Electric shower - low pressure = no hot water - why??

[JB277]

I've got a Triton Bezique electric shower that's fed off the cold water supply. Very occassionally the warning light saying 'low pressure' comes on and the shower does not heat the water - I've checked the manual and it says this will happen. Fine. I've got two questions however:
1) Why does the shower need a certain level of pressure to be able to heat the water / why does it cut off the heat at too low a pressure? - the (cold) water coming out of the shower is not appreciably at any lower pressure to me when it says low pressure.
2) What causes the drop in pressure from the mains (I live in a third floor flat)? Is it to do with the cold weather (it only happens when the weather's very cold) - does the cold freeze some of the water in the system and cause a slight pressure drop?

I'd be very grateful if anyone could enlighten me as my girlfriend keeps whingeing about having to have cold showers in the morning... Cheers.

 

[breezer]

electric showeres rely on back pressure, so if the pressure is not enough in the first place it will not be able to operate.

Being on the 3rd floor does not help, it means that that everyone wants hot water for baths showers etc, and so you being "at the end of the line" you get whats left (ok not quite that bad) but you do get whats left of the pressure, seeing as all the other people lower down have pinched it

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lynda, moderator

post locked as poster asked same question here