My baseboards sound like a babbling brook is running through my house and I've been told that's because there is a lot of air in the system. I've watched a bunch of videos on how to bleed a system and I understand the basic principles (I think) but I wasn't sure how to do it exactly on my own system.

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The basic steps as I understand it are:

1. Attach a hose to the spigot (labeled with a red arrow) and run it to a sink/drain.
2. Close all of the zones but one (I only have one zone but it feeds four different parts of my house).
3. Open the water supply valve to add cold water into the system (I don't have anything that automatically adds water to the system like I've seen on newer systems).
4. Let water run through the system until all the air bubbles are removed (I was told it can take 5-10 minutes).
5. Turn the water supply valve off, close the zone I was working on, and move to the next zone.
6. Repeat steps 3-5 until all four are done.

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Is this correct? I'm assuming I want the system off for this while I'm flushing water? How do I know what the pressure will be when I turn the system back on again? And what is the spigot on the bottom below the circulator pump for?

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Thanks so much for your help; I'm just trying to become a little more handy around the house and a little less dependent on others.

https://imgur.com/a/Z8K7Bzz