"John Aston" wrote in message
...

Page 39 in Viessmann's Vitodens 200 manual http://tinyurl.com/6cqtt shows
a
mixing valve between the pump and the boiler. With a low load, the rads
could be off and the flow from the boiler could be very low. Why would
this
arrangement be acceptable without a low loss header?


Re-reading that manual page, the design assumes that the flow though the
radiators will be greater than the flow through the UFH.