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Last radiator on system heating slow

The house I bought this year has a 1 pipe steam system. I replaced all the radiator vents and they all seem to heat up evenly except the one in my bathroom. It usually wont get too hot or it wont heat at all if its not running long. The one in my bathroom is the last one on the run. I noticed there is one main vent on the system which is just before the piping turns up to goto the bathroom radiator. It seems like it's venting a little bit. If I put my hand a little above the vent when the system is running I can feel a very little bit if air blowing out of it. I'm wondering is it's plugged or the wrong size. It's a Vent Rite #35. I have an adjustable vent rite on the radiator in there set to the highest vent setting (8). The house has 2 radiators up stairs and 6 downstairs including the bathroom.

Any ideas? I think it's a bad main vent and I can't find any other ones.


  • Jamie HallJamie Hall Member Posts: 12,716
    Bigger main vent. The VentRite just won't cut it. How long and what diameter is the main?
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • tkos115tkos115 Member Posts: 73
    I haven't measured it but it looks to be a 3in pipe or close to it. I'll measure tomorrow and fin out for sure though.
  • FredFred Member Posts: 8,418
    tkos115 said:

    I haven't measured it but it looks to be a 3in pipe or close to it. I'll measure tomorrow and fin out for sure though.

    length of each main is equally important, in addition to diameter to properly size the main vents.
  • tkos115tkos115 Member Posts: 73
    I finally got time to measure the steam pipes. They are 3in diameter. Assuming I measured correctly. The main coming out out of the boiler and running the length of the house on the front is approximately 21ft. The pipe main off of that when it turns 90 degrees to the left running down the other side is about 12ft. And then the main that's off that from the other 90 degree turn is about 24ft long. That's the one that has the main vent on it and also has the return main attached to it.

    Hopefully that all makes sense in how I described it. I also attached a pic to show the way the vent is mounted.

  • FredFred Member Posts: 8,418
    So are you saying there is just one pipe off of the boiler header and that it is a 57ft loop around three sides of the basement?
  • gfrbrooklinegfrbrookline Member Posts: 610
    If you have a 57' main I would put 3 big mouths on a 90 degree elbow, facing away from the return drop down so it will drain, That will mass out the venting capacity of the 3/4 tap which I am assuming you have.

    What vents do you have on the other radiators? Once you have the mains vented correctly you may need to slow down the radiator vents to keep everything balance.

    It sounds like a lot of upfront cost but if you vent the main fast and the radiators slowly you will cut your boiler run time down significantly and have a much more balanced system. You will see a return on investment over time in the fuel bill and wear on the burner.

    You also want to make sure your pressutrol is set correctly, .5 cut in, 1.5 cut out, or lower if your have a vaporstat.
  • tkos115tkos115 Member Posts: 73
    My other vents are groton brand. I think my upstairs ones are a 6 and a D. I only have two upstairs rooms. Downstairs I used mostly 4s on the first 2 radiators and a 5 on the farther downstairs ones. Also how would I go about adding in more main vents to the piping? Do I have to cut the pipe that's there and splice in a different piece, or can I use a T/adapter on the existing spot where the 1 main vent is now to add more? I will post a better pic of that pipe as well shortly.

    Sorry for the late response.. holidays... haha
  • tkos115tkos115 Member Posts: 73
  • tkos115tkos115 Member Posts: 73
    My pressuretrol is currently set at .5 cut in and 1.5 cut out. It was set closer to 4 cutout when I bought the house.
  • SteamheadSteamhead Member Posts: 13,771
    If that's a 3/4" pipe where the vent is mounted, you can elbow over away from the joist, install several tees and mount your vents on them. If headroom is tight you'll need to install a shorter standpipe.

    But I'm concerned that the elbow at the end of the main is turned at such an angle as to cause water to back up in the main. If so, it'll bang.
    All Steamed Up, Inc.
    "Reducing our country's energy consumption, one system at a time"
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Baltimore, MD (USA) and consulting anywhere.
  • tkos115tkos115 Member Posts: 73
    It does look weird in the picture but I checked it out and it is angled fine and wont cause the water to back up. I haven't had any water hammer problems so far this year. I have roughly 6in of clearance to work with between where the valve threads into the pipe and where the floor starts. I'm going to go to the hardware store in the next day or two and try to rig something up for a few valves.

    Thanks for the help and insight everyone and happy new year as well!
  • tkos115tkos115 Member Posts: 73
    I know this is a very late update but I have since corrected the problem. I installed a "Bigmouth" air vent and it helped a lot. I may eventually add a tee and put another one in if I can figure out how to fit two of them.
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