Isolation relay for Ecobee smart thermostat: NC or NO?
You're correct, you'll use the normally open (NO) terminal. When the thermostat calls for heat, it will energize the relay coil. The energized coil will close the contact in the relay between common (C, COM, etc.) and normally open (NO).  

When the coil is not energized, the contact connects the common (C) terminal to the normally closed (NC) terminal. When the coil is energized, the contact connects the common (C) terminal to the normally open (NO) terminal.

NOTES:   


When looking at schematics/wiring diagrams, all the components will be shown in the "normal state" ("shelf state").  That is to say, they'll be shown as if they were laying on a storage shelf, with no power sources connected to them.


The relay you linked to should work, however, it's probably a bit overkill.  If you're only switching the thermostat terminals from a boiler, you're probably talking about milliamps @ 24 VAC. The relay in the link can switch 10 amps @ 240 VAC.