Your Vote? Baseboard swivel vents open or closed?

You need air rising thru the radiator to create convection currents and circulation in the room. If not it will be warm near the cover and no where else.

If the radiator gets HOT from closed covers, the water temp rises in the boiler and it will shut down on it's normal temp control so you accomplish nothing.

Normally the burner cuts out at say 180 F and in at 140-160 F. W/O the boiler dissipating heat thru the rads it just gets to the 180 F sooner and cycles off the burner. Excess starting and stopping wastes a lot of gas and air up the chimney. Boilers are sized to run steady (er) instead of lots of stop/starting.