Yeah I am sure that something like that could be a possibility, not overly experienced with hydronics either, but still not incredibly difficult for the op to rule out, just feel the pipe.
If the flow control is set up as Fail Open then I would think so. If I were installing this system I would set it up as such so I could still have heat with a failure...but then again on a central domestic water it could be days or weeks before you would notice a failure with the hydro-air set up. Same deal on heater control valaves on many automotive hvac systems. They are usually Fail Open. You would still have cooling but it would not operate as well. I wonder how many auto techs misdiagnose this and replace big ticket items or just add refrigerant? Then you have the shady auto techs that know good and well what the problem is and trump up the diagnosis from a $15 part to a compressor replacement.