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The new flat has no gas, so hot water is provided by an immersion… - 'Twas brillig, and the slithy toves did gyre and gimble in the wabe [entries|archive|friends|userinfo]
Thomas

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[Friday 28th May 2010 at 9:02 pm]
Thomas

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The new flat has no gas, so hot water is provided by an immersion heater. I've had this thought for a while now that I could save a lot of energy by switching it off most of the time, and maybe just running it for a bit first thing in the morning. Now I've got the smart meter I could see just how much power the heater draws, and so decided to crank the numbers and work out what I'd save. Here goes...


The specific heat capacity of water is 4.18 J g−1 K−1. The tank has a capacity of 115 litres. A litre of water is equal to a kilogram, so to raise the temperature of the tank by 1° C takes:

4.18 × 115 × 1000 = 480700 J C−1

A Joule is equal to 2.7778×10−7 kWh, which gives:

480700 × 2.7778×10−7 = 0.1335 kWh C−1

The recommended temperature for a hot water tank is 65° C, and a reasonable mains water temperature is 15° C, so to heat a full tank of water takes:

0.1335 × (60 − 15) = 6.01 kW

My current electricity tariff is 11.52 pence per unit (1 unit = 1 kW of electricity consumed), so heating a full tank costs me about 70 pence. This all assumes 100% efficiency everywhere, so the actual cost is a bit more.


Anyway, back to the original question. Logically, for my plan to save significant amounts of money I'd have to lose all the stored heat from the hot water tank while the immersion heater is switched off (i.e. the losses from the tank would have to be at least 6 kWh per day). Recently I did leave the heater off for 24 hours (give or take), so armed with these numbers I should be able to work it out.

And the answer is: not significantly. To reheat the tank after 24 hours took about 40 minutes. That's a lot less than 2 hours. The actual energy consumption for that was:

(40 ÷ 60) × 2.86 = 1.91 kW

That's pretty good, given that the rated heat loss from the tank is 2.3 kWh per day (though it's currently quite warm here which will help).

I think I'd still save some electricity - the rate of heat loss will be higher the hotter the tank is, so it'll take more energy to continuously maintain the tank temperature - but probably not a massive amount. Maybe next I'll get hold of a thermocouple or something and log the temperature loss over 24 hours.

Of course, this does rely on me not going horribly wrong somewhere in the maths :)
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Comments:
[User Picture]From: olego
Friday 28th May 2010 at 8:42 pm (UTC)
I did read this post :-), but I still have a question:

You mentioned both a hot water tank and an electric shower. Does it mean that you have both? What kind of water flows into the electric shower: warm from the tank or cold from the street? Why would you want a hot water tank, then? Alternatively, since you're stuck with a hot water tank, why use the electric shower instead of reaping the benefits of having hot water in the tank? See my confusion?
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[User Picture]From: boggyb
Friday 28th May 2010 at 9:02 pm (UTC)
Right, here's how the plumbing is arranged in this flat:

The hot water cylinder feeds, via a pump, the hot water taps for the bath, bathroom sink, and kitchen sink. The pump is needed as the hot water cylinder is not at mains pressure - the tank is less than a metre off the floor, so without the pump there's very little water pressure. The pump runs automatically when I use the hot water.

The bathroom has an electric shower fitted above the bath. The shower is fed by the cold water main only. It's controlled by changing the flow rate (so I can get lots of pressure or lots of heat, but not both together) and by switching the heating element between off, half or full power.

The cold water taps are all at mains pressure (there's no cold water tank in the flat).

I'm a bit surprised that the shower isn't fed by the hot water cylinder, but I suppose this means the shower unit is less expensive and easier to install.

So, to sum it up: I have both, cold water flows into the electric shower, the hot water tank provides water for washing (both myself and in the kitchen - I don't have a dishwasher), and showers take less time than a bath.
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[User Picture]From: tau_iota_mu_c
Saturday 29th May 2010 at 4:47 am (UTC)
UK plumbing is weird.
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