High-rise plumbing design and plumbing codes
High-rise buildings have very complex and challenging plumbing systems. I find that there is not much plumbing code language that specifically addresses how a high-rise building should be designed to save energy, save water, and provide the owner and occupants with a sustainable and safe installation.
All of the basic plumbing code language for backflow prevention, minimum and maximum pressures, and waste and vent stacks for buildings over three stories tall applies to high-rise buildings. But, the code is silent in many other areas in which money is wasted and equipment wears out in high-rise buildings, leaving building occupants without water and basic hygiene.
I have investigated many problems in high-rise buildings, and the problems have included:
• booster pump sizing
• maximum and minimum pressure zone issues
• water heater pressure issues
• domestic hot water return pumping issues
• drainage and vent stack issues
The model plumbing codes are mostly silent with the exception of a few things, like minimum and maximum pressures and waste and vent stack sizing. The code is a minimum, so it does not address many of the issues that make good engineering sense for an energy efficient or sustainable plumbing system. One of the most common problems I find in high-rise building plumbing systems is pressure zone problems. Problems occur when the plumbing designer or design-build contractor fails to pay attention to the minimum requirements for pressure and maximum requirements for pressure in a plumbing system.
The code is a minimum document and most high-rise buildings are built by developers who want to build a building for the cheapest first cost without any concern for energy or maintenance costs. A developer wants to build it as cheap as possible, get it certified as being green, and sell it to someone else who will have to deal with the energy and maintenance costs over the life of the building. Often, these developer-built, high-rise buildings use four times as much energy as buildings with properly engineered domestic water booster pump systems.
A typical developer-built, high-rise building will have a single or duplex booster pump in the basement. The pump serves the entire building with pressure reducing valves on all of the lower floors where the supply pressure will exceed 80 psi. This type of system design with a single booster pump package and pressure reducing valves is a very energy and maintenance inefficient plumbing system. But, many owners are stuck with this design if they buy a building with this design concept.
Pressure reducing valves
Most of these developer-built buildings have pressure reducing valves or what I call “energy wasting valves” on the bottom floors. On the top 10 floors, there are no pressure reducing valves. This design will waste hundreds of thousands of dollars in energy over the life of the building. I have always said, if you have pressure reducing valves in your design, it is not a very green plumbing system design.
I would like the green certification programs to be revised to never award certifications to a building utilizing PRVs because of their massive energy waste. They should actually take points away for every floor operated downstream of a PRV in a high-rise water distribution system. There are some exceptions for Pressure reducing valves to reduce normal water pressures down to lower pressures for make-up water to hydronic systems, final rinse water in dishwashers, etc.
There are different quality levels of valves and pumps, and, often in the developer-built buildings, the valves and pumps may be of the lower quality, less expensive types that tend to fail more often. The pumps in this type of design tend to have seal failures and leaks; the lower quality pressure reducing valves typically experience wear on the seats, especially as the pressure differential grows. Even high quality pressure reducing valves will experience problems with significant pressure differentials across the valve seat.
The pressure differentials can be addressed by reducing the pressure in stages, which the developers typically do not want to spend the money on. When the pressure reducing valve wears out, this is often referred to as wire drawing. This causes more money to be spent purchasing replacement parts and more money to be spent on labor to replace the pumps and pressure reducing valves. Wire drawing of valves occurs when high velocity water shoots across the seat of a control valve and any sediment or scale in the water can score the less expensive (softer) valve seats. After a short period of time, it looks like someone took a hack saw, or wire saw, and cut a groove in the valve seat. As the valve continues to wear, it loses its ability to maintain downstream pressure, and, during periods of non-use, the downstream pressure can reach the same pressure as the upstream pressure. This can lead to exploding toilets, bursting pipes, flooding, leaking faucets and toilets, which is a significant waste of water when the supply pressure goes from 60 pounds per square inch to, say, 360 pounds per square inch.
