There is a puddle under the water heater in the morning and nothing at noon. Or you hear a hiss at the discharge pipe for a minute after somebody showers, and then silence. Most people replace the temperature and pressure relief valve, and most people get the same drip back inside a month.
The valve is doing its job. Something is raising the pressure in your plumbing past the point where the valve is set to let go, then the pressure falls back and the valve closes again. On a house in St. George that something is usually thermal expansion in a closed system, and the fix is a tank the size of a basketball, correctly charged.
What is actually happening
Water expands about 2 percent by volume heating from 60°F to 140°F, and water is essentially incompressible. Those two facts together are the whole story. When the burner or the element fires, the water in the tank has to get bigger, and it cannot be squeezed.
In an open system it does not matter. The expanding water pushes gently back out through the cold inlet, into the service line, and into the street main, which is effectively infinite. Nobody notices and nothing drips.
Close that path and the expansion has nowhere to go. Pressure in the house climbs, fast, and it can reach the relief valve's setpoint of 150 psi within a single heating cycle. The valve opens, passes a small volume, and closes. The water cools, the pressure falls, and the system sits quietly until the next heating cycle. That is the periodic drip that stops on its own.
Why your house is probably closed
Three devices close a system, and any one of them will do it: a check valve, a backflow prevention assembly, or a pressure reducing valve at the service entrance. Each of them is a one way door. Water comes in, nothing goes back out.
St. George requires approved backflow assemblies on all outside watering systems. The city publishes that requirement on its own backflow page, along with the annual test and the hose bib vacuum breaker requirement. So in this city a large share of homes are closed systems as a consequence of an ordinance, not as a consequence of anything the homeowner chose. A pressure reducing valve does the same thing, and plenty of houses here have one.
The plumbing code requires thermal expansion control on a closed system for exactly this reason. In practice that means an expansion tank, teed into the cold water line near the water heater, with a rubber bladder separating a volume of water from a volume of air. The air compresses. That is the entire mechanism. It gives the expanding water somewhere to go that is not your relief valve.
If you have a backflow assembly on your irrigation, or a pressure reducing valve at the front of the house, and no expansion tank at the water heater, you have found the problem before anyone has opened a toolbox.
The check anybody can do
Buy a pressure gauge that screws onto a hose bib. They are inexpensive and they live in a drawer for years.
Thread it onto an exterior hose bib, open the bib, and read the number with no water running anywhere in the house. That is your static pressure. Write it down. Then leave the gauge on and let the water heater run through a heating cycle with nothing drawing water, and read it again. Utah's residential code caps static pressure at 80 psi under IRC P2903.3 and requires a pressure reducing valve above that. Thermal expansion control itself is IRC P2903.4.
On an open system those two numbers are nearly the same. On a closed system with no expansion control, the second number is dramatically higher, and a gauge with a lazy hand on it will hold the peak so you can see how far it went. That reading is the argument. It is also the number the expansion tank has to be set to, which is the next section.
The precharge rule, which is where most installations go wrong
An expansion tank arrives from the factory with air in it at a fixed pressure. The precharge has to be set equal to the measured static supply pressure of the house before the tank goes into service. Not close to it. Equal to it.
Here is why. The bladder separates air from water. If the air side is charged lower than the water side, the incoming water pressure squashes the bladder flat against the top of the tank the moment the tank is put in service. The tank is now full of water with no air cushion. It looks installed. It is doing nothing, and it was doing nothing on day one.
A factory precharge of 40 psi on a house running 80 psi static is the textbook version of this, and it is the most common installation error in the trade. The tank gets hung, nobody puts a gauge on the Schrader valve, and the relief valve keeps dripping. The homeowner then gets told the water heater is bad. Bradford White puts a consequence behind it in its own warranty, which excludes leakage or malfunction caused by installation "in a closed system without provisions to manage thermal expansion."
Setting it correctly takes a bicycle pump or a small compressor, a tire gauge, and the static pressure reading from the section above, and it has to be done with the tank isolated so there is no water pressure fighting the reading. It is five minutes of work that decides whether the part functions at all.
