Water temperature range in a hot water cylinder

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Discussion Overview

The discussion revolves around the temperature distribution within a hot water cylinder used in solar water heating systems. Participants explore the dynamics of heat transfer, the behavior of water at different temperatures, and the implications of insulation and system design on temperature gradients.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant suggests that in a solar water heating system, heated water rises to the top of the cylinder, creating a temperature difference between the top and bottom due to the motion of water molecules.
  • Another participant argues that if the tank is insulated, the temperature could equalize to that of the solar panel output, potentially reaching high temperatures that could affect materials like lead solder.
  • A different participant notes that the top of the cylinder is generally hotter than the bottom due to the density differences caused by temperature, and seeks to estimate the temperature difference based on heat transfer rates.
  • One participant questions whether the primary reason for the temperature difference is the rising hot water, suggesting that solar panels may deliver hot water to the top of the tank, similar to an oil boiler system, and provides specific temperature readings from their own thermal store.

Areas of Agreement / Disagreement

Participants express differing views on the mechanisms behind temperature distribution in the cylinder, with some emphasizing the role of rising hot water and others considering the effects of insulation and system design. No consensus is reached on the exact dynamics at play.

Contextual Notes

Participants mention various assumptions regarding insulation, heat loss, and the specific designs of their systems, which may affect the temperature readings and dynamics discussed.

Who May Find This Useful

This discussion may be useful for individuals interested in solar water heating systems, thermal dynamics, and the practical implications of system design on temperature management in hot water cylinders.

BobBob20
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I am thinking about solar water heating. The way that the system gathers heat is by moving any heated water into into the cylinder where it rises to the top and cold water is taken from the bottom of the cylinder and moved through the heating panel.

Because the hot water will always rise there will always be a temperature difference between the water at the top and the bottom.

But because of the motion of the water molecules the heat differential will disperse.

So, for example, If the temperature is 50°C halfway up the cylinder, what will be the highest and lowest water temperatures in the cylinder?

Thanks.
 
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I don't think it's that simple. If the tank is insulated there is no heat loss so the temperature of the whole tank could eventually approach that of the output from the solar panel. I've heard that pressurised systems can get hot enough to melt lead solder and that special solder as to be used!
 
Everything I have read says that the water in the hot water cylinder will be hotter at the top than at the bottom of the cylinder according to the way the temperature affects the density of the water.

I am looking for a way to estimate what the difference in temperature will be between the top and bottom of the cylinder.

I guess it will be a balance between the rate at which the hotter water rises versus the rate at which the hotter water loses its heat energy through conduction to the cooler water.
 
Hot water rises yes but I don't know if that's the main reason why the top of a thermal store is hotter. I thought solar panels delivered hot water to the top of the tank like a boiler/furnace?

My thermal store is heated by an oil boiler that way. Hot water goes in the top at about 75-80C and heats the top of the store until a thermost about half way down exceeds 60C. The return water taken from the bottom of the store seems to be about 50-55C (it's cooled by the return from the UFH). The boiler provides about 20-30C uplift.
 

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