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This month, we continue with the checklist for commissioning and troubleshooting domestic hot water (DHW) systems. We discuss temperature-actuated or thermostatic balancing valves (TBVs), digital monitoring of balancing valves, plastic pipe temperature and pressure limits and pressure-reducing valves (PRVs). Heating Thermostat
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Temperature-Actuated or Thermostatic Balancing Valves
Temperature-actuated valves are commonly known as thermostatic-balancing valves and are an improvement over manually adjusted balancing valves in DHW systems.
TBVs use a thermostatic element that reduces the flow as the temperature of the water flowing over it increases. If the temperature is cold or ambient, the valve will be fully open; as the thermostatic element senses a rise in hot water temperature, it closes — forcing circulated hot water down the hot water main to the next branch.
The TBVs slowly close automatically when the set temperature is reached, forcing the circulated hot water down the hot water main to the next branch until each branch is hot. This is how the TBVs automatically balance the hot water distribution system. The last branch should be without a TBV or include a check valve to allow flow when all branches are satisfied.
When the water in a branch starts to cool down, the TBVs slowly open to allow hot water to flow through the branches again to keep them warm.
This thermal shutdown can prevent systems with standard TBVs from being used for thermal disinfection. TBV manufacturers realized this and offered a new TBV design, allowing high-temperature hot water to bypass the normal TBV operation by designing the balancing valve to start to open back up again at a high temperature.
This feature requires the TBVs to allow a trickle of flow to pass through; when the system temperature is set to a high disinfecting temperature, they open back up to allow flow with disinfecting temperatures. The high disinfecting temperatures are also scalding, so this system requires close design and construction oversight.
Designers must ensure the proper commercial heating equipment capable of reaching disinfecting temperatures, system temperature controls and pipe insulation type and thickness is installed on all circulated hot water pipe.
These special TBVs used in systems designed for thermal disinfection account for a certain amount of heat loss in the system. When designing systems with thermal disinfection-capable TBVs, system temperature is critical to their proper operation. The system design must account for heat loss in the main and branches so that system temperatures do not fluctuate wildly. The accuracy of the thermal element and the flow rate through the TBV can affect system temperature performance.
In general, TBVs work better than manual balancing valves and can ensure an even distribution of hot water throughout a large or high-rise building with multiple branches. TBVs are ideal for large buildings with a central-circulated hot water distribution system with multiple hot water branches, such as hotels, apartment buildings, condo buildings, nursing homes, hospitals or any similar building.
These valves save time and increase accuracy versus manual balancing valves, which take many hours to be properly balanced by a hydronic-balancing contractor. The convenience of the TBVs comes after installation. The system balances itself and minimizes the time required to manually balance the system and ensure proper hot water flow to all parts of the system.
Many design engineers use basic thermal-balancing valves and rely on chemical disinfection to control bacteria and microorganisms in the water distribution systems. When using chemicals, care should be taken to not exceed the piping, valve and equipment manufacturers’ maximum recommended chemical exposure; otherwise, the chemicals can cause oxidation damage to plastic pipe and corrosion in metal pipe.
Digital technology is now being introduced to all parts of the hot water distribution system to allow for tie-in to building management system software for monitoring and adjusting digital mixing valves and balancing valves. They accurately monitor, control and record temperatures for recall as part of a water management plan. Digital control valves can maintain hot water distribution and return temperatures above the Legionella growth temperature.
Caution for TBVs Using High-Temperature Thermal Disinfection
If a maintenance person is not trained properly and randomly turns the water heater temperature up in response to no-hot-water calls or it-takes-too-long-to-get-hot-water reports, that can cause all the TBVs to open and allow dangerously hot water to flow through the entire hot water distribution system. This can expose many people to a scalding potential.
A simple installation of a TMV conforming to ASSE 1017 or CSA B125.3 should control any over-temperature situation, and install a lockable bypass valve around the mixing valve for use during the thermal disinfection process.
Using TBVs for thermal disinfection allows high-temperature water to be circulated throughout the system where an unsuspecting person, child, elderly person or disabled person can be in harm’s way. Not everyone understands this issue or is aware of the potential danger associated with higher water temperatures, including untrained maintenance personnel.