Pipe pressure ratings
There are different categories of high-rise buildings as far as design of the water system is concerned. The taller the building is, the higher the pressure rating will be required for the pipe, valves and fittings. Each pipe material has different pressure classifications. It is important to make sure you are using the correct pressure classification for a high-rise building. Often, in these developer built buildings the water riser is rated at a higher pressure, and all piping downstream of the pressure reducing valve is rated at the lower pressure rating. When the pressure reducing valve on a lower floor fails the higher pressure is exposed to the lower pressure rated pipe, valves, fittings and fixtures. The ideal or green and sustainable plumbing design would not use pressure reducing valves (energy wasting valves)
The plumbing designer should determine the minimum required pressure for the governing fixture on the top floor and make sure the minimum pressure will be maintained during peak flow periods. The designer must also consider the maximum allowable pressure in the plumbing system, which is 80 PSI. If the most demanding (governing) plumbing fixture at the top of the building requires 35 PSI minimum at the top of a pressure zone and the highest pressure allowable per code at the bottom of the pressure zone is 80 PSI a building with floors 10 feet apart can have about nine or 10 floors per pressure zone depending on the friction loss and pipe sizing. The elevation head pressure and allowable friction loss associated with the pipe sizing will dictate how many floors should be in a pressure zone. In the above example, if the pipe sizing is generous, then there can be 10 floors. If pipe sizing is smaller and velocities and corresponding friction losses are higher, then only nine floors should be in a pressure zone. For example, elevation head loss is 0.433 per foot of elevation. If the floors are 10 feet from finished floor to finished floor the elevation pressure loss for every floor will be 4.33 pounds per square inch. The key here is to have enough water pressure on the top floor of a pressure zone to allow the fixture to operate as designed.
The codes address the minimum pressures at the fixtures, and they address maximum static water pressures in the water distribution system where they require water pressure reducing valves or regulators where the water pressure exceeds 80 pounds per square inch (psi). The plumbing fixtures listed in Chapter 6 of the model plumbing codes list minimum flowing pressures or residual pressures at each fixture. For example, a bathtub, shower or bidet requires a minimum of 20 psi flowing pressure. That would likely be about 22 psi static pressure. A water closet, siphonic type, flushometer valve requires a minimum of 35 psi residual (flowing) pressure. The newer 1.6 and lower water closet tank type fixtures requires 20 psi where the older higher flow models required a minimum pressure as low as 8 psi. Many newer models of water conserving water closets rely on the water supply pressure to force the flow into the bowl and trap as part of the design and operation of the fixture with minimum pressures as high as 45 psi. If the minimum and maximum pressures are not adhered to, problems are likely to occur.
As with any type of building that has full time maintenance staff some of the problems associated with plumbing system failures begin with the hiring of the maintenance staff. If the building owner does not hire properly trained or certified maintenance staff, the owner should at least pay for or encourage their staff to seek out training that addresses the care and maintenance for the systems they are hired to maintain. I have seen many failures where the maintenance staff contributed to the failure because they did not know what they were doing. For example, firing a boiler before opening the water fill valve. Then, the person realizes they forgot to fill the boiler with water then opening the fill valve and let cold water rush into a red hot boiler. Boom! The owner just bought a new boiler.
Domestic water pressure booster pump start-up
Another common failure is when there is a prolonged power failure and the tenants in the building are allowed to drain the water from the piping system before the power is restored. The maintenance staff must shut down the domestic water booster pumps so that they do not start-up with a water-empty riser. I have seen the results of pump start-ups that were not done properly and it can blow the piping apart.
If a pump starts up with no water in the downstream piping, there is no backpressure on the pump, so it will operate at the end of the pump curve. This increases the flow volume significantly and the velocity increases also. It is not unusual to see water velocities close to 12-15 feet per second, which will pack a whale of a punch when the water in the pipe gets to a fitting like an elbow or tee. The rule of thumb for water hammer is 60 times the water velocity is the potential water hammer pressure spike that can occur.
Many failed large diameter pipes have led to significant flood incidents with tens of millions of damages in high rise buildings when pumps have been started-up after a period of down time. If there is a power outage or even a planned outage for draining the piping and replacing pressure reducing valves, which occurs many times per year with the PRV designs, the maintenance personnel must be trained to follow pump manufacturer’s start-up procedures when a domestic water booster pump system has shut-down long enough for the water to be drained from the riser (more than a few minutes). Some pump controls are designed to require a manual start-up after a power failure for this reason.
Water hammer will occur when a pump starts up and must be controlled by turning off the pump during the outage and closing off all the valves. When the power is restored, the maintenance staff must open a faucet or two at the top of the building (or utilize an automatic air vent) to vent out the air from the high points in the system and then partially open one valve on the booster pump package and manually run one (smaller) pump until water comes out of the fixture(s) at the top of the building. This will slowly introduce water into the system to prevent the water hammer associated with all of the booster pumps coming on with an empty pipe, which can blow apart the pipe or fittings and flooding a high-rise building.
