Advice needed - replacing immersion heater cable.

[Beamish]

Hi all,

As it's winter now, I turned up the heat on our immersion heater yesturday, but noticed that the wires are in very bad condition... all very brittle, and the green/yellow earth has got dark brown patches on it, also it looks as though someone has spread marmite on the cables I have switch it off, and it won't be used until I replace the cable.

Firstly, the current cable looks similar to the type of cable used in a domestic 13a extension lead... surely the cable should be of a much better quality ?.... but what should I replace it with ? ... I think 10mm2 would be overkill, and I'm sure it would be too thick to put in the heater connections, Is there a specific cable I should get for this job.

More Info..... In case you need it...

The house was built around the mid 50's, but has been rewired since (probably late 70's)... The Immersion Heater is switched on 24/7 (In case that has any impact on the cable I should use)... The reason why it's on all the time is because we have a coal fire as the only source of heat, so during the summer we don't light fires, just use the immersion for hot water, during the winter i usually turn it up a few degrees. The immersion tank is on the first floor, the damaged wire runs through the floor boards down to the switch (ground floor).... My second question... Should I :

a) Replace the entire cable from immersion to switch
b) Just replace the upstairs portion of the cable using a suitable blank connector
c) Take this opportunity to move the switch upstairs near the immersion tank

Sorry for going on and on.... but I guess the more info you have the better the reply I'll get.

Thanks.

 

[brown-nought]

Firstly, you are right the cable should not look like marmite!. Do not use it.

You described the cable was like an extension lead flex Does the flex go all the way back to the switch down stairs or is it connected to a cable that goes down?

Normally we call round cables to appliances FLEX and flat cables used to wire to lights and sockets etc CABLES.

Special immersion flexes are available but they are not designed to be run though walls/floors.

 

[Beamish]

Thanks for the reply,

From your definition, the wire I called "Cable" is definately "Flex"...

That means it's round... just like an extension lead flex.

I'm not 100%, but I'm fairly sure the "flex" runs straight down to the switch... I can have a proper look again tonight though if it would help ?

From your reply, it sounds as though you may be suggesting that I'd need to remove the switch on the ground floor, connect the supply to the switch up to a new cable which I'd then run up through the floor, then connect the upstairs end of this to to immersion flex, then connect that to the immersion heater ?

Is that correct ? if so what should I use for the connections, and what rating cable/flex should I use ?

Thanks again.

 

[brown-nought]

The most simple arrangement for the immersion circuit would be for a cable to run from a separate way(MCB/Fuse) in the Consumer unit through a 20A DP switch next to the immersion. A heat resistant flex is then run from the switch (flex exit on bottom or front) to the immersion. (An extra DP switch might be added in a more convenient location if needed.)

It doesn't sound like this is what you have. Flex should not have been used other than to connect from a local switch (or cable connection outlet) to the immerssion.

The making on the flex makes me belive it is either very old and or has been over loaded.

There are a range of issues with just replacing the flex and it is probably worth getting a professional to look it over.

1) The heating element may be faulty causing overloads
2) The flex is run incorrectly, damaged and may be too small.
3) The correct size of the supply cable needs to be determined. (Its not just a matter of going to a DIY store and selecting a cable with right max load other rating/derating factors need to be considered.
4) Wiring from 70s should be checked over - its not only that its old but its had possibly 25+ years of DIY bodging.

I've also assumed here that this is a normal domestic style heater of 3.x Kw.

BN

 

[Dustyy]

You may also wish to assess whether you need to have your immersion on constantly. I often tend to use the immersion for heating the hot water cyclinder, especially in the summer and find that having the immersion on for only 20-30 minutes gives plenty of hot water, which remains at an adequate temperature for a at least 6 hours.
Adjusting the amount of time the immersion is left on would obviously reduce electricity consumption and lower your electric bill.

 

[Beamish]

Thanks for the answers / comments,

brown-nought - The current immersion switch already has it's own MCB on the consumer unit (Currently switched off after I noticed the state of the flex), I haven't taken the front off the immersion switch yet, so I'm not sure of the cable thickness and whether it's suitable for the MCB rating. Either way, I think I'll get a qualified electrician round to check for the problem as you mention there could be many factors.

Dustyy - beacuse using the immersion is the only way of getting hot water when the coal fire isn't lit, we've spent a while determining the best use of it..... we spent two months (as you suggested) turning the immersion on/off when needed, an another two months just leaving it on, our electricity bill, over these periods was much cheaper leaving it on constantly, as it is only actually topping up the heat, rather than heating the tank from near cold when it's turned on.

Thanks again for all the reply's .... now to find a reliable sparky !!

 

[kai]

Please note that Immersion heater needs adequte Earth Bonding Arrangements, especially if it's located in bathroom airing cupboard, as a few are.

A recommended component in any 3KW immersion circuit is a MK "K330_WHI" , 13 amp fuse-connection unit, between the circuit cable and the immersion flex, as it allows lighter flex to be used than otherwise would be the case. i.e. 1.5mm instead of 2.5mm between FCU and Element(s).

Note that MK is one of the makes to use, as they are very good quality build.

 

[Damocles]

Come on guys, sounds like an extra long flex was used so that the switch could be downstairs. The immersion heat has rotted the flex where it connects to the immersion. I would run 2.5mm cable from switch back to inside the immersion cupboard. There fit a flex outlet plate to the wall, and a heat resistant immersion flex to the immersion.

Could be loads of other things wrong, but then I could be president of the USA and writing posts here because the war was no fun any more.

what is rating of breaker/fuse on immersion circuit? I would expect 15/16A.

 

[Beamish]

Again, Many thanks for the reply's...

I had a good look at the layout of wiring for this last night.....

Damocles - Spot on.... The breaker is 16A...
from this the existing "cable" runs to a cupboard beside the coal fire, where it is connected to the immersion switch, from this switch a standard "flex" has been used which runs directly through the ceiling and connects to the immersion heater. But also, another "cable" is connected from the immersion switch and goes to a water pump switch, from this another standard "flex" runs to the water pump beside the coal fire.

So, as the rating is 16A I will do as Damocles suggested, and run a 2.5mm2 cable from the switch into the immersion cupboard (Not in bathroom) then fit a flex outlet and use heat resistant flex to the immersion.

But I will also replace the standard flex to the water pump with a heat resistance one, as the pump also gets warm (not as hot as the immersion, but still warm).

A neighbour, 2 doors away had a fire break out in their immersion cupboard 2 years ago on christmas eve... Both houses are the same... (ex council) and as such would have been re-wired at the same time and in the same way, by the same contractors.

Thinking about it now, I should have checked this flex out 2 years ago... I just can't stop thinking about what might have happened had I not have gone to turn the immersion up this winter !! ... Looking at the flex it certainly wouldn't have lasted much longer !!

I think I'll now write a standard letter warning of the potential problem and drop copies in to all houses in my street.

Thanks to all who commented.... a great forum !!

 

[ban-all-sheds]

You should probably go via an FCU to the shower pump...

 

[Beamish]

sorry, I obviously didn't make it clear, but it's not a shower pump, it's the central heating pump.... i.e. when the fire is hot enough we switch it on to pump the water round the rads.....

I know, I know, very primitive !!

I guess the same applies with the FCU though ?

 

[ban-all-sheds]

Yup.

 

[securespark]

Apologies if someone's said it but use Butyl for the connection to the Imm Htr element.