Shower Air Lock/Pump issue

17 Jul 2015
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United Kingdom
Hi all, hoping for some advice. Have got a pump in our bathroom that feeds the shower and a basin, and is gravity fed from the loft. (the pump is below the hot water cylinder which is also in the bathroom, header tanks in the loft) We've recently been having an issue with what i think is an air lock or the pump pulling air but only on the hot water side.

The piping set up has a T just after the outlets of the pump, where a smaller bore pipe comes out of both hot and cold to feed the basin mixer tap, and the main pipes carry on to the shower. So occasionally we go to turn on the shower and find that, when the mixer is turned to hot side, the pump 'splutters' and turns on and off and cant push any hot water through. However, if we turn on the basin mixer tap, the pump/hot water works just fine. And if we turn off the pump (switch in the cupboard), hot water will gravitate through to the basin tap, but will not gravitate to the shower (i.e nothing comes out). The cold water to both shower and basin always works fine and the pump always kicks in.

I believe the whole system was put in ~5 years ago, and we have owned the house for about a year. In that year, it has on perhaps 2 or 3 occasions had this problem, but it would clear within ~30 seconds. In the last month or so its been happening every 2 or 3 days, and sometimes takes up to an hour of turning on and off the mixer tap whilst the shower is on (and pump is spluttering) until all of a sudden it just kicks in and works. We have found that leaving the pump off when using the basin and only turning it on to use the shower has helped to stop it happening so often, but it still happens.

I had a plumber come take a look and his solution was to change the pump to a negative head pump (pretty expensive as it turns out!). But i feel that if this was a head problem then a) why is it suddenly a problem now when for the past however many years it has worked (almost) fine. b) why does the basin work fine? and c) why the cold water never has a problem? He wasnt able to answer those questions...

Apologies for the essay, as you can tell I'm a beginner when it comes to heating/hot water systems and wanted to give all the info. Any ideas/advice appreciated.
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