UFH Mixer Valve Query

[calama]

Hi I am wondering if the mixer valve would normally affect the flow rate? If I set the temperature to anything above 45deg then the flow meters barely register. But if I set it to minimum then I get 2lt/min at the flow meters. I can't get the correct temperature at the right flow rate to get a large room heated up. 

I would expect the mixer would just mix the hot input and cold return without affecting the flow...am I wrong or is it defective?

Thanks for any advice.