Two immersion heaters with switched FCUs got very hot

[Bear2Biker]

Hi all, as the elements are at the same height (so seemingly act similarly) is it possible that they are wired in series despite the the labeling, this would still draw fairly close to 12+ amps for a considerable time, giving the browning effect on the FCU. I have a Gledhill upright and the element is positioned at mid height (which I thought wasnt best) it seems it is shaped with an angled downward loop to get to about 4-5 inches from the bottom of the tank.

 

[Harry Bloomfield]

, as the elements are at the same height (so seemingly act similarly) is it possible that they are wired in series despite the the labeling, this would still draw fairly close to 12+ amps for a considerable time, giving the browning effect on the FCU. I have a Gledhill upright and the element is positioned at mid height (which I thought wasnt best) it seems it is shaped with an angled downward loop to get to about 4-5 inches from the bottom of the tank.

In series their resistance would be double, so the current and wattage would be halved. Instead of (assuming 240v) 3kw/12.5amps each, the series draw would be 1.5Kw/6.25 amps.

 

[Bear2Biker]

Harry, thanks for correcting me, brain not in gear( it was a bit early for me), I just remember the OP saying the supply cables seemed to go from 1 to the other, but no that draw probably wouldn't discolour the FCU even if it was on 24/7, I've looked in mine (which isnt currently opperational) & there are no baffles but there does seem to be an additional 1" entry at the base (closed off) - possibly for a circulation pump if required. The OP's pic does show it as quite long so I suppose 2 heaters at that height would actually heat quite a bit of water in the top length in the short term and it is after all supposed to be auxillary to the boiler coil.

 

[pedr0]

They were definitely in parallel, 2.5mm from CU, into one FCU, into second FCU and then into circulation pump.

I contacted Gledhill and the guy there said that the two immersions are indeed at the same height and therefore heat the same water space (implying that there isn't any clever arrangement inside this horizontal tank to do the sink/bath thing). He said that there are two in there to allow for customers who have Economy 7 or solar or what not. Not sure I buy that because these are emergency afterall - why would you be planning ahead to use your emergency on Economy 7? But maybe my opinion of him is too coloured by his assertion that it wouldn't be possible for me to run them both at the same time anyway because the "software wouldn't allow it"!

With your help I've realised that my situation is quite simple:
* A single 2.5mm radial circuit fed by a 16A MCB connected to both immersion heaters in parallel
* The circuit is insufficient to run both immersions at the same time
* Both immersions heat the same volume of water (i.e. the top half)
* The advantage of running both immersions at the same time would be to heat the top half of the tank quicker

I'm frustrated that the installer didn't lay two radial circuits or a single one that was capable of supplying sufficient power for both heaters. However, it is only for emergencies so I'll get over it.

Conclusion: I will be replacing the FCUs for a single dual immersion heater DP 20A switch to prevent them both being run at the same time.

 

[Harry Bloomfield]

He said that there are two in there to allow for customers who have Economy 7 or solar or what not. Not sure I buy that because these are emergency afterall - why would you be planning ahead to use your emergency on Economy 7?

That doesn't sound to unreasonable to me. Not everyone has gas heating, so E7 would be their option. Certainly worth while for solar which is effectively free, so the more use can be made of it the better.

With your help I've realised that my situation is quite simple:
* A single 2.5mm radial circuit fed by a 16A MCB connected to both immersion heaters in parallel
* The circuit is insufficient to run both immersions at the same time
* Both immersions heat the same volume of water (i.e. the top half)
* The advantage of running both immersions at the same time would be to heat the top half of the tank quicker

That about sums it up.

I'm frustrated that the installer didn't lay two radial circuits or a single one that was capable of supplying sufficient power for both heaters. However, it is only for emergencies so I'll get over it.

Who ever did that, ought to be shot.

 

[pedr0]

Not everyone has gas heating, so E7 would be their option.

Yes, but then they would have made a poor choice designing their system with this indirect horizontal cylinder whose elements can only heat up the upper half of the tank!

 

[Harry Bloomfield]

Yes, but then they would have made a poor choice designing their system with this indirect horizontal cylinder whose elements can only heat up the upper half of the tank!

