As lindnova said, it depends on the demand from the water/air heat exchanger as well as your incoming water temperature, pipe sizing, and flow speed. Properly sized, you should have at least 160 degrees returning to the boiler if you're starting with 180 coming out of it. The higher return temperature, the more efficient things will run since the temperature change will be not as drastic. If you want to radiantly heat your floor above a crawlspace, use a side loop off of the main loop with it's own tiny circulator to control the flow with a thermostat and then insulate between the pipe and the cold side of the crawlspace. Don't forget your vapor barrier either. A metal heat transfer plate to hold the pipe up against the floor will help you maintain a more even heat since PEX doesn't like transferring heat very well.