This is the kind i have on my gas furnace.
Automatic air vent valve leaking water.
Air vents automatic air vents should be isolated after original fill of closed hydronic systems.
Air or other gas may collect within plumbing for water delivery systems to taps and basins particularly with good main supply pressure this air is usually flushed through with the water flow and does not cause a problem although in some hot water.
Float type automatic vents on hot water heating systems are normally left open to automatically purge air from the system.
There is water coming out of the top brass venting thing above the tank.
Based on tapping the expansion tank it does not seem waterlogged bottom 2 3rd of tank sounds hollow or empty.
Alot of times a leak can be traced back to the relief valve opening.
The taco automatic coin vent installed on water or steam radiators the taco automatic coin vent is the ideal air purging device.
Taco automatic air vents include.
Leaky hot water heating system vent repairs how to repair a leaky hot water air vent or bleeder valve.
If you still have air in your system after that point you have a system leak which is causing the fill valve to add water and entrained air.
Expansion tank air valve vs vent leaks reader question.
The coin vent features special hygroscopic cellulose discs in the valve body that when dry allow air to be released from the system.
Here we explain how to repair or replace both manual heating system air bleed valves and automatic float type air vents that are leaky.
An automatic bleeding valve or air release valve arv is a plumbing valve used to automatically release trapped air from a heating system.
I found information on the expansion tank noted in the subject line on your web page.
Guide to repairing an auto air vent aav.