Electric Shower

WickedWizard · 8 Oct 2009

Hi all
I have a creda 8.5kw shower,10 months old which has started playing up.
When you turn it on to the setting we normaly have it on it does not get hot, the full power light does not come on till you turn the dial over half way, this results in cold water blasting out.
I am getting a reading of 235v to the shower & 237v at the consumer unit.
The engineer that came out under guarantee said there was not enough voltage at the shower, so it was working as a 7.5 kw.
I thought it could have been one of the micro switches, anyone else have any better ideas?
Cheers

kevplumb · 8 Oct 2009

better off posting it in the sparks forum

long as you don't get a reply offa the rule book preacher

sooey · 9 Oct 2009

Eh Kev if you think that t*t is bad on 'rules' don't get him started on topping murderers.

WickedWizard · 9 Oct 2009

Could a mod please move this to Electrics plz

Goldberg · 9 Oct 2009

WickedWizard said:
I have a creda 8.5kw shower,10 months old which has started playing up.

The manufacturer will repair or replace it under warranty, if you tell them it's faulty.

Agile · 9 Oct 2009

As I see it this is correctly located here as its a water heating appliance.

Its designed for a supply of 230v.

If its only getting 135v then the power will be just over a quarter of the full rated power.

The fault will be associated with the distribution circuits however, running it for a short while will cause 2 kW of heating at the fault point which should become visible pretty quickly!

Then the electrician can be called.

Anyone who is competent should easily be able to sort out the problem which is probably a cable terminal not tightened properly.

Tony

Goldberg · 9 Oct 2009

Agile said:
its a water heating appliance.

No, it's an electric shower. It has a tiny weeny bit of plumbing, and a lot of electricity.

Its designed for a supply of 230v.

No, its designed for a supply of 230VAC +/- 10%.

If its only getting 135v then the power will be just over a quarter of the full rated power.

No, it will be 56% of the rated power of 8.5kW.

The fault will be associated with the distribution circuits however, running it for a short while will cause 2 kW of heating at the fault point which should become visible pretty quickly!

Indeed do - it might even be as visible as this:

Then the electrician can be called.

Called what?

WickedWizard · 9 Oct 2009

Sorry my bad I meant to put 235v & 237v

Goldberg · 9 Oct 2009

Goldberg said:
WickedWizard said:

I have a creda 8.5kw shower,10 months old which has started playing up.

Click to expand...

The manufacturer will repair or replace it under warranty, if you tell them it's faulty.

Agile · 9 Oct 2009

Goldberg said:
If its only getting 135v then the power will be just over a quarter of the full rated power.

Click to expand...

No, it will be 56% of the rated power of 8.5kW.

Oh dear, you fell for that one didn't you Isaac ?

Did you leave school before GCSE science ? Ohms law ?

Do I have to spell it out or can you work it out for yourself now?

Tony

Goldberg · 9 Oct 2009

Agile said:
Did you leave school before GCSE science?

Yes. GCSEs were introduced in 1986, which is after I left school.

Ohms law?

Do you mean Ohm's Law? If so, what about it?

Do I have to spell it out or can you work it out for yourself now?

Spell what out?

If you're asking if I want you to explain your flawed working, then no I don't. I have no interest in how you came to make the mistake.

However, I had no expectation of you managing to work out the power dissipated, since I haven't forgotten this debacle:

Agile said:
Goldberg said:

Agile said:

I could repair a shower but the reality is that many fail because the input connections cannot stand the current.

Click to expand...

That's just misleading nonsense - overheated connections are caused by installer error.

Click to expand...

Many of the cheap shower units I see have miniscule terminals and conductors to carry 45A.

45A needs about 10 mm² to safely carry the current !

electronicsuk · 9 Oct 2009

Ok, it's sort of irrelevant as the OP made an error with the voltages, but lets settle this. The shower is rated at 8500w, and this rating is, more often than not, at 240v, so we'll take that as the nominal voltage.

P = I * V, therefore I = P / V, so I = 8500w / 240v = 35.42A current draw for an 8.5kW shower @ 240v

We can then use V / I = R to get the element's resistance, so that's 240v / 35.42A = 6.78 Ohms. This figure will not be static in reality as the elements's resistance will change as it heats up or cools down, but this is a close enough approximation to work with.

With the resistance of the element now known, the rated current at 137v would be I = V / R = 137v / 6.78 Ohms = 20.21A. In terms of power, this would be P = I * V = 20.21A * 137v = 2.768kW.

This is not 56% of the rated power output, nor is it just over a quarter of the rated output (not by my definition of 'just', at least), although it's closer to that than 56%!

To the OP: Call back the engineer, they're having you on, as 237v is WELL within tolerance for supply voltage. Your electricity supplier is obliged to provide 230V +10/-6% (216 to 253V), and a further 4% drop is permitted on top of this for losses in the distrobution circuit(s) in your home.

TicklyT · 9 Oct 2009

Back to the original problem......

Some showers have two heating elements, just switching on one for a cool shower, and both for a hot shower.

If the first element has failed, the cool settings will be cold, and the hot settings will be cooler than they used to be.

Taylortwocities · 9 Oct 2009

Is that voltage measured with the shower running or not running.??

If there is a prob with wiring then the voltage could drop a lot when the shower is running.

Goldberg · 9 Oct 2009

electronicsuk said:
2.768kW.

This is not 56% of the rated power output, nor is it just over a quarter of the rated output (not by my definition of 'just', at least), although it's closer to that than 56%!

Quite right, and I'm big enough to concede that I was wrong.

It is of course 56% ^2, which is about 32%.

electronicsuk said:
Your electricity supplier is obliged to provide 230V +10/-6% (216 to 253V)

Quite right; I was also wrong about the -10%.

At least we now know that Agile considers it acceptable to point out errors by poking fun in the manner of a simpering and mocking playschool brat:

Agile said:
Did you leave school before GCSE science ? Ohms law ?

Do I have to spell it out or can you work it out for yourself now?

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Electric Shower

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