low pressure

[Gary68]

Hi,
I have 15mm pipework to the downstairs bathroom. We have a mixer tap/shower on the bath but very low pressure on the hot tap, the cold is mains fed. The pressure on the hot is so low that we cannot use the shower. Any ideas how we can improve the pressure?
Thanks

 

[Burnerman]

The cold water pressure is forcing the hot water out of the way - both feeds need to be at the same pressure, so ideally the cold water feed should be gravity too. 22mm pipe work would help matters as well, if possible.
John

 

[Gary68]

changing the pipes is not an option, the walls are newly tiled. Is the only other option to change to a combi boiler?

 

[Burnerman]

It sounds a bit drastic - couldn't you get into the 15mm mains feed, cap it and reconnect into a gravity pipe from the loft tank?
John