What causes immersion heaters and thermostats to go wrong

[STI]

Im flumoxed. A plumber has recently changed both the heating element and thermostat in the immersion heater. About a week after the lady called me because the water was not getting hot. The plumber said it must be an electrical fault, so the lady called me.

I checked the element and thermostat and the new stat had blown. This was replaced. Due to the short life of the thermostat i checked the voltage at the stat 241v i checked the wiring was all clean at the terminals, i checked wiring at the switch everything was fine. The CU end showed no signs of loose connections and it had at no time tripped. The IR came in at 187Mohms.

Today i receive another call and the hot water is not working again. So assuming at this stage the thermostat or element has gone again what could be causing such short life spans of either of these parts.

Thanks in advance

 

[ericmark]

Where the tank is plastic the stats have a non resettable fuse so if they fail once they need replacing. This was because if it fails to switch off the plastic tank can fail and there have been fatalities.

However these stats should only be used where the immersion heater is the sole method of heating the water. Where there is a second form of water heating then the stats with re-settable buttons are used as it could well be the second heating source that is faulty. In these cases a tank able to stand boiling water is used as a header tank so if the water does boil the tank will not fail.

As standard plumbers merchants will supply non re-settable type in interest of safety. You have to ask special of re-settable type which are always used where solid fuel heating is used.

 

[TicklyT]

If the thermostat temperature is set too high, the overheat cutout may operate before the 'stat.

 

[STI]

Ericmark, thanks for the response. There is a second source of heat for the water, an oil AGA. The stat in the immersion has a reset button and until i go back tomorrow i dont know if this needs to be reset. However i understood that this only cuts out if the water temp gets too hot. As the lady is complaining the water isnt getting hot enough, and claims to have adjusted the temp setting on the stat i dont think its the reset button. However i may find out more tomorrow.

The tank is a standard copper one.

 

[STI]

If the thermostat temperature is set too high, the overheat cutout may operate before the 'stat.

The plumber had set the stat temp to full on, I turned this down on the new one and the lady says she has tried turning it up but the water still is not heating up enough. I can only see one reset button and thats on the stat housing i cant see anything on the new element.

 

[RF Lighting]

Is the immersion heater her sole means of heating the water?

 

[333rocky333]

. The IR came in at 187Mohms.

Where was that measured.
Did you test between the the element to earth though.
Is this a job for Homeserve, seriously my girlfreind up there has had 4 visits now and still no hot water

 

[STI]

. The IR came in at 187Mohms.

Where was that measured.
Did you test between the the element to earth though.
Is this a job for Homeserve, seriously my girlfreind up there has had 4 visits now and still no hot water

That was the cable result, from CU to switch to terminals with the stat and element disconnected. I havent tested the element to earth. I did a continuity check on it and that came in ok.

Dont think the plumber was homeserve .

 

[STI]

Is the immersion heater her sole means of heating the water?

No the AGA heats it too but not sufficient for baths etc. The AGA is still heating it to its "normal" level

 

[RF Lighting]

I wondered if that might be the case.

I've had this a couple of times on systems heated by a back boiler and an immersion for if the fire wasn't lit.

The old immersion goes faulty after years of use, and a plumber fits a new one complete with new stat and cutout overload, as required for all modern stsytems with plastic header tanks.

The water was gets heated by the back boiler (or aga?) to a temperature above the setting of both the rod stat and the cutout. That causes the stat to open, and also the overload to open.

Once the water cools, the stat closes again as normal, but as the cutout is a safety device it will not automatically reclose. It may need resetting, or replacing.

If the header tank is metal you can remove the safety cutout. If it is plastic, you're a bit stuck.

 

[STI]

I wondered if that might be the case.

I've had this a couple of times on systems heated by a back boiler and an immersion for if the fire wasn't lit.

The old immersion goes faulty after years of use, and a plumber fits a new one complete with new stat and cutout overload, as required for all modern stsytems with plastic header tanks.

The water was gets heated by the back boiler (or aga?) to a temperature above the setting of both the rod stat and the cutout. That causes the stat to open, and also the overload to open.

Once the water cools, the stat closes again as normal, but as the cutout is a safety device it will not automatically reclose. It may need resetting, or replacing.

If the header tank is metal you can remove the safety cutout. If it is plastic, you're a bit stuck.

RF Thanks and sorry to have to ask but are we talking about another cut out switch other than the one that is on the stat control (the little reset button not the temp dial) and if so where would that cut out be sited, i cant see anything in the element/stat housing nor any leads/cables/wires going anywhere out of the cupboard where the tank is, other than the supply cable/flex.

 

[bernardgreen]

If the header tank is metal you can remove the safety cutout. If it is plastic, you're a bit stuck.

I have heard of thermostatic sensors fitted to plastic tanks to warn the house owner that the cold water tank is starting to heat up. This means the back boiler or Aga has to shut down or quantities of hot water run off to cool the system.

Could that be a better way to protect the plumbing from a run away immersion heater ? I expect regulations will say NO to that idea.

 

[StelleElectrical]

Ive seen a few where we change the stat and it works for maybe a day or two,then get a call to say its not working we go back test it again and everything is sound.Problem is sometimes to be the off peak meter so the hydro would get called.