If a potable water system has a backflow device, pressure-reducing valve or check valve where it enters the building, it is a closed system. Long ago, we had open systems where thermal expansion simply pushed water back into the municipal water system and we were blissfully unaware of thermal expansion pressure issues. Pressure-reducing valves had an internal bypass relief that allowed increased pressure caused by thermal expansion to push through the internal relief and move back into the service line.

When our local water company began installing dual-check backflow preventers, we suddenly found gas-fired water heaters developed collapsed flue tubes, which caused the byproducts of combustion to spew forth from their access doors. The hot and cold water lines were cocked at odd angles, too, looking like a bowlegged cowboy! Customers were less than thrilled when told they needed a new water heater.

Lots of confusion reigned supreme regarding how to deal with the thermal expansion-generated increased pressure. At that time, in the early 1980s, available potable water thermal expansion tanks were not rated to 150 PSI, so a secondary relief valve had to be installed. In addition to TXTs (thermal expansion tanks), there was a ballcock valve that would relieve pressures in excess of 80 PSI and they are still available. Eventually, potable water TXTs became widely available with a 150 PSI rating. Unfortunately, very little information was available, with much of it misleading in regards to properly sizing the TXTs.

As pros, we must design for worst-case conditions. No matter where you live, you will see 40° F incoming cold water from municipal systems during winter months. Although tank-style water heaters come with a theoretical 120° F thermostat setting, ASSE codes recognize thermal stacking happens where layers of water as much as 30° F above the thermostat setting can occur, which makes the upper limit of 150° F code compliant. Then, along come homeowners who jack up the thermostat setting because they run out of hot water! Plumbing codes limit water pressure to 80-PSI before a pressure-reducing valve is required. In theory, we should be sizing our thermal expansion tanks for a 110° F temperature rise and thermal expansion charts provide a multiplier of .01787, which for a 50-gallon water heater equals .89, or 9/10ths of 1-gallon.

Clearly, the homeowner has done his research and makes valid points regarding the conflicts within the Uniform Plumbing Code that was adopted by his local Authority Having Jurisdiction. My reply:

The obvious need is controlling the pressure generated by heating water and its expansion within a sealed system.

Semantics! Where a properly sized thermal expansion tank does control thermal expansion, a secondary relief valve, even if its specific purpose is to relieve excessive pressure, does not (in my opinion) control thermal expansion — it relieves excessive pressure. While the two are related, they are not the same.

Three Issues:

1. The relief valve will release the buildup in pressure over and above its setting, however, providing the system remains idle with no dripping faucets or running toilets, the entire potable hot and cold potable water system remains stressed due to the now elevated pressure. This will shorten the life of the system and components — like the water heater.
2. Given the drive for conserving natural resources, the relief valve option wastes water whereas a thermal expansion tank contains the expanded water, which returns to the system the next time water is used. It makes no sense to be wasting a gallon, or more, of potable water every day.
3. Repeated weeping through a relief valve causes a buildup of minerals via evaporation around the washer and seat, and given the ever-so-slight opening between spring-loaded rubber washer and the valve's seat, any granular products attempting to pass through will, combined with mineral deposits, cause the valve to constantly weep, wasting untold amounts of potable water. Given that your secondary relief valves are located outdoors, that could go undetected for an extended period of time.

Here's a column on thermal expansion I wrote for PM Engineer magazine.

Words have an impact, and from this master plumber’s perspective, it begs the question: are we controlling or limiting thermal expansion? We do control thermal expansion when a properly sized TXT is installed on a closed potable water system. If, on the other hand, we are installing a secondary pressure relief valve that limits system pressure to 125 PSI, that is a pressure-limiting device and we are not actually controlling thermal expansion.

From the installation instructions from the water heater manufacturer our Florida homeowner has in his home: “If this water heater is installed in a closed water supply system, such as the one having a back-flow preventer in the cold-water supply, provisions shall be made to control thermal expansion. DO NOT operate this water heater in a closed system without provisions for controlling thermal expansion. Your water supplier or local plumbing inspector should be contacted on how to control this situation.”

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I have always purchased commentary editions for plumbing and mechanical code books because each code rule is followed with commentary on how to interpret the code. For example, following the IPC 607.3 Thermal Expansion Control code passage, the commentary addresses uncontrolled pressure increases due to thermal expansion and states that elevated pressure above 80 PSI causes faucets and other system components to have shorter than normal lifespans. Also addressed are the stresses placed on water heater tanks due to frequent pressure fluctuations caused by uncontrolled thermal expansion.

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There's that word "control" again. The IPC (commentary edition) uses the same word in 607.3 and goes further regarding thermal expansion in 607.3.1 when discussing pressure-reducing valves by stating the PRVs with pressure bypass can no longer be used as a means to relieve thermal expansion, and the downstream pressure cannot exceed the reduced pressure setting due to thermal expansion. That's not realistic, even with a properly sized thermal expansion tank as there will certainly be a small rise in a closed system’s pressure. PRVs with internal bypass are mostly a moot point anyway due to municipal water companies installing dual-check valves or testable BFPs where more than a few apartments (two in our case) exist connected to the same water service. Interesting to see the code touch upon why the repeated opening/closing of the T&P valve raises a safety concern. In 607.3 the code book commentary does note a thermal expansion tank is typically installed to control thermal expansion.

I have always purchased commentary editions for plumbing and mechanical code books because each code rule is followed with commentary on how to interpret the code. For example, following the IPC 607.3 Thermal Expansion Control code passage, the commentary addresses uncontrolled pressure increases due to thermal expansion and states that elevated pressure above 80 PSI causes faucets and other system components to have shorter than normal lifespans. Also addressed are the stresses placed on water heater tanks due to frequent pressure fluctuations caused by uncontrolled thermal expansion.

The question, it seems, is if utilizing a secondary relief valve set to 125-PSI constitutes uncontrolled thermal expansion and that repeated cycles between 80 PSI to 125-PSI will have an adverse impact on the plumbing system’s components.

I wonder too about the wisdom of locating the secondary pressure relief valve outdoors. The potable water that wastes, the potential for contamination (think flooding during hurricanes or heavy rains that potentially submerge the relief valve), and eventual fowling or failing partially open, which may well go unnoticed for an extended period of time. Bugs no doubt will appreciate the source of water! Discharge from the water heater’s T&P relief valve must terminate with an air gap and these secondary relief valves also must also terminate with an air gap.

Some of the secondary pressure relief valve manufacturers have disparaging content regarding TXTs that doesn’t hold up to my decades of TXT experience. It is common for properly sized TXTs to last the lifetime of the tank-style water heater and when a new water heater was installed, the TXT was also replaced.

If the AHJ allows pressure relief valves or TXTs, then it’s your call as to which one you believe best serves your customer’s needs. Even better, educate the homeowner(s) and let them decide.