Immersion heater problem

[STI]

Hi, I have an immersion heater that has a single element (physically) but is connected to economy 7. On the wall is a switch that states small or large, so I assume this means on small only some of the element heats up
Recently our hot water has not been getting hot, just warm. So that's my first point of confusion the "system" seems to be partially broken. Seems a bit like saying slightly pregnant.

I have tested the stat and that does not appear to be working.
I have tested the element and I am getting a reading of>99.99 I am using A MEGGER 1552 MFT, those who know this kit will know as well as giving a reading it has a open closed circuit indicator when set to Ohms. This indicator keeps switching open to closed and back again when showing the >99.99 reading.

So I am guessing at the moment that my element has gone too. But what I cant understand is why am I getting warm water, if its not working it should be cold. Its been doing this for 3 days so I would have cleared any residual warm water in the tank.

Can anyone advise

 

[AndyPRK]

Do you have an RCD in the house?

I little concerned your testing indicates a faulty stat and element which is unliky so questions your test method.

 

[STI]

Yes we do have an RCD switch protecting amongst others the immersion circuit.
Testing method has been

disconnect from supply

For stat remove all connections. Place probes on both terminals with temp dial set to furthest point then move temp dial to opposite furthest point. No difference in reading

For element remove all flex connections.
Test across element terminals I get the readings previously stated

The RCD has not blown, BUT when I tried to switch on the boost timer this did blow the main switch, not the RCD.

This made me think if the element does have "two stages" when I use the timer boost does this use the full element, which then leads to the breaker blowing but explains why when I am on ECO 7 I am still getting a small amount of warm water and the breaker isn't blowing as it only uses the smaller section of the element.

 

[AndyPRK]

So maybe the stat is ok.

What do you mean by the main switch tripping?

If you have a dual element you should see 4 element ends and terminals.
If normal single only 2.

 

[Taylortwocities]

First, set your MFT to measure resistance (not the low ohms setting).

Measure across the immersion element. For a 3KW heater the element should be in the region of 17.5Ω

Next measure across the stat. At lowest setting the reading should be open circuit. At highest temp setting you should see a short circuit (0Ω ).

That should check if the components are OK. Come backl when you've done that.

 

[STI]

Yep done that.

The stat reads 0.47 across the whole range of the dial setting

The element reads >99.99

By the main switch I mean the main breaker on the CU i.e. the switch that isolates all of the CU.
Only two connections on the element (plus an earth)

I have just connected the element to my PAT machine and I am getting an IR result of less than .10 so not a dead short but certainly well on its way.
I guess this is why I am getting some warmth.

 

[Taylortwocities]

That should tell you that the element fails the Ir test.

 

[Robin0577]

Sounds like your element may have split open and part of your current path is now via the water rather than the element wire itself, not an uncommon occurrence with immersion heaters, especially in hard water areas.