Potterton Performa 24 hot water issue

[Claireh]

Hi, myself and my plumber are confused with a problem with the Potterton Performa 24.
Our tenant had an issue with heating & hot water, the water to water heat exchanger was replaced, change over valve replaced and emittance valve. heating is now all ok but the hot water is a mystery!
Downstairs the kitchen sink has a flow rate of 10ltrs per minute and is at 47 degrees. Upstairs in the bathroom the sink has a flow rate of 8 ltrs per minute and is 39 degrees to start with and then gets cooler. The bath has a flow rate of 10 ltrs per minute but the water is barely luke warm, even if turned down to a trickle.
Any ideas why the hot water is fine downstairs but not upstairs? anything else we can try?
Many thanks

 

[petej]

I once had a similar thing and it turned out to be a shower mixer that was passing cold water into the hot system I only found out when I turned off the supply to a multipoint water heater to change a hot tap washer and cold water was still present at the hot tap !. Is there a temperature mixing valve fitted under the bath (These often go faulty) or a shower mixer over the bath ? Perhaps the problem it is more noticeable near the fault.