Shower pump - different sized pipework for hot and cold feed

[dan1w]

I'm about to install a shower pump to supply my bath. At the moment I have a 22mm cold feed running directly from the cold tank to the bath.

I have a load of 15mm pipe to use up, so would like run the hot feed from the pump to the bath in 15mm, since the floorboards run under the walls I want to run this into the loft and back down into the bath as it's far easier.

So the cold feed would consist of 22mm to pump, reduced to 15mm into pump then back up to 22mm to the bath, this would be running beneath the floor boards.

Hot feed would consist of 22mm to pump, reduced to 15mm then run up to loft and back down.

This would obviously create some difference in flow/pressure between the feeds when it reaches the thermostatic mixer. I don't know how these taps work to control the temperature so if anyone could tell me whether this scenario would create problems I'd appreciate it.

 

[kbdiy]

How does the hot feed run to the bath at the moment? What size?

 

[dan1w]

22mm as well on the hot water feed for the rest of the house.

 

[kbdiy]

So why can't you just 'insert' the pump in the 22mm of both hot and cold feeds so that both are still delivered to the bath in 22mm and only 15mm in and out of the pump?

 

[dan1w]

Just because I don't want the pump coming on when the hot tap is used in the kitchen or cloakroom, nor those taps intefering with the pressure available to the shower when it's being used.

I've no idea what the acceptable pressure ratio is for my mixer, but I've seen there are some available which go up to 10:1. I've tried to get my head around what the pressure difference might be in this scenerio but it gets a bit complicated. It seems seperate equalisers are available which is an option, running the whole lot in 15mm is probably the cheaper/easier and less risky option.

 

[dan1w]

Just in case anyone else is ever seeking an answer for a similar question, I went ahead with the original idea and it works fine, I didn't even need to adjust the temp knob on the thermostatic mixer.

I left the crappy flexi hose in situ on the cold feed as a sort of flow reducer and because I'm lazy. I had to usebthe cold feed to help prime the hot by slowly turning the mixer from cold to hot. Now I can have a decent shower.