Still wheezing

Patrick North

Hi all,
Even after adding new main vents (thanks Steamhead!) and replacing radiator vents with new Hoffman 1A's, it still seems as though my rads are struggling to expell air. WGSH notes that a system should be all but silent in operation, but when the boiler kicks on my rad vents sound like a pressure cooker ready to blow. On one rad, even the valve hisses and struggles a bit. System pressure is set nice and low (one # give or take), though I don't have a fine level gauge to know exactly how low.
Some additional details that may or may not be important:
It's a three story house with two boilers- one serves the second and third floors.
Both boilers are significantly oversized.

Any ideas? I'm thinking the risers to the second/third floors may need to be vented as they aren't attached to any radiation on the first floor and therefore aren't vented there. What about adding more main venting? Waste of time, or can't hurt? More/larger radiator vents? Hate to lose the adjustability of the Hoffmans.

Thanks for any suggestions you have.
Patrick