Pumped showers run cold, but only in the winter!

[mef13]

Subject says it all: and no, it's not that the water in the tank is cold, it's at 55 degrees!

We have four showers fed from a single pump. In the sumer all is good. As the loft cools (that's where the pump is) the shower temperature falls even though there is clearly hot water available. In Sept/Oct running two showers at once restores the hot flow, though one alone will only get tepid.

Right now a lukewarm shower is all that's possible. As I said, the tank is full of hot water. The outlet for the pump is about a third of the way down the tank, but the extract water is at full temperature.

Ideas, please?

 

[HERTS P&D]

Faulty pump, what pump is it.

Andy

 

[mef13]

It's a Grundfos, can't remember the exact model number. It was installed about four years ago.

Problem started towards the end of last winter, but went away in the summer.... now it's back!