UFH extension, and existing Circulating Pump running?

Hi folks,
I have an extension in which there is fitted underfloor heating, connected to it's own pump & manifold and room thermostat. The old part of the house has regular radiator heating and hot water tank with it's own circulating pump and thermostats. I have noticed that the circulating pump is also activated whenever the UFH zone is activated. Is that normal?

The valve for hot-water and the rads are closed, so the pump is not doing any useful work, just moving water from the boiler and back again. It's quite noisy though!

The UFH pump is running OK and the floor heats up fine, so that part seems good.

I just don't understand if it's normal for the primary circulating pump to be running. I cannot find any wiring diagram on the internet to help, but AFAICT whenever the boiler is powered up it sends power to the pump irrespective of the stats. Can anyone please help demystify?

Yes, both the UFH manifold pump and the main CH pump should run. The manifold pump is just there to circulate water around the UFH pipes, the CH pump circulates the heated water from the boiler to the manifold. If it's noisy, get a new one that can modulate its speed to the load on the system.

Makes sense - thanks!