Strange Immersion Heater Wiring

[stem]

This is the wiring diagram of the hot water system supplying our offices. The hot water cylinder has two immersion heaters both operated by the same timeswitch, but certain aspects of it seem a bit odd to me. Your comments / thoughts would be appreciated.

Untitled

• stem
• 27 Mar 2014
• 1

I would have expected the bottom element to be controlled via the timeswitch to come on during the 'off peak' rate and the top element wired to a permanent supply as a daytime top up as required, but they are connected together to operate from the same timeswitch. Is this normal?

Both elements are both fed from one 32A MCB wired in a 'ring' using two lots of 2.5mm T&E to the contactor connections. Seems a bit unusual to me, but I guess it makes it easier to get the wires into the terminals.

Then the immersion elements are connected in a second ring directly via DP isolators and not fused down. Presently we are just using the top immersion set to be on 24/7, so it's protected only by the 32A MCB

 

[phatboy]

Makes sense to me I think.

The top element can be on whenever you want, but if the off-peak supply kicks on for the bottom element, the power to the top element is automatically removed.

 

[kbdiy]

Surely not - both elements are wired in parallel so there is no individual control.

Question is - does the property have off-peak supply now or has it been converted to standard supply so someone rewired the elements as shown?

 

[JohnW2]

Makes sense to me I think.

Not to me really - my expectations are more like the OP's.

The top element can be on whenever you want, but if the off-peak supply kicks on for the bottom element, the power to the top element is automatically removed.

Eh - are we looking at the same diagram?! What I see is both the elements fed (via respective isolators) from the output side of the contactor - so it will only be possible to have either of them on during the 'timed on' period', and if both isolators are 'on', both elements will be powered during that timed period. Am I missing something?

Kind Regards, John

 

[EFLImpudence]

I don't think so.

Surely the top element should be wired direct and, if the timer is capable, there is no need for the contactor.

 

[stem]

Thanks everyone for your input, here are my replies / comments

Makes sense to me I think.

The top element can be on whenever you want, but if the off-peak supply kicks on for the bottom element, the power to the top element is automatically removed.

That's how I would expect it to be wired, but the reality is that both isolators are not only wired together, but in a ring.

Surely not - both elements are wired in parallel so there is no individual control.

Question is - does the property have off-peak supply now or has it been converted to standard supply so someone rewired the elements as shown?

Exactly, they are wired together and yes the property does have off peak supply. It's a fairly new building constructed in 2006. There is no gas supply to the premises and the electrical installation is 'as built'

Eh - are we looking at the same diagram?! What I see is both the elements fed (via respective isolators) from the output side of the contactor - so it will only be possible to have either of them on during the 'timed on' period', and if both isolators are 'on', both elements will be powered during that timed period. Am I missing something?

No, you're not missing anything, that's how it is.

Surely the top element should be wired direct and, if the timer is capable, there is no need for the contactor.

Good point, I'll check out the timer switch rating. If it's only used to operate one element, then it may well be up to the job on it own.

 

[JohnW2]

Eh - are we looking at the same diagram?! What I see is both the elements fed (via respective isolators) from the output side of the contactor - so it will only be possible to have either of them on during the 'timed on' period', and if both isolators are 'on', both elements will be powered during that timed period. Am I missing something?

No, you're not missing anything, that's how it is.

Glad you agree - but, for the record, "it were me", not phatboy, wot wrote that

Kind Regards, John

 

[stem]

Eh - are we looking at the same diagram?! What I see is both the elements fed (via respective isolators) from the output side of the contactor - so it will only be possible to have either of them on during the 'timed on' period', and if both isolators are 'on', both elements will be powered during that timed period. Am I missing something?

No, you're not missing anything, that's how it is.

Glad you agree - but, for the record, "it were me", not phatboy, wot wrote that

Kind Regards, John

Sorry, I was a bit over enthuastic with the cut & paste. Now amended. Credit where credit's due

 

[ekmdgrf]

My interpretation is based on the assumption (ha) that the timer has an override, so during the timed period it gets blasted by both elements and during the day, if a top up is required, one can manually turn the timer on and then isolate the bottom element. This seems a bit rubbish really. One probably has plenty of time during the night to heat the water on one element and during the day, one might well forget to turn the bottom isolator back on.

