Best pump for my shower

31 Jul 2012
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United Kingdom
I would like some advice on the best pump for my shower. I can't work out if I need a negative head pump

For financial reasons I'm reusing the existing mixer tap with shower fitting. (the one with pop up diverter)

I was going to run a dedicated 22mm supply to the bath/shower supplied via a Surrey flange. The cold is mains supplied via 15mm pipe. The shower head is aprox 500mm below the base of the cw tank.

The current problem is that when I pull up on the diverter valve to use the shower I loose the hot water at the shower head. It seems that the the cold water mains feed is overcoming the hot water feed. I only get cold water at the shower head and it sounds like it is also forcing its way though the hot feed and into the tank. But if I drop the shower head to bath level it equals out slightly and the water gets warm.

I am looking at installing a positive pressure CT85+ single impeller salamander pump. Just on the hot. I dont think i need a negative pump, my rational is that because I would run the water to temperature through the tap first then open the diverter to operate the shower. the pump would already be running and the hot feed would not be overcome. Thus no negative pressure to operate it.

Money is short and pumps are expensive much more so if it's a negative head pump. Hence my reluctance to use one.

Sorry to ramble! In short I have 2 questions

1. Can I get away with just fitting a positive pressure pump?
2. can I fit it under the bath
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Good morning,

If you have mains pressure to one side of a mixer and you want to pump hot to the other side you would need to install a negative head pump that matches your mains pressure as close as you can. The reason for this is that if you have a positive head pump and mains cold, you will find that the mains pushes back of slows the water over the flow switch in the pump and causes the pump to either not activate or pulse during use. The other point that you mention is that there is 500mm between the cold tank and the shower head. This would put the shower head into negative head, you would need to have 600mm to ensure there is at least 1l/30 secs natural flow through the pump to initiate it.

hope this clarifies the situation

so yes - to a negative head pump.

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