Immersion heater time taken to heat a tank of water

[Horatio1960]

Hi all

Last year our hot water tank sprang a leak, so had to replace the tank, which is a standard direct 900 x 450 which had a lower side mounted immersion heater, old and new tanks were/are foam lagged

We have no other means of heating except an immersion heater

It has a timer and used to be on two hours in the morning and an hour before evening, never had a real problem with running out of hot water then

Replaced the tank which was only available with a top mounted immersion element, so had to fit a new longer element

I set the new to the same temperature as the old one (55 degrees) but found it wasn't heating the water enough for two showers, water was just warm, not hot

So changed it to 60degrees, that appeared slightly better temperature wise but didn't heat as much water as the old set up

So changed the times on to four hours in the morning, this was better but still only heats the top section of the tank and lasts two showers or so

So my question being, and I realise there is no set answer but am looking for a ball park figure

How long should an immersion heater take to heat a tank of water up as four hours seems way over compared to the previous tank and should it heat the whole tank

I am trying to determine what if anything is the problem

Many thanks for any feedback

 

[333rocky333]

Did you see the element fitted, I once found a short element in a top mount tank, presumably because thats what he had on the van at the time

 

[EFLImpudence]

It could be a duff one I suppose.

 

[ban-all-sheds]

How long should an immersion heater take to heat a tank of water up as four hours seems way over compared to the previous tank and should it heat the whole tank

O-Level/GCSE physics.

The specific heat of water is 4.19J/gK.

For every kW of heater you have you'll be able to put 1,000 joules per second into the water.

So if you know how much water there is, and what the incoming temperature is, and what the target temperature is, it's a trivial calculation to work out how long it will take to get to that temperature.

 

[AndyPRK]

Yeah. You may have a short element

 

[JohnD]

The longest immersion heater you are likely to find is 27" long. Yours may be shorter. It cannot heat water below itself. The water stratifies with the hot rising to the top and the cold staying underneath. The thermostat is even shorter than the element so will switch off the heater when the top part reaches target temperature.

"Trivial calculation" my arse. But an immersion heater warms the water at a rate of about one litre per minute, until the thermostat turns it off.

I presume from what you say that you have no boiler so are totally reliant on the immersion heater warming the top half (or so) of the cylinder, is that right?

 

[Risteard]

Did you see the element fitted, I once found a short element in a top mount tank, presumably because thats what he had on the van at the time

It must have got wet on the roof rack.

 

[ericmark]

To get a full tank of water you need the Willis system an Irish invention they only pretend to be thick they are very clever. With the Willis system it heats from the top and thermo syphon and venturi effect slowly heats the whole tank. It means even 10 minutes after turn on you get piping hot water and it will heat the whole tank surprised some one from Derry did not tell you?

With a heater in the top of the tank it is heating all the way up the immersion heater not just at the bottom it will cause some thermo syphon circulation but the bottom of the immersion heater will be in cool water gradually as it turns off and on it will cause the water to circulate but it would need to be left on 24/7 to heat the whole tank as as it reaches the set temperature it will switch off the water will mix and then it will switch on again. So the simple calculations as to kW and size of tank will not work as the immersion with start cycling on and off before whole of water is heated.

As well as length of the heater also depends on length of thermostat clearly if it is not at the bottom of the pocket it will turn off early. Legionnaires' disease
* 70 to 80 °C (158 to 176 °F): Disinfection range
* At 66 °C (151 °F): Legionellae die within 2 minutes
* At 60 °C (140 °F): Legionellae die within 32 minutes
* At 55 °C (131 °F): Legionellae die within 5 to 6 hours
Means low temperatures are not a good idea, I would say 65°C is about right as a balance between scolding ourselves and protecting against Legionnaires' disease. So turn that temperature up to start with. Also set timer onto 24 hours for a couple of days to find out if you can possibly get enough hot water. I question if ever worth switching off as water never get cold even when turned off for 24 hours so switching on and off with a timer only really helps with economy 7 and the like.

 

[Horatio1960]

Thanks all for the feedback

To try and answer all in one go

I saw the element fitted as I fitted it

I am wondering if its a duff one, it heats up the top but not lower down

Whilst a calculation would tell me what is supposed to happen, it rarely does in practice, hence why I was looking for comparisons or others findings

It is definitely a 27" element I had to get a new long one as the old was a lower side fitting 14"

Thanks JohnD re one minute per litre, thats just what I wanted, so a 117l tank should reach the required temperature in approx 2 hours, which is what I experienced before changing the tank and immersion

This one is on for four hours and I would estimate it heats the top third only

Clearly something wrong but what

I suspect the element, would you agree ?

Or am I missing something?

The element is definitely working, just not as it should

The immersion element came with the thermostat fitted, I do not think I removed it before fitting, might it be worth checking that first to see if by chance they fitted a short element

If a short thermostat it would switch off too early wouldn't it?

