There must be one, or two, somewhere. They control the flow of water through the CH and HW circuits so they only get heated when required. If you dont have them, both circuits would be heated whenever the boiler was alight.I'm not aware of any motorized valves.
How were hot water times/temperatures controlled before the Usense was installed? There must have been valves to stop CH and HW heating up when not required?
With an Opentherm system it's normal to have a diverter valve which allows either the HW or the CH to be heated, not both at the same time. The reason for this is that HW required the flow temperature to be constant at about 70C, while CH will vary the temperature as required. A diverter valve may have been installed in the boiler (its a Viessmann option); check with the installer.
As I said, the Usense is designed for use with some Baxi combi boilers. Like all combis, they have a diverter valve built in and automatically disconnect Opentherm when a tap is turned on, so the boiler runs at max output. The HW feature on the Usense is there for preheating only.