Two immersion heaters with switched FCUs got very hot

[pedr0]

(Edit: Please also see the last post on the previous page for photos as installed.)


I think it must be this (or equivalent model from 5 or so years ago) https://www.gledhill.net/products/alternative-energy/stainlesslite-horizontal/:

 

[JohnW2]

Oh! My mistake. Hot water is not tapped from the end - it's taken from the actual top (i.e. the long edge of the cylinder) when the tank is lying down. Harry's request for photos forced me to crawl in there and take a better look.

Thanks, That makes much more sense.

The two immersions look as if they are at roughly the same 'height', about half way up the side of the cylinder. If that is the case, then both will behave similarly, and both will only primarily heat the water above them - so the water in the bottom half of the cylinder will always remain relatively cold - i.e. you'll never have more than about half of a cylinder's worth of properly heated water.

With a 'standard' vertical cylinder, the main immersion is usually close to the bottom, so that nearly all the water in the cylinder (above the immersion) gets heated.

Kind Regards, John

 

[Harry Bloomfield]

The only pipes that are attached all enter/exit at the highest point.

The inlet pipe is maybe pipe to the bottom internally.

You are correct about the level of the two heaters, both the same, which suggests that you could turn either or both on, the only difference is the speed of heating up the water..

 

[pedr0]

the only difference is the speed of heating up the water..

Probably in theory, but in practice, I think that when only 1 is used, it only heats a small volume of water because the local temp in the tank causes that heater's thermostat to turn the heater off. I guess over a *long* period of time, the heat would dissapate through the rest of the tank and the thermostat would turn the heater back on. But in practice it feels like one heater heats up only a smallish volume of water.

 

[JohnW2]

The inlet pipe is maybe pipe to the bottom internally.

One would imagine/hope so.

You are correct about the level of the two heaters, both the same, which suggests that you could turn either or both on, the only difference is the speed of heating up the water..

Indeed.

Kind Regards, John

 

[JohnW2]

Probably in theory, but in practice, I think that when only 1 is used, it only heats a small volume of water because the local temp in the tank causes that heater's thermostat to turn the heater off.

Per the theory (and, usually, in practice) it shouldn't be like that. The water heated by one element should rise to the top of this cylinder and then spread out across the whole width of the 'top', with the layer of 'floated up' hot water gradually getting deeper as more heating occurs, until ultimately the whole top half of the cylinder will contain heated water.

However, that doesn't alter the fact that, with the immersions where they are, there will never be appreciable heating of water below the middle of the cylinder.

I suppose it's possible that there is something very complicated going on within that cylinder, but I can't really think what it could be that would appreciably alter anything I've said above. To get the water in the bottom half of the cylinder appreciably heated would seemingly require water to be actively pumped from top to bottom, against the temperature gradient.

Kind Regards, John

 

[DetlefSchmitz]

It isn't intended that the whole of the tank is heated electrically. The elements are for emergency use. The boiler coil is at the bottom of the tank for normal usage.

 

[JohnW2]

It isn't intended that the whole of the tank is heated electrically. The elements are for emergency use.

That makes sense - but, in that case, isn't two elements (to heat half of the tank) a bit OTT?

The boiler coil is at the bottom of the tank for normal usage.

One would certainly hope so (and that's what the cutaway drawing illustrates).

Kind Regards, John

 

[RandomGrinch]

That makes sense - but, in that case, isn't two elements (to heat half of the tank) a bit OTT?

Agreed, and I might be wrong; but in a 'regular' set-up, might one of the elements be connected to off-peak, and the other a daytime 'boost', and so the two elements would never usually be on together?

 

[JohnW2]

Agreed, and I might be wrong; but in a 'regular' set-up, might one of the elements be connected to off-peak, and the other a daytime 'boost', and so the two elements would never usually be on together?

Indeed so, but if the elements were for 'regular use' (i.e. as the primary means of water heating, one would not put both in the (vertical) middle of the cylinder, for obvious reasons. One would expect the 'daytime boost' one to be fairly near the top (so as only to heat a small amount of water with expensive electricity) and the main one near the bottom (to heat the entire cylinder of water with cheaper electricity.

