Tesla 7" dual safety thermostats failing

[YorkshireMidge]

I keep having issues in a flat we own with Tesla dual safety immersion thermostats item 21275 from Screwfix on the Economy 7 circuit/heater. The boost circuit doesn't get used enough to draw any conclusions for that one.

The E7 stat seems to last barely longer than the one year warranty Tesla put on them, and quite a few reviewers of the product on the Screwfix site have reported exactly the same, or even earlier failure. I've got the point where I keep a spare at the flat and I'm just about to change it again today as the occupant is reporting unusually hot water, suggesting the contacts have fused.

I just wondered if anyone had any recommendations for anything better/more reliable as these Tesla ones are quite widely sold and there doesn't seem to be be too many more options on the market. I accept these are switching a fairly hefty current for several hours on a daily basis - but I would expect better.

Thanks!

 

[Madrab]

If you're having such an issue with them failing just after a year then I'd take it up with Tesla. I'm sure they don't make their stats so poorly that they always fail just after a year. Have you had the circuit checked too to ensure there's no issue with that?

Its not an Ariston HW heater is it?

 

[CBW]

https://www.bes.co.uk/immersion-heater-7-thermostat-19765/ any good? Maybe screwfix aren’t sending the faulty ones back and reissuing them?

 

[YorkshireMidge]

If you're having such an issue with them failing just after a year then I'd take it up with Tesla. I'm sure they don't make their stats so poorly that they always fail just after a year. Have you had the circuit checked too to ensure there's no issue with that?

Its not an Ariston HW heater is it?

It's a Tesla immersion heater element. I've had a good check around and can find nothing wrong circuit wise. The only other thing on the E7 circuit is a single storage heater and not had a minutes problem with that and it has been in for seven years.

I've had a dig about in my records and found this - which is a photo of the last one that failed and it is dated October 14 2022. So this latest one has actually made it to 19 months. If this was the only time it had ever happened, I'd have likely disregarded it and put it down to a loose screw connection. I'm pretty sure I replaced the cooked wiring.

 

[YorkshireMidge]

https://www.bes.co.uk/immersion-heater-7-thermostat-19765/ any good? Maybe screwfix aren’t sending the faulty ones back and reissuing them?

The BES one is the same item - TS220.

 

[CBW]

The BES one is the same item - TS220.

Thanks, I didn’t realise.

 

[Madrab]

It's a Tesla immersion heater element

The TS220 is a stat not an element, is the immersion element a Tesla too then? Is it the element or the stat that's burning out, seems to be the stat?

Have you checked the immersion element and checked its current draw? Seems like it's cooking the terminal which would indicate a current over draw or the cable isn't rated for the current it's carrying?

 

[oldbuffer]

Perhaps two possibilities worth considering:
1. As Madrab has suggested, loose / inadequate wiring causing arcing and overheat. The cable from the wall outlet to a 3 kW immersion heater should be 3 core, 2.5mm squared, heat resistant, flexible cable.
2. Limescale build up in the base of the hot water cylinder leaving the element and thermostat tip in scale rather than water causing them to overheat.

 

[ericmark]

Interesting what you say, I want to change mine for a longer immersion and thermostat, but I was having a problem with the reset tripping, I turned down temperature and no problems since.

However I am using DC and variable load, so I have expected problems.

The first step is test is it using 3 kW or is there a fault and it is using more? The display on mine shows how much it uses, so know I am within limits.

Is there RCD protection? If the element is damaged it should trip the RCD, but if missing the current draw could be well over 3 kW.

 

[YorkshireMidge]

Just an update as I've been across to the flat this morning. Dug out the stat and there were no signs of overheating and it was functioning correctly at that point. So I opened it up* and it was actually like new inside - shiny copper and no signs of overheating whatsoever. So it's a bit of a mystery. Water temp was noticeably hotter than I expected but not way hot. So possibilities seem to be:

1. Occupier usually takes a bath on a morning, but has been taking a shower instead with the current mini-heat wave - so it's possible she has just felt (correctly) that the tap water temp is much hotter during the day than she is used to. I have dropped it down slightly.
2. Contacts had stuck together (as some reviewers on the Screwfix site have reported) but subsequently released at some point

*Important to move the red temp control to one or other end of its travel when doing this, as it comes off a splined spindle when you open the case up, and you need to be able to put it back in the right position.

So I made a decision for the sake of tenner and the fact I had one, to swap it for my new spare anyway just to be on the safe side, and I'll see how it goes.

The TS220 is a stat not an element, is the immersion element a Tesla too then? Is it the element or the stat that's burning out, seems to be the stat?

Have you checked the immersion element and checked its current draw? Seems like it's cooking the terminal which would indicate a current over draw or the cable isn't rated for the current it's carrying?

Correct - immersion element is Tesla too. I did a quick resistance based calculation and it seemed OK - I don't think I have a meter that will go above 10A current.

