What does a relay do on a boiler?

Regarding this, what does a relay do on a furnace?

The electric relay in a furnace is responsible for relaying electrical power to the heating elements and the blower motor inside the furnace cabinet. The heating elements provide the heat to warm the air, and the blower motor sends this warmed air through the ductwork.

Secondly, how do you know if Aquastat is bad? Check circulator terminals: If voltage is everywhere it should be and a relay is clicking constantly OR a transformer is buzzing, those component(s) are probably bad. Separately at RELAY CONTROL SWITCHES using a water pump pressure control switch as an example we discuss clicking and chattering relay switches.

Besides, how does an Aquastat relay work?

The aquastat monitors the water temperature in the boiler through the use of a copper bulb that stays in contact with the water inside the boiler and that shuts down the boiler automatically if the low- or high-temperature set point is exceeded.

Is it normal for relays to get hot?

Warm yes, hot no. The prior descriptions are correct - a relay coil is typically an electromagnet. Again to simplify: Heat dissipation (i.e. temperature rise) is an indication of electrical power. Too much heat = too much resistance and/or or too much current and/or too much voltage, since P=IV & V=IR.