The question is so vague that I'm a little reluctant to wade in here, but without knowing what control function you are trying to achieve here (what part of the system is the aquastat even sensing the temp of?), it's hard to steer you. Got a system schematic of what you are trying to do?
There are a lot of badly designed radiant floor systems using tankless HW heaters out there, and some sorta-OK versions. Is this something you cooked up yourself? Is this a suspended-tube, slab, staple-up, or some other type of radiant floor?