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Can a main vent be too big?

Big_Al Member Posts: 20
The main steam vent on my new old house is an ancient looking Trane unit. It doesn't seem to vent much and the radiators get hot sort-of one at a time. The main loop in the basement is about 75 feet of 3" pipe, connected to a fairly new 175,000 BTU gas boiler. I can't seem to find any published data to tell me what size vent to use for what size loop . . . only things like use this one for a small system and the other one for a large system.

I decided to go big. I ordered a Hoffman 75 vent to replace what's there. Does this sound right? Is it a problem to vent a main too quickly?


  • Brad White
    Brad White Member Posts: 2,398
    I would think

    that so long as the main in question vented at the same time as any of the other mains (all get hot at the same time) and within a few minutes or less, there should not be a problem.

    If one main were vented ahead of another and the vents then closed and pressure setpoint is met, the lagging main would just have to wait for the next cycle... if it ever comes.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"

    -Ernie White, my Dad
  • Big Al
    Big Al Member Posts: 35

    Thanks! The other "main" is a 15 foot long piece of 2" pipe that serves two of the smaller radiators. It doesn't even have a vent other than the ones on the radiators, and they heat up very quickly.

    I am also hoping that the Hoffman 75 will help as a better vacuum breaker. Now, when the thermostat finally shuts the boiler off, the (Watts adjustable) radiator vents sometimes really whistle.
  • Steamhead (in transit)
    Steamhead (in transit) Member Posts: 6,688
    I would pair up that H-75 you ordered

    with two Gorton #2 vents at the end of that 3-inch main. You can build a "tree" of pipe to hold them.

    With the venting capacity this will give you, the rads near the end of that main will get steam more quickly.

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  • Big Al
    Big Al Member Posts: 35

    Tonight I'll put the Hoffman 75 I ordered at the end of the main. I'll also put a nostril-sized Heat-Timer VariValve on the sluggish radiator at the end of the line. Both should vent the main. If that radiator then gets hot before the others, that's OK. It's a little small for the room anyway. If the varivalve is too big, I can always turn it down or put the old vent back.
  • Big Al
    Big Al Member Posts: 35

    The Hoffman 75 at the end of the main and the VariValve at the last radiator seems to have done the trick. The lagging last radiator on the main heats up really fast, just like I wanted, and the rest of the system is right behind it. It seems like verything seems to heat up more quickly. I hope it saves me some energy that way. And . . . if the master bedroom heats up better, maybe I can program a lower setback temperature at night. Sweet!

This discussion has been closed.