Long 15mm pipe runs

Probably a silly question.

I've got some very long, unnecessarily long, 15mm pipe runs around my house, am I right that they will reduce the flow rate at the taps at the other end due to friction? I understand the pressure will be more or less unchanged because of physics but the flow rate isn't the same thing.

One of those pipes feeds my combi boiler, would that reduce the hot water flow rate as well? Edit: for clarity this means the cold water feed for the boiler is 15mm and roughly 20m longer than a direct shorter run could be.

Finally, would going to fatter pipes help at all?

 

No witha combi boiler leave the pipes at 15 mm increasing will just cause problems, and will increase the time lag to get HW at the taps

 

To the boiler, not from the boiler. I could have been clearer.

 

15mm will be ample, just modify the route for tidiness sake.