No. The plumbing code sections we checked don't require a bypass in the valve. IPC 607.3 requires thermal expansion control on the water heater's cold supply where a PRV, check valve or backflow preventer closes the system. A built-in bypass can relieve some expansion into the main, but Watts says it does not replace a relief valve.
What a bypass PRV is
Water expands when it's heated. A PRV holds pressure back from the house, so it closes the system. Watts explains the problem: "During no-flow periods in a system, pressure reducing valves, backflow preventers, and other one-way valves are closed, thus eliminating a path for expanded water to flow back to the system supply. That is when system pressure increases"
A bypass PRV adds a small check valve inside the regulator that opens the other way. Watts describes it on its LF25AUB-Z3 line: "The standard bypass feature permits the flow of water back through the valve into the main when pressures, due to thermal expansion on the outlet side of the valve, exceed the pressure in the main supply." Watts' LFN250 sheet gives the mechanics. The bypass check valve holds against the street main pressure, "However, when thermal expansion pressure increases to just 1 lb. to 2 lbs. higher than the main pressure, the check valve opens passing the expanding water back into the supply main at the rate it is expanded."
Zurn Wilkins describes its Model 600XL the same way. The valve's "direct acting integral by-pass design prevents buildup of excessive system pressure caused by thermal expansion." FEBCO puts a number on it: "This feature prevents downstream pressure from rising to more than 10psi above the supply pressure."
The feature is a model choice. Zurn lists a "less integral by-pass check valve" option for its valves, so you can buy a PRV with one or without one.
What the code requires
The requirement is at the water heater, not in the PRV. IPC 607.3 says: "Where a storage water heater is supplied with cold water that passes through a check valve, pressure reducing valve or backflow preventer, a thermal expansion control device shall be connected to the water heater cold water supply pipe at a point that is downstream of all check valves, pressure reducing valves and backflow preventers."
That sentence triggers on a PRV, a check valve or a backflow preventer. It then requires a "thermal expansion control device" on the heater's cold supply. The tank has to be sized "such that the pressure in the water distribution system shall not exceed that required by Section 604.8," which is the 80 psi static limit.
IPC 604.8, the PRV section, covers when a PRV is required and how it must be built: an approved valve "with strainer," and one "designed to remain open to permit uninterrupted water flow in case of valve failure." Neither 604.8 nor 607.3 mentions a bypass. This page quotes the IPC. Your city's adopted code and amendments control on a permitted job, so ask the building department if you're unsure which applies.
Where a bypass falls short
Every manufacturer that describes the feature also lists limits.
It isn't a relief valve. Watts' LFN250 sheet warns: "While this feature limits the conditions causing relief valve dripping, it in no way replaces a pressure relief valve which is necessary to protect against other causes of excessive pressure."
It works only under some supply pressures. The same sheet says: "Effectiveness of the thermal expansion bypass feature is limited to systems where the street main pressure is less that the allowable setting of the pressure relief valve." Watts' LF25AUB-Z3 sheet says "The bypass feature will not prevent the pressure relief valve from opening on the hot water supply system with pressure above 150psi." And FEBCO: "Bypass will not work if inlet pressure is above 150psi."
It still leaves you to provide relief. FEBCO's troubleshooting section says the system is closed when "a check valve or backflow preventer is installed in the supply piping," and that "You must make provisions for pressure relief protection of your plumbing system and components. The use of a relief valve or potable water expansion tank may be required."
Read together, those lines point to a practical problem. A bypass sends expanding water back into the main. If a check valve or backflow preventer sits in the supply piping ahead of the PRV, that path is blocked. That's our reading of the manuals, and it's why FEBCO still calls for relief protection. If your line has a check valve or backflow preventer ahead of the PRV, don't assume the bypass on your valve gets to do its job.
How to tell if your system is closed
Two checks help. First, look at what's on your supply line: a PRV, a check valve, a backflow preventer. Any one of them can close the system. Our page on do I need an expansion tank walks through that. Second, use FEBCO's test for a system that seems to have too much pressure. Its instruction: "try briefly opening the cold water tap. If the increased pressure is caused by thermal expansion, the pressure will immediately be relieved and the system will return to the set pressure."
Zurn's Model 600XL maintenance guide describes the same pattern. Its troubleshooting entry for pressure that "periodically builds up" says the cause is "Thermal expansion of water as it is being heated," and "Pressure will be relieved the next time a fixture is opened until the next time the heater turns on." Its remedy is a device on the system, either an expansion control relief valve piped to a suitable drain or an expansion tank, set at the PRV's outlet pressure. Zurn adds: "This will not prevent pressure rise but should limit it to a safe level."
Watts describes two families of solutions on its thermal expansion page. Containment solutions "allow for thermal expansion while containing thermally expanded water in the plumbing system," and relief solutions "discharge thermally expanded water at a pressure setting that is below the setting of the water heater's temperature and pressure relief valve." An expansion tank is the containment kind, so nothing has to be piped to a drain.
What to do
If you're buying a new PRV, a bypass model is fine, and it can reduce nuisance dripping in the right conditions. Treat it as a bonus. The reliable answer is the one the code names: an expansion tank or another approved device, downstream of the PRV on the water heater's cold supply. Watch your relief valve for weeping. A gauge reading that climbs after each heating cycle tells you the system is closed. Our pages on why a relief valve leaks and why pressure suddenly gets too high cover the symptoms.
Where the valve goes matters for all of this, and where a pressure reducing valve goes shows the layout with the backflow preventer and expansion device. For the basics of the valve itself, see do I need a pressure reducing valve.
HQ Plumbing & Air can test your pressure, replace a PRV, and check or add an expansion tank. We're licensed and insured, and there's no charge for the estimate. You get the price up front, and we answer 24/7.
Short version: a bypass is a helpful feature, not a substitute for expansion control. The tank does the reliable work.