Immersion Heater

[Wrangler3]

We take advantage of overnight cheap electricity to use washer, dryer etc. Can any one tell me how many hours it takes for the immersion heater to come to full heat as I am getting paranoid about using un neccessary power! Our tank is about 3' 6" high and at the moment we have it on from 0130 to 0630 and that gives us water for the whole day. Could I reduce this by one or even two hours without reducing the availability of hot water??

[ummmm]

We've got a really old boiler. It takes about 30 mins to heat enough water to have a bath. Which is equal to all four of us having a shower.

Are you turning it on separately or with your central heating?

[Wrangler3]

What is this thing you call central heating?!! No it is just the boiler we heat, there's two of us in the flat and we have 2 showers/baths each a day and need it for washing up etc. Heating not in the equation!

[ummmm]

It's hard to say. You don't need it on for that long to have a full tank of hot water.

Is it on a timer?

[The Builder]

I guess having it come on at 3:30 wouldn't hurt. You'd still have a tank of hot water. You'd probably find that the saving would hardly be noticeable though because the element has a thermostat that switches it off when it's up to temperature.
Because of the thermostat, the element is only on intermittently anyway ..........

[gingejbee]

From cold, a 3 kw. immersion should heat a copper cylinder in less than an hour. If in doubt, just give it a try - nothing drastic will happen!

[jake-the-peg]

Bit of a guess here but that sounds like about 166 litres. 166 Kg

now we get to more guesses

Say you want to go from 5 degrees to 35 degrees that's a 30 degree change

The energy to do that is 4200x166x30 = 20 MJ (4200 is the specific heat capacity of water)

If your immersion heater is 3Kw that's 3,000 Joules per second

dividing one by the other gives you 6700 seconds or about 11 hours.

That's heating the entire tank from cold - if your tank is only heating up 15 degrees it's half that. If your immesion heater is 6Kw it's half that.

You see about the guess work.

Also if you have central heating switched on it depending in the system it may be taking its heat from the hot water tank which cools the water.

Not enough information I'm afraid but perhaps you can see how you can do the rough estimates

[jake-the-peg]

Whoops - sorry 1.9 hours

Not enough coffee yet this morning

[Wrangler3]

Thanks for all your input guys. If I understand you (all) correctly, the actual heating up of tank to full heat only takes an hour approx? After which the thermostat keeps topping it up?? If this is the case, I can surely then just put it on for an hour/two hours say in the cheap period, then let the thermostat keep it at heat??? Sorry to be obtuse!

[jake-the-peg]

Probably as I said earlier there are a lot of variables.

Personally my central heating draws heat from the heat exchanger in the cylinder so if the central heating's on it does have a significant effect.

[oranger]

My old boiler can get a full head of steam in about 2 secs, bless her cotton socks.