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Let me offer some other thoughts.

Most engineers have hissing vents.

Systems are all different, and each radiator is usually a different size, is at a different temperature and has different vent openings. Some radiators' steam is more wet than others. And the vents are at all different temperatures.

A large radiator with a small vent may never see its vent close. Its steam will have condensed and cooled before it reaches the vent. That vent may stay open; it may only see steam on the coldest days. A small radiator's large vent may get hit with steam and close its vent quickly every time it heats.

The vent condition, orifice size, steam moisture content, radiator location and condensate production will all affect the sputtering of vents. You have to work with the system and find the venting arrangement that gives you the best performance in the typical conditions.

This is where the "art of steam heating" comes into play. Good engineering isn't just math and charts. Anyone can do that.

Size your vents strictly from a chart and they will probably deliver some surprises.

The most important consideration for having clean water is to stop adding fresh water! Fresh water has plenty of oxygen which causes corrosion and makes rusty/muddy water. It sounds counterintuitive but the more clean water you add, the dirtier your boiler will be.

Find and fix those leaks! Before I knew better I had a leaky radiator. It was on the top side between sections so it only leaked steam so I didn't really care as it didn't make a mess. During the cold spells I was adding a couple gallons of makeup water weekly. I thought this was normal. I always had muddy water no matter how often I flushed it.

I fixed that radiator and now I only have to add makeup water once or twice per season!

RectorSeal 8-way also helps considerably. It has some magic chemicals in there but it is really good at adjusting the pH of the water. A pH of around 10 will essentially stop corrosion inside the boiler so the water stays clean much longer.

Every now and then I drain a bit out of the boiler drain and float LWCO and it comes out completely clean. No sediment at all.

Just be aware that the directions on the 8-way are more for commercial boilers. Use 0.75-1oz per gallon of water in your boiler. The 8-way will also do some cleaning so you may find your boiler water getting cloudy. That is all the scale and junk that has built up over time. After the first year or two of using 8-way in my boiler it stays nice and clean and clear.

Diablo Cast Iron toothed blades are the best thing that happened to me…ever...dont tell my wife.

Float type have a brass float with alcohol bottle inside , when steam comes in contact with the bottle the alcohol inside will flash and push out the bottom and seal the vent . If the system is flooding the water level will lift the float and seal the vent .. You can hear the float rattle inside when handling .. The bimetal style will only close with steam , not rising water .. The bimetal will sometimes act like a reed and vibrate into noise ..

I respect you based on your cool vent testing posts, but here you give no reasons for your argument. You just say "it isn't properly functioning". I'm sorry, but appeals to authority are not a strong argument for me. I have heard "I've been doing this for 30 years" way too many times from people who shouldn't have been LOL

Here are my reasons in case there are any you find fault with:

• when a system is running for a reasonable amount of time, it is pretty much full of steam with some small amount of air remaining (in a not-completely-steam-filled radiator for example.)
• When the steam is no longer produced, regardless of the pressure during running time, the pressure will reduce as the steam collapses. Until it gets to 0 gauge pressure there will be no sucking sound (I think we agree to this point), no matter the initial pressure
• OK now the relevant part: The remaining steam will collapse and there will be a significant vacuum that must eventually return back to atmospheric pressure (0 gauge)
• So air must enter the system
• It can only enter via the main vent(s) and/or some number of radiator vents (I'm discounting the vacuum breaker since that is very rare on a residential system in my experience)
• Some of those vents might open sooner than others and it's not always possible to know which will open first
• Some of those vents might be more noisy or be in a more noticeable area of the house, resulting in the noise which is the topic of this thread
• None of the above indicates any problem with the system (I'm not counting running pressure of the system, which we all agree should be low, but I'm not counting it because it is not relevant to the amount of air that must be inhaled into the system every time the boiler stops making steam)

First, you don't really need clean water. It's fine to let it get a little brown. Toward the end of the heating season you can add a couple gallons and flush one out to reduce sediment buildup.

But the key is to increase the pH of the boiler water. See my video:

https://youtu.be/Bx8NryCUkng