Spoke to Vaillant Technical this morning, helpful but really couldn't throw any light on the problem but did say D.015, is the actual pump speed.
I would suggest changing to delta P (not sure it thats 2 in D.170??) but take a snaphot as above in dT mode, take a set of readings and try a few different dPs say 400mbar max and 100mbar, minimum and see does d.015 pump speed change and the dT change.
These are the different options, (D.170), which one to use??
0: Without bypass, Δp-const.
1: W/o bypass, Δp-const.,kick
2: Bypass, Δp-const.
3: Spread ΔT
4: Fixed pump stage
Its becoming clearer now, at last! option 2 where you can set the pump head between 100&400mbar is the pump in constant pressure,CP,mode
Option 4 where you can set the pump between 100&400mbar is the pump in constant curve, CC, mode.
D.170 setting Description
2: Bypass, Δp-const.
Constant pressure pump control type
The internal heating pump works in
constant pressure in the heating installation.
The pressure level can be
selected between 100 and 400 mbar
via D.171.
The internal circulation valve is only
opened according to requirements in
order to maintain minimum circulation.
If required, a maximum opening
pressure can be set via D.174.
3: Spread ΔT
Spread pump control type
The internal heating pump works at
a temperature spread between the
flow and return temperature. The target
is the spread level of the heating
system and not the pressure supply.
The target value for the spread
is specified via D.172.
The work area of the heating pump
for the spread regulation can be set
via D.173 and D.174.
4: Fixed pump stage
Pump curve pump control
The internal pump works on a selected
pump curve. The volume flow
and the pressure for the heating
system result via the pump curve
and the system resistance that is
present. The target value for the
pump curve is specified via D.175.
There is no control system for pressure
or temperature; the pump works
on a fixed pump curve.
This pump mode is preferred for
homogeneous heat transfer if a low
loss header, decoupling module,
hydraulic cascading and a buffer
cylinder is installed.
Heating system with radiators
Underfloor heating systems
Heating system