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steam air venting

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jsstwo
jsstwo Member Posts: 3
in THE MAIN WALL
Hi, is it possible to over vent the main/return on a steam boiler?

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  • Jamie Hall
    Jamie Hall Member Posts: 23,286
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    You can spend more money than you need to... but in general, no.

    With this comment, however: if the system has more than one main, and the mains are considerably different in length, you may have a balance problem if the mains -- particularly a shorter one -- are significantly over vented in relation to the longer ones. The situation isn't particularly drastic, so the general comment -- not really -- is a pretty good place to start.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,837
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    No. As long as the vent closes once the steam gets there. But why would you want to do that?
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • jsstwo
    jsstwo Member Posts: 3
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    i thought the faster u vent air from the main the faster the steam reaches the radiators. the less run time and should save money. am i incorrect in thinking this way?
  • Jamie Hall
    Jamie Hall Member Posts: 23,286
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    jsstwo said:

    i thought the faster u vent air from the main the faster the steam reaches the radiators. the less run time and should save money. am i incorrect in thinking this way?

    You are correct, -- up to a point. At some point in the venting, you will find that there is no decrease in the time it takes the steam to reach the end of the main -- even if the main were wide open. This is because the steam "front" (if you will) can only move along the main as fast as it can heat the main up to where it has pretty well stopped condensing, and that is determined not by the venting, but by the size and weight of the main and by the insulation around it, if any. For instance, 2 inch schedule 80 weighs 5 pounds per foot. Iron has a heat capacity of about 0.1 BTU per pound per degree Fahrenheit. So... if we are to heat a 50 foot length of schedule 80 from 70 to 212, it will take almost 4,000 BTU to do the job. If that is the only load on a 100,000 BTUh boiler, it will take a bit more than 2 minutes to do the job -- and that assumes that the pipe is well insulated and there are no radiators in that length. And no amount of venting can speed that up.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • dopey27177
    dopey27177 Member Posts: 887
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    Finally some some one said that insulation on the steam is beneficial and will allow steam to get to the radiators faster. In addition the insulation will save about 3500 BTUs. That don't save that much in one cycle. Add to the 3500 BTUs as many as 10 cycles on a cold day and you now have 35,000 BTUs. Thats no chump change when you factor that out to 1/3 the heating days in areas where winter heating can be 100 days. This example is based on an average in my old house in Brooklyn, New York.
    Multiply the 35,000 BTUs by 33 days and you will have saved 1,155,000 BTUs.

    Number 2 oil has about 144,000 per gallon.
    Divide that into 1,1555,00 BTUs and the savings is about 6 gallons of oil.

    Natural gas has 100,000 Btus per therm.
    1,155,000 divided by 100,000 and the savings is about 1,155 therms.

    Does not save that much on the face but think of the long term, many people live in their homes 10 to 20 years.

    The cost of insulating that steam main is like a federal bond that keeps on paying a dividend year after year.

    This, my case, my home in an area that does not have long severe winters.

    As to the vent valve thing, over venting steam mains is a waste of money because air will not flow down the steam main faster than the resistance of the pipe will allow it, the same goes for the flow of steam.

    In most cases in newer homes (homes designed to operate at 2 PSI max pressure) the steam main rarely is larger than 3" from the boiler and generally reduces to 2" at the end of run so whether you have one or two steam mains in the building you do not need a vent valve that removes more than 4 cubic feet of air per minute.

    Remember the vent valves on the radiators vent air too.
    Also thermostat location is very important.

    Balancing your venting at the radiators is more important than super sized main vent valves.

    Jake
    Steam: The Perfect Fluid for Heating and Some of the Problems
    by Jacob (Jake) Myron
    ethicalpaul
  • jsstwo
    jsstwo Member Posts: 3
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    thank u for the insight
  • STEAM DOCTOR
    STEAM DOCTOR Member Posts: 1,968
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    Properly sized main vents and properly sized radiator vents, are both important. Balancing starts at the main. Undersizing main vents and balancing only at the radiator level, will leave you like a dog chasing it's tail 
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,576
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    The goal is to for steam to fill all the supplies first, and then rise up to each radiator simultaneously.
    To this end, the capacity of the main venting must exceed that of the aggregate of all the radiator vents.
    This is why slow radiator vents are to be preferred over fast ones.
    After the system has been running for a while, any slower rads on taller run outs can have an increase in vent capacity.—NBC 

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