6kW hot water cylinder

[nicck]

Currently a 2 x 3kW hot water cylinder has been installed.

Originally it was on a radial 2.5mm cable with a 16a MCB. This tripped after a few minutes.

Electrician has been in to fix it.
It is now on a 20a MCB which has not tripped yet. However the cable is very hot.

6kW draws about 26amp which is too much for a 2.5mm cable in conduit.
Also 20amp MCB could be a problem.

Solution may be to run a thicker cable, it's about a 4-5m run. And up the MCB.

As an alternative could the radial 2.5 be turned into a dedicated ring with a second 2.5 cable run from same MCB to the first 3kW element, the second element spurs of the first. The MCB could be uped to 32a?

 

[EFLImpudence]

It is not normally possible to have both elements on at the same time.
Has it been wired incorrectly allowing both?

Just increasing the rating of MCBs is not a wise procedure after tripping.

2.5mm² cable and 16A MCB is the usual circuit for immersion heaters.

 

[JohnW2]

It is not normally possible to have both elements on at the same time. ... Has it been wired incorrectly allowing both?

I wouldn't say necessarily 'wired incorrectly'. As with many other installations I've seen, the 'secondary' ('boost' or whatever), 'upper' immersion in my cylinder is manually switched. Although one would not normally do it (although I suppose some people might to speed up the water heating), there's nothing stopping me turning it one when the 'main' (lower) immersion is on. If the OP's situation is like that, maybe he has overlooked the fact that he has left a manual 'boost' switch on?

However, if his 2.5mm² cable is 'getting very hot', this may imply that both immersions are 'routinely' coming on simultaneously - which, as you suggest, is probably not what the person who wired them really intended to happen (i.e. the wiring may well be 'incorrect'!).

Having said that, if it's Method C, then 6kW is actually within the rating of 2.5mm² cable, so I would not have expected it to get exceptionally hot.

Just increasing the rating of MCBs is not a wise procedure after tripping.

As a generalisation, I'd agree. However, in this case there's nothing intrinsically wrong with increasing the rating of a MCB to one which is still adequate to protect that cable. Indeed, if one is available, upping to a 25A MCB would very probably be acceptable. However, a 20A MCB (and even {just} a 215A one) would still be undersized if the 6kW load is really 'intentional'.

IF he wants to retain the ability to run both immersions simultaneously, the OP's suggestion to turn the 2.5mm² radial into a 2.5mm² ring (aka running a second 2.5mm² cable in parallel with the existing one, as far as the first immersion), with upgrading the MCB to a 32A one, would obviously be electrically acceptable - but if he's going to run a new cable from CU to cylinder, it might just as well be a new ('replacement') 4mm² one, rather than an 'additional 2.5mm² one.

Kind Regards, John

 

[AndyPRK]

There are modern pressurised cylinders fitted with 2 heaters these days.

This is not some E7 system. Just 2 heaters due to the height / capacity.

I often wondered how they cope / wired!

I would go for 4mm cable myself.
Or 2 2.5mm on 2 Mcbs

 

[SUNRAY]

Currently a 2 x 3kW hot water cylinder has been installed.

Originally it was on a radial 2.5mm cable with a 16a MCB. This tripped after a few minutes.

Electrician has been in to fix it.
It is now on a 20a MCB which has not tripped yet. However the cable is very hot.

6kW draws about 26amp which is too much for a 2.5mm cable in conduit.
Also 20amp MCB could be a problem.

Solution may be to run a thicker cable, it's about a 4-5m run. And up the MCB.

As an alternative could the radial 2.5 be turned into a dedicated ring with a second 2.5 cable run from same MCB to the first 3kW element, the second element spurs of the first. The MCB could be uped to 32a?

If an additional 2.5mm² cable is being installed and there is a spare way in the CU then I'd seriously look at separate 16A radials for each, or change the cable to 4mm²

 

[SUNRAY]

Or I could read all of the replies...

 

[AndyPRK]

A rare case of respondents agreeing!

But only because they came to the conclusion themselves!

 

[nicck]

Thanks for the advice.

The system is

http://www.thermaq.co.uk/unicyl/

It is a Therma Unicyl twin coil, unvented. 2 x 3kw heaters.

I think it needs checked to makes sure it is installed correctly.

I could run a 4mm cable, or a second 2.5mm. I will look at what the consumer unit has free. There may be unused MCBs there.

 

[JohnW2]

It is a Therma Unicyl twin coil, unvented. 2 x 3kw heaters.

What capacity?

Kind Regards, John

 

[nicck]

Not sure of capacity, does not say on it.
Can make an estimate. Will be back on site later today.

 

[JohnW2]

Not sure of capacity, does not say on it. Can make an estimate. Will be back on site later today.

Fair enough. If you can tell us the rough dimensions, that would be good enough.

I also forgot to ask - are you using this just for providing hot water (from taps etc.), or is it being used for other purposes (i.e. heating).

Kind Regards, John

 

[nicck]

It is hot water, kitchen bathroom and shower.

 

[JohnW2]

It is hot water, kitchen bathroom and shower.

Thanks., As for the capacity, this should help you to work out (from the approx dimensions) which one you have ....

I'm a little uncertain of the significance of capacity being quoted "at 3 bar" since the volume/capacity of the cylinder should not change significantly with changing pressure!

Kind Regards, John

 

[nicck]

Height 94cm diameter 55 cm, external
Cylinder dimensions there are attachments that make it a bit taller.

 

[JohnW2]

Height 94cm diameter 55 cm, external ... Cylinder dimensions there are attachments that make it a bit taller.

Fair enough. I therefore imagine that it is probably the smallest of their Twin Coil ones, then (height in table = 110cm), hence 150 litres.

That's what I rather expected, and it lead me to wonder whether you would often (if ever) need the second 3kW immersion running at the same time as the first one. Even if you managed to use up all the hot water in the cylinder (unlikely), to heat 150 litres of water from 'cold' (say 18°C) to, say, 62°C would require 7.7 kWh of electricity, which means that one 3 kW immersion would achieve that in about 2.6 hours. Only if you wanted the water to heat more quickly than that would you need the second immersion on at the same time as the first one.

A common reason for having two immersions (one low in the cylinder and the other high) is that the upper one can be used (on its own) occasionally, as a 'boost', to just heat a relatively small amount of water (that in the top part of the tank) - something which is only really necessary if one's 'main' heating is done using cheap electricity at night (with, for example, an E7 tariff).

To put this is context, my main HW cylinder is 140L, hence very similar to yours. It does have two immersions, but the second (upper) one is virtually never used. With our usage, the total energy used by the immersion is around 4 kWh/day in mid-summer and around 6 kWh/day in mid-winter (when the cold water is colder and heat losses greater) - corresponding to an 'average power' (over 24 hours) of about 170 W (0.17 kW) in summer and 250W (0.25 kW). The total duration of 'immersion on-time' (controlled by thermostat), for my one immersion, varies from about 45 to about 90 minutes per day.

This graph of my HW heating which I often post here might give you some idea, but for any particular household, one's usage of hot water is obviously the main determining factor. (the very recent high peak in my HW usage is explained by my two daughters having been staying here - hence multiple ludicrously protracted showers and undoubtedly some baths as well!)....

Kind Regards, John