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The rate of flow of heat between two systems is measured in watts (joules per second).

The formula for rate of heat flow is ∆Q/∆t = -K×A×∆T/x, where ∆Q/∆t is the rate of heat flow; -K is the thermal conductivity factor; A is the surface area; ∆T is the change in temperature and x is the thickness of the material (∆T/x is called the temperature gradient and is always negative because of the heat of flow always goes from more thermal energy to less).

Simple for the OP to calculate, I hope

Originally posted by **I have spoken**
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Thanks, I hadn't thought of doing a thermal conductivity calulation. I've just done one, and I get a temperature drop of 8.3 Celcius over 23 hours. I'm a bit surprised that so much heat is being lost through the insulation, I thought convection around the loop through the boiler might be the biggest problem. Time to buy an extra insulated jacket maybe, but also possibly turn up the thermostat to avoid legionella.

My input numbers were

Height = 1m

Diameter = 0.42m

Insulation thickness = 0.04m

Conductivity = 0.03 W/mK

Air temp = 25 Celcius