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I've found a few posts before about this however they all seemed to involve additional underlying issues (poor main venting, excessive pressure, etc).
This will be the second season with my 331 sq. ft. 1-pipe counterflow system. New Peerless 63-03 118 MBH(in) last October, dropped header with 2x2" risers feeding 2" mains. Steam quality appears to be good and all radiators get steam at roughly the same time and the system is balanced and makes ZERO noise during the heat-up cycle. No pressure build on the 0-5 PSI gauge at all except on recovery from setbacks.
I initially had NO main venting (it was a coal conversion). It worked "OK" but response time was slow on second floor. I tapped the ends of the mains directly for 3/8" and put in 3" nipples. I had two Vari-Vents that I bought early last season and found they were no good for radiators (too fast, threw off balance). But they were basically an open 1/8" pipe (that's what it felt like when I blew into them). So instead of wasting $40 I decided to try them as main vents - bushed the nipple down to 1/8 and screwed them in. They work great, time for live steam to get from header to 2nd floor rads might be 2 minutes. The 2nd floor risers are vented through the rad vents - only one radiator per riser.
I have all-new Gorton 5's and 6's on all radiators except one, which has a 4. No problems at all during heat-up.
It's when the heat call is over and vacuum forms in the system, that one or more of the Gortons open up and make a loud sucking hiss sound. I actually went as far as countersinking the Gorton orifice to improve aerodynamics and putting a thick sock over the vent to try to dampen the airflow/noise, none of which worked too well. Which radiator opens first seems to be random. This becomes annoying when the bedroom is up for the draw at 4:00 AM.
Is this a main venting issue? I feel this should be happening in the basement on the main vents, not in the living space. Do typical main vents (Gorton #2's, etc) normally perform the vacuum relief function? I don't want to buy them and change out the Vari-Vents, which otherwise are serving me well, if it won't help.
Any experience or suggestions?