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In South Africa a "geyser" is a type of water heater that typically consists of a cylindrical water tank of 100 to 200 Litres capacity heated by an electric heating element of 2kW to 4kW capacity.

A geyser blanket is an insulator that is wrapped around a geyser to reduce wasted heat loss.

Insulating the outlet pipes makes sense since there will be heat losses there, but does insulating the whole geyser actually make any difference to your electricity bill if the geyser feels cold on the outside (in other words, the geyser is insulated by design)?

Geyser blankets are said to make significant savings:

  • I am changing the world

    A geyser blanket is proven to save up to 1 800kw/h per year! (depending on the size of your geyser)

  • Home Insulations

    Wrapping your geyser and hot water pipes with think pink aerolite or isotherm is a practical and efficient solution is to minimize thermal energy loss. The result will be a monthly electricity saving of up to 21 percent!

This 2006 article claims that a well-designed geyser doesn't need a geyser blanket:

This test indicates the energy losses incurred by an electric geyser over a 24-hour period. SunTank’s outstanding energy efficiency performance in this test gives hope that other industry players will follow the lead and save the water heating industry the embarrassment of having to retrofit geyser blankets. SunTank’s managing director Yoram Gur Arie says, “We have set certain integrity guidelines for the company that enable us to face customers with pride. How can anyone sell a new geyser and expect the customer to immediately pay additional fees for an installation of a geyser blanket in order to achieve the desired performance? I call on the geyser manufacturing industry to rewrite performance standards and regain some lost reputation.”

Assuming all modern geysers follow the same standards of being insulated by design, does a geyser blanket save the consumer any electricity?

  • geysers are tankless water heaters that heat on demand, there is no heat reservoir and any elements that do vary in temperature will likely be copper which doesn't have a lot of heat capacity (it doesn't cost much to heat) – ratchet freak Sep 1 '13 at 11:05
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    One reference above, "I am changing the world" is accused by my Avast software of attempting to plant a Trojan. It also invented a new unit of something, kw/h per year... – DJohnM Sep 1 '13 at 19:45
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    In the UK, the term "geyser" is not associated with electrically heated water tanks. A typical older UK house has a water tank on the upper floor heated by hot water from a gas or oil powered boiler which circulates through a helix of copper tubing inside the tank to heat the water for hot taps at sinks, basins, baths etc. The hot-water tank invariably has a supplementary electric "immersion heater" for use when the primary source of heat is not in use. All such water tanks are nowadays supplied with moulded-on insulation - In the UK you can't easily buy an uninsulated hot water tank. – RedGrittyBrick Sep 1 '13 at 21:34
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    kw-h per year is a sensible unit: kw measures power, kW-h measures energy, then kW-h per year is total energy saved per year. – ChrisW Sep 1 '13 at 23:42
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    Using a hyphen as in kW-h is just as wrong as kW/h. The SI standard allows for "kW·h" or "kW h", while kWh is also commonly used. – Tor-Einar Jarnbjo Sep 2 '13 at 9:32
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Yes, by simple physics, it will save the customer some electricity.

The rate at which heat is lost by a hot water tank is proportional to the difference between water temperature and air temperature. And it's also proportional to the U-value of the covering. Adding insulation will decrease the U-value, and so will decrease the rate at which heat is lost.

Hot water tanks are typically controlled by a thermostat. So sufficient electricity is used, to replace the energy lost by heat loss from the tank, and the energy used by drawing off hot water from the tank.

Therefore, by reducing the heat loss from the tank, thermal insulation reduces the electricity consumption.

Source: For details of the calculation, see for example the UK government's Standard Assessment Procedure for Energy Use in Dwellings, SAP2009 (pdf) - here's Table 2, showing how the rate of energy loss typically decreases, with increasing thickness of insulation.

enter image description here
(reproduced under fair use, Crown copyright 2010)

Note that your quote says up to 1800 kWh/y will be saved: for a very large, very badly insulated tank, the savings can be very high: 6 kWh/d is certainly plausible, and that's a rate of heat loss of 250W. Let's take a large, really badly insulated hot water tank with a surface area of 5 square metres, and base U-value of 2.0, a water temperature of 70° Celsius, and an air temperature of 20° Celsius. That gives a heat loss rate of 5 x 2 x (70 - 20) = 500 W. So if the insulation halves that (and it should do better than that), that's more than 1800 kWh saved per year.

The energy savings = energy loss before - energy loss after

In each case, the energy loss = (water temp - air temp) x surface area x U-value x duration

Insulation will slightly increase the surface area. And it may also change the time-profile of water temperature and air temperature around it. So the specific number is hard to calculate accurately in advance - it can be approximated, with fair accuracy, by making some simplifying assumptions.

The U-value changes according to the following equation:

( 1 / new U-value ) = ( 1 / old U-value ) + ( 1 / U-value of insulation )

e.g. if the old U-value was 2.0, and the U-value of the insulation is 1.0, then
the new U-value = 1 / ( 1 / 2.0 + 1 / 1.0) = 1 / 1.5 = 0.67

(source - as above, SAP2009)

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    The OP says, the geyser feels cold on the outside. If the insulation in the geyser is already very good (or perfect) then the savings from adding a blanket would be negligible (or zero). – ChrisW Sep 2 '13 at 21:46
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    I've discovered the SANS 151 Standard which is being used by the SABS as a heat loss specification: law.resource.org/pub/za/ibr/za.sans.151.2010.pdf On page 16 they have a list of maximum permissible standing losses, column 4 seems to be the column I'm interested in. Assuming geysers perform to this standard, would adding a geyser blanket have a significant reduction in losses and how much would the actual electricity saving be with the reduction in losses? – Jan Vladimir Mostert Sep 3 '13 at 8:37
  • The question remains, does the saving still come down to as quoted: "A geyser blanket is proven to save up to 1 800kw/h per year! (depending on the size of your geyser)" – Jan Vladimir Mostert Sep 3 '13 at 9:49
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    So it saves electricity for badly insulated tanks. According to the SABS standard, the daily permissible loss for a 550l tank (which is massive) is 6.05kWh, for a 150l it is 2.59kWh, how much will the losses be if a geyser blanket is added? – Jan Vladimir Mostert Sep 3 '13 at 11:15
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    @ChrisW "feels cold on the outside". A metallic surface at 30 C still feels cold, but the saving still may be measurable in 20 C enviroment. – Suma Sep 3 '13 at 12:35

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