Shower stops after about 10 seconds; v cold outside

[closey7]

I have a combi boiler and a Mira 'Exposed Thermostatic Mixer' shower unit.
Has always worked well; approx 8 years.
This morning, very cold outside - approx -8 degrees outside - water at required temperature came through but it stopped flowing after about 10 seconds at the warmest setting.
Similar problem later when trying again.
Left Central Heating on for a few hours and it now seems to be working ok.
The Mira User Guide states -
"Thermostatic Shut-down - for safety and comfort the thermostat will shut off the mixing valve within 2 seconds if either suppy fails (achieved only if the blend temperature has a minimum differential of 12 degrees from either supply temperature)."
What exactly does that mean?!
Would the boiler and or the shower thermostat be adversely impacted by very cold water?

 

[martygturner]

So do the other taps work.....

How is the shower fed...straight from the mains or via a tank ?

it seems that the shower will shut off if the temperature of the blended water is outside of 12 degrees from either inlet. So think about it if the cold water is freezing then the blender will only need a tiny amount to cool the hot water..it probably cannot meter that small amount so shuts off. You could try turning down the hot water temp so there is not such a miss match

 

[JonathanM]

"Thermostatic Shut-down - for safety and comfort the thermostat will shut off the mixing valve within 2 seconds if either suppy fails (achieved only if the blend temperature has a minimum differential of 12 degrees from either supply temperature)." What exactly does that mean?!

That seems very badly worded.

I think what has happened is what @martygturner says. Because the cold water is so cold, too little cold is being used to be measured, so the shower thinks the cold supply has totally failed. If that is the case, then perhaps another way around it would be to increase the temperature of the hot water, so that more cold is needed, and then the shower won't think that the cold supply has failed.

I think what the 12 degrees bit means is this. Say your shower is set to 40 degrees, and the cold supply fails, the shower won't shut off unless the hot water temperature is more than 52 degrees. Likewise, in the summer, say your shower is set at 30 degrees, and the hot supply fails, the shower won't shut off unless the cold water is less than 18 degrees. I think that is what it is trying to say!

EDIT: If you do increase the hot water temperature, I would test the shower temperature thoroughly the first time, before stepping in.

EDIT2: It may be that when the central heating has been on for a long time, you feel warmer and you don't turn the shower temperature up as much, so more cold water is used, and the shower doesn't think it has failed.

EDIT3: I don't know what model you have, but the Mira manuals I have checked recommend a minimum 55C hot water temperature, with an absolute floor of 50C. But check your own manual.

 

[JonathanM]

Another test might be to turn the shower temperature down, and see if it still cuts out.

Anyway, cold snap is over, so hopefully this problem sorts itself!

 

[closey7]

Thank you very much for your useful comments. The problem certainly seems to have been caused by very cold in-coming water.

 

[Johntheo5]

Just wondering about thermostatic showers in general used in conjunction with a combi. If you set the shower to its max setting of say 40C? and the combi to its minimum DHW temp of 35C?, will the shower also shut down even though no cold water required.