boiler downstairs, warm water upstairs only if the tap is throttled

[alistair.samuelson]

Moved into new house, boiler is fairly new - 2-3 years old. Downstairs its possible to get extremely hot water of the kitchen tap but upstairs, the only way to get fairly hot water is to turn the tap on but only partially. Not enough and it seems like the water doesn't get heated at all. Too much and its only tepid. Its the same in the bathroom for the hot tap at the sink and the shower.

The cold water into the boiler is fed directly from mains.

Any ideas on how to solve this?

[Cod Roe]

What is the flow rate from the different taps, time how long it takes to fill a bucket. Are they similar? What are your pipe sizes, is it all

15mm or does it go up to 22mm? If you wait long enough with the bathroom tap on full does it eventually get hot (talking minutes here)?