How can I diagnose a water heater leaking through TP valve?

Question

My natural gas water heater leaks about 1/2 gallon of hot water through my TP valve every day. This started about a month ago. Last week I replaced the TP valve with one that had identical specs as the old one, and it still leaks. There is a thermal expansion tank installed just above the heater.

The replacement valve I installed is CashAcme 23577-0150. The valve indicates "500,000 BTU/hr (ASME) Max Heater Input" and the water heater plate indicates 750,000 BTU/hour so maybe I just need a higher-rated TP valve, but I want to make sure that I am understanding the ratings correctly before I try a valve that is rated differently than the one I took off originally. The new valve also says 105,000 BTU.

While I was replacing the TP valve and had the heater off and water disconnected, I tried to drain the heater using the drain valve at the bottom of the heater. I had a difficult time getting water to come out the drain. Perhaps the heater has accumulated some sediment and that is preventing the drain from working. The plate on the water heater indicates it is 18 years old. Perhaps this is reason enough to replace the heater.

I turned the water temperature down slightly and it didn't seem to make much difference. The temperature of the hot water at the faucet does feel less hot, but I didn't measure it with a thermometer. If I have installed the correct valve, my guess would be that the thermostat that controls the temperature may be faulty or not working properly. The unit has a "digital" style control with lights that indicate the desired temperature. Perhaps the problem is with the electronics or perhaps it may be with the temperature sensor/sending unit.

I am very confident in my ability to just replace the water heater, but obviously cost and time are a concern. How long should water heaters last? What steps can be taken to diagnose the issue? Are replacement parts typically easily available that would fix this problem? Should I just give up and replace the water heater at this point?

Answer 1

The most common reason for a T/P to operate is excess pressure, which can happen with an expansion tank if the expansion tank has gone bad (or if it's too small, but that is less common, especially for "used to work.") It can also be because you have a PRV (pressure reducing valve - regulator) that's not working or adjusted right and is allowing overall pressure to be too high.

A crude test of the tank is to see if water comes out the air valve on the expansion tank, which indicates a failed bladder. Slightly less crude test is to depressurize the water side of the system and see if there's air pressure in the tank bladder, as there should be.

A pressure gauge is required to diagnose an operating pressure relief properly. Preferably one with an indicator of maximum pressure reached since last reset, so you don't have to sit there and watch the thing, or have a camera do that for you (modern method.) With absolutely no effort at seeing if a better deal could be had I found one for under $12 (plus shipping, so...) on a very quick look.

An 18 year old water heater is old, to be sure, and you might want to replace it for that reason, but the age of the heater (and the crud on the bottom from not draining it regularly) almost surely is not why the T/P is operating, IMHO. If it's located where the inevitable leak indicating that the time has come won't be a big deal (I see a lack of drain pan, but it also looks like bare concrete) then I would not rush to replace it, but I might shop occasionally to have an idea of what you'd do when you found it leaking, without panic-buying something, at least.