The Relief Valve That Started Dripping After the Meter Change

Why this matters

A homeowner called because the relief valve on a six-year-old gas water heater had started dripping into the pan, a few ounces at a time, and they wanted the heater replaced because "it's failing." Nothing about the heater had changed. What had changed was the water meter: the utility had swapped it about five weeks earlier for one with a built-in check valve, which turned an open system into a closed one. A heater replacement would have dripped exactly the same way. A gauge left on a hose bib overnight told the whole story, and the fix was an expansion tank with its precharge set to the house's static pressure.

Safety before the diagnosis

The relief valve is doing its job, so nothing gets plugged. The homeowner had asked whether they could cap the discharge pipe "until the plumber comes." The answer is never. The temperature and pressure relief valve (T&P) is the device that keeps a heater from becoming a pressure vessel with no outlet; capping its discharge, or a valve or plug anywhere on that line, removes the last protection against a tank rupture. The tech said so on the phone and again on arrival, and checked the discharge pipe ran full size to its termination with a plain end, as The T&P Discharge Pipe and the Rules Other Trades Break describes.

Discharged water is scalding. Water leaving a T&P can be at full tank temperature. The tech kept hands and face away from the discharge outlet whenever the burner was running, and did not lift the test lever until the diagnosis was done.

What the tech saw

• The drip came in episodes. Dry in the morning, a short dribble in the late morning after showers, more in the evening after dishes and a bath. Dry overnight, when nothing was drawing water.
• The pan held about a cup of water each evening by the homeowner's account.
• The heater was correct. 50 gallon atmospheric gas, thermostat at the "hot" mark, outlet temperature measured at the kitchen faucet after a minute of flow: 122 F.
• No expansion tank, no pressure reducing valve (PRV) anywhere on the service.

Candidates, killed one at a time

1. A worn or fouled T&P seat. A seat with debris or scale on it leaks all the time, cold or hot, at any pressure. This valve was dry overnight and dry first thing in the morning, when nothing was drawing water. A leak that comes and goes with the heating cycle is pressure or temperature, not a seat. Replacing the valve would have bought a week of dry pan, at most, from a fresh seat that opens a little later, and then the same drip.

2. Overheating. The temperature element of a T&P opens at a rated temperature well above normal operation, commonly 210 F. If the heater's gas control were failing to shut off, the tank would overheat and the valve would discharge hot water on temperature. The outlet read 122 F, the thermostat cycled the burner off as it should (watched from a cold start to shutoff), and nobody had reported scalding water. Temperature was not the trigger.

3. High street pressure. With no PRV, the house sees whatever the utility delivers. A gauge on the laundry hose bib with nothing running read 62 psi: inside the 80 psi maximum static pressure at fixtures that IPC Section 604 and UPC Section 608, as adopted, both set, and far below the 150 psi pressure setting typical of a residential T&P listed to ANSI Z21.22/CSA 4.4. Street pressure alone does not open the valve.

4. Thermal expansion in a closed system. Water heated in the tank expands; heating a 50 gallon tank from incoming water at about 50 F to 120 F adds a little over half a gallon of volume. In an open system that half gallon pushes back out through the meter into the street main and nobody notices. In a closed system there is nowhere for it to go, water barely compresses, and pressure climbs on every heating cycle until something gives: the T&P, a toilet fill valve, a faucet cartridge. That fit the pattern exactly: drip after draws, when cold makeup water refilled the tank and the burner heated it, and dry overnight once the tank was hot and satisfied.

The gauge reading that showed it

To prove candidate 4 rather than infer it, the tech left a test gauge with a maximum-reading (telltale) pointer on the laundry hose bib overnight, valve open, and asked the homeowners to use water normally that evening and morning.

  HOSE BIB GAUGE LOG, LAUNDRY, 50 GAL GAS HEATER

  Day 1  16:10  static, nothing running ....  62 psi
                telltale pointer set to ......  62 psi
  Day 1  19:40  after dishes, burner running   118 psi
  Day 1  20:05  burner off, pan wet .........  146 psi
  Day 2  07:50  before anyone up ............  139 psi
                telltale pointer reads ........ 150 psi
  Day 2  08:30  after 2 showers, burner on ..  71 psi
  Day 2  09:05  burner off, T&P dripping .... 149 psi

Two things in that log settle it. The telltale reached 150 psi, the T&P's pressure setting, from a static start of 62 psi, with no change at the street. And pressure stayed high overnight (139 psi at 7:50 a.m.) with nothing running, which only happens when the water cannot flow back out through the meter. In an open system the house drifts back to street pressure within minutes of the burner stopping. The morning showers dropped it to 71 psi by drawing water out, and the next heating cycle drove it right back to the relief point.

