Twin immersion heaters

Chris Pearson:

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davezawadi (David Stone):

What is wrong with the B32 and two 2.5s Sparkingchip? Overload is impossible as I'm sure you realise? Ring connection is not necessary Chris, although it doesn't hurt and satidfies the overload point which worries some people. I expect only 1 heater at a time, but both would work safely. However it would be easier to have a Bath/Sink switch and just use the existing 2.5 cable. Job done. Local isolation/switching on/off is not necessary, although you may find it convenient, along with a timer. Ensure no non-return valves have been fitted anywhere back to the water main.

If the B32 trips off you have lost all heating. With two B16s and two separate circuits you have a more reliable system.

I disagree. You have a less reliable system, because the chance of at least one circuit failing is doubled.


What you have is redundancy so that in the event of failure, only half the heating is lost.

I don't quite understand your logic Chris. With just one B32 M.C.B. if one element becomes faulty and trips the M.C.B. you have lost both elements for heating. With two separate supplies, via two 16 Amp M.C.B.s and supply circuits there is more inbuilt reliability. If one fails the other can still be used.


Z.

Am I the only one using a BS4343 'ceeform' on his immersion heater? ?

Sparkingchip:

Don’t fit plugs and use sockets, they will melt.

I agree that standard 13A socket is a bad idea - in particular the fuse & its contacts seem to be a weak point in a lot of modern designs...


Am I the only one using a BS4343 'ceeform' on his immersion heater? ?

davezawadi (David Stone):

What is wrong with the B32 and two 2.5s Sparkingchip? Overload is impossible as I'm sure you realise?

A concentric heating element is one device for which protection against overload should seriously be considered unless RCD protection is provided, especially where the normal operating current is close to the tabulated current rating of the cable (considering all correction factors). It's one of the very few components, where the fault of negligible impedance doesn't always happen.


A possible fault on a concentric heating element is a short to the element casing, with a connection to either Line or Neutral broken.


If this occurs on the Neutral side, then the element will run at overload, dumping its current back down the protective conductor. If there is an RCD, great, it will operate. Otherwise, if you are lucky, the element will be stressed and "blow" again before anything overheats. But if you're unlucky, conductors can overheat and protective devices still not operate.


In the case we are discussing, assuming the heating element is no more than 13 A, it's probably only an issue for reference method 103, possible 101 at a push but at that current I'd expect the element to blow itself again  - so David's point is probably correct for this application. If the heating element is rated higher than 13 A, however, we start to run into the potential for a problem.


I thought it worth sharing the benefit of experience, just in case anyone has a situation with a marginal CSA on a heating element circuit (without RCD protection) in future ...


 

DC:

At the mo, it will be one immersion coonnected to the existing circuit, so no notification needed?.  I may have the the upper one appropriately connected later.


The heating guys are fully qualified and taking care of building regs and the admin on that side.

Pleased to hear that the plumbers will notify their work!

Zoomup:

davezawadi (David Stone):

What is wrong with the B32 and two 2.5s Sparkingchip? Overload is impossible as I'm sure you realise? Ring connection is not necessary Chris, although it doesn't hurt and satidfies the overload point which worries some people. I expect only 1 heater at a time, but both would work safely. However it would be easier to have a Bath/Sink switch and just use the existing 2.5 cable. Job done. Local isolation/switching on/off is not necessary, although you may find it convenient, along with a timer. Ensure no non-return valves have been fitted anywhere back to the water main.

If the B32 trips off you have lost all heating. With two B16s and two separate circuits you have a more reliable system.

I disagree. You have a less reliable system, because the chance of at least one circuit failing is doubled.


What you have is redundancy so that in the event of failure, only half the heating is lost.

DC:

Thanks - just saw another thread on 13A capacity of today's kit versus 1960s and 1970s - the consensus seemed to be to keep 3kW rated appliances off ring mains, despite the nominal rating.  I suppose an domestic electric fire or fan heater at that rating would have a room to cool, immersions are normally in cupboards and the wiring cannot dissipate heat so well.

Yes it was quite a common trick to put an immersion heater as a spur from a ring final circuit. Plug in 3KW fan heaters were not unknown either

DC:

Thanks - just saw another thread on 13A capacity of today's kit versus 1960s and 1970s - the consensus seemed to be to keep 3kW rated appliances off ring mains, despite the nominal rating.  I suppose an domestic electric fire or fan heater at that rating would have a room to cool, immersions are normally in cupboards and the wiring cannot dissipate heat so well.

The main issue is how long the equipment is going to run for without a chance to cool down.

davezawadi (David Stone):

What is wrong with the B32 and two 2.5s Sparkingchip? Overload is impossible as I'm sure you realise? Ring connection is not necessary Chris, although it doesn't hurt and satidfies the overload point which worries some people. I expect only 1 heater at a time, but both would work safely. However it would be easier to have a Bath/Sink switch and just use the existing 2.5 cable. Job done. Local isolation/switching on/off is not necessary, although you may find it convenient, along with a timer. Ensure no non-return valves have been fitted anywhere back to the water main.

If the B32 trips off you have lost all heating. With two B16s and two separate circuits you have a more reliable system.


Z.

DC:

Thanks - just saw another thread on 13A capacity of today's kit versus 1960s and 1970s - the consensus seemed to be to keep 3kW rated appliances off ring mains, despite the nominal rating.  I suppose an domestic electric fire or fan heater at that rating would have a room to cool, immersions are normally in cupboards and the wiring cannot dissipate heat so well.

We normally use a heat resisting flex for the final connection to an immersion heater. 2.5mm2 T&E cable is fine for a 3kW immersion heater supply up to the local switch. . A 20 Amp switch with a neon indicator is better for the final connection rather than a fused connection unit or 13 Amp plug. Modern switched fused connection units can vary in quality. Some will be o.k. for a 12.5 Amp load, other will burn out quickly. P.S. previous post edited.


Z.