shampoo sinks.
Document the following:
If no, document and notify building owner of the potential scald hazard.
d. ASSE 1069, Performance Requirements for Automatic Temperature-Control Mixing Valves.
Regarding gang shower TMVs. For each valve, record the following:
e. ASSE 1070/ASME A112.1070/CSA B125.70, Performance Requirements for Water Temperature-Limiting Devices.
Water temperature-limiting devices shall control and limit the water temperature to fittings for sinks, lavatories or bathtubs and are intended to reduce the risk of scalding. They are intended to supply tempered water to plumbing fixture fittings or be integral with plumbing fixture fittings supplying tempered water. The device shall be equipped with an adjustable and lockable means to limit the setting toward the hot position.
For each ASSE 1070 device, document the following:
f. ASSE 1071, Performance Requirements for Temperature-Actuated Mixing Valves for Plumbed Emergency Equipment.
These devices are intended to provide tepid water for emergency fixtures including eyewashes, eye/face washes, drench showers and combination units. They are intended to be installed in systems complying with ANSI Z358.1.
These TMV devices consist of a hot water inlet connection, a cold water inlet connection, a mixed water outlet connection, a temperature-controlling element and a means for adjusting the mixed water outlet temperature while in service. The device shall also have a means to limit the maximum outlet temperature under normal operating conditions.
Provisions shall be made so that the temperature control is vandal-resistant and cannot be inadvertently adjusted. In addition, they have a feature that bypasses cold water in the event the outlet temperature rises above the set point.
Next month, we will continue with commissioning and troubleshooting domestic hot water systems with checklists and a deeper discussion on water temperature, pressure and velocity effects on various piping materials (temperature vs. pressure ratings), flexible connectors in hot water systems, pressure-reducing valves in hot water systems, and many other topics.