Then the meter. The tag on the new meter in the curb box, read by the tech from above the box with the lid off and without touching the meter, listed an integral dual check. The homeowner found the utility's notice of the meter change in the recycling: dated five weeks earlier. The drip had started, by their account, "about a month ago."

Why the old setup never dripped

For six years, every heating cycle pushed its half gallon of expansion back through the old meter into the main. The water heater installer had found an open system with no check, no PRV and no backflow preventer, and correctly fitted no expansion tank. The system did not change because anyone in the house changed it; it changed when the utility swapped a meter, which utilities do for their own reasons, often with a check valve built in to protect the main from backflow.

The customer had not been told the system was now closed, and nobody would have thought to tell them. That is the most common way a house becomes closed without a plumber present.

The fix

Sizing. Both model codes, as adopted (IPC Section 607, UPC Section 608), require in substance a means of controlling thermal expansion where a check valve, PRV or backflow preventer closes the system, and an expansion tank is the usual one. The tank was sized for a 50 gallon heater at 62 psi static and the relief setting by the method in Expansion Tank Sizing for Closed-Loop Domestic Water Heaters. That card owns the arithmetic; this one takes its answer, a tank of the common size for that heater and pressure, as given.

Isolation before installing. Gas control set to pilot, cold supply valve to the heater closed, a hot tap opened and left open until it stopped flowing, which proves the tank side of the shutoff at zero before the cold line was cut; the hose bib gauge sits on the house side of that shutoff and cannot show it. The cold line at the heater carries the full overnight pressure until it is relieved, and cutting into it at 140 psi throws hot water from the tank side.

The install. The tank went on the cold supply between the heater's cold shutoff and the heater, supported by a strap to the wall so its weight full of water does not hang on the pipe. The full procedure is in How to Install a Thermal Expansion Tank and Set Its Precharge.

The precharge. Before any water reached the tank, its air side was set with a tire gauge and a pump to 62 psi, the measured static house pressure. The factory charge had read 40 psi. Left at 40 against 62 psi of house pressure, the bladder would sit compressed by street pressure alone, and the tank would have little room left for expansion before the T&P opened again.

Restore. Cold supply opened slowly with the hot tap still running to purge air, joints checked dry at 20 minutes, hot tap closed, gas control returned from pilot to on, burner lit and seen running.

The T&P. The valve had discharged dozens of times in a month. A seat that has lifted that often can stay weeping from scale, so the plan was to watch it and replace it only if it kept dripping with the pressure controlled. It stopped.

How they confirmed it

The telltale gauge went back on the laundry hose bib for another night with normal use. Static 62 psi at 4:30 p.m.; telltale at 7:30 a.m.: 74 psi. The relief setting is 150 psi; the peak of 74 psi left a margin of 76 psi. The pan stayed dry for the next week, checked by the homeowner and photographed for the ticket.

The note to the customer

The tech left a short written note with the invoice:

Your water heater is not failing. Five weeks ago the water utility replaced your meter with one that has a built-in check valve. That makes your plumbing a "closed system": when the heater warms water and it expands, it can no longer push back into the street, so the pressure rose until the heater's safety valve opened to relieve it. That is the valve doing its job. We installed an expansion tank to absorb that expansion and set it to your house pressure of 62 psi. Please do not ever cap or plug the pipe from the safety valve. The tank's air charge should be checked at each water heater service, and if the safety valve starts dripping again, call us before replacing anything.

The ticket recorded the before and after telltale readings, the static pressure, the precharge as set, the meter tag, the utility notice date and a photo of the dry pan.

References

• IPC 2021 Sections 604 and 607 and UPC 2021 Section 608, as adopted by your jurisdiction: the static pressure limit at fixtures and thermal expansion control on a closed system.
• ANSI Z21.22/CSA 4.4, the standard residential relief valves are listed to.
• See related: How to Install a Thermal Expansion Tank and Set Its Precharge; Expansion Tank Sizing for Closed-Loop Domestic Water Heaters; How to Replace a Temperature and Pressure Relief Valve.