Boiler problem

[Jayd1974]

Hi I am working on a heatline 24c combi boiler I have hot water and heating when I turn hot tap on I get hot water but if I open tap upstairs at the same time I don't get any water I can only get hot water out of 1 outlet a at a time I have clean plate heat exchanger check Diverter valve pins move ok I have check inlet water filter on hydro block have changed flow turbine and water pressure main is ok can anyone help with advice it's only 2 bed house with 4 rads 24 kW should be fine to

 

[Alex10]

Measure your cold mains flow rate as close to water entry as possible.

Imagine you measure 8 lpm you run you tap up stairs at 7 lpm you will only have 1 lpm left to come out of your second outlet.