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Vent size for under floor radiators

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In my house, we have underfloor radiators (about 8 of them) that heat the first floor. The rest of the home (2nd floor and attic) have standard 1 pipe radiators. The thermostat is on the 1st floor. The system is warmer on the 1st floor, ok on the 2nd floor, and cold on the 3rd floor. The underfloor radiators are 2 pipe - they feed in high and drain low, and drain through a steam trap. The steam traps are vented after the steam trap, but no other vent (that I can see) on the underfloor radiators. The underfloor radiators are vented with either Gorton #1's or Ventrite #35's. I'm wondering if these gortons and ventrites are too large and I should put a smaller vent on the underfloor radiators, so the 1st floor vents slower and the 3rd floor can heat up. Pictures attached and seeking suggestions.




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  • Long Beach Ed
    Long Beach Ed Member Posts: 1,204
    edited December 2022
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    Sounds like you've been reading.

    Balancing a system is an exercise in trial and error. Venting is a big part. Radiator sizing is another. As is steam quality and quantity. We'll assume that your near-boiler piping is correct, your water's clean and you're producing dry steam. Then we'll assume that you piping is correctly configured, sized and that the radiators are properly sized to each room's heat loss. We'll assume that any newly tightened spaces have had their radiators resized to compensate.

    Go on to the venting. Your steam mains should be made to vent quickly, with steam reaching the end of each main at roughly the same time. This is why many three story buildings have vents on the ends of their risers up on the top floor. You can usually meet this goal through proper venting.

    You possibly overlooked one of the very reasons your first floor cabinet radiators have steam traps and separate return piping. They can be throttled. You can turn down the heat on the first floor by partially closing the two-pipe radiators' steam valves. That would direct steam to the upper floors. Why not give that a try before changing anything.

    You may find that the first floor radiators were specifically oversized to allow the resident to chose the balancing day to day. Do they have direct exterior cold-air inputs or did they ever? That would also account for oversizing.

    You have a nice system there and those traps appear to have been maintained.

  • Dsisson
    Dsisson Member Posts: 92
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    The first floor radiators are forced air. There is a fan room that blows through the first floor radiators. I can't control the speed of the fans, but I can close the air vents on the 1st floor (although it seems a waste to contain the heat below the floor). There are valves to shut off the 1st floor (underfloor) radiators, but I assumed they are just on/off like a regular radiator. Do you mean that I can close them partly?

    There isn't any exterior cold air input (actually, there used to be but it's completely blanked off since before I bought the home).

    The system is running max 8 oz of steam and everything does seem to be working properly. I do believe that the near boiler piping is correct. No issues with water hammer or noises. All radiators get hot, but some much faster than others (I've been working with vent sizing upstairs with success, but the third floor is still colder than the other floors). I do have gorton #2's at the ends of the mains. I suppose I could add more gorton 2's if necessary, but again, no obvious issues short of floors with differing temps.
  • Jamie Hall
    Jamie Hall Member Posts: 23,310
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    Yes, you can turn the valves on the first floor radiators off partly -- one of the beauties of two pipe steam. If those returns for those radiators feed into the F&T traps, in fact, fiddling with the vents on those lines is pretty pointless. While F&Ts make miserable vents, they do vent...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Long Beach Ed
    Long Beach Ed Member Posts: 1,204
    edited December 2022
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    When fiddling with the venting remember that before you congratulate yourself for the nostril-sized vent which gets that second floor radiator hot, remember that can be the cause the third floor's not heating.

    Balancing encompasses more than just venting quickly. You often must hold back air to force steam to fully circulate to all radiators equally.

    Once a radiator begins condensing steam, it draws a vacuum which sucks more steam to it. Radiators which are not condensing at that point may not see steam until the entire system's pressurized.

    As Bro. Jamie states, you can throttle the downstairs radiator elements, and that may leave more steam for your third floor needs.

    You may also try reducing the steam pressure if you find a stubborn riser. There's a small possibility that some water's backing up into returns.
  • Dsisson
    Dsisson Member Posts: 92
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    Great advice, thanks guys. I have been doing some research as to correct radiator vent sizing. I can tell that lots of my radiators vents are too large (heat timers or gorton c's). I'm going to swap some around to dial it in better and invest in some smaller gortons or ventrite adjustable valves. And thank you for the advice about the 1st floor valves. I'll give it a try and see what happens.