"Theo" wrote in message
...
dennis@home wrote:
its quite easy to do the maths.

its done the same as electrical resistance.

however its easier to use something like

www.vesma.com/tutorial/uvalue01/uvalue01.htm

OK, let's try:

Hot water cylinder 450mm dia, 1.2m tall (first one on Screwfix)
pi*d*l = 3.14*0.45*1.2 = 1.7 sqm

According to the calculator, a 'wall' with 60C inside and 20C outside:

Uninsulated (1mm of steel): U=5.55 W/m^2 K
+25mm of polyurethane: U=0.85
+80mm of glassfib U=0.31

deltaT = 60-20 = 40
Uninsulated, power transfer ('loss') = 1.7*5.55*40 = 377W = 3306 kWh/year
+25mm PU = 1.7*0.85*40 = 57.8W = 506 kWh/year
+80mm GF = 1.7*0.31*40 = 21.1W = 185 kWh/year

The problem is getting a real figure for the added insulation.
That number is plucked out of the air, its not the real figure.

- adding the jacket saves 321 kWh/year of gas, at 4p/unit about GBP12.84.
(+combustion losses)

So it'll payback in 9 months - assuming you keep the water hot all day.

Don’t buy that with real commercial storage hot water
tanks and its never 20C outside all year round anyway.