Immersion Heater Thermostat Knocking

[sparkypants]

Evening all.

Just had a callout to an immersion heater making wierd knocking sound within 30 seconds or so of being switched on. Seems to be the thermostat in the dry pocket and if I hold the thermostat then the knocking stops.

Polarity, voltage etc correct. Top 22" immersion heater...

Need help!

 

[telectrix]

replace element.

 

[sparkypants]

A local plumber has replaced the element recently and 4 thermostats!

 

[telectrix]

was it knocking before the wet wipe replaced it?

 

[Engineer54]

Just wondering if the plumber replaced like for like with these thermostats?? If you put your finger on them and it stops the knocking, perhaps the probe is too thin in it's housing tube?? When the elements are on and nearing full set temperature there can be a fair amount of turbulence around the element/thermostat area. Maybe enough to vibrate the shell of the tank enough to give this knocking sound from the probe in it's housing tube...

To be honest this is just guess work, i know that large elements and their associated heating of surrounding water can in some cases vibrate thin copper tanks. So maybe this is what's happening here??

 

[alli]

How strange. Could you wrap a bit of PTFE tape around the top of the thermostat so it locates securly into the dry pocket to stop it knocking. Though why the thermostat would be knocking in the first place is beyond me!

 

[morph]

could you use a small piece of cleverly carved cheese?

 

[UKMeterman]

You could try some thermally conducting grease as sold at Maplins for computer heat sinks..

 

[Knobhead]

You could try some thermally conducting grease as sold at Maplins for computer heat sinks..

Nice idea!

 

[Marvo]

I would try bending it slightly to make it a tighter fit in the tube. Can't help thinking if the knocking noise is loud enough to warrant replacing the element and thermostat it must be loud enough to annoy the customer. There may be a more serious problem. I would consider removing the element again and having a good look inside. Maybe the plumber missed something.

 

[Engineer54]

That would be fine when there is a solid, butted against a solid, but not in this case where a thin probe is located in a housing tube larger than the probe. The PTFE tape would be a better idea in this case...lol!!

 

[UKMeterman]

It could be banging due to air, or does the cylinder need descaleing and the element is in 6" of chalk? Have you tried a 500V IR test?

 

[micknew]

Seems to me that the answer is that the thermostat tube has a small hole in it, tiny drops of water are getting into the tube and causing small 'explosions' (knocking) as the water vapourises and escapes between the thermostat and the tube.
Replacing the whole unit is the answer.
The fact that holding the stat down stops the knocking suggests this is the answer to the problem.

 

[Engineer54]

Seems to me that the answer is that the thermostat tube has a small hole in it, tiny drops of water are getting into the tube and causing small 'explosions' (knocking) as the water vapourises and escapes between the thermostat and the tube.
Replacing the whole unit is the answer.

Why then, if putting your finger on the thermostat stop the knocking?? The explosions (knocking) would still continue surely?? ...A good theory though.

 

[micknew]

It would stop the knocking if held down because the stat could not move.
The vapourised water would just escape, it would only have to be a tiny leak caused by corrosion.
I have changed hundreds of immersion heaters and have never experienced this my self, cannot think of any other possible answer.