Some tankless water heater has a minimum and maximum circulator flow rate however I have not seen this addressed in any radiant heat open or closed designs. Why? AOsmith has this note in their owners manual. "The recirculation pump is to provide no less than 2 GPM (7.5 L/min) and no more than 4 GPM (15 L/min) through each activated unit in the system."

I am guessing and not sure of your exact idea but I think the unit overheats and short cycles on low flow rates and can't maintain temp on the higher flow rates.

I tried putting a Bosch tankless on radiant floor system 15 years ago and it constantly short cycled and wore out forcing me to switch back to using tank systems.

Haven't gone back to tankless sense. Tankless creates a dimension of complexity when used for more then just showers and sinks.

Radient and instant hot water circuit make tankless difficult. Sending hot water back to a tankless sometimes freaks them out.

Domestic water heaters (tank or tankless) are not designed or certified for living space heating applications such as hydronic radiant heating. And, as such, many building codes and many building inspectors don't allow them to be used for living space heating applications. Of course, that doesn't stop some folks from using them anyhow for living space heating applications.

I believe your quote is talking about a DHW recirculation loop, where the pump circulates hot water to far away fixtures. Nothing to do with heating.

"I have not seen this addressed in any radiant heat open or closed designs."

"The recirculation pump is to provide no less than 2 GPM (7.5 L/min) and no more than 4 GPM (15 L/min) through each activated unit in the system." This is a strange statement? Yes, there are min and max flow rate requirements, but you wouldn't want to use a tankless water heater for a DHW recirculation loop either...only a tank water heater.

And you wouldn't want an "open" hydronic radiant heating system either. So you might want to rethink what you are planning.