Immersion heater time taken to heat a tank of water

[JohnW2]

You can't seem to buy the dual ones any more - I can't understand why.

They're not all that easy to find but, as I wrote:

They still exist (in UK), but are not widely socked (e.g. click here ).

Modern economy seven setups have two immersions in the side of the tank, one top and one bottom.

As I've said, that's what I've had for decades. The bottom horizontal element probably heats more of the water than does a top-entry (including 'dual') one - and, and BAS says, if a single element dies, one only has to replace one element. The 'modern tanks' I've seen all have (like mine) one in the side near the bottom and the other as top entry.

Kind Regards, John

 

[ericmark]

I did quick calculation and got approx 22°C temperature rise per hour which ties in with what others have found. However the thermostat must limit the maximum temperature of the water we all did the experiment as school boiling water in top of test tube with a lump of ice weighted with solder still at bottom of the tube.

Now the old heater warmed the water at 27 inches down only. The new heater warms the water all the way up the heater, this will produce a gradient of heat rather than a block of hot water. Without some means of causing circulation you will not have a tank 3/4 full of water at 60°C but a tank with 3/4 full of water at a varying temperature with the top at 60°C the 3/4 point could well be at 20°C.

The heating of the water will produce some circulation but it depends on exactly where the heater is. I found this out brewing beer in a 25 litre fermentor use a heat belt and even though placed at the bottom the bottom still did not reach the 20°C I was aiming for in the centre near the bottom. Using a demo underfloor heating tile the whole fermentor showed same temperature. With the belt what I got was a cone in centre of cooler beer, since the yeast also helped circulation it was not cold but not as warm as the rest.

In the cistern the heater is normally at an angle so there will be a gradient of temperature reflecting that angle of water not heated at the end of two hours. But there will be some circulation and gradually the thermostat will cool down again and switch on again for a short time and over the day the whole top section will warm up.

Step on is set timer to 24 hours if that works then great. If that does not work then you need some way to mix the water.

in the basic system you can see how it shows not a line between hot and cold water but gradually getting warmer. A pump fitted between the cold fill and hot take off would circulate the water. Likely it would only need to work for a few seconds, in large houses they have a pump to circulate water so you have hot water in seconds at the tap rather than having to run nearly a sink full of cold water before the hot water arrives. I had this in a house I was staying at and in the summer the hot pipes were making the rooms too hot. So he turned off the pump. However he had two cisterns and this would allow 6 of us to shower in the morning, turning off the pump resulted in a cold shower for third person. The pump was not fitted to circulate within the cistern but clearly it did.

Since the airing cupboard is not heating up as much it would seem the insulation is better so leaving on 24/7 should not cost more than a few pence extra.

 

[kai]

I also heard that the Dual ones were less reliable than their side entry counterparts - hence they fell out of fashion in the Eighties.

 

[JohnW2]

I also heard that the Dual ones were less reliable than their side entry counterparts - hence they fell out of fashion in the Eighties.

That's possible. The common availability of cylinders with two immersion bosses probably had a lot to do with it.

Kind Regards, John

 

[Horatio1960]

Instead of the four hours setting I put it on at 7pm yesterday, by this morning at 10am there was very little difference for all that, the airing cupboard was slightly warm, the upper part of the cylinder (testing the plug on the side close to the top and the hot water outlet pipe) was as hot as it was yesterday after four hours, the lower plug nearer the bottom was tepid at best

 

[JohnW2]

Instead of the four hours setting I put it on at 7pm yesterday, by this morning at 10am there was very little difference for all that, the airing cupboard was slightly warm, the upper part of the cylinder (testing the plug on the side close to the top and the hot water outlet pipe) was as hot as it was yesterday after four hours, the lower plug nearer the bottom was tepid at best

I reckon you've got a duff (or incorrectly set) thermostat.

Kind Regards, John

 

[SimonH2]

I recall having read comments about "accelerator pipes" helping. Basically, surrounding the top fitted element in a pipe that fits nearly to the bottom of the tank. I think the idea is that it brings cold water up from the bottom of the tank, creating a thermosyphon similar to the Willis system mentioned earlier. Assuming a suitable piece of pipe could be found, it might be worth trying - it doesn't involve any external plumbing changes.
Does anyone have any experience doing this ?

My thoughts are along the lines of ...
If you are bringing cold water up from the bottom of the tank, it's going to mean that the warm water coming out the top of the pipe isn't as hot as it can be where the water stratifies as it probably is now. Therefore, by the time the top of the cylinder is hot enough for the stat to turn off, more of the cylinder is hot.
Ie, in simplistic terms, say the water came out the top of the pipe 20˚ hotter than it went in at the bottom, and the bottom of the tank was at 10˚. Initially, the top of the tank would only reach 30˚, and would remain like this until the warmer water has reached the bottom of the tank. So instead of having a tank where the bottom is still 10˚ but the top may be 40 to 50˚, you've a tank full of water at 30˚. By this time, the water will be coming out the top at 40˚, and you won't turn off a stat set at (say) 60˚ until the bottom of the tank reaches 40˚. I suspect that with a good thermosyphon, 20˚ is optimistic - if there were only 10˚ difference, then you'd need the whole of the tank to reach 50˚ before you triggered a stat set at 60˚.

With a bare element, while it will create water movement, it'll draw in water from around the same height as the bit of element that's heating it. So while the stat is getting close to switching off, the top of the element is actually heating already hot water at the top of the tank, rather than cooler water that's been drawn up the pipe from the bottom of the tank.

Sit's back and waits for the corrections

 

[SimonH2]

Oh yes, and a few pipe thermometers like these could make diagnosis a bit easier - though they aren't much use for sensing the temperature profile down the height of the tank. Will at least tell you what the top of the tank has got to.