Why does pressure rise after a Watts pressure regulator?

Watts's instructions say that if the downstream system pressure is higher than the set pressure under no-flow conditions, the cause could be one of three things:

1. 1Thermal expansion.
2. 2Pressure creep.
3. 3Dirt or debris on the seat.

The sheet explains only the first in detail, so this page does too. Creep and debris are covered below with what the sheet does and does not say.

Watts describes thermal expansion when water is heated in a closed system. Its LF25AUB-Z3 instructions discuss supply pressure above 150 psi and check valves or backflow preventers in the context of that model's bypass. The 150 psi figure is not a universal threshold for a system to be closed: another device can block return to the supply at lower pressure. Our page on what a backflow preventer does explains why those devices exist.

The Building America Solution Center makes the same point. It notes that use of a pressure reducing valve creates a closed system, and that thermal expansion can lead to increased pressure in some cases. EPA's WaterSense sheet tells builders to consult local plumbing codes to see if thermal expansion, which can occur from installing a pressure reducing valve since it creates a closed system, needs to be addressed.

### What does Watts say to do about it?

Watts says you must make provisions for pressure relief protection of your plumbing system and components. It says a relief valve such as the Watts 530C, BRV, Governor 80 or 3L, or a potable water expansion tank such as the Watts DET, PLT or DETA, may be required. Our page on whether you need an expansion tank explains how a tank works, and our page on a leaking water heater pressure relief valve covers what a dripping relief valve means.

### How do you test for thermal expansion?

Watts suggests briefly opening a cold water tap. Its instructions say that pressure caused by thermal expansion will immediately be relieved and return to the set pressure. That response is a useful clue, rather than a unique diagnosis: releasing water can also lower pressure that accumulated through a leaking regulator seat. Record whether the pressure rises again during a heating cycle or also climbs while the heater is not running, and have the regulator and expansion control checked.

Watts offers a pressure test gauge, model 276H300, to help you determine if you have high water pressure. Attached to a hose bibb, it registers the highest pressure reading over the period of time it is left on the system. That helps if the spike happens at night after the water heater fires, when nobody is watching the gauge.

The 276H300 retained-hand guide explains why a red maximum can remain high after current pressure falls. Preserve both observations and the testing conditions; a retained peak alone cannot distinguish expansion from another pressure event.

Some Watts valves handle part of the problem themselves. The LF25AUB-Z3 sheet says the regulator has a built-in thermal expansion bypass feature, and that this feature prevents downstream pressure from rising to more than 10 psi above the supply pressure.

Read that number carefully. It is a ceiling, not a guarantee of your set pressure. Here is an example with our own numbers. If the supply is 70 psi and the valve is set to 50 psi, the sheet's limit means the house side could still climb toward 80 psi, since the limit is tied to the supply pressure, not your set point. If your plumbing code or your water heater maker asks for a tank or a relief valve, the bypass does not replace them. Watts still says you must make provisions for pressure relief.

Watts lists pressure creep and dirt or debris on the seat as causes but does not describe either in the troubleshooting section. Here is what the rest of the sheet supports.

• Dirt on the seat. The maintenance section says to clean the strainer, which means shutting off the inlet water pressure, removing the strainer plug, and removing and cleaning the strainer. It also gives steps to replace the seat disc. Watts's installation notes say to flush the lines before installing the valve to remove loose scale, dirt and other foreign matter that can damage or clog it. Our page on how to clean a Watts pressure reducing valve strainer covers the strainer steps.
• Pressure creep. The sheet lists it without a detailed definition. Pressure that slowly rises with no water running, including when the heater is not heating, calls for checking whether the regulator seat is holding. Opening a tap may lower that pressure temporarily too. A plumber can test the valve and determine whether servicing the seat or replacing the valve is appropriate.

Our page on signs your pressure regulator is going bad lists common symptoms, and what to do when water pressure suddenly gets too high walks through other causes.

### What target pressure keeps the system calm?

The Building America Solution Center says keeping the pressure at 60 psi can help maintain the performance of water fixtures, reduce dishwasher and washing machine noise, and reduce breakdowns in a plumbing system. EPA recommends incoming service pressure between 45 and 60 psi. If your valve is set near the top of that range and thermal expansion pushes it up, you have less room before the house side reaches 80 psi than a house set at 45 psi. Our page on what home water pressure should be covers the range.

### Why does the number matter?

EPA's sheet says high water pressure can damage plumbing piping, fixtures and appliances, and can lead to increased water consumption, premature equipment failure and risk of leaks. It also says most U.S. plumbing codes require pressure regulating valves on domestic systems where the pressure of the supplied water exceeds 80 psi. So a valve set at 50 psi that reads 80 or more with no flow is a problem to solve, not a quirk to ignore.

### What should you tell the plumber?

Bring the facts you can collect in ten minutes. Note the pressure reading with no water running, then the reading after you briefly open a cold tap, and whether it fell back to the set value. Write down the model on the valve's nameplate, and whether the house has a check valve, a backflow preventer, an expansion tank or a relief valve. If you left a 276H300 gauge on a hose bibb, note the highest reading it recorded. Those details let the plumber sort thermal expansion from a worn seat without guessing.

Call when the house-side pressure stays above the setting after the cold-tap test, when a relief valve or the water heater's pressure relief valve is dripping, or when you are not sure whether your system is closed. HQ Plumbing & Air is licensed and insured, gives free estimates, quotes an upfront price, and offers 24/7 service. Also see how to adjust a Watts pressure reducing valve for the set-point steps.