Pressure zones to avoid high pressure rated pipe, valves and fittings
There are several ways to design the water distribution system in a high-rise building. The most efficient system design utilizes a separate booster pump for each pressure zone in the building up to about 40-60 stories in height depending on floor to floor height). When the height of the building gets much higher, the pressure rating of the pipe, valves and fittings must be higher and the cost goes up. In ultra-high-rise designs, they often use a water supply pipe to a suction tank for a higher zone domestic water booster pump and piping system to begin with a higher pressure zone, which may feed the suction tank for even higher pressure zones if needed. There are many additional issues with these types of ultra-high-rise systems that may include by-pass valves, isolation valves, overflow drains, relief valves, etc. This allows lower pressure pipe, valves and fittings to be used on the lower floors of ultra-high-rise buildings.
For buildings that utilize 300 psi fitting and below (40-60 floors max depending on floor to floor height) multiple booster pumps could be used with a booster pump package for each pressure zone with no pressure reducing valves. This saves a ton of energy, as compared to a single large booster pump that is designed for the full flow and head requirement for the building. It would be like carrying two 5 gallon buckets of water up 100 flights of stairs so that someone can take a sip of water then pour all of the rest of the water down the drain or out a window. There was a lot of energy wasted carrying all that water up to the top floor, when only an six ounce cup of water was needed to satisfy the demand for a drink. This is what happens when a very large booster pump package is installed at the base of a high-rise building and pressure reducing valves are used to reduce the pressure on the lower floors. It wastes tons of energy!
I stayed on the top floor in a high-rise hotel in Florida not long ago, and I got up at 3:45.a.m to catch an early flight. When I got in the shower, there was plenty of hot water, but no cold water. When I drove around the complex to the front lobby, I noticed all of the irrigation zones were flowing at the same time, which were apparently on the cold water booster pump. This must have used up all of the pressure and the hot water system was apparently on a separate booster pump. I stopped by the front desk to let them know about the problem, and suggested they adjust the timing on their sprinkler zones.
The maintenance guy showed up while I was checking things out and said he would go check the boiler and turn the hot water temperature down. I explained the hot water temperature was not the problem, it was the cold water system was being starved by all of the sprinkler zones flowing simultaneously. At this moment, he reminded me that he had been working there for over five years and he worked his way up the building maintenance so he knew what he was doing. He assured me it was a boiler thermostat problem. I said I checked the two-handled sink faucet and no water came out. It was only the gurgling sound of air rushing in so in was probably not a water heater temperature problem. His eyes gazed up at the ceiling as he tried to comprehend what I just told him. He mumbled something like he was still going to check the boiler anyway as he walked away.
I sent a letter to explain the problem and the hotel chains chief engineer explained to me that they changed the cartridge in the room and that should take care of it. I had to follow up with a letter to explain to the hotel engineer what the real problem was.
There have been lots of cases where material substitutions have been made. It is important for the engineer, contractor and owner to verify the pressure ratings, thermal expansion properties and temperature ratings of the substituted materials. Verifying helps to make sure the piping is installed in accordance with the pipe manufacturer’s instructions.
Do not circulate through pressure reducing valves
I have seen many high-rise building designs where the water heaters are located in the ground floor or in a penthouse. In these poorly designed buildings, as the water riser goes up in the building it has pressure reducing valves on the lower floors.
When the hot water system is distributed this way, there is no way for circulation of the domestic hot water across the pressure reducing valve. In many cases the inexperienced designer, contractor or developer discovers this after the building is built. Then a re-pressurization pump is added on every floor to inject the hot water return back into the hot water return system so the water can flow back to the water heater. These systems are the worst energy violators of all. Pressure reducing valves usually don’t last very long in hot water service and are constantly being replaced. The best solution is to provide local water heaters. If you are going to use central water heaters, the solution is to design the system with the water heater within the pressure zone.
Many of these issues should be considered for inclusion in the green codes or energy codes. At the very least, when designing or constructing high-rise buildings we should strive to address these issues before the building is built.
Ron George, CPD, is president of Plumb-Tech Design & Consulting Services LLC. Visit www.Plumb-TechLLC.com.