When does a hot water decide to heat water

[Chefwong]

My own wikipedia is keened on knowing...

I tried googling this to no avail.
Assuming that on any HW tank, the top 2/3 is hot water and the bottom is cold.
At what point does it trigger the system to heat water. When it drops below the set temp.
Or X Variable, Y Variable ?

 

[Reach4]

On a gas water heater it is the temperature right behind the control -- near the bottom of the tank.

 

[Sponsor]

 

[Chefwong]

Got it. So the temp probe/sensor is ~right behind~ the valve body.
Thx

 

[Reach4]

you can see the probe at 4:00 into the video.

 

[hj]

The incoming/replacement cold water is directed to the bottom of the tank by the dip tube. However, not all dip tubes extend to the bottom of the tank, so the point where the incoming water will cool the thermostat enough to turn the heater on varies according to the make, model, and incoming water temperature. ALL thermostats have a differential which determines HOW low the temperature has to fall below the setting to activate the heater, and this differential has a +/- leeway due to manufacturing and assembly differences. The water ABOVE the thermostat is "hot". Below the thermostat is varies from cold to hot depending on how long the heater has been inactive allowing convection currents to mix the water in the tank to arrive at some state of stasis.

 

[Jadnashua]

An electric WH typically has two heating elements and two thermostats, and most of them will not activate both at the same time. The top one runs when its thermostat senses cold, then when that's satisfied, it enables the bottom one to turn on.

On my indirect, the sensor is about in the middle of the tank up the side and is just pushed into a tubular well, held in by a spring, but in it's case, it has a heating coil from the boiler running nearly the whole height of the tank, so it can transfer lots of heat fairly quickly. That's one reason why you can typically use a smaller indirect than a stand alone tank...faster heat recovery and larger heat source.