Rushing Air Vent, sometimes

I have a feeling you're hearing escaping steam, not air. Replace the rad vent. But also check your main vents, at least to see if they're properly sized.

Get a cheap stethoscope to check main vents.

He cant hear he said bc of basement noises. As someone who is deaf in one ear, I often run into this problem.

Right. We don't expect it to typically be hot when it's first coming out. At the start of a call for heat, it's all cool or maybe warm air escaping the main vent. Then when the steam reaches the main vent and heats it to "steam hot" the main vent should close and allow nothing out for the remainder of the call for heat.

This is why I suggested "following the steam" along your main by touch so you can know when it's getting to your main vent(s) and you can see if they are allowing air to pass and then closing to steam like they should.

The same will happen with radiator vents, but often they will not even see steam on a normal call for heat because the thermostat is often satisfied before some or all radiators are completely full of steam with the radiator vents closing.

(I edited this post to add "main" to everywhere I was talking about main vents because I had some confusion at times in your thread about when you were talking about main vents vs radiator vents)

How long are those mains shown by the black lines?

Agree with @Steamhead. You could also get a Ventrite 1 for that bathroom radiator and slow it down a bit but if you are wanting more heat in that bathroom then that wouldn't be desirable.

Seems like a pretty small system. Can I ask what the EDR steam rating is on the boiler (or just the boiler model #)? Have you ever totaled up your radiator EDR's?

Yes your radiators should total roughly equal or slightly less than 358. If you are significantly oversized it can make keeping vents quiet a bit more of a challenge. I'd start by going to Gorton #1's for the main vents, however, I'm still a little confused by the fact that the air rush issue on that bathroom radiator only happens in milder weather and not when it is colder. Usually the difference in warmer weather is that the boiler is coming on less frequently and therefore more often the pipes are quite cold at start of cycle. Under this condition the main vents actually come into play less than they do when the pipes are warm since the steam condenses quite a bit warming the pipes and thus advances more slowly. This can create more condensate so I would check the pitches of all your mains and definitely your return lines. You want to be sure there is no place condensate could pool in a low spot and start blocking air that is trying to escape the main vents at the end of the returns. Do you notice any hammer?

@ethicalpaul My point was that there is more total condensate produced from the time the boiler starts steaming to when steam first reaches the radiators in a cold system than a warm one due to the boiler running longer to heat those pipes, and he is heating up his entire returns as well.

@klmcc1894I'm seeing a disconnect on the description of the problem here since initially it sounded like the issue only occurred during warmer weather when the boiler might be running only once every 10 hours. Later when monitoring you said it did not occur on a cold cycle . Also, you said the water level drops more when the problem is occurring. I assume this is not a permanent drop resulting in need to add water by hand or feeder since that could only be a leak or steam escaping the system through vents, valves or the boiler itself up the chimney. That would also seem odd to be more of a warm weather issue than a cold weather one. If a temporary drop then more water must be out in the system or the return is slowed somehow.

Just to check again when you hear this air rush noise is the radiator filled (all sections) or not? Is vent hot or cold? Is the boiler still on or has it shut off?

That simplifies things. Your vent on that radiator is shot (stuck open). Get a new vent for it. You have Ventrite #1's everywhere else so that would be a good choice. Should reduce your water loss as well.