We've been having a ton of water hammer with our gas fired steam system with the start of the heating season here in NJ, more often and more violent than I recall the last 2 winters we've lived in this house. It seems to come from near/around the pipes by the boiler, as oppose to the radiators themselves. I had the boiler cleaned/serviced before the season began and the tech mentioned that it was very dirty.

I noticed today when the system was fired that the water in the sight glass dropped below visual level, the glass was empty! I turned down the thermostat to have the system stop firing and water returned to the sight glass where I had set it (about half way up).

Some water loss in the sight glass while the system is on is expected, but all of it?? This can't be normal. What could cause this and how do I correct it? I know that a boiler fired with too little water is dangerous.

In addition, as I was right next to the boiler while fired, it seemed/sounded like the vertical pipe(s) coming from the top of the boiler which lead to the mains, was filled with gurgling/boiling water. Is this possible???

Btw, I do have a low water cut off and automatic water feed. The auto water feed light turned yellow when the water in the sight glass dropped, but did not feed any water.

 

Sounds as if the boiler water needs to be skimmed,others will jump in with more advice.

 

Yep, she needs skimmed. You have oil on the surface of your water. This is the number one reason for unstable water levels. See the link below.
Is this a new installation or was there some re-piping done recently?
Technical Menu
Follow the link down the right column on skimming. Also see the FAQ link for more info on steam.