How often should an electric immersion heater turn on/off?

[stoduk]

I've got a vented hot water cylinder with two immersion heaters (economy 7 and boost). I've noticed recently that when heating overnight I can hear the element turning on for only 10 seconds or so before turning off again, repeating this cycle soon after. Last night I noticed it around 5am, so it will have been heating for quite some time - TBH I'm surprised in summer that it still needs to turn on by then.

Can anyone comment on whether this behaviour is expected or not? Perhaps I'm just noticing what it has been doing for years!

It is a standard element with a built-in thermostat so I would expect it to get to temperature and turn off for quite a while (I'm assuming these are bi-metallic strips so they don't react that quickly AFAIK). I've just checked and the temperature setting was near the top of the (10-70 ºC) range, so I've turned that down.

TIA!

 

[flameport]

so I would expect it to get to temperature and turn off for quite a while

That's what should happen.
Turning on/off every few seconds probably means it's completely clogged with scale, and isn't actually heating the water.

 

[stoduk]

Thanks for your response. Scale could well be a problem, the water is incredibly hard around here. I doubt having the thermostat set so high was useful as that will have accelerated scale deposits. The water is still heating up overnight, but maybe it is less efficient at transferring heat if there is an insulating layer of scale - which could explain the faster cycling.

I'll see how turning the temperature down affects things, but perhaps it is time to invest in a water softener!