Your boiler heats its water to 180 degrees.

That's hot.

If it's pumping out heat to your house, you need that hot water running through your baseboard to radiate enough of a comfortable temperature. 

But this causes heat expansion, and creates air in a hydronic system, which you do not want.

To catch air and pull it from your system, the air manifold must be installed properly.

Honeywell makes a product called a Spiral vent that pulls micro bubbles out of the system and deposits them into an expansion tank, keeping them out of the loop and your system running air tight.

If the loop isnt installed right, then on cold nights when your boiler runs longer, it creates more air, more heat expansion with no way to be pulled out of the system.

Call a professional to test this, because when you need your boiler the most, it may konk out leaving you in the cold.