I have installed a underfloor heating system that is running off the tankless hot water system. The heating box is of the closed loop type.
At the moment I have 5 zones being used from the manifold.
The loops are from 200ft to 300ft. Of 1/2in pex.
When running one zone, the shortest, the flow meter shows just a little bit better than 1/2 GPM.
This is measured by the flow meter on the Watts manifold.
When opening up another zone the flow goes down to half or less.
I am running about 30psi at the moment, when at first I was at about 15psi and the flow was quite a bit less.
If I open any more zones I don't think I will have any flow at all.
The pump is an Armstrong E9B.
Flow range: 0 to 38gpm
Head range: 0 to 42.4ft.

Who designed the system?
The system designer is responsible for this problem.
If you just put this together by yourself and did not design the sustem then you will have a problem especially when you run on the ragged edge of 30 PSI as it is to high.
Also your loop size is to large. It is always recommended to break large loops up to multiple smaller size.

Do a Google for your type of heating system for further info. Google is loaded with info.