bungie
Dump the duo valve, use a ball valve as per the instructions. you are restricting the flow over the thermostatic coupling. Sounds like I might know summit LOL …. remove the duo valve and fit a ball valve 
DISCLAIMER
All advice is given with-out seeing the job, and hence all advice MUST be taken as advice with limited knowledge on the exact situation. NO responsibility can or will be taken. And yes, I am a licensed Plumber and Drainer with my own business in Brisbane Australia
