"RJH" wrote in message
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On 26/09/2018 01:11, 543dsa wrote:


"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.


It is based on a set of reasonable assumptions so far as I can see. It's
called modelling . . .

Not possible to model the real life U value of the
added insulation given that it has to have the
pipes and overflow relief valve going through it.

Sure, you can certainly use worst case numbers
but even with the external temperature, it isnt
really that easy to actually measure what that
is over the whole of an average year etc.

As I said initially, it makes a lot more sense to
actually measure the reduced loss with the
heater turned off, with and without the extra
insulation, but you do have to buy it to test it.

- 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.


A lot quicker if the immersion is used regularly.

How often it is used has no effect on the payback period.
What is being calculated is the loss from the cylinder that
is saved with the extra insulation and how long it takes
to pay for the extra insulation.

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

I think the context here is domestic tanks indoors.

Yes, but that doesn’t make it any easier to calculate.

A business that stores hot water outside in an uninsulated tank isn't
going to last long enough to even begin to discuss insulation.

Sure. But we arent talking about uninsulated tanks, we
are talking about insulated tanks and whether EXTRA
insulation like the one in the url will pay for themselves.

And plenty of domestic tanks are outdoors,
because that doesn’t waste space indoors.