Different hot/cold water pressure

[larold]

I'm a bit of a newbie at diy, especially plumbing but am working on my bathroom and learning as I go along. I'm installing a new bath and shower, and have been googling about pipes etc. Prior to purchasing a new shower, I bought a water pressure tester guage. It measured 6.75 bar on the cold tap and 3.1 on the hot tap. From what I can see I have an unvented system, but if I understood what I have read on the internet correctly, shouldnt my hot and cold water pressure be the same if it is an unvented system? Does it mean there is a problem?
I also have a question about the pipes. It seems the main water pipes are 22mm and the feeder pipes to the shower, bath, sink etc are 15mm. What is the reason for this? What would happen if I used 22mm pipe to an appliance rather than 15mm? Would it give me greater pressure? Or perhaps too much?

 

[cso30]

Hi,

An unvented system will have a pressure reducing valve on the cold main feed to the cylinder.

Feeding an appliance with 22mm feeds won't increase the pressure but will increase the flow. I would say run the mains in 22mm and feed the appliances in 15mm, just check the size of the shower connections they should be 15mm but maybe 22mm.

 

[swidders]

If you're concerned about the differential pressures on the hot and cold runs, you could insert a pressure reducing valve near the whole house stopcock.