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I have an inline water heater. Once it gets hot it will stay that way for 20+ minutes at a time easily.

When the shower gets used it feels like their is a floor in how cold the water will get - if I turn the cold up past a certain point it runs ice cold.

This waterheater has been in a while. I've already flushed the lines with vinegar and tripple checked the gas and gas regulator.

What more should I look at? My guess would be the water flow control mechanism but I really don't know.

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Are you using a single handle mixing valve, or are there independent hot and cold handles in the shower? –  Tester101 Oct 16 '13 at 12:14
    
@Tester101 - I actually have removed all of the single handle mixing valves in the house. I don't trust them. –  Tim Brigham Oct 16 '13 at 12:49
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1 Answer 1

Tankless/on-demand (is this the same as inline?) water heaters only heat if the water flow rate is above some minimum, typically around 0.5 gallons per minute. As the water temperature is made more cold, less and less hot water is going into the mix. At some point, the hot water flow decreases below the threshold and the water heater turns off, yielding cold water flowering through the hot water pipes.

Your water heater may have an adjustable flow rate threshold, and if so, it could be decreased. Another solution would be to decrease the temperature setpoint to be closer to the desired final temperature (though you may need the hotter water for dish or clothes washers).

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Thanks. What I find odd is this behavior only started a couple weeks ago. For the last couple years this tankless water heater has worked perfectly. Something obviously changed. –  Tim Brigham Oct 16 '13 at 17:39
    
Could the setpoint have changed? Does the water seem hotter now? It is frustrating that this is how the units are designed, but there isn't much that can be done except install a (perhaps small) point of use heater with a tank. –  Pigrew Oct 16 '13 at 17:55
    
I don't think so. When I initially installed this water heater I set the temperature to maximum and the flow control to minimum (for our energy efficient washing machine). –  Tim Brigham Oct 17 '13 at 13:02
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