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I wonder if it is possible


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To have a thermostat on a water heater so precise that it would keep water to the 10th of a degree?


There's an old movie called HOW TO MURDER YOUR WIFE. It starts off with Charles ("I am Mr Ford's man") giving you a guided tour of the premises.


"This is Mr Ford's townhouse."


"This is Mr Ford's rooftop garden."


"This is Mr Ford's shower, set to precisely ninety-eight point SEVEN degrees - Mr Ford's body temperature."


I thought that was a very cool bit of business. I don't think they had the ability to do that back in the 60s when the movie was made, but I wonder if they could do it today?

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Every thermostat has a  factory set hysteresis, which is a particular number of degrees above or below the setting at which the burner or element turns on or turns off.  


I suppose it would be possible to set the hysteresis  very tightly around the nominal setting, but the heating element or burner would be turning on and off so rapidly that a very large fraction of the energy would be wasted. 


 Usually hysteresis is about 3 deg F plus for shut off and about 1 degree F minus for turn on.  So set at 68 deg, it turns on at 67 and off at about 71 for a four deg range of variation. 


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We used to install these if absolute control was required but I only used them for air, not water.



A proportional control would be better but it requires more equipment and more cost. I also think most folks would find 98.7 too cold for a bath or shower.

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