I am often asked when I did my first high-rise design. The answer is: my senior year of college. I took a course called “High Rise Plumbing Design.” It was a graduate-level course, so I needed special permission from my advisor to take the course. I was the only undergraduate crazy enough to take this class.
When I read the class syllabus, I assumed most of the class would involve designing the drainage, waste and vent systems. How wrong I was! About 80% of the class was spent on water-piping systems. Taking the class, it made perfect sense. It was easy to design the drainage system. The water piping was a different story. There were many options in the design of the water-distribution system. The key was balancing the pressure to get the proper flow at the proper system pressure.
While the education was great for high-rise design, it also applied to any multistory building where the pressure needs to be properly controlled. It could even apply to a single-story building in mountainous areas with fluctuating water pressure.
Part of the class studied the plumbing codes. The one fact that remains is today’s plumbing codes are as bad as older plumbing codes (when I took the course) regarding the regulation of water pressure. For all practical purposes, the plumbing codes have no useful requirements for controlling water pressure. If anything, they have incorrect requirements.
One of the reasons for inadequate pressure-control requirements in the codes is the lack of understanding in the plumbing world regarding pressure. Everyone knows that as you raise water in a pipe, the pressure at the top is lower than the pressure at the bottom. OK, that is a simple basic law of physics. They also know that when you pump water, the pressure increases. Finally, they all know that when you put in a valve to lower the pressure, the pressure is lowered on the outlet side. They also know that the valves are called either pressure-reducing valves, pressure-control valves or automatic-control valves. After that, everything gets muddy.
The IAPMO Uniform Plumbing Code requires the water pressure to be lowered to 80 psi or less with the installation of a pressure regulator. If the valve is 1 1/2 in. or less, a strainer is required ahead of the valve. An expansion tank is required downstream of the valve to control thermal expansion. The exception to this is when it is designed by a professional engineer.
The ICC International Plumbing Code also limits the pressure to 80 psi, with an exception for risers. The code requires a pressure-reducing valve with a strainer installed ahead of the valve. The valve must be designed to fail in the full open position to prevent the uninterrupted flow of water.
Both codes require the valves to be accessible for service and repair. But that’s it. There are no requirements for valves to isolate the pressure-control valve. To service or repair the valve, you would have to shut off the water to the entire building. That makes absolutely no sense.
A better understanding
The lack of code requirements and the lack of understanding has resulted in many system failures dealing with pressure control. However, since the plumbing codes basically leave it up to the plumbing engineer, this is an area we must take over and do correctly.
What the plumbing codes never seem to acknowledge is that PRVs, PCVs or ACVs require maintenance. These are valves that cannot just be installed and then ignored. All too often we have building owners that assume these valves should work forever. It is the engineer’s obligation to identify the maintenance schedule for these valves. I rarely recommend going more than five years without servicing. When the water quality is poor, I recommend a shorter period of time.
Many times, extra valves are purchased so the valves can be swapped out. That allows the removed valve to be rebuilt and then reinstalled in some other location. Hence, a schedule for dealing with all the valves in the building.
I don’t know how you would service a pressure-control valve without having shutoff valves to isolate the valve. This should be a code requirement since I have experienced value engineering that includes removing the shutoff valves to pressure-reducing stations.
The two code requirements that are ridiculous are the expansion tank in the UPC and the uninterrupted flow of water in the IPC. In most commercial buildings, the piping system is so large and the flow very regular that an expansion tank serves no purpose. If there is a concern for the failure of the valve, there should be a means of either isolating the valve and water supply or relieving the pressure. Which leads to the uninterrupted flow of water in the IPC. If the valve fails, you don’t want high pressure, maybe 150 psi, in the piping system downstream of the valve. That can do significant damage to the piping system, plumbing fixtures and building.
When reading both codes, one has to assume that the writers of these sections were thinking about single-family dwellings, not multistory buildings. It also appears that they were thinking about high-street pressure, not pressure created in the building by pumps or elevation.
Clear the decks
Rather than an uninterrupted flow of water, I would prefer to isolate the piping system if the pressure is too high. Of course, that means a second control valve, going by various names depending on the manufacturer, that shuts off the water supply when the pressure gets too high. The beauty of this piping arrangement is that the control valve will get serviced immediately when it fails. No one likes going without water.
Another issue not addressed is the range of flow rates through the valve controlling the pressure. If you install a valve that regulates the pressure at a flow rate between 20 and 80 gpm, what happens when the only water required is the flushing of a water closet? A second valve with a lower flow rate would have to be installed parallel to the larger valve. This is ignored by the plumbing codes and, all too often, ignored by the engineer. Remember to consider all ranges of flow in the piping system.
Finally, I have seen too many multistory buildings that have an upfeed for the cold water supply and a downfeed for the hot water supply. That sounds like a good approach since the cold water goes up from the street to the water heaters on the upper floor. Then the hot water is fed down from the water heaters. However, from a practical standpoint it is a nightmare of an installation.
You are asking the contractor to balance a system that often is nearly impossible to get right. The hot and cold water pressure should be about the same. If you are controlling the pressure with ACVs or PRVs, the upfeed has to be set to match the pressure in the downfeed.
However, the high use of hot and cold water on a given floor level will screw up the balancing on the other floor levels. Think about the change in pressure as you use various fixtures. It always is easier to have the supply piping either all upfeed or all downfeed. Yet, the plumbing codes never get into that level of regulation for water pressure. They leave it up to us.
You can almost ignore the plumbing code when it comes to requirements for controlling water pressure. This is where engineers earn their keep by properly designing water-piping systems.
Be sure to do it right.