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?
Best Answer
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.