i think i understandn now what you are trying to do, but by circulating your baseboard water through the HX (if i have the picture in my head correctly) you have in fact created a "closed loop" system for your baseboard, and with that you will be needing an expansion tank for any temp changes or pressure changes
I think you can elimenate the HX all together and just tee off your furnace feed to your baseboard, control this with a zone valve on your thermostat and also forget the pump (for now) your furnace pump will probably handle it all, if not you will need to have the zone pump come on with zone valve
you dont want the pump running and not moving water, this would burn out the pump