Two immersion heaters with switched FCUs got very hot

[Harry Bloomfield]

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?

No, you shouldn't be using both at the same time. MCB's do not instantly trip on rating + .1amp. How quickly they trip, depends upon excess load and time it is on for. A 2.5mm cannot be fed from a 32amp MCB, but if you let us know about the length of the 2.5mm, and identify the route of the cable (buried in plaster, clipped to surface, buried in insulation), someone will be able to advise the maximum size of MCB which can be fitted.

 

[pedr0]

Thanks Harry, though that's unwelcome news.

The cable goes from the cellar to the 3rd floor/loft of a victorian terrace house. I'm not sure of the length. It runs through stud walls - not buried in plaster.

 

[JohnD]

Yes, no.

 

[Harry Bloomfield]

Thanks Harry, though that's unwelcome news.

The cable goes from the cellar to the 3rd floor/loft of a victorian terrace house. I'm not sure of the length. It runs through stud walls - not buried in plaster.

It seems a bit odd that with two immersion heaters, if they have wired it from scratch for the two. I wonder if it was originally a cylinder with just one immersion heater, then the new cylinder installed but with the two?

Not my areas, but I think the most you can put on 2.5mm T&E is 26amps, but that sounds like a long run and volts drop might come into it too.

 

[JohnW2]

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

How quickly they trip, depends upon excess load and time it is on for.

Indeed. However, if each immersion takes 12-13A (i.e. 24-26A total) then, looking at the curves, a B16 MCB ought to trip in something like 16 minutes if the two immersions were on simultaneously. If one was starting from a fairly 'cold' situation (which I would have thought one would be, if one felt the need to switch both on), I would have thought that both would be 'on' for a fair it more than 16 mins before the thermostat switched them off.

pedr0: can you remember for how long you had both immersions on simultaneously?

Kind Regards, John

 

[pedr0]

I wonder if it was originally a cylinder with just one immersion heater, then the new cylinder installed but with the two?

No. This was a whole house rewire and completely new plumbing throughout as well. I can at this stage though question the quality on the contractor

 

[pedr0]

pedr0: can you remember for how long you had both immersions on simultaneously?

I would have thought quite a long time. The intention was to heat up the whole tank - so I would have planned to leave it for an hour or so. And I think I did that on at least two or three occasions.

(Edit: Actually I might have even though of legionella, so might have deliberately left it on for a couple of hours. And yes, most of the time I'd have been heating it up from totally cold)

In my investigations this morning though I found that one of the FCUs is damaged by heat on the inside

 

[JohnW2]

I would have thought quite a long time. The intention was to heat up the whole tank - so I would have planned to leave it for an hour or so. And I think I did that on at least two or three occasions.

That's what I would have expected and as I said, I would also have expected that if you started with the water fairly cold, it would taken quite a time (well over 16 mins) before either of the immersion's thermostats switched them off.

It's therefore strange that the MCB did not trip but, unfortunately, MCBs cannot really be tested. If you end up getting an electrician involved, you might explain this story to them and ask whether it might be advisable to replace the MCB.

In my investigations this morning though I found that one of the FCUs is damaged by heat on the inside

That doesn't surprise me. As is often said here, immersions really shouldn't be supplied via FCUs, since they are not really intended toi carry 3kW loads for long periods.

Kind Regards, John

 

[Harry Bloomfield]

No. This was a whole house rewire and completely new plumbing throughout as well. I can at this stage though question the quality on the contractor

Most certainly you can. Can you recall them to do the job properly, at their expense? Would the cable route even be accessible now?

Choice is to add a second 2.5mm plus a second 16amp MCB, or replace 2.5mm with a 4mm and change present MCB for 32mp.

 

[Harry Bloomfield]

It's therefore strange that the MCB did not trip but, unfortunately, MCBs cannot really be tested. If you end up getting an electrician involved, you might explain this story to them and ask whether it might be advisable to replace the MCB.

Delayed tripping might possibly be due to the volts drop/ less current, in the cable and/or mains voltage being a touch low.

 

[JohnW2]

Delayed tripping might possibly be due to the volts drop/ less current, in the cable and/or mains voltage being a touch low.

I very much doubt that is the explanation.

The cable apparently goes "from the cellar to the 3rd floor/loft". If fairly straight, that might be as little as 10 metres, but let's assume 15m of 2.5mm² cable. At 26A that would result in a voltage drop of about 7V, about 3% of 230V (hence about 3% reduction in current). Even if the supply voltage were, say, 220V (213 after VD), one would still get a current of something like 24V A, which you still trip a B16 fairly quickly (well under 30 mins).

Kind Regards, John
Edit: Typo corrected

 

[Harry Bloomfield]

The cable apparently goes "from the cellar to the 3rd floor/loft". If fairly straight, that might be as little as 10 metres, but let's assume 15m of 2.5mm² cable. At 26A that would result in a voltage drop of about 7V, about 3% of 230V (hence about 3% reduction in current). Even if the supply voltage were, say, 220V (213 after VD), one would still get a current of something like 24V, which you still trip a B16 fairly quickly (well under 30 mins).

Think you meant 24amps? At 213v, the wattage might be as low as 2.5Kw x2 = 5kw = 21.73amps - add to which something was getting hot, so even more volts drop involved.

 

[JohnW2]

Think you meant 24amps?

I obviously did - now corrected. Thanks for noticing!

At 213v, the wattage might be as low as 2.5Kw x2 = 5kw = 21.73amps - add to which something was getting hot, so even more volts drop involved.

It obviously depends upon the voltage to which the "3kW" of the immersion relates.

If it were 3kW at 230 V (i.e. about 13.04A at 230V), that would correspond to about 12.08A at 213V (hence 24.16 for two).
If it were 3kW at 240V (i.e. 12.5 A at 240), that would become 11.09A at 213V (i.e. 22.18A for two, not far from your 21.73A figure).

For a start, if one was starting from 'fairly cold', I would expect ~22A to trip a B16 before either of the stats had turned an element off.

More to the point, the worst case I'm discussing relates to a supply voltage at CU of 220V (and 15m of cable), and I would think it extremely unlikley that the OP's supply voltage is as low as that (and similarly low on each of the occasions he's turned on both immersions).

More realistically, if one assumes a supply voltage no less that 230V (hence about 223V with VD), the total current would be about 23.2A, even if the 3kW related to 240V. In my house, with a typical supply voltage of 245V, the total current would (assuming ~7V VD) be about 24.8A, even if the 3kW related to 240V.

Kind Regards, John

 

[Harry Bloomfield]

It obviously depends upon the voltage to which the "3kW" of the immersion relates.

It was marked on the label, attached to the immersion heater - 240v 3kw.

 

[JohnW2]

It was marked on the label, attached to the immersion heater - 240v 3kw.

It was - I forgot that. In that case you know which of my sets of alternative figures to look at (in fact, the one's I was 'majoring' on).

I still very much doubt that, with any credible assumptions, a B16 should, with both immersions on (starting from cold), fail to trip before one of the immersion's stats operated.

I don't think I would want that MCB to remain in service in my house, but others may feel differently.

Kind Regards, John