The situation can be resolved by using the TBV and then using chlorine in mild to moderate amounts as approved by the piping, fitting, valve and equipment or appliance manufacturers.
Water heaters should be turned off when circulating and dispersing water treatment disinfection chemicals to ensure even disinfection. Heat causes rapid oxidation of some chemicals, such as chlorine, monochloramine and chlorine dioxide. With the hot water system turned off and flushed of hot water, both the cold and hot water systems can be chemically disinfected.
Plastic Pipe Material Temperature and Pressure Limits
When considering plastic or composite pipe approved for domestic water distribution systems in buildings, it is important to understand that with many plastic or composite piping, as the temperature goes up, the material softens and causes the pressure rating of the material to go down.
Manufacturers often publish this information in technical documents, listing the standards that the piping conforms to and the temperature and pressure relationships at various temperatures. If you have a project with high temperatures, high pressures or both, and the manufacturer does not publish the information, get the information from the manufacturer directly.
The plastic and composite pipe materials listed in the approved materials list for water distribution pipe in the 2024 model plumbing codes are as follows:
Chlorinated polyvinyl chloride/aluminum/chlorinated polyvinyl chloride (CPVC/AL/CPVC);
Cross-linked polyethylene plastic tubing (PEX);
Cross-linked polyethylene/aluminum/cross-linked polyethylene (PEX/AL/PEX);
Cross-linked polyethylene/aluminum/high-density polyethylene (PEX/AL/HDPE);
Polyethylene/aluminum/polyethylene (PE/AL/PE);
Polypropylene plastic pipe or tubing (PP).
Generally, all plastic pipe and some plastic composite pipe used for DHW distribution systems have a pressure and temperature relationship. The higher the temperature, the lower the maximum allowable pressure because the plastic becomes softer.
Potential Hot Water Distribution Temperatures
In low-rise construction, pressure and temperature are generally not a problem if the pressure is below 80 pounds/square inch (psi) and below a residential water heater’s thermostat temperature limit of 160 F.
In commercial storage-type water heaters, the standard for combination gas control valves requires the maximum temperature on the thermostat to be 180 F, with each mark on the thermostat representing approximately 10 degrees, plus or minus 10 degrees. Then, the water heater standard for storage tank-type water heaters allows the temperature from the bottom to the top of the water heater to vary by 20 degrees because hot water rises to the top.
The combination of the tolerances of the thermostat standard and the water heater standard allows a 30-degree temperature fluctuation in uncirculated storage-type water heaters. It is a good idea to include a TMV to smooth out these temperature variations.
Water heaters with an energy input of more than 200,000 BTU/hr. are considered boilers. Typically, boilers have a thermostat temperature adjustment that allows the thermostat to go up to 200 F, the upper-temperature range for heating water systems.
• TMVs for stable hot water system temperatures. If a commercial water heater or boiler is installed in a building where the hot water temperature can fluctuate and cause the maximum temperature limit-stop to be out of adjustment or exceed the maximum temperature rating for a plastic piping system, it is important to protect the downstream hot water distribution piping by installing a TMV on the hot water distribution system.
Many piping system failures can be traced back to untrained maintenance personnel trying to turn up system temperatures to account for improper storage temperatures, equipment sizing shortfalls or compensating for parallel heating that has failed. In addition, tall buildings (more than 45 stories) or mega-tall buildings have pressures that can easily exceed the maximum working pressure at a given temperature for standard 125 psi or 150 psi pipe and fittings.
• PRVs in DHW systems. Recently, developers of high-rise, tall and mega-tall condominium buildings have looked at using PRVs and plastic and sometimes composite water distribution pipe to reduce first costs for construction. In some cases, I have seen design professionals give in to the demands of developers who push to reduce the first cost, making the building very inefficient and unsustainable from a maintenance and energy cost perspective.
Developers want to construct a building quickly and cheaply and then sell the building before they must deal with the increased utility costs and maintenance headaches.
