Just fitted some insulation
over a hot water cylinder which already had about 25mm of foam
insulation. Of course, the many pipe entries meant it was not easy to
line up the edges of the bags, and I had to cut a hole for the cylinder
thermostat, as the instructions say it should not be covered.
After fiddling around and making sure it wasn't covered, I wondered why
it shouldn't be covered. Surely it doesn't make any difference if it's
covered of not if it's on contact with the metal of the tank.
Oh, and the cylinder insulation doesn't seem to be designed for
top-entry immersion heaters (which we have) as much of the insulating
action seems to rely on the top of the tank being covered completely
with overlapping edges. I had to ensure the immersion heater and
connecting cable wasn't covered.
Not really possible to do the maths in isolation.
You'd have to actually measure how quickly it cools
with the electricity turned off, with and without the
extra insulation, to see how long it would take to
pay for itself and if it actually would.
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 glassfibre: 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
- adding the jacket saves 321 kWh/year of gas, at 4p/unit about GBP12.84.
So it'll payback in 9 months - assuming you keep the water hot all day.
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.
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.
Yes, but that doesn’t make it any easier to calculate.
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.
On 25/09/2018 20:22, email@example.com wrote:
On jackets I've fitted here have been minimal air gaps especially none
at the top of the cylinder where the panels overlap giving a thicker layer.
There is a table at
which suggests payback of less than 1 year in the OPs case were he had
25mm of existing foam insulation. The table figures assume the jacket
cost £25 but the OP paid £10 so their 2 year figure needs to be reduced.
Depends how much you have already but as a rough rule of thumb if you
doubled the thickness using the same material then the rate of heat loss
through it would he halved. However, conduction along pipes in and out
of the hot zone make it somewhat less effective than that.
£30 spent on better insulation round the hot water tank pays for itself
with the fuel savings in the first year. It also means that your hot
water stays warmer for longer when the heating isn't on.
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