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.
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lynda, moderator

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[Steve]

Electric showers rely on a certain amount of water pressure. They heat the water using electric elements. The slower the water flows over these elements, the hotter the water becomes as it passes through. Therefore if pressure drops too low, you will get scolded. not good for Triton, or you, for that matter. This is why electric showers have a low pressure cutout.

Bear in mind also, the temperature dial on the shower only alters the water pressure to the heater, thus altering the temperature. It does not alter the power to the heater, contrary to popular belief.

Drop in mains water pressure - will be exaggetated by living high up, caused by people using high-consumption water things like washing machines, baths and electric showers , especially in flats below yours.