why not Thermo valves with Webasto water heater

[Falcoron]

REF; Webasto top -C water heater.
Is there a reason why Webasto state in the install manual why thermostatic valves cannot be used on the radiator's fed by this heater?
I will follow the document but just wondering why this would cause any issues.
Plus, does this means all rads. are at full tilt all the time?

Sorry if i am missing something just cant get my head around why this is.
cheers

 

[snowbird30ds]

You would probably need one open all the time so the flow is not fully shut off, I believe home central heating is the same.