I'm not entirely sure it was sane to wire it using dual 2.5mm conductors. While it's technically a ring, I'm not sure it's the intended application. An immersion heater (two in this case) yanks a lot of current for an extended time. If one of your 2.5mm conductors comes loose, that's going to get messy. I have a feeling that BGB in general prohibits parallel conductors but makes an exception for 13A sockets, party for historical reasons and partly to abate a fear that dispensing with the 32A ring puts us on a slippery slope to adopting Schuko sockets That said, I'll defer to those wiser than myself to comment on whether it truly is a bad thing.

 

[stem]

The timer does have an override, so the method of operation you suggest is possible, I suppose the danger is that we forget to turn the bottom isolator back on, or don't turn the time switch off when we leave.

The other problem is that if I'm away from the office no one will bother, so I was hoping I could set up something that could be left to operate without intervention.

My plan was to leave the top element on 24/7. This element does just keep the water at the top of the cylinder hot and is sufficient for our needs. Then I was going to have the bottom element come on during the 'off peak' rate and heat up the whole vessel. Then the top element controlled by it's internal thermostat, will just top up than as and when required during the day.

 

[JohnW2]

My plan was to leave the top element on 24/7. This element does just keep the water at the top of the cylinder hot and is sufficient for our needs. Then I was going to have the bottom element come on during the 'off peak' rate and heat up the whole vessel. Then the top element controlled by it's internal thermostat, will just top up than as and when required during the day.

That's more-or-less what one would expect, particularly if you set the thermostat of the top element a bit lower than the other one (since it could be that you may well not need the top element to power up very often, particularly in the early part of the day - that's my experience). As has been said, that can be easily achieved by just having the top element connected directly (through isolator) to the supply, and by having the bottom element switched by an adequate timeswitch (without a contactor).

That's essentially the setup I have, although I don't leave the top element on 24/7 - I switch it on manually, if required (which is very rarely, thanks to a very well insulated cylinder). However, as I said, I could probably achieve much the same by leaving it on 24/7 with a lowered thermostat setting - since the thermostat would then very rarely turn the top element on.

Kind Regards, John

 

[SimonH2]

Another factor is that you (ideally) shouldn't be using a time switch - you should be using the switched facility from your meter so that you are sure to only run the heating when on the lower rate.

Does you meter have a time switch, or a radio teleswitch ? If it's a timeswitch then there's no guarantee it's right (there was a bit in, IIRC, Which recently about someone finding he'd been massively overcharged because the clock was wrong). If it's a teleswitch then there isn't a fixed time for it to operate.

It all smacks of having been done by someone without too much clue

 

[EFLImpudence]

Another factor is that you (ideally) shouldn't be using a time switch - you should be using the switched facility from your meter so that you are sure to only run the heating when on the lower rate.

Because of the set up the E7 must switch the whole installation so a timer is essential.

 

[JohnW2]

Another factor is that you (ideally) shouldn't be using a time switch - you should be using the switched facility from your meter so that you are sure to only run the heating when on the lower rate.

That would have merit, if it were possible, but it certainly isn't with my E7 installation. I have no 'switched facility from my meter' - there is simply a time-switch (with sealed connections, and set to roughly the right time ) which switches the meter (again, obviously with sealed connections), for the entire installation, between low and high rate metering - there is only one 'output' from the meter.

I suppose I could ask to be provided with an extra output from the time-switch, to operate a contactor (the thought of having that conversation with a call centre makes me tremble ) - but I don't know how they would react and/or what they would charge me for the pleasure.

Kind Regards, John

 

[AlanE]

My thought would be that the property has a dual rate, night and day, rate meter rather than an economy 7 for heating only.
If storage heaters are fitted they may will be switched by the supply company but with a night and day rate meter the whole property can use cheaper electricity at night.

However would have expected the top immersion to be wired to the input side of contactor so water can be topped up without disturbing the timer or the off peak immersion heater switch..