Gone through multiple Mira Thermal Switches

[Gwynston]

I have a Mira Sport Thermostatic instant heater shower that's only 5 years old. Last summer the thermal cutoff switch got triggered, so the shower wouldn't heat up. Took some googling and a steep learning learning curve for me to diagnose the issue, but I was successful in sourcing a replacement Mira TCO switch and overcoming the fiddly job of fitting it in place of the old dead one.

However, last month the same thing happened again. I repeated the replacement process but it's happened AGAIN after just two weeks! So something is triggering this and I'm trying now to diagnose what. I can't see any obvious signs of water ingress or corrosion, but the last two times, there was a definitely a pattern:

Some time not long after the shower has been used and switched off (once it was just a minute after, another time it was 20 minutes later) the shower suddenly turns itself on, makes a sudden surging noise and then cuts out. Last year when it happened, I was aware that the water was boiling hot at this moment, so this is obviously what's triggering the TCO.

So the mystery is what is causing this spontaneous switching on and overheating?

I guess it might be avoidable if we always switched off the isolating string switch in the bathroom? But that's simply not always going to happen with 3 teenagers in the house! And besides, I don't want this to occur when someone is still in the shower, as I almost got scalded when it first happened last year.

 

[flameport]

Caked with limescale internally / other parts failing.

If it's boiling the water it's dangerous.
Replace the whole thing before it causes injury.

 

[Harry Bloomfield]

As above, it is choked up with limescale, which causes inadequate cold water flow. As a result of inadequate flow, the water gets too hot, which causes the TCO to trip.

It might be choked internally, or at the spray head. Soaking in lemon juice/vinegar/limescale descaller will dissolve it.

 

[denso13]

Caked with limescale internally / other parts failing.

This, other relays/parts can fail causing the water to boil and trip the thermal switch.

 

[CountryFan]

Has the shower head and or hose been replaced at all? Some folk like these fancy looking ball filled types

 

[Johntheo5]

I have a Mira Sport Thermostatic instant heater shower that's only 5 years old. Last summer the thermal cutoff switch got triggered, so the shower wouldn't heat up. Took some googling and a steep learning learning curve for me to diagnose the issue, but I was successful in sourcing a replacement Mira TCO switch and overcoming the fiddly job of fitting it in place of the old dead one.

However, last month the same thing happened again. I repeated the replacement process but it's happened AGAIN after just two weeks! So something is triggering this and I'm trying now to diagnose what. I can't see any obvious signs of water ingress or corrosion, but the last two times, there was a definitely a pattern:

Some time not long after the shower has been used and switched off (once it was just a minute after, another time it was 20 minutes later) the shower suddenly turns itself on, makes a sudden surging noise and then cuts out. Last year when it happened, I was aware that the water was boiling hot at this moment, so this is obviously what's triggering the TCO.

So the mystery is what is causing this spontaneous switching on and overheating?

I guess it might be avoidable if we always switched off the isolating string switch in the bathroom? But that's simply not always going to happen with 3 teenagers in the house! And besides, I don't want this to occur when someone is still in the shower, as I almost got scalded when it first happened last year.

The TCO contains two sets of contacts, wired in series, one set of contacts open when/if the water temperature reaches 48C and is self resetting, the other is set to around 72C, if that opens then the TCO must be renewed, (and fault found). the 48C set can be heard operating every time you switch off the shower, you will/should hear a click about 10sec after the shower is switched off. Power is only supplied to the heating elements by two micro switches which operate when the start button is pressed and when sufficiently high pressure pushes a diaphragm/lever to physically close these micro switches, this corresponds to a minimum flowrate of around 3LPM. It may be limestone is the problem but no harm to carry out a bucket/flow test, turn the temperature control to fully cold and with the shower head in place in its holder, hold a buket under it for exactly 30 secs, measure this, X2 to give the flowrate in LPM, should be ~ 8.0LPM but a minimum of say 5/5.5LPM should be enough (pressure) to close the micro switches.
(My) present mains temp is 12C which will provide 4.61LPM at 40C for a 9kW unit or 5.0LPM for a 9.8kW.

 

[Gwynston]

Thanks for the responses.
I'm not sure that it's limescale - we're not in a hard water area. e.g. we get hardly any limescale in our kettle and this shower is only 5 years old. The previous shower lasted 18 years and was still working - I only replaced it because it wasn't holding a very consistent temp.
I don't get how some of the suggestions would lead to the behaviour I've seen - i.e. the shower spontaneously switching itself on a few minutes after use and overheating.

