Question regarding electric water heaters

banmido12345

I've just moved into an all electric flat in London with what seems to be an immersion heater. The brand is Telford with a capacity of 120 litres. It also has a layer of green foam for insulation I think. I've just been told by the plumber it is better and more economical to leave it on 24/7 as it takes more energy to reheat from cold water. I've had a look on several forums and there seems to be a debate that leaving it on constantly will only cost a little extra. I tried turning the hot water heater on for half an hour before my shower yesterday but the water was only lukewarm. Another huge inconvenience is I'm on pay as you go electricity (with edf I think) so I don't think I have economy 7. Any advice on how to use my water heater would be much appreciated.

snowscreamer

We had to live with an immersion-only hot water tank for a few weeks whilst we saved to get a hot water tank that could be connected to our boiler. We tried several configurations and worked out that the cheapest was heating up the tank once per day (definitely don't leave it on all the time - that will cost a fortune - those things are powerful and drink electricity!)

You can calculate how long it will take to heat up from cold:

4.184 kJ to heat one litre of water by one degree Celsius
Heating by approx 50 degrees (from 10deg to 60deg) - therefore 4.184 * 50 = 209.2 kJ to heat one litre of water by 50deg
Heating 120 litres by 50 degrees - 209.2 * 120 = 25104 kJ

1 kJ = 1 kWs
3600 kWs = 1 kWh

25104 kJ = 6.97kWh to heat up your entire hot water tank from 10deg to 50deg

What is the power rating of your immersion heater? Assuming 3kW (standard) then it will take 6.97 / 3 = 2.3 hours to heat up fully.

Our tank is 50% bigger than yours (180 litres) and we had it on for a couple of hours a day in the end and that seemed to work OK. The water wouldn't have been entirely cold the following day as it was quite a new tank so well insulated and we would only use a small amount of water each day so tank was still warm.

I suggest you try 2 hours a day. That will use 6kW of energy per day. Certainly if the tank is completely cold you'll need 2.3 hours - half an hour is nowhere near enough.

Smiley_Dan

I've just been told by the plumber it is better and more economical to leave it on 24/7 as it takes more energy to reheat from cold water.

There is no debate. Assuming you are simply consuming electricity from the grid, he/she is incorrect.

lstar337

Make sure you are not on E7, post a picture of your meter if you are not sure.
How is the rest of the flat heated? (You will probably need advice on this also)
If you put a jacket on your tank it will lose less heat and will require less electricity.

macman

As above, the longer it's on, the more it costs. However a properly lagged tank will lose very little heat, and what does escape heats the house in winter. The system should have a timer so you can run it from say 5-7am on cheap rate.
There is no such thing as 'PAYG' electrcity, if you have a PPM, you can still have E7, so look at your meter-is it dual rate?

Andy_WSM

banmido12345 wrote: »

I've just been told by the plumber it is better and more economical to leave it on 24/7 as it takes more energy to reheat from cold water.

I would hazard a guess that science was not his strongest subject at school then.

Get a timer for it. Set it for an hour in the morning and that will likely be enough for a shower and to wash the dishes.

Also worth popping to ScrewFix / Wickes / B&Q and buying an extra jacket for it to go over the top of the foam insulation. My tank has an extra jacket and it is surprising how warm the green foam feels beneath the jacket - which would obviosuly be wasted heat in the warmer months of the year (otherwise it just goes towards heating the space you live in during the Winter).

banmido12345

I've just tried heating the water on for around 2 hours and the water seems to be at the correct temperature but only if I just turn on the hot water tap without mixing with cold (old shower head) But the problem is the hot water pressure isn't exactly great, but if I mix with cold water then the temperature is too low. I also can't seem to find a thermostat nor a timer on my heater as well. Forums won't let me post any links as a new member

lstar337

banmido12345 wrote: »

Forums won't let me post any links as a new member

Post them with spaces and someone here will fix them.

banmido12345

http:// s29.postimg. org/6y9i5w2c3/image. jpg
http:// s29.postimg.norg/migvwfcgj/image. jpg
Here are the images of the meter and the heater

Andy_WSM

banmido12345 wrote: »

http:// s29.postimg. org/6y9i5w2c3/image. jpg
http:// s29.postimg.norg/migvwfcgj/image. jpg
Here are the images of the meter and the heater

Think I can post them:

Andy_WSM

It's not an Economy 7 meter.

The theromostat is inside the cover on the round box with the cable going in to it at the bottom of the tank.

And, yes, definitely buy it an extra jacket