How to tell whether the tank you already have is dead
Expansion tanks are wear items. The bladder ruptures, or the precharge slowly bleeds off, and after that the tank fills with water and the system behaves exactly as if there were no tank at all. The relief valve starts weeping again, months or years after somebody fixed it.
Two field checks. Tap the tank with a knuckle from the top down. A working tank sounds hollow at the top and dull at the bottom, because that is air over water. A tank that is dull all the way up is waterlogged. Second, and definitive: depress the Schrader valve on the end of the tank with a key or a screwdriver tip. Air should come out. If water comes out, the bladder has failed.
A tank that passes water at the Schrader valve is replaced, not recharged. There is no repair for a torn bladder, and pumping air into it does nothing because the air and the water are no longer separated.
One more thing worth checking while you are there: a waterlogged tank is heavy, and it is hanging off a copper or PEX tee. A tank of any size needs its own support rather than hanging on the pipe alone. A failed tank that tears out its own fitting turns a five minute part into a flood.
What the pressure is doing to everything else
The relief valve is the loudest symptom, not the only one. Every pressure spike is applied to the whole system: faucet cartridges, toilet fill valves, supply hoses, the softener valve, the ice maker line, and the water heater itself. Fixtures that fail early in a house that looks well maintained are a reason to put a gauge on the hose bib.
There is a second reason not to ignore a weeping relief valve here specifically. St. George water runs hard, at the 18 grains per gallon the city tells residents to set their softeners to, with a maximum of 22. Water passing slowly across a valve seat, over and over, leaves mineral behind. A relief valve that has been weeping for a year often will not seat cleanly again even after the pressure problem is solved, so it gets replaced along with the fix. That is replacing it once, for a reason, rather than replacing it every few months and calling that maintenance.
Four things not to do
Do not cap the discharge pipe. Do not plug it. Do not install a valve in it. The relief valve is the last protection between a stuck thermostat and a pressure vessel in your garage, and a discharge line is required to have no valve and no threaded end precisely so that nobody can do this.
And do not simply replace the relief valve and stop there. If the cause is thermal expansion, the new valve will do the same thing the old one did, because it is supposed to. Fix the pressure, then replace the valve if it has been weeping long enough to have lost its seat.
Sizing the expansion tank is not a guess either. It depends on the system volume, the measured incoming static pressure, and the temperature setpoint. Those are three numbers a plumber can take in the time it takes to walk the house.
What we’d do at your house
We put a gauge on a hose bib and read your static pressure before we say anything about your water heater.
If there is a pressure reducing valve or a backflow assembly on the service, we will tell you that your house is a closed system and why that matters.
We set the expansion tank precharge to your measured static pressure with the tank off the system, and we write that pressure on the tag so the next person knows.
We support the tank rather than hanging it on the pipe.
If your existing tank passes water at the Schrader valve, we replace it and show you the water coming out, because that is the whole diagnosis in one second.
We will not swap a relief valve, call it fixed, and leave the cause in your wall.
Sources
What this page says comes from the documents below, read in full. It describes them. It is not legal advice. Confirm with your association, your city, or an attorney before you act.
- Thermal expansion mechanism, closed systems, the 150 psi relief setpoint, and the precharge to measured static pressure rule 01-plumbing-corpus.md §1.5; briefs.txt brief 16
- Waterlogged expansion tank diagnosis (tap test, Schrader valve, replace rather than recharge) 01-plumbing-corpus.md §1.5
- City of St. George backflow information page (approved assemblies required on all outside watering systems, annual test, hose bib vacuum breakers; Water Services, 620 East Waterworks Drive, 435-627-4802) 01-plumbing-corpus.md §7.3
- T&P discharge piping rules (no valve, no threaded end, gravity drain, visible termination) 01-plumbing-corpus.md §1.4
- City of St. George Water Distribution Division softener guidance, 18 grains per gallon, maximum 22 05-environment-water-geology-corpus.md §1.1; playbook.txt §4
- Downstream effects of pressure on fixtures, and T&P weeping as a symptom of a missing or waterlogged expansion tank 01-plumbing-corpus.md §1.5, §1.8