Topic: A question about water heaters and thermodynamics

[marcparis]

I'm working on a document for a manufacturer of a somewhat innovative water heater and they make a claim which doesn't make much sense. I don't have the background to know what's right, but I thought some of you might have an idea.

The claim is that water is heated faster because the shape of the water heater favors mixing of water in the tank. This surprises me, because from what I know, it's important to have the cold water intake at the bottom of the tank, and to keep the heated water at the top of the tank where the outlet is found. This means that the hottest water is at the point where it is removed from the tank.

I also imagine (but here's where my knowledge is particularly weak) that the heat transfer is faster and more efficient from the heating element to cold water than it would be from the heating element to warmer water. By mixing the water, the heating element is in contact with warm water, so this design would seem to be less efficient, rather than more efficient.

Have I got this very wrong?

[MessyAndy]

You need to have the heater at the bottom of the tank, just like the element is in the bottom of the kettle, because the hot water will rise and set up a convection current., by the time it gets to the top the thermal energy will have spread out slightly, thus the top is not hotter than the bottom.

[Bunsen]

Yeah.  Without being able to actually see the actual design, I'd tend towards agreeing with MessyAndy.  Unless they are using some kind of coil heating element that heats very quickly but is insanely energy inefficient, I'm not sure how much the shape of the reservoir would actually improve anything.