Hot water cylinder with immersion tripping rcd.

[Toddius]

Hi guys.
I have a newish (4 years old) hot water cylinder with 2 immersion heaters fitted.
It has worked fine but yesterday when switching it on it tripped a rcd. I have disconnected the mains from the element put it in terminal strip and tried the switch again and it doesn't trip so I would guess at the issue being within the immersion.
Could this be the thermostat or is it likely to be something else. Thanks in advance

 

[dancinplumba]

most likely the element shorting, need a new one

 

[townfanjon]

Yep probably imm heater , if you are competent and feel safe to do it , just to make 100%, swap the boost stat into the bottom imm heater and try that

 

[Toddius]

Cheers for the replies. Will swap the thermostat over tomorrow.i thought it might be the immersion. Going to be a absolute pain draining the tank if the immersion does need changing.
The drainage point is very tight to get to.

 

[townfanjon]

Try the stat you maybe lucky.
I am assuming this is an unvented cylinder , if it is , just to make you aware you have to be qualified to work on these cylinders .

 

[Tim S]

You can test the thermostat and the element using a multi meter. If the element is ok you should get a resistance reading of around 19 ohms for a 3kw element

 

[Murdoch]

How do the "disconnected" cable cores look?

On older installations the heat from the immersion & tank will make the insulation deteriorate and then the live or neutral could come in contact with the cpc or metal parts .....

 

[Toddius]

Thanks again.
Iv checked the resistance. I have open circuit on the thermostat (grabbed a new one from plumbase) that changes to 0.6 ohms when temp is set to 65.

On the heater element I have 19.6 ohms.

Murdoch The cable looks fine.

Both of our elements don't work now. One went about about two years ago called the guy who fitted the new cylinder when it was Asked him to come back and I'll pay for what needs doing he said just use the other one.

It's been bad luck from the start. Electrician who come to connect it nearly set our house on fire. He plugged it into a plug top timer. Me and my partner were sitting downstairs and could smell smoke. The whole thing melted.

 

[Toddius]

Try the stat you maybe lucky.
I am assuming this is an unvented cylinder , if it is , just to make you aware you have to be qualified to work on these cylinders .

This is what it is. Probably makes more sense to you than me.

 

[steadyon]

It's a vented direct cylinder.

If you have the Economy 7 (or similar) electric tariff, the lower immersion would be connected to this to heat up the whole cylinder at cheap rate (round about 1/2 normal rate) electricity. The upper immersion would be connected to the normal tariff electricity supply, and used to re-heat the top 1/3 of the cylinder if you ran out of hot water. If you have been using the top immersion only, then if you do have Economy 7, you've been paying twice as much as you need to to heat your water.

I'd advise you to have the cylinder drained and both immersion heaters replaced with good quality (Incalloy or Titanium) ones, then make sure that each is properly connected to an appropriate electricity source.

 

[Toddius]

Thanks steadyon.
So. About the economy 7. We don't have the economy 7 tariff anymore. We come off that. When they disconnected and changed the meter outside they disconnected the feed to the hot water heater spur and the old storage heater spurs.all the economy 7 items. That's why we got a electrician to come and connect the stuff back up.

We have had someone round and he's going to drain the cylinder(with a water hoover) and change the elements. He said for me to get the elements. Can you recommend any.

We have a electrician coming too as the previous one has connected a electric shower to a fused spur with No switch. I'm sure he wanted to kill me off. Haha.
It's actually very scary and off putting how many poor tradesmen there are.

 

[Toddius]

Also would just like to say thank you. The advice is very much appreciated.

 

[Tim Watts]

It's not always reliable to test the element with a multimeter for this type of fault - the earth leakage may only be apparent at higher voltages or may vary as the element heats up. But if you do and you get a resistance from L or N to the earth terminal of < about 7600 **ohms, then it's fairly conclusive that the element is knackered.

** 230V divided by 0.03A

 

[steadyon]

If both immersions are connected to the same supply point in the consumer unit, then if one is already on, and the other is turned on, that would be enough to trip a circuit breaker. Each immersion will take about 12 amps when on, so two require 24 amps. If they are both on the same 16 amp circuit breaker, it should trip it. Might also be enough to trip the RCD.

I generally use "Backer" brand immersions. They do titanium and Incoloy. More expensive than ordinary copper ones, but they do last longer in hard water areas (I live in London). If you get an Incoloy one, make sure the pocket the thermostat goes into is also Incoloy, not copper.

I understand the Tesla ones are also good, but I have no personal experience of using them.

 

[Murdoch]

^^ connecting both elements to a 16 A mcb would not trip an RCD .... unless damage to the cables occurs