Hot water cylinder insulation

[Jeff Layman]

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.

[Graham.]

Mine is an old bi-metal type with a knob on the front to set the temperature (in deg F)

[Chris J Dixon]

With existing tank insulation, can anybody do the maths to work out how much difference the extra lagging will make?

I might well try to do the same, if there is any significant benefit.

Chris

[543dsa]

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.

[tabbypurr]

results depend heavily on what airgaps are left, and those are hard to control.

nt

[dennis]

wrong as usual

its quite easy to do the maths.

its done the same as electrical resistance.

however its easier to use something like

[543dsa]

We'll see...

Not when you don?t have the numbers to use.

Pity that that doesn?t tell you the actual u value of the original tank, or the added blanket given the impossibility of allowing for how well it is fitted around the pipes and thermostat etc.

[Theo]

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. (+combustion losses)

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

Theo

[543dsa]

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

Don?t buy that with real commercial storage hot water tanks and its never 20C outside all year round anyway.

[tabbypurr]

last time I looked at a jacketed cylinder there were gaps all over the place. ROI is thus much worse, but by how much I couldn't say.

NT

[spuorgelgoog]

For under a tenner, and with bank interest rates so low, it's hardly a big decision.

Owain

[RJH]

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

A lot quicker if the immersion is used regularly.

I think the context here is domestic tanks indoors.

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

[543dsa]

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.

[alan_m]

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.

[Martin Brown]

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.

[John Rumm]

Worth keeping in mind that if you have the cylinder in an airing cupboard, you may actually want some heat loss to heat the cupboard.

[Martin Brown]

Fair point but I doubt if you could insulate a tank in such a cupboard well enough to cause a problem. Not a lot of room in the ones I've seen.