HW Cylinder Stratification

[Johntheo5]

Should the cylinder top temperature be exactly the same as the temperature at the bottom with a bottom horizontally mounted electrical heating element at the end of reheating??

I have a 150L OV 150L HW cylinder with 3 methods of heating, solar with very closely wound coils (3 in parallel) that take up only ~ 40L of the bottom cyl vol, a oil fired coil that heats 100L and a top mounted heating element that heats 28/30L. I have 3 X PT1000 temperature probes, one is mounted just above the top of the solar coil, one above the cylinder coil and the third one just below the cylinder top.
If I start with a fully cold cylinder using the solar coil only, the cylinder will heat up completely uniformly with the 3 probes reading almost exactly the same, to within 0.5C or less. If I start with a cold cylinder using the cylinder coil only then the two top probes will read exactly the same, the bottom probe will eventually rise just a few degrees, if I just use the heating element it will obviously only heat the top 30L with no increase in the bottom two probes. I have often seen the top probe reading 60C, the cylinder probe say 40C and the solar probe 25C depending on the heating combination and HW usage. If I then just use the solar coil, the cylinder coil probe will read/remain at exactly 40C (and the top probe 60C) until the solar probe is almost 40C, these two probes will then rise together until they reach almost 60C, the whole cylinder then rises uniformly.

Hence my query.

 

[Madrab]

The convection currents, being how a coil heats the water in a cylinder, will only circulate up and then down to where the heat source is, then be heated and rise again, that circulating water current won't drop below the heat source, if that makes sense.

Therefore if the heating coil is a 1/3rd of the way up then the water below it won't warm up by much, due to the stratification effect and the fact that heat rises, it will rise a little as some heat will still radiate and spread below but again once that warms it will rise.

 

[Johntheo5]

Yes, but what about the bottom mounted electric heating element which is a very high energy device, would you expect to see the same behaviour from this from the same conditions.

 

[Madrab]

Sorry, was a little confused , it was mentioned that

solar with very closely wound coils (3 in parallel) that take up only ~ 40L of the bottom cyl vol, a oil fired coil that heats 100L and a top mounted heating element that heats 28/30L

Is that solar coil fed by solar vacuum, i.e. a wet system solar coil? I presumed from your OP that the only electrical element was the top immersion?

Any heat source (electrical or otherwise) @ the bottom of the cylinder should heat everything above it, again by convection, so once the stat at the element reaches its setting and turns off then the water above it should be at least that temp. The convection current from that heat source cuts through any stratification layers that may have formed in the water column above it eventually equalising the temps.

Stratification is really only in play when a cylinder is fully heated and it then begins to be used. As cold water enters the bottom, the hot water stays at the top and they don't mix directly. Hence why most of the HW from a cylinder comes out the tap at a constant temp, only when it starts to get cold, it then tends to go quite cold quite quickly.