Immersion Heater Tripping out on Over Temperature

[AdrianUK]

This is a new thread created as a breakaway discussion from the "Testing that the controller is live & working" over on the Electrics UK forumn.

That thread can be found at : https://www.diynot.com/diy/threads/...ler-is-live-and-working.562216/#ixzz6kDBGF2jV


I'm currently having great fun with our immersion heater.

I'm having repeated problems with the over temp cutout operating 'early'.

I'm still in the process of debugging this & currently have a thermocouple in the tank (yes, actually in the water) which I'm logging every 30s. The logging software will also record the max & min temperatures seen during the logging period.

I have the thermostat set at 60 deg & that temperature agrees with the logging data. It would appear to have a hysteresis of around 5 deg.

But, and here is the puzzle, the over temp cut-out still operates randomly. My next step is to log the current pulled by the immersion so that I can get a precise collation between the temperature of the water & the thermostat operating. My current suspicion is that the over temp cutout is operating way before its quoted design of 85 deg.

A note about that particular thermostat ... the over temp cut-out can't be reset until the thermostat is COLD. I thought I'd found the problem when I found the over temp cutout tripped & it wouldn't reset (tank is also heated by the central heating). I removed the thermostat ready to replace with another & decided to have one more go before I threw it away ... it reset.

The thermostat has been replaced, twice, so it isn't just a bad 'stat (although it could be a batch problem).

 

[AdrianUK]

To answer a question on the other forumn & for those that may want to do this, here is how I fitted the thermocouple into the hot water cylinder:

I've replaced the elbow on the hot water exit pipe from the top of the cylinder with a 22 x x 15 tee.

Into the 15mm end (which is now vertical) I've fitted a 15mm to 1/8" FBSP reducer.

Into this reducer is fitted a standard 3mm x 1/8" BSP thermouple compression fitting and into this is a 300mm long stainless steel thermocouple.

The sensing tip is set about 150mm down from the top of the dome of the cylinder, but still away from the immersion heater element.

 

[terryplumb]

I seem to recall an issue with your particular thermostat brand. Did you buy them from Screwfix by any chance ? I can't for the life of me remember if somebody told me ,or it was a bulletin for recall.

 

[AdrianUK]

I seem to recall an issue with your particular thermostat brand. Did you buy them from Screwfix by any chance ? I can't for the life of me remember if somebody told me ,or it was a bulletin for recall.

Yes, as it happens, it is from Screwfix.

The heater is the Titanium 27" (75262) & the thermostat is the 89445.

Thanks for the tip... I'll do some research....