There may bleeder valves on the line somewhere. Have to follow the pipes.

 

5 Steps to Healthy Steam Radiators

5 Steps to Healthy Steam RadiatorsA simple checklist for solving five common problems with your steam radiatorThe simple anatomy of a radiator* This is from an

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If image radiator, you should add a vent even if not steam. And somewhere in the water pipe, an air scoop. Floor baseboard with fins may not have any vent though it is good to have a vent for removing air.

Heating Help: The Wall

The #1 Online Resource for HVAC Answers. Ask and answer questions about HVAC systems, both old and new.

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Also try above forum.
First gather as much info about your system. Boiler model. Hot water/steam. Photos. What kind of radiator. Sight glass or pressure/temp gauge on the boiler. How water is supplied.

Above is floor baseboard with bleed nipple. Name? Not a pro, just years of looking.

 

About the floor baseboard bleed valve, it will work better if the finned pipe in the radiator is not quite level with the bleed valve at the high end There may be some vertical wiggle room in the pipe runs below the floor to adjust the level.

If you don't have bleed valves on baseboard radiators then you have to use the brute force method, isolating the zones and running a continuous stream of tap water turned on high through the zones one at a time (basement drain valves needed). It may take a few minutes to be reasonably sure the water flow has become established and bubble free. Then go on to the next zone.

Steam radiator air valves might not work well for FHW systems since the latter run on higher pressure and break the valve mechanism..