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My shower water temperature reaches what feels like an early plateau (warm enough to get in), but then continues to get warmer at the same handle position for the next 3-5 minutes. The obvious explanation would be that the cold pipes are initially keeping the temperature down, but this is the first place I've ever lived where I noticed this happening. Is it related to modern shower valves? Tankless water heaters? Or was I just slower to get in the shower for the previous 40 years?

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    What type of water heater do you have? How far from the water heater is the shower? What is the temperature of the water entering the home? Is there a thermostatic mixing valve installed?
    – Tester101
    Sep 16, 2015 at 13:10
  • It is an older (unknown vintage) tankless heater for the entire house--are you asking about the brand? Or are there subcategories/designs? (Sorry, it came with the place, so I've never learned anything about it or tankless heaters in general.) The water heater is about 30 feet from the shower. The water runs through uninsulated copper pipes in a "mild weather" crawl space (Los Angeles). Based on that wiki link, no, there's no thermostat. I installed a low-end shower valve 3 years ago--it had a tab or something to restrict maximum hot water flow, but I removed it. Thanks for any insight!
    – Phil Esra
    Sep 16, 2015 at 19:19
  • Sorry--regarding the water temp when it enters the home: I don't know, and I'm not sure how the tankless heater regulates it. Too hot to comfortably use only the hot water tap once it comes up to full temperature (about 1+ gallon until full temp at the kitchen sink, about 6-8 feet from the heater).
    – Phil Esra
    Sep 16, 2015 at 19:27

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Assuming a common mixer valve, if the faucet washers are not neoprene, warming may cause the one in the cold valve to expand, choking off the cold water.

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