I raised the question of how to control flow rates on Vaillant ASHP at Introducing HeatpumpMonitor.org - a public dashboard of heat pump performance - #28 by Zarch which @Zarch suggested was more approriate here. Mick pointed out the relevant installer level configuration settings of the VWZ AI controller at:
Conf. heat. build. pump
Conf. cool. build. pump
Conf. DHW. build. pump
On my system this controller is built into the uniTower, rather than being a separate unit, but settings are identical. Values for the settings are “Auto” or manual from 50-100%PWM: %PWM is % Pulse Width Modulation which is the method used to control the pump motor voltage (hence power). Defaults on the uniTower are “Auto” for heat and cool but 65% for DHW.
There is little published by Vaillant on these setting butI did find some discussion ion Page 30 of https://www.vaillant.co.uk/downloads/aproducts/renewables-1/flexotherm/flexotherm-400vinstallation-and-maintenance-instructions-919862.pdf which is a GSHP. Quote for “automatic mode” description is:
At the factory, the nominal flow is automatically achieved by volume flow regulation. This volume flow regulation allows for efficient operation of the building circuit pump because the pump speed is adjusted to the hydraulic resistance of thesystem. Vaillant recommends that you retain this setting.
So the reason for my wondering about this was that my 3.5kW Arotherm+ is installed with a secondary pump in series on the flow of the external unit. Manual monitoring of the rates shows that during heating cycles the flow rate is at the maximum 860l/hr that the 3.5kW boiler plate specifications allow (currently target FlowTemp=32C). During DHW, the flow rates are 1186l/m (target 50+15=65C) which is way beyond the maximum flow rate specified. As Conf. DHW. build. pump is 100%PWM on my system it seems like there may be need for some playing around!