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Pipe slope

Pin Member Posts: 61
I read from the forum that the pipe slope needs to be at least 1 inch for every 20 feet of piping.

Can someone explain the reasoning behind this?? And the consequence if less than this?

What if the supply line does not have enough slope (for example, 1-2 inch for 55 feet of piping)?



  • Slope

    According to Dan in "The Lost Art of Steam Heating," the following slope (pitch) is necessary:

    Main Counter Flow- 1inch in 10 feet.

    Main Parallel Flow - 1 in 20 feet.

    Laterals to radiators-  1/2 inch in 10 feet

    The reason for the slope is so the condensate (water) will drain back to the boiler.

    I always find that it helps to think of it as a long children's slide in a park with a steady stream of kids using it. If the slide has enough constant slope, the children will easily slide from on end to the other. However if slide has dips or flat spots, the children will slow down, bunch up and collide with each other. Condensate reacts to dips and flatspots the same way. In addition, due these "bunching up" and collisions,  the condensate reacts with the steam travelling in the top of the pipe and this causes water hammer.  The slope figures above  are what is considered necessary to maintain an problem free steady movement of the condensate back to the boiler. Less slope than these figures you're likely to encounter operating and water hammer problems.

    All of this is explained in great detail in "The Lost Art of......." so if you're considering making modifications I would strongly suggest you get a copy before doing anything.

    - Rod
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    Pipe slope question.

    I thought that for drain pipe the recommended slope was 1/4" per foot or 1" to 4 feet. So I was surprised at the recommendation of much shallower slopes for condensate return pipes. Perhaps the steeper slope for drain pipes is because of the need for solids to move along too. But I made a sink from plywood in my darkroom that has a slope of one inch in 8 feet and the water does not all drain out and I use a squeegy to get the last little bit out, and there are no solids involved.

    So why do the shallower pitches work in steam systems? Do they allow for a little bit of water to remain at the bottom of the pipes?
  • Jamie Hall
    Jamie Hall Member Posts: 17,080
    Can't compare

    sanitary waste drains with steam lines.  Apples and oranges.  The code requirements for sanitary waste drains are indeed there so that if there are solids, the velocities are high enough to move them.

    The pitch for steam lines has been determined over the years -- more than a century -- to be adequate from the condensate to flow properly.  Note that counterflow has twice the pitch of parallel flow (and also note that a counterflow steam main should be a larger diameter).  If the pipes are installed and hung properly, no water should remain in them.  That's a big if: you would be astonished at how often I see both mains and returns with sags.

    The consequence of not enough pitch is that condensate may not drain properly.  This may produce water hammer (sometimes quite spectacular water hammer) or even, in some really bad cases, no heat at all as the water sits there and the steam (or air, on a return) can't get by.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Pin
    Pin Member Posts: 61

    What does it mean to have counter flow main or parallel flow main?
  • Parallel Flow & Counter Flow

    A Parallel Flow system means that the steam and the condensate both travel in the same direction, in this system the highest point of the main is at the boiler end and the lowest at the return end,  On a counterflow system, the lowest part of the main is on the boiler end. The steam travels up the main away from the boiler and the condensate travels in the opposite direction back to the boiler -hence the name "counter flow". Normally there isn't a  return attached to the main on the end farthest from the boiler.

     On a 1pipe system the lateral pipes  going from the main to each radiator are counterflow.

    Steam being a gas will travel (flow) from area of higher pressure to areas of lower pressure where as condensate (water) needs slope in order to flow.

    - Rod
  • Slope in Steam Systems

    You are pretty limited on how much slope you can put on a main in a steam system as you have to maintain certain parameters.  You first start with, in most basements, a limited height.  Let's say we have a basement with 8 foot vertical height. You then have to consider the boiler's water line height above the basement This can vary between about 24 inches (Peerless, Weil McLain) to 31 inches (Megasteam)   So if we take the 96 inches (8 ft ceiling height) and subtract the 31 inches we now have 65 inches between the ceiling and the waterline.  Then we have to consider the "A" dimension which "should be at least 28 inches" (Page 82 "The Lost Art..." ) so subtracting that figure we are now down to 37 inch envelope between the ceiling and the lowest point on a steam carrying pipe.  This gives us a walking clearance from the floor of   96 (ceiling height ) -  37 (envelope) of 59 inches. 

    Now we consider the length of the main which we will say is 50 feet long. That means 50 ft out and 50 feet back in the dry return = 100 feet of steam pipe.  Since this is a parallel flown system, we must maintain 1 inch in 20 feet so that  we need a drop of 5 inches from the start of the main to the end of the return.  Measuring from the ceiling that brings the walking clearance on the pipe down to 91 inches though since pipes normally don't touch the ceiling and we have to factor in the width of the pipe plus we need to clear a 12 inch structural beam we better`subtract another 17 inches from the 91 which gives us still  74 inches of walking clearance.

     So what are we going to pick?  Using the minimum of what we need in the envelope for slope and have 74 inches of walking clearance in the basement (plus we have way more slack  in a big  "A" measurement )? Or should we use the maximum of the envelope and while we will have very good slope, we only have 59 inches of walking headroom? A "no brainer" - minimum slope as we want to have a usable basement. A lot of old basements are much less than 8 feet so to try and retain  the basement as a viable  living area, using minimum slope becomes really imperative and to get a few extra inches of clearance they do away with a long dry return and drop straight to the floor. and then you have the choice of tripping over a wet return pipe or burying it.

    - Rod
  • ERNIEB19
    ERNIEB19 Member Posts: 2
    pipe slope,

    Hi wnat to thank you for your posting aboy the hight and measurements of th eceiling clearence, have a question, now what about when the slope on my old house is wait to much ? I want to bring the return pipe up , still living about 2-3 inch slope , i only have a bout 25 ft or returning  , could I do this change ?
  • wam525
    wam525 Member Posts: 14
    pipe slopes in a 2 pipe steam system

    So, i've got a 2 pipe steam system which has a lot of water hammer. i realize the need to have the main steam line slope up to the radiators at a slope of at least 1" in 20 ft, but what about the return steam line? For the pipe in the basement, they are often level, and in some places they slope in the opposite direction! It seems that this was the way they were put in. So does anyone know what the correct slope is for the return line in a 2 pipe steam system.
  • JStar
    JStar Member Posts: 2,752

    The returns can be level and still work. Back-pitched....not so much. Are the returns hot?
  • Supplies at negative slope?

    I would think the supplies from the boiler should be sloped towards the returns through the radiators. This will keep the steam, and the condensate flowing in the same direction.

    Part of the equation for making the water flow downhill, using a gentle slope, (and payday is Friday); is the behavior of water, which is hotter than that going down your sink.

    If you heat a test tube full of water, I think you will see that the meniscus changes its shape when well hot, but not boiling.--NBC
  • wam525
    wam525 Member Posts: 14
    pipe slopes in a 2 pipe steam system

    well, no actually, and maybe that's the problem. There are 3 radiators in a large kitchen and two were added after a big remodel. I think the plumbers didn't know much about steam, and used long runs of copper for both the steam and return lines. After years of water hammer, i suspect the traps are dead too, but the radiators still get hot. I'm sure i'm wasting a lot of oil though.
  • jumper
    jumper Member Posts: 1,710

    This thread shows why steam heat became a lost art. A steam heated building ideally is designed to accomodate the heating system. Extra pitch helps a lot.