Newly installed UFH, flow rates drop after a bit of use - what does this mean?

[ADringer]

We've have wet UFH installed and just started to run it now the screed is dry enough. I've set all the flow rates for the zones according to the design, but when I come back after a short time two of them have dropped back to 0. I can open the flow gauge further and get it back up, but then drops again shortly.

 

What does this mean? Air in the system? A leak on the loop?

 

Thanks.

[JohnMo]

Good chance it's air.  You will need to bleed the system.

[ADringer]

1 minute ago, JohnMo said:

Good chance it's air.  You will need to bleed the system.

Cool, that's what I was hoping. Thing is I just found a leak in one loop where the builders had retroactively installed perimeter insulation and must have nicked a pipe 🙄. Was hoping that these other loops were just air.