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Have a two story house with kitchen, main bath, guest toilet and laundry room on first floor, and two bedrooms upstairs with one full bath each. The water heater is a 55 gal gas operated, located in the crawl space. There is an expansion/pressurized tank on the hot water line of the house as well.

Yesterday, out of the blue and without any warning, the shower in the main bath went from hot to lukewarm then cold after about 30/45 seconds of running. The sink in the same bath would put out scalding hot water, as did the kitchen sink and any other outlet in the 1st floor. Turning the mixer valve control to max hot puts out warmer water, just enough for one to shower.

Checked upstairs and one of the bathroom showers displays the same symptom, while the other one works fine. Again, all faucets in the 2nd floor put out hot water as expected.

I checked the anti-scald on both shower mix valves and these don't seem to have them (Price-Pfisters - MFD 2007), but after that I'm out of options. Today both showers continue to do the same, while the third one upstairs (the one least used) works fine.

Where should I start looking? Is it possible that both mixer valves would have gone bad at the same time? What else could be causing this?

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  • Would you have noticed that the upstairs shower problem started the same day, or did you only notice it after the downstairs shower experienced problems? Swapping out the mixing valve is going to be the fastest way to identify if it's the problem.
    – BMitch ♦
    Mar 8, 2016 at 16:07
  • BMitch, thanks for your response. I will look into the process to change the cartridge. Valve is PricePfister and I know they are lifetime warranty.
    – Andres Gutovnik
    Mar 8, 2016 at 23:03
  • Folks, I wanted to let everyone know that I was able to fix this issue by replacing the cartridge. PF will ship these free for the first owner of a home (I'm technically the second owner here, since I bought it from the guy that built the subdivision and had my house as his personal residence for the past 7 yrs or so).
    – Andres Gutovnik
    Jul 8, 2016 at 14:42
  • Two cartridges went bad at the same time?
    – rogerdpack
    Nov 9, 2020 at 6:55

2 Answers 2

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Wait, there's an expansion tank in the HOT water side? That's wrong, it's supposed to be on the cold-water side of the hot water heater. Expansion tanks do not have heaters in them, so the water in them is cold! Try this theory on for size: You turn on max-hot, until the water runs nice and hot. The lengthy hot-water pipe from downstairs is now full of hot water. Then you set it for mixed-warm, which flows a lot more water overall, and reduces system pressure. What does the expansion tank do? Equalize to that new pressure - the air in the tank expands, by pushing cold water out of the expansion tank into the "hot" line. It gets upstairs 30 seconds later.

How do you test this? First, when it gets cold, touch nothing - just let it keep flowing for 120 more seconds. See if it goes back to normal if you wait long enough. If so, I'd blame the expansion tank being on the wrong side.

Or check the tank's temperature - room temperature - then trigger the problem - and shut off everything and run back down and check the tank's temperature again. If it's warmer, it just ingested some hot water - which means it recently pushed out some cold water.

Otherwise, I'd do some other experiments. Shut off the cold supply to the shower. Now if it does the hot-cold bounce, it can't be the mixing valve! Turn the tub to hot max. Let it get hot and watch for that bounce. None? Throw open the sink faucet to COLD and flush the toilet, to simulate the extra flow of setting the shower to "warm". Watch for the bounce again. If it still happens, that means the (good) valve isn't getting good hot water.

What would make it suddenly break? Probably something minor upstream that wouldn't affect healthy plumbing. Maybe loose cruft in your piping (probably upstream of the hot water heater). Maybe in street piping. Maybe the water company changed supply pressure.

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    Hi Wolf, I may have oversimplified the explanation. The expansion tank is in fact on the cold side of the Hot Water heater... I will look into the test of cutting cold water and seeing what that does. Will report back
    – Andres Gutovnik
    Mar 8, 2016 at 23:04
  • You may not be able to shut off the cold water to test the hot as the shower valve probably is a pressure balancing valve. No cold water pressure would made the hot drop to zero as well. That's so when someone flushes the toilet your shower doesn't suddenly get hotter.
    – Mysterfxit
    Mar 8, 2016 at 23:18
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    I tried closing the cold water line to the shower but I wasn't able to do so since it did what Mysterfixit suggested. However I was able to close the valve on the expansion tank side, which made the shower run on full hot for as long as needed... Still a bit baffled by this, but I will see what the outcome is once mixing valves are replaced.
    – Andres Gutovnik
    Mar 13, 2016 at 16:57
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It's possible some debris in the hot water line fouled the two most used shower cartridges. The least used one was spared because it wasn't in use when the debris passed through.

Do you have galvanized steel piping? If so a chunk of rust may have broken loose inside upstream and wound up in the shower valve.

Has any work been done recently in the hot water lines?

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    Mysterifix, piping is copper/brass (based on color). House is 6 or 8 years old at the most... I doubt rust is the issue. However, there were deposits on the AC drain line, so maybe some debries were formed somewhere and got loose? There wasn't any work done on the property at all.
    – Andres Gutovnik
    Mar 8, 2016 at 23:07

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