8 steam rules every tech should know

Good summary! I especially like the comparison with air conditioning — that may help some of the younger techs. see what is happening — with one exception; as you note, steam systems breathe, while sir conditioning and heat pumps, although they work on the same principle of latent heat, do NOT breathe!

Most — say 99.9% — steam systems have vents to allow air in the system to escape to the atmosphere as steam is raised by the boiler. Those same vents allow air back into the system when the boiler shuts off and the steam condenses (which it does very very rapidly!). That's intentional. There are a very few steam systems which are intended to not have that happen — to have lower pressure (a "vacuum") when not steaming — but they are very rare indeed.

There are a lot of ways air can, and does, get into hot water — hydronic — systems. It's not supposed to, and if the system is designed and installed well it won't happen. But… not all systems are. Makeup water — which always contains some dissolved air — is one way. Operating the system at too low a static pressure, or with the point of no pressure change on the outlet of a circulating pump, may result in portions of the system being below atmospheric pressure and then even a slight leak will allow air in (watch out for this one: vacuum leaks don't always leak under pressure!).