Immersion Problem - Element or Thermostat?

[trevmai]

Today i went to a tenants house who complained about her water not heating up on the economy 7 element in the immersion heater. So i powered up the economy 7 element, it hissed like a kettle for about 30 seconds then the hissing went very quiet. I put my tester across the thermostat and it showed 230v on both sides. I know the power is there but the water was not heating up. I then turned on the booster element and it hissed like a kettle for about 10 minutes then calmed down, this heated the water.
Normally when i've had to look at immersions if the breaker trips while it is powered up, the elements nackered. This time the breakers not tripping and there's power going through the thermostat - but no hot water!
Any ideas what this could be

 

[electronicsuk]

If there's power on the other side of the stat and the cable to the economy 7 element is good (i.e. power at the element) then the answer should be obvious

The breaker will only trip if the element has gone short circuit (likely down to earth), yours may have failed open circuit hence no tripping.

 

[RF Lighting]

What sort of tester do you have?

Can you carry out a continuity test across the element prongs? (with the wiring disconnected)


Also try an IR test L>E & N>E (should be no reading)


I have seen a failed imm element do exactly as you describe, i.e. not trip the MCB and sizzle for a bit, but not actually heat the water.

The metal bit the actual element winding is housed in had split open. Gawd knows how it didn't trip the breaker. I can only think there was power flowing to earth through the water (hence the sizzling), but <16A

 

[ricicle]

If you put your meter across the thermostat and you had 230V then the contacts are not closed!!

 

[RF Lighting]

I read it to mean he tested each side of the thermostat to earth

 

[partpdoctor]

hi
the easy way to do it is this....
1 disconnect the neutral at the imm heater...
2 power up the element and start checking...
3 if the thing was right you should get 240v to earth at the stat (both sides) AND at the neutral of the element....
4 if you get a supply at both stat terminals and nothing at the neutral of the element, this will confirm that the element has gone open circuit cleanly without "blowing" to earth...

hope this helps...