Junkiepanda, Jeff has given you some very important information. Get the water pressure gauge he recommended. The second hand on the gauge is often referred to as the “tattletale“ hand. You can watch the gauge all day, but the “tattletale“ will show you the highest pressure over a 24 hour or longer period. It will stop at the highest pressure and not reset itself. High water pressures usually occur at midnight and after. Put the gauge on the heater’s drain valve to see exactly what’s happening at the water heater.
I remember my experience with something similar 50+ years ago as a young plumber. Customer installed new water heater. T&P valve started dripping when heater was heating. Customer had never experienced this problem before. He replaced two T&P valves before calling water heater’s warranty department. The manufacturer had sent out two companies prior to calling my company. Each had simply installed new T&P valves that had not corrected the concern.
Lucky for the customer we knew there are only two factors that affect T&P valves, elevated pressure (150+ Psi) and elevated water temperature (210*F). A couple of good ways to know which is the culprit is by the mount of water released. A valve relieving because of temperature will relieve a lot of water quickly. And the water will be very hot and steamy.
With a pressure problem, the valve only releases enough water to maintain the 150 Psi. This is usually a drip. As Jeff pointed out, water is not easily compressed.
Let’s skip past the next two hours it took me to find the reason for the elevated water pressure. My problem was a RPV (Reduced Pressure Valve) installed at the home’s water meter. Over the years it had gotten covered over and the home owner didn’t actually know it was there. The PRV was originally installed some 30 years before when this was a new area with 150 Psi water pressure. The city utility actually came out and removed the old valve because it was no longer needed.
PRV are of two types, with and without bypass ports. Valves without bypass ports will act as check valves and not allow the pressurized water to back into the city water main. Valves with a bypass will allow water to flow back into the water main. This PRV‘s bypass port had gotten stopped up preventing the expanded water’s pressure to back past the valve.
This is a common problem when water is turned off to make repairs. After removing the PRV the water heater concern went away. The water gauge confirmed this was the problem. But, before you put up your tools check one more thing.
With the water pressure gauge still installed, make sure the PRV is maintaining 60 Psi 24/7. Remember, plumbing fixtures, valves and piping manufactures limit their warranties to 80 Psi or less. All the plumbing codes I’ve familiar with also limit the pressure to 80 Psi.
As earlier noted, an expansion tank may address the T&P function. However, an “inquiring minds (need to know)“ may force you to find the valve causing the problem. My bet is it’s a PRV, RPZ, DC, check valve or other device located at the meter. When you fixed the leak on the sprinkler system, you stopped the slow relief of over pressured water. Compare the drip from your T&PV now to the leak on your sprinkler piping. Was it the same of perhaps more?
I hope this mini series helps. But the water pressure gauge is the key to identifying your problem. Also, we plumbers call the water pressure gauge several different things, gauge with dead man’s leg, lazy hand, “that two headed gauge,” gauge with max indicator, and sometimes “that damn special“ water gauge! This is a tool all plumbers should have in their tool boxes.
Let us know what you find!
