Hot Water Cylinder Temperatures

[John.g]

150 litre cylinder, top 100 litres are heated by a coil (average temp 70C), the bottom 50 litres (or the whole 150 litres in summer) are heated by a solar coil. There are PT1000 probes fitted in numerous places and the HW zone valve is controlled very accurately by a spare output from the solar controller.

With no solar gain, the cylinder behaves the same as any cylinder, ie if the whole cylinder is cold then the cylinder (100 litres) will heat from the bottom up completely uniformly until it reaches its SP of 60C, the probe is located about a quarter way up the coil, the cylinder top temperature will also be exactly 60C. If water is drawn off then the zone valve opens when the lower probe reaches 55C and again will re heat uniformly until it reaches exactly 60C, the cylinder top temperature will also be exactly 60C. But if there is hot water being continuously drawn off at a rate lower than the cylinder coil input, then the (lower) probe will still shut the zone valve at exactly 60C but the cylinder top temperature will now be 64/66C.

This raises a question in my mind, why doesn’t the cylinder top temperature be higher than the lower one even when heating up the (or any) cylinder from cold as the coil flow is from top to bottom with the hottest water at the top and one might expect the water to heat from the top down?, the temperature differences when water is being continuously drawn off does make sense strangely enough. I believe some manufacturers have the coil flow from bottom to top.

 

[Chuck]

If I've understood your question correctly...

Assuming stratification is preserved the temperature drop along the coil stays relatively constant while the hot/cold boundary moves down the cylinder until it reaches the top of the coil. At that point the top turn of the coil stops losing heat so the next turn down rises in temperature and the hot/cold boundary in the cylinder moves down to the next turn, and so on to the bottom of the coils. In the absence of the thermostat, most of the tank ends up at 70 degrees with the hot/cold boundary at the bottom of the coil, the return temperature from the coil has risen to match the flow termperature, by which point the heat transfer has dropped to zero.

 

[John.g]

I still find it a little strange that if the whole cylinder (100 litres) is heated to 60C, you then run off a batch of say 30 litres of water, some of the top coils will then still be immersed in water at 60C and all the 50 litres or so of water above the top coil will also still be at 60C, the zone valve opens and introduces water at 70C into the top coil so there is still a deltaT of 10C which one would think should impart some heat to the 60C water and very gradually increase the temperature of the 50 litres of that 60C water which is above the top coil.
Perhaps the reason is that there is also 50 litres or so between the coil flow and return connections and that all the heating takes place here until that 50 litres also reaches 60C and then the whole 100 litres would increase in temp if the zone valve was allowed to remain open?.
I wouldn't have taken any notice normally only that I did see the temp at 64C with a continuous small flow rate.

 

[Gasmk1]

this may explain it How a Twin Coil Cylinder Works - https://www.viridiansolar.co.uk/resources-3-7-1-twin-coil-cylinder.html

 

[John.g]

That explains quite well how a twin coil cylinder like mine works. My oil fired boiler is switched off for ~ 6 months and all the hot water is supplied by the bottom coil, in the spring/autumn when the oil boiler is switched back on you may have the top 100 litres at 60C or lower and the bottom 50 litres at 15/20C. When the sun comes up/out, another probe mounted just above this bottom coil switches the solar circ pump on when the solar collector temp is 7C higher and will stop the pump when this temp is 3C higher than this probe in the cylinder (called the store temperature) the 50 litres will continue to rise until it reaches 60C or whatever the upper layer temperature is and will then continue to raise the whole cylinder temperature to its max (while on solar) of 75C if sufficient solar radiation is available. The hot water outlet temperature is limited to 60C by a TMV.

 

[Chuck]

Perhaps the reason is that there is also 50 litres or so between the coil flow and return connections and that all the heating takes place here until that 50 litres also reaches 60C and then the whole 100 litres would increase in temp if the zone valve was allowed to remain open?.

Are you assuming that the water heats by conduction? This is not likely to be the case. The hot coil normally sets up a convection cell so the water heated by the coil rises to the top of the tank and the displaced cooler water follows a path that takes it past the coil. Tanks have to be quite carefully designed to make sure this happens.

 

[John.g]

I still can't think of a logical explanation re my first paragraph in post #3 as convection should dictate that there should be some rise in the cylinder top temperature