A plumbing system boosting the cold water pressure by a large booster pump package sending the cold and hot water to the roof, where it gets heated and then distributed in a down-feed system through PRVs in all the lower floors has, in many cases, proven to be an unsustainable design. The hot water flowing through the PRVs is very aggressive and wears out the valve seats much faster, requiring constant maintenance.
The ideal high-rise plumbing system design would use a separate pressure booster pump package with multiple smaller pumps staged to come on as demand increases (three to five pumps each) and a smaller express riser with a pressure rating for the pressure at the base of the riser serving each pressure zone. This design increases the number of pumps but reduces the energy costs by four or more times over a big single or duplex booster pump combined with PRVs on all the lower floors.
Each booster pump package should be sized to supply the minimum required pressure at the top of a pressure zone to satisfy the demand for the governing fixture. Then add the pressure drop for each floor using the floor-to-floor height and the elevation pressure gain to ensure the bottom floor does not exceed 80 psi. This typically results in pressure zones of about eight to 10 floors, depending on the governing fixture at the top.
Smart and Efficient Design Does Not Use PRVs
Engineers should avoid using PRVs on cold water systems, especially on hot water systems, with a few exceptions. Some engineers on high-rise and tall buildings make the mistake of using PRVs on DHW systems.
I have always said that pressure-reducing valves are energy-wasting valves.
Design professionals should avoid boosting the pressure with one big booster pump package and then reducing the pressure on lower floors with PRVs. I have taught classes on high-rise system plumbing design, where I explain big booster pumps and PRVs in tall buildings:
“Using big booster pumps to raise the pressure followed by pressure-reducing valves or PRVs to reduce the pressure is like driving down the street with your foot pressed all the way down to the floor on the accelerator, and then trying to control your speed with the brake. That will use a lot of energy, wear out your engine (the pump) and wear out the brakes (PRVs). When the PRVs fail, you will get full pressure in the downstream piping.”
Hot water is very aggressive. If you try to reduce hot water pressure, it will usually wear out the seat on the PRVs in a very short time. When these worn seats leak overnight and there are no fixtures flowing, the downstream pressure can reach the full upstream pressure. In very tall buildings, this often exceeds the pressure and temperature rating of the downstream pipe, fittings and valves, equipment and appliances.
At this point, the building owner will soon learn the weakest link in the piping system. PRVs have their place at the building water service entrance when the utility pressures are higher than 80 psi and at specific pieces of equipment, like a dishwasher final rinse or equipment that specifies a low pressure, but not in a high-rise building to establish pressure zones.
I often see plumbing designs using big booster pumps to increase the pressure. On lower floors, they use PRVs on cold water lines and sometimes on hot water distribution systems, which are not sustainable. The results are often increased operating costs for the energy waste on pumps and PRVs and maintenance costs for replacing worn-out valve seat components.
If an owner scrimps on or neglects maintenance, it can bankrupt a company when piping bursts and floods dozens of floors, causing massive cleanup costs and replacement of valuables. Many of these high-rise building floods can approach $100 million in damages.
The engineered design must consider all the parameters, such as the elevation pressure and system temperature, to ensure the piping system’s maximum temperature at a given pressure. If the pressure can increase because of a failed upstream PRV, allowing extreme pressure in the downstream distribution piping, a PRV should be considered for installation downstream of each PRV station if series or series/parallel PRVs are used so as not to exceed the piping manufacturer’s pressure and temperature ratings.
PRVs and TMVs should be considered to ensure they do not exceed the maximum temperature at the highest-rated pressure.
Most piping manufacturers issue technical manuals, similar to Table A, that cover the maximum pressure and temperature ratings for their pipe.
Table B is a manufacturer’s hydrostatic temperature and pressure ratings for cross-linked polyethylene (PEX) piping.
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Sf113e Thermal Fuse Next month we will continue this series with a discussion of hot water temperature maintenance cables, the design and set-up of a domestic hot water distribution system to control Legionella bacteria growth and minimize scald risks, and the flushing and disinfecting of domestic hot and cold water systems.