Yes, seems a strange design. I can understand them providing one element as a back up for a failure, but two and at the same level ???

 

[JohnW2]

Yes, seems a strange design. I can understand them providing one element as a back up for a failure, but two and at the same level ???

Indeed - it does seem very odd. As I said before, it presumably would have be perfectly possible to have one immersion fairly near the bottom (in one of the 'sides' of cylinder when it is lying horizonal).

Kind Regards, John

 

[Harry Bloomfield]

Indeed - it does seem very odd. As I said before, it presumably would have be perfectly possible to have one immersion fairly near the bottom (in one of the 'sides' of cylinder when it is lying horizonal).

The manufacturers sheet only suggests their is one immersion, perhaps the owner specified two?

 

[pedr0]

No - I've got the 250l version. In the tech specifications table in that image (very small print) it shows that 2 are standard for that size tank. So just a bit of a weird design. It would heat up quicker using 2, but the gledhill support guy (whom I don't hold in high regard) really tried to convince me that the two should never be run at the same time. "Well, it's like a kettle - you wouldn't want two of them boiling in there. It'll overheat" (seemingly oblivious to thermostats)

 

[JohnW2]

No - I've got the 250l version. In the tech specifications table in that image (very small print) it shows that 2 are standard for that size tank. So just a bit of a weird design.

Very weird. As Harry said, it's fair enough to have immersion(s) as backups in case one's other means of heating fail OR to have two, for situations in which the only heating is (dual-tariff) electric - but certainly in the latter case, and ideally also in the former, one immersion should be near the bottom (when installed horizontally) - it seems daft to have a situation which allows one to electrically heat only about 125 litres of water in a cylinder holding 250 litres!

It would heat up quicker using 2,

It would, and having two in the tank the size of yours might be sensible. It's larger than most people have (I think that mine, 140 litres, is pretty typical) so your tank (almost twice as big) would take a while to heat even half of its contents with just one 3 kW immersion (well, given the dilly locstion of yours, it would take roughly the same amount of time to heat half of your as it does heat all of mine!)

but the gledhill support guy (whom I don't hold in high regard) really tried to convince me that the two should never be run at the same time. "Well, it's like a kettle - you wouldn't want two of them boiling in there. It'll overheat" (seemingly oblivious to thermostats)

Quite. Provided only that the electrical supply to them was satisfactory, you could have 10 (each with their own thermostat) if you wanted - that would heat the water very rapidly, and the thermostats would ensure that nothing would come to any harm (or the water 'boil'!)

Kind Regards, John

 

[Harry Bloomfield]

No - I've got the 250l version. In the tech specifications table in that image (very small print) it shows that 2 are standard for that size tank. So just a bit of a weird design. It would heat up quicker using 2, but the gledhill support guy (whom I don't hold in high regard) really tried to convince me that the two should never be run at the same time. "Well, it's like a kettle - you wouldn't want two of them boiling in there. It'll overheat" (seemingly oblivious to thermostats)

Oh, OK. I wonder if Gledhill might have their own installers and their installers wire them the way your is wired? It's totally daft to have two, and only being allowed to use one at a time, completely pointless.

 

[JohnD]

I wonder if it's a standard vertical one that they have sold for horizontal use.

 

[JohnW2]

I wonder if it's a standard vertical one that they have sold for horizontal use.

I doubt it, because (a) if it were used vertically, the hot outlet would be in the wrong place and (b) it's hard to be sure from the pics we've seen, but it doesn't look as if either of the immersions would be as close to the bottom as one would like if it were vertical.

Kind Regards, John

 

[JohnW2]

It's totally daft to have two, and only being allowed to use one at a time, completely pointless.

As I've said, it would probably not be totally daft, even if only used for very occasional 'standby use', since that tank is almost twice the size of what many people have, with a single 'standby' 3 kW immersion.

... and only being allowed to use one at a time, completely pointless.

... now that IS totally daft. I can but presume that whoever wired them did not know enough about 'electrics' to understand the consequences of what he/she had done!

Kind Regards, John