New pump fitted, shower wont get hot.

[AndyP1102]

For about 5 years I have had a triton dove mixer shower with a 1.5 bar pump. The pump is situated in the airing cupboard next door to the bathroom and fed from the cold water storage tank in the loft and the hot water cylinder in the airing cupboard. There is a pressure cylinder on the hot water system set to 2 bar instead of a hot water header tank.
My problem is that the pump recently started to wear out (started squealing & wouldn't cut in unless I squeezed the flexible feed hose for the hot supply) and yesterday I replaced it.
I have fitted a Wasp 50 pump from B&Q which has the same 1.5 bar rating as my old pump and once fitted and switched on it kicks in fine and after the initial commissioning runs fairly quiet. However the water from the shower head won't get hot no matter that the temperature control is on full heat. I can feel the pipe between the pump and mixer bar is red hot and the hot water connector on the mixer is hot but the water from the shower head stays cool. I have removed the mixer bar and checked the filters for blockage but they are totally clear. If I turn the pump off and just let the systems natural pressure drive the shower I get full control of the temperature as normal, gets hot when turned up and cold when turned down but obviously the flow is greatly reduced. As soon as the pump is switched back on it kicks in and the water goes cool again. Have even tried turning the pump around so the cold and hot feeds switch sides in case one of the impellers were not operating but no change. Any thoughts?

 

[stem]

I have seen two things cause this.

1. An airlock in the hot pipe, although if the pipe is hot along its entire length this seems unlikely.

2. The hot and cold connections to the mixer are reversed. In this case, because the mixer is thermostatic it detects that heat is required and opens the hot inlet up, but if it's incorrectly connected to the cold supply from the pump only cold water comes out.

I suppose you may have disturbed some sediment during the replacement that has blocked the hot pipe at the mixer. It may have a filter somewhere that needs cleaning.

 

[AndyP1102]

I have seen two things cause this.

1. An airlock in the hot pipe, although if the pipe is hot along its entire length this seems unlikely.

2. The hot and cold connections to the mixer are reversed. In this case, because the mixer is thermostatic it detects that heat is required and opens the hot inlet up, but if it's incorrectly connected to the cold supply from the pump only cold water comes out.

I suppose you may have disturbed some sediment during the replacement that has blocked the hot pipe at the mixer. It may have a filter somewhere that needs cleaning.

Thanks for the response.
1. As you say seems unlikely when feels hot all the way through the pipes.
2. Is connected the same way as it has always been and never had this problem before.
3. Possible, but the only filter visible is clear and hot will come through if pump is switched off but still connected.
This is really baffling me. According to the mixer bar instructions troubleshooting page this can be caused by the flow being too fast and can be rectified by fitting a pressure regulator valve. If this is the case then why did I never need one previously and where do I fit it. On the cold or hot and before the pump or between pump and mixer?