Immersion heater 7" dual safety thermostat cut out tripping?

[ericmark]

It was tripped when I moved in, and resetting it, it tripped again so I as a temporary measure turned the temperature down, down in fact to 45ºC which is too low, the TS-220 15A 240 V~ was replaced, and the setting left as it was supplied, Tesla UK Part No 21275 which looks identical to old one, ran for around 3 weeks, then found the reset had tripped, powered from an iboost+ it was OK until I set it to boost overnight, using the off-peak.

I have always considered if the reset trips, then time to renew to be on the safe side, but so soon after fitting, I wonder what else can cause it to trip? Not inclined to swap a second time, but maybe reduce the temperature a little, but wonder if missing something.

 

[Johntheo5]

Is it a top mounted (vertical) single or double element immersion or a side mounted immersion element?

 

[ericmark]

Top mounted single immersion.

 

[Johntheo5]

A top mounted single element immersion should have its thermostat the same length as the element, if your element length is only 7" then it will only heat ~ 25/30L but it still shouldn't trip on the H/L stat, if you pull the stat out and stick a bit of wire down through the stat pocket until it bottoms out and measure its length then that will give a good idea of the element length and the correct length of stat.

 

[ericmark]

Rather difficult to remove the thermostat, so turned the temperature down, annoying as that was the whole reason for changing it to start with.

 

[Johntheo5]

Have you any wiring schematic?, is there a possibility that the Iboost is bypassing the stat leading to stat H/L operating?

 

[ericmark]

No wiring is correct, and the over temperature trip had tripped before we moved in, and before iboost+ was fitted, otherwise I would be questioning the DC output of the iboost+.

Since the output of the iboost+ is often just 500 watt, it has likely masked the problem, and setting it to heat overnight has likely tipped the balance.

When the thermostat is open circuit, the iboost+ reports "tank hot" and the same when the over temperature trip has opened.

 

[Johntheo5]

If the Iboost voltage is DC then as long as its not> 230V it works the same as AC since the heating element is resistive only.
If the HL stat is operating at its set teerature of ~ 90C then the HW outlet one would think should also be very hot.

Other thoughts are, is the HW outlet taken off the top of the cylinder or at the side? which might cause a air pocket, causing the stat to sense a lower/false temperature.

 

[ericmark]

There is a pipe centre top of the cylinder, but yes, have wondered about an air lock. There is always air in water, so can't see any bleeding would help. It would just return.

 

[Johntheo5]

Some HW cylinders have a Surrey Flange especially where a pump is used with a vented cylinder, just ensure someone didn't just stick a copper stub pipe down into the cylinder which certainly won't work like a Surrey Flange.

If you still have the old stat you could stick it in a "7ins" high pot or whatever and heat it up to see if the control stat is actually OK.

 

[ericmark]

Problem found burnt connection, think best idea is new immersion heater as also other issues. Plumber coming Monday.

 

[Johntheo5]

Maybe a loose connection on the new stat assuming no signs of burning when you removed the old one. Ensure you get as long a element as possible without fouling the (if installed) heating coil and also a stat to match its length, obviously measure the old elememt length and stick a bit of wire down the stat pocket to measure its length.

 

[ericmark]

The old far too short (9") immersion replaced with new 27" one, work just finished, got a plumber in to do it. Clearly heating up well

so will see how it works over next few weeks, but should be even able to have a bath. From cold it took 4.22 kWh to heat the tank, will be interesting to see how much for a week.

 

[Johntheo5]

With this top mounted immersion you will only heat the full immersion length of the cylinder from a fully cold cylinder.
20/25% of the HW must then be drawn off before the stat will cut back in, so the Iboost will not realize its full potential savings if anything less than this is used during the day.
Also, if 70/75% of the HW is drawn off, just enough to get the the stat to start calling at 50C (with stat setting of 60C) then the top 75% will/can still be at 60C and the remaining 25% at mains temp then the actual final HW temp will be far higher than 60C because the vertical element will heat the cylinder without any destratification and you might end up with 75% at 70C and the remaining 25% at 30C on stat cut out, maybe worth checking the HW temperature occasionally and especially after a night boost to see how it compares with the (assumed) SP of 60C.