In between writing these replies ericmark's reply came in, I will reply seperately to that

Thanks again all

 

[JohnD]

If you measure 27" down from the top, diagonally, you will see how far it reaches. Nothing below that will be warmed at all.

The cylinder will always be hotter at the top so the thermostat will cut out before the lower part is fully hot. I have a cylinder with gas boiler and use the immersion as a stand-by, it will only do a very meagre bath. I think if you wanted two showers you should leave the heater on during and between them, so it can make a bit more hw. You could try leaving the immersion on overnight so it is fully hot, then running a bath or into buckets to measure how much water you get before it runs cold. Your previous cylinder, with the heater at the bottom, was I think much better.

It will bring water to temp faster in summer than in winter, as the incoming water is warmer so its temperature does not have to be raised so far. 5C to 55C in winter is a 50C rise; 15C to 55C in summer is only a 40C rise so about 20% quicker. Additionally your summer shower will mix in more cold water, so the hot will last longer.

 

[ban-all-sheds]

"Trivial calculation" my arse.

Err - yes.

If we assume a 3kW heater then that will put 180,000 joules per minute into the water.

So if we know how many l, i.e. kg, of water there is in the tank, call it x, then in each minute the heater will raise the temperature of the water by

(180000 ÷ 4.19 ÷ x ÷ 1000)°C

But an immersion heater warms the water at a rate of about one litre per minute,

So would a torch bulb, it just wouldn't raise it by very much.

Raising a temperature at a rate of so many litres per minute is a nonsensical statement.

 

[JohnW2]

"Trivial calculation" my arse.

I guess that's in the eye of the beholder - it only involves multiplication and division. As BAS said ...

The specific heat of water is 4.19J/gK. ... For every kW of heater you have you'll be able to put 1,000 joules per second into the water. .... So if you know how much water there is, and what the incoming temperature is, and what the target temperature is, it's a trivial calculation to work out how long it will take to get to that temperature.

The specific heat of water means that 4.19 Joules is required to heat 1g of water through 1° C - alternatively 4186 Joules to heat 1kg of water through 1 C.
A typical 900 x 450 mm cylinder has a capacity of 117L. Let's guess that about 90L of that (hence 90 kg of water) gets heated by a 27" element. I will also assume that the water is to be heated from 15° C to 60° C - and increase of 45° C. I will lso assume a 3kW immersion. On that basis, taking it in bite-sized steps ....

1 Joule will heat 1kg of water by (1 / 4186) degrees.
hence: 3000 Joules will heat 1kg of water by (3000 / 4186) degrees
hence: 3000 Joules will heat 90 kg of water by [3000 / (4186 x 90) ] degrees
hence a 3kW immersion (supplying 3000 Joules per second) will head 90 kg of water by [3000 / (4186 x 90) ] degrees per second
To heat 90 kg of water by 45 degrees therefore requires 45 / [3000 / (4186 x 90) ] seconds ...
... or 45 / [3000 / (4186 x 90) ] / 60 minutes
... which can more simply be written as (45 x 4186 x 90) / (3000 x 60) ... which works out as 94.2 minutes.

But an immersion heater warms the water at a rate of about one litre per minute, until the thermostat turns it off.

A very good rule of thumb. As above to heat water 90 litres of water from 15° C to 60° C should take about 94 minutes.

Kind Regards, John

 

[JohnD]

For the benefit of anyone who didn't understand, or is being deliberately obtuse,

"But an immersion heater warms the water at a rate of about one litre per minute,"

means

"warms from typical incoming watermain temperature, to target hot water temperature"

And "one litre a minute" really IS a trivial calculation!

If BAS will be kind enough to perform his "trivial calculation" he can tell us what a normal 3kW immersion heater will do to one litre of water in one minute.

The amount of water in the tank is less useful, since the hot water accumulates at the top and stratifies by temperature. The cylinder is not the same temperature throughout, it will always be cold at the bottom.

Edit
Too slow!

 

[JohnW2]

The amount of water in the tank is less useful, since the hot water accumulates at the top and stratifies by temperature. The cylinder is not the same temperature throughout, it will always be cold at the bottom.

That's obviously fairly true, but don't forget that there is conduction (both in water and the copper cylinder) as well s convection, particularly if hot water is not used for some time. I wouldn't want to have my hand in the water even at the very bottom of a cylinder if the water "up above" in the cylinder were at, say 65° C (and had been for some time)!

Kind Regards, John

 

[ban-all-sheds]

If BAS will be kind enough to perform his "trivial calculation" he can tell us what a normal 3kW immersion heater will do to one litre of water in one minute.

For the benefit of anyone who didn't understand, or is being deliberately obtuse, it will raise the temperature by 180000/4.19/1000=43°.