Pressure Regulator needs an Expansion Tank
Here in the Greater Phoenix, AZ desert, a whole-house pressure regulator valve, also known as a pressure reducer (valve) is sometimes employed to reduce high water pressure supplied by the water company to a safer level for use inside the home, typically agreed to be between 40 and 80 PSI. In most areas, an expansion tank or expansion device must be installed whenever a pressure regulator is installed and there is a tank-style water heater. The expansion tank protects the plumbing components (including the pressure regulator itself) from premature failure due to high pressure resulting from a closed system.
One of the characteristics of a pressure regulator, aside from lowering the pressure, is that they only allow the water to flow one way - into the home. If the water tries to flow out of the home (back to the street) it cannot. This is known as a "closed" plumbing system. In a closed system, the pressure of the water inside the water heater will increase as the water is heated, because the water is trying to expand but cannot. Because the pressure is same everywhere in the plumbing system, this means that the pressure inside the pipes increases, as does the pressure placed on the various plumbing fixtures, and the back-pressure on the pressure regulator.
An expansion tank, usually installed above the water heater, will help prevent the system pressure from increasing by allowing the expanding water to expand inside of it. If the water is allowed to expand in this way, the pressure will not increase. On homes without a pressure regulator the expanding water would normally push back into the water supply pipes, which can absorb the pressure increase.
We can summarize the previous paragraphs as follows: A plumbing system that uses a tank to store heated water and has a pressure regulator installed without an expansion tank forms a closed system which will cause the water pressure in the home to increase due to thermal expansion, which may cause premature failure of the various plumbing components including the pressure regulator itself.
It is worthwhile to note that some plumbing valves in particular will cause extensive flood damage if they fail. Think about what you would wake up to if the refrigerator water dispenser valve (the one in the freezer door) failed and flowed all night long. Or if one of the sink shut-off valves under a sink started leaking. The toilet filler valve, which is one of the cheapest quality valves and most prone to failure is somewhat protected, unless the toilet is backed up. In that case you would notice the toilet running and after a few years you would wonder why the fill valves needs replacing every year. My point is that the various plumbing valves in your home were designed to hold back normal home water pressure, commonly agreed upon among home inspectors and building professionals as 40 to 80 PSI.
Before I continue let me stress that a home inspection is NOT a code inspection, and a home inspector should not make a habit out of quoting code. I am simply offering an opinion but for the sake of this article I would like to show that it is indeed back-up by code. I come across MANY pressure regulators installed without expansion tanks and have had to explain to experienced and licensed plumbers why it is wrong. The following code snippets are from the 2012 code cycle, but haven't changed significantly in recent years.
My thought process when educating a client (or licensed plumber) as to why an expansion tank is needed with a pressure regulator is as follows:
I observed that the home had a whole-house pressure regulator installed but did not have an expansion tank. Because the builder (or owner, or plumber) installed a pressure regulator they have acknowledged that water pressure above 80 PSI must be hazardous. Thermal expansion can increase the water pressure in the system. Whether or not a code reference supports my opinion, it is just common sense to protect the home from flood damage by limiting the maximum water pressure to 80 PSI, as is generally accepted to be the upper limit for residential water pressure. The following code references apply:
2012 UNIFORM PLUMBING CODE
608.0 Water Pressure, Pressure regulators, Pressure relief valves, and vacuum relief valves.
608.2 Excessive Water Pressure. Where static water pressure in the water supply piping is exceeding 80 psi (552 kPa), an approved-type pressure regulator preceded by an adequate strainer shall be installed and the static pressure reduced to 80 psi (552 kPa) or less. Pressure regulator(s) equal to or exceeding 11⁄2 inches (38 mm) shall not require a strainer. Such regulator(s) shall control the pressure to water outlets in the building unless otherwise approved by the Authority Having Jurisdiction. Each such regulator and strainer shall be accessibly located aboveground or in a vault equipped with a properly sized and sloped bore-sighted drain to daylight, shall be protected from freezing, and shall have the strainer readily accessible for cleaning without removing the regulator or strainer body or disconnecting the supply piping. Pipe size determinations shall be based on 80 percent of the reduced pressure where using Table 610.4. An approved expansion tank shall be installed in the cold water distribution piping downstream of each such regulator to prevent excessive pressure from developing due to thermal expansion and to maintain the pressure setting of the regulator. Expansion tanks used in potable water systems intended to supply drinking water shall be in accordance with NSF 61. The expansion tank shall be properly sized and installed in accordance with the manufacturer’s installation instructions and listing. Systems designed by registered engineers shall be permitted to use approved pressure relief valves in lieu of expansion tanks provided such relief valves have a maximum pressure relief setting of 100 psi (689 kPa) or less.
2012 INTERNATIONAL PLUMBING CODE
607.0 Hot Water Supply System
607.3 Thermal expansion control.
A means of controlling increased pressure caused by thermal expansion shall be provided where required in accordance with Sections 607.3.1 and 607.3.2.
607.3.1 Pressure-reducing valve.
For water service system sizes up to and including 2 inches (51 mm), a device
for controlling pressure shall be installed where, because of thermal expansion, the pressure on the downstream side of a pressure-reducing valve exceeds the pressure-reducing valve setting.
Happy Inspecting everyone!