Perhaps two possibilities worth considering:
1. As Madrab has suggested, loose / inadequate wiring causing arcing and overheat. The cable from the wall outlet to a 3 kW immersion heater should be 3 core, 2.5mm squared, heat resistant, flexible cable.
2. Limescale build up in the base of the hot water cylinder leaving the element and thermostat tip in scale rather than water causing them to overheat.

The cable from wall outlet to heater is the correct 2.5mm butyl rubber stuff specifically for that purpose (wish they would make cable clamps bigger to take it!).

We're not prone to limescale in that area and the element is about 7 years old, but I'll bear that in mind if I have further issues.

Thanks all.

 

[Johntheo5]

You said above that its a "E7" stat, does this mean that its only heating the cylinder for a timed night period?
Is there just a single side mounted element&stat in the bottom of the cylinder or is it a top mounted element&stat?
What is the cylinder capacity?.

 

[AdrianUK]

I too have had issues with Tesla immersion heaters & thermostats, although in my case this is with a top mounted 27" heater & 11" stat. I have written about this in a previous thread but I cant find it at the moment.

My issue was with the anti-boil over heat portion of the stat randomly tripping for no apparent reason. This happened on a number of occasions & resulted in me adding an internal thermocouple to the cylinder so that I could log what was happening. Data was captured for several 'trips' but each time the logged temperature was nowhere near 95C.

Eventually the stat failed & the water boiled. Yes, it boiled even though the tesla thermostat has a manually reset over-heat cutout! Having pulled the thermostat out & inspected on the bench, the reasons for its behaviour came clear - the thermostat itself was not the cause - but the manufacturer supplied link wire which joined the output of the thermostat to the heating element. When removed, both connecting terminals of the thermostat where found to be tight, although there was obvious discoloration & distortion on the stat case due to heat.

This link wire terminated, at the thermostat end, in bare conductor & at the heater end in a crimped 'fork'. Closer examination of the crimp revealed that it was very poor - not correctly closed & only making contact with some of the strands.

My conclusion is that heat from this poor crimp was being conducted back along the short link wire (which also suffered thermal damage) & into the body of the thermostat. It was this conducted heat which was causing the mal-operation of the over-heat stat & eventually lead to the total failure of the both the normal & over-heat portions of the unit.

The thermostat was replaced with new & the link wire replaced with a piece of 90C 2.5mm2 wire. Everything was good for another two years & checked on every 6 months or so - no signs of overheating. The logging has been kept in place.... I have observed that the opening temperature of the thermostat is slowly creeping upwards..... upto about 15 deg higher than set on the scale, with the odd incursion to about 20 deg higher....

 

[YorkshireMidge]

You said above that its a "E7" stat, does this mean that its only heating the cylinder for a timed night period?
Is there just a single side mounted element&stat in the bottom of the cylinder or is it a top mounted element&stat?
What is the cylinder capacity?.

Yes - well, the modern equiv of E7 anyway. It's a five terminal smartmeter now (which replaced one of those old radio control Economy 7 switches).
There is second immersion heater and circuit which I referred to as boost which is wired off the nomal supply via a switch in the kitchen. That hardly ever needs using so I haven't seen the same problem on the stat for that.

The cylinder is either the 117L or 140L (can't recall which) bought from these guys https://www.plumbnation.co.uk/economy-7-copper-vented-cylinders-397-0000

Not so big that it would cause problems I wouldn't have thought.

 

[YorkshireMidge]

@AdrianUK That's interesting. I wonder if I had a similar problem as you back in 2022 (and possibly prior occasions). One side of the immersion had a screw terminal on it. The other had a short bit of link wire - and it is that you see well baked in my photo. I definetely replaced it at that point with a piece of 2.5mm core from a spare bit of the butyl rubber cable I used to connect the heaters to the wall outlets.

Interesting too that the opening temp is steadily rising in your case and that could also be a factor in mine this time around esp. when mixed with the current mini-heatwave.

Thanks.

 

[AdrianUK]

The reason for this being at the front of my mind was that, for the first time in over 2 years, the immersion stopped working again in the early hours of Friday morning. At the bottom of this email is the log file (plot1) from the last week. The deep 'V' shaped dips are someone taking a bath

This plot, upto about 280 x 10^3s, is typical week. At around 290 x 10^3s something interesting happens & the opening point jumps to around 88C ...... then returns to normal for a few cycles & then does it again. At around 410 x 10^3s is the final cycle where the temperature exceeds 90C & the over-temperature cutout operates.

Again I removed the stat & inspected. This time, no signs of overheating - there is no baking of either the thermostat housing or the link wire. Opening the thermostat casing revealed nothing - the contacts are not welded. There is some pitting to the main contacts but that's to be expected after 2 years of switching 13A many times per day.

I have replaced that stat with another. Its behaviour over the last 24hrs is quite interesting - its already exhibiting the 'jump' in opening temperature from around 65 x 10^3s (plot 2). I'll be watching carefully over the next week.....