"Aidan" wrote in message
om...
"John Aston" wrote in message
t...

Doesn't an automatic bypass valve stop this problem?

No. I don't think I was clear.

The first paragraph referred to a heating system with a mixing valve,
but only one pump, i.e., the way NOT to do it. The mixing valve was
between the pump and the boiler. At low loads, most of the flow
produced by the pump was re-circulated around the heating circuit
leaving only a small amount to return to the boiler. The boiler did
not like this.


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?