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.
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.