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4 rads not heating up on the coldest day in 10 years

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OK I have a single pipe steam system and last night 4 radiators wouldn't heat up and the the boiler kept working, I shut down the heat for a while today and put it back on and they seem to be fine. The pitch all seems fine. I dont know if the water didnt get back to the boiler blocking new steam or if there was a vent issue. My main vent is a #35 vent rite. The Pressurtol Main is just below 2lbs and the differential is on 5. Any idea and Also why would the extra cold day make it not work but any other cold day (even 15 degrees) it seems to work fine?

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  • Fred
    Fred Member Posts: 8,542
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    You need more main venting than that Vent-Rite 35. Do you have enough pitch on those radiators back towards the steam supply pipe? Are the Run-outs to those radiators pitched back towards the Main enough? What kind of vents do you have on those radiators? how old are they? Are they adjustable? Sometimes if they are venting to fast they will get a little water in them and not let air escape or they could stick shut. If you can take them off when the boiler is not running, shake them out. If they are old, it may help to soak them in vinegar for a few hours (probably best done on a warm day).
  • itzasean
    itzasean Member Posts: 71
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    They all are pitched properly. 2 of the 4 have a riser going directly into them without any runouts. The only issue I know of is the other 2 share a riser but on the top its a Tee that goes 5 feet into one Rad and the other side of tee is only about 8 inches. Both pitched.. Why would this start acting up on the coldest day but every other day seem ok? Also what size Main would you use?
  • Jamie Hall
    Jamie Hall Member Posts: 23,421
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    Do I read that the pressuretrol main is on 2 and the differential is 5? I do hope that that is 0.5, not 5, and that it is a subtractive pressuretrol, not an additive one. If it's additive, and the settings are as you typed, your pressures are much much too high, and one possible explanation is that the pressure locked those four vents shut. Many vents won't drop open until the pressure drops to a pound or less.

    Otherwise... are there any parts of those runouts to those radiators which are exposed to really cold conditions? It may sound bizarre, but I have seen steam pipes or returns carrying condensate freeze enough to block flow, in spite of steam being present.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Fred
    Fred Member Posts: 8,542
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    The Main vent depends on the size and length of the Main. For a 2" Main, the rule of thunb is one Gorton #2 or equavalent (3 Gorton #1's or 2 hoffman 75's, etc) for every 20 feet of Main. If you need more than 1 vent, you can put them on a nipple and aTee or elbows and mount them at the same location on the end of the Main.
    As far as why those radiators started acting up, my guess is during milder weather, the radiators didn't get completely filled with steam and the vents weren't put to the test until the coldest day. Shake them out, if they are adjustable, don't set them to the highest setting and if they stick closed after the initial heating cycle and don't let the air out of the radiator on subsequesnt heating cycles, replace them.