"John Aston" wrote in message
...

"IMM" wrote in message
...

snip

With condensing boilers headers reduce efficiency. With a condenser you
don't want very hot water feeding right back into the return. This
raises
the return temp which reduces efficiency. You want the return temp as
low
as
possible.

snip

Won't a modulating boiler adjust its
flow temperature and velocity to reduce
the return temperature though? I thought
that that was the purpose of the
modulating control.

Firstly there is two types of modulating control. Modulation to maintain a
flow set point, as on most combi's. If set to 70C when approaching 70C the
burner lowers to keep the heat moving out. The second is load compensation,
which senses the flow and return temps. When the temps close up it sees the
building is getting up to temp and lowers the flow temp and hence the return
temp. They are very different.

The Siemens BLC1.A controller I gave details can be set from load
compensation to weather compensation. It does it externally to the boiler
by switching with integral boiler anti-cycle control.

Velocity? Pump speed. Some do, but these are on very expensive boilers. In
some cases this may be a disadvantage as the pump when modulated down may
not reach the far points of a largish system.