I've mentioned my setup earlier, and it's just like that, with each of the immersions on their own 16A circuit. As I implied, I do very occasionally have both on together, on the rare occasions (usually only Christmas!) when I have 'a houseful of people', in order to rapidly heat nearly a whole cylinder of water (particularly if one or more of that 'houseful' decide to 'have a bath' ).

Kind Regards, John

 

[Harry Bloomfield]

Probably in theory, but in practice, I think that when only 1 is used, it only heats a small volume of water because the local temp in the tank causes that heater's thermostat to turn the heater off. I guess over a *long* period of time, the heat would dissapate through the rest of the tank and the thermostat would turn the heater back on. But in practice it feels like one heater heats up only a smallish volume of water.

I cannot really see how that would work, unless...

Your documents only show one immersion heater, yet you clearly have two and the look like manufacturer installs. I wonder if the cylinder might be somehow compartmentalised so each immersion heater only heats it own side of the cylinder? The quantity of heated water would be very restricted anyway, because the elements can only heat the upper section of the cylinders, due to their positioning. The only way to get a near full tank of hot water, is via the heat exchanger pipes, from your gas or oil main heating system.

 

[pedr0]

TBH it's an observation based off a only a couple of weeks worth of using the immersion heaters while the gas is broken. So I haven't got enough evidence to be confident about it. Perhaps I just haven't left it for long enough when I'm using just one heater.

 

[JohnW2]

I cannot really see how that would work, unless... Your documents only show one immersion heater, yet you clearly have two and the look like manufacturer installs. I wonder if the cylinder might be somehow compartmentalised so each immersion heater only heats it own side of the cylinder?

Quite so - as I wrote ...

I suppose it's possible that there is something very complicated going on within that cylinder, but .....

... but I find it very hard to believe that will be the case - not the least because, if one wanted such functionality, it could be achieved more simply by having one of the immersions higher up (maybe in one of the 'sides' of the cylinder, with it installed horizontally).

The quantity of heated water would be very restricted anyway, because the elements can only heat the upper section of the cylinders, due to their positioning. The only way to get a near full tank of hot water, is via the heat exchanger pipes, from your gas or oil main heating system.

Indeed so.

Kind Regards, John

 

[JohnW2]

TBH it's an observation based off a only a couple of weeks worth of using the immersion heaters while the gas is broken. So I haven't got enough evidence to be confident about it. Perhaps I just haven't left it for long enough when I'm using just one heater.

I think that has to be the case. Assuming nothing very strange is going on inside the cylinder (see my recent post), if you left one (either one) of those immersions on for long enough, all the water in the upper half of the cylinder would be heated up to the desired (thermostatically-controlled) temperature.

I don't know what size of cylinder you have but if it is, say, 180 litres, then in theory (ignoring losses) a 3kW immersion should heat the upper half of the contents (90 litres) from, say, 18°C to 62°C in about 92 minutes (i.e. about 1½ hours). However, one would be able to draw a small amount of hot water (which had 'risen to to the top') much earlier than that.

Edit: ... or, I could have added, if one had both immersions on (i.e. 6 kW total), it obviously should only take about half that time (46 mins, about ¾ hour) to heat all of the water in the top half of a 180 litre cylinder (i.e 90 L) from 18°C to 62°C .

Kind Regards, John

 

[pedr0]

two immersions would either have to each be on different (16A) dedicated circuits from the CU (which is how mine are) or both being supplied by a single 32A (radial) circuit serving the immersions and little/nothing else.

My investigations have revealed that both immersions are on the same 16A radial circuit. The only other thing on that radial circuit appears to be a secondary circulation pump.

Does that mean that I must not run both immersions at the same time?

And if not, can the circuit be upgraded to 32A simply? The cable is 2.5mm t&e

Edit : and also, if they each draw 12a,why didn't the cu trip when the were both on at the same time?