The Caleffi technical publications are excellent free resources that every one in hydronics should follow to learn or be reminded about better ways to build heat delivery systems. The Heatspring free courses are a must see, plus you get a certificate.
Low Temperature Heating Fundamentals - Heatspring
https://www.heatspring.com/courses/the-importance-of-low-temperature-distribution-systems
Magazine:
http://www.caleffi.com/usa/en-us/technical-magazine
Video Webinars
https://www.youtube.com/playlist?list=PLuuV0ELkYb5VE0I4evUZ30b5U78CRlRdg
Taco Flo-Pro Articles and Videos
http://flopro.taco-hvac.com