Bleeding a baseboard hot water heating system

Well, hopefully your baseboard has a coin vent or keyed vent at each room or drop into the floor(check under end covers)...here, simply fill the boiler and bleed through the vents until air is out of the system.

If this is not the case then hopefully you will find a shutoff above and below (or shutoff flanges..look close, actually on the side of the circulator flanges) the circulator you are going to replace (if this is case...super easy to fix, just shut shutoffs...replace circulator..slightly tighten nuts/bolts and slowly open shutoffs to release any air from installation..then tighten full on, then open valves...no need to bleed further!)....IF you find the shutoffs as described you will probably also find a boiler drain above the circulator. Here, you would replace the circulator then you would keep the shutoffs shut and fill the boiler from the fill valve (most have a fast fill feature...a lever that allows quick fill...but be careful not to leave unattended..can overfill/overpressurize and cause the boiler relief valve to go off) and then you would attach a hose to the boiler drain and purge the system of air (BUT only if you think you need to)...this takes some time...do not rush it...throttle up...throttle down until you can... Say... Take a full bucket of water and with the hose you are purging air with set into it...get little or no air when the hose is submerged for a minute or so! Then shut the water fill valve (P.S.I. On boiler should be at 12-15 P.S.I. Maximum when finished), turn on the shutoffs and then turn on the boiler.

Further, if there are vents and the boiler drain and shutoffs mentioned... Use both to rid the system of all air!

If there is not...well I recommend installing the setup described...just cut into pipe...install shutoffs above and below circulator (super easy replacement of circulator in the future..so you won't have to go through this in the future) and install a boiler drain above the top of the circulator and follow instructions as presented!

If I can help more just let me know...IF my answer was helpful, please RATE MY ANSWER below! Thank you!

Excellent answer, very helpful.