Our water pressure is pretty poor to be honest, so maybe the response from Johntheo5 is most relevant? I'll give that flowrate test a go, although the pressure issue can actually fluctuate a lot. Might it be that if the pressure is insufficient at switch off time, the 48C switch isn't correctly disengaging 10 sec later?
The shower head does likely need descaling, so I'll give that a try too.

I'd really like to not have to replace a 5-y-o €300 shower, but I'll soon have spent €100 on replacement TCO switches anyway! And the safety factor is obviously an important consideration too...

 

[Johntheo5]

Strangely enough, or maybe not, I had a 9.0kW Mira Sport for ~ 18 years, now replaced with a 9kW Triton T80Z, due to ongoing external waterworks the mains pressure was frequently quite low or non existant for hours each day so I installed a lever valve (with a DCV at the mains), to use the CWSC, to my amazament I could have a one element shower at 40C from mains of ~ 15C even during the summer while this work was ongoing, the flowrate was still ~ 2.6/3.0LPM, the "pressure" at the shower was only about 1/1.5M, 0.1/0.15bar, yet it was sufficient to close the microswitches. The Triton will give maybe half this flowrate under gravity and certainly will not close the microswitches

If the microswitches arn't opening with no flow (shower switched off) then the TCO should operate even faster than 10sec, have you listebed for this click after switching off?.

Also suggest you get someone to switch off the shower while you watch the elec meter and see if the rapidly flashing LED slows down (if a smart meter) or the spinning disc slows down if the old type meter, this will tell you if the microswitches are opening or not, maybe worth renewing the microswitches even if unlikely to be causing the problem.

 

[Gwynston]

I did a flow rate test a couple of times with the dial set to fully cold and got between 5.4 and 6 LPM. So it varies, and likely drops below that from time to time (we often see noticeably lowered pressure on the kitchen mains tap, especially in summer).

I also tested with the dial set to one of the warmer settings, and although it's not actually heating because the TCO switch is presently gone, the passage through the extra plumbing dropped the rate by around a half. I can't actually test the 10s microswitch trigger because there's no heating until I get a new TCO.

I'm currently descaling the shower head, so I'll test the flow rate again when that's done.

 

[Johntheo5]

Present (mine) mains temp is 12C, a showering temp of 40C (assuming a 9.0kW shower) requires a flowrate of 4.61LPM, if the showering temp reaches 48C (TCO, auto reset trip), then the flowrate must fall to 3.58LPM, if the showering temp reaches 72C (TCO final and no reset trip) the flowrate must fall to 2.15LPM, since even 48C is too hot for most, then its more than likely IMO that the temperature while showering was not much > than say 45C, flowrate 3.9LPM which IMO points to the shower "switching off" normally but the microswitches staying eneregised for whatever reason leading to the TCO 48C contacts opening first (quite normal) and the 72C contacts then opening, most certainly not normal.

My Triton, on installation tests, flowed 7.5/7.8LPM with dial to fully cold.

 

[Gwynston]

Tested flow rate again with limescale-free shower head. Got perhaps about 10 - 15% more, but it's not a very exact measurement.
I'm none-the-wiser and have a new TCO switch ordered and on the way,,,

 

[Johntheo5]

Some time not long after the shower has been used and switched off (once it was just a minute after, another time it was 20 minutes later) the shower suddenly turns itself on, makes a sudden surging noise and then cuts out. Last year when it happened, I was aware that the water was boiling hot at this moment, so this is obviously what's triggering the TCO.

That 20 minute behaviour certainly doesn't indicate that the elements stayed energised after switching off the shower. The heating can including the two elements contain 0.23L of water (I measured it years ago on a similar 9kW scrapped shower) so this 0.23L will be totally evaporated in less than 1 minute with 9.0kW heat input, I would expect the elements to burn out in very short order if this were the case. Of course the TCO cutting out is doing what its supposed to do so maybe thats why no burn out.
Suggest you isolate the shower at the consumer unit, remove the cover and probe with a multimeter set to ohms between the output terminal of the TCO and the supply terminal on any one of the heating elements, if microswitches still made then you should get a reading of almost 0 ohms, maybe 0.3 or so, if you probe to the output side of a heating can you should get ~ 11.7ohms, if you get a OC reading then microswitches not closed. You can't just keep throwing new TCOs at the problem.

Also, another test is with the water isolated, press the start button (shower power restored and with a new TCO) the low flow neon should illuminate if microswitches open.