Another one claimed by greed....

[Manalto]

I also have them at the manifolds for my in-floor radiant for the kitchen and master bath.

Radiant is wonderful.

[1918ColonialRevival]

Yes, it's called a bleed valve. Each radiator will have one at the highest point. You'll need to bleed the air out of each radiator, starting with the ones that are highest and furthest away from the hot water source.

One thing to add to this. Make sure the bleed valves are securely tightened into each radiator. You may find you have a couple that need to be replaced, which isn't difficult.

Balancing the system is a bit of an art. Expect to spend at least a couple of hours doing it.

James, don't worry - after you do it a couple of times, you get the hang of it!

[mjt]

Yes, it's called a bleed valve. Each radiator will have one at the highest point. You'll need to bleed the air out of each radiator, starting with the ones that are highest and furthest away from the hot water source.

Blogpost explaining this and showing me bleeding the air out of a couple of radiators.