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Easily Confused

HeatingHelp Administrator Posts: 654
edited December 2017 in THE MAIN WALL

Easily Confused

Words can confuse people, especially when talking about hydronic heating.

Read the full story here


  • Phil_17
    Phil_17 Member Posts: 178
    Unfortunately, yes. I am rather confused...

    Above, you start by talking about velocity, and mention 4 fpm. Using that as a starting point, I presume that you are going to hold that constant, and talk about how the GPM will RISE as the size of the pipe increases. Instead you try to convince us that the "GPM will slow down", which is not correct if you are holding the velocity constant. For the same velocity, a larger pipe will be moving much more GPM than the smaller one. If you hold the GPM constant (which is much closer to the way things would work in most situations), then the velocity would "slow down" as you go from the smaller pipe to the larger one, which is what I think you were trying to get at, but somehow you got it confused :-(

  • KC_Jones
    KC_Jones Member Posts: 5,750
    I am hoping that response is tongue in cheek.
    2014 Weil Mclain EG-40
    EcoSteam ES-20 Advanced Boiler Control
    Boiler pictures updated 2/21/15
  • DanHolohan
    DanHolohan Member, Moderator, Administrator Posts: 16,568
    Sorry for the confusion, Phil. Here's what I said:

    Velocity is about a certain amount of distance (let say feet) and a fixed amount of time (one second). Suppose water is moving at four feet per second. You can see that in your imagination, right? But how many gallons are we talking about here? We didn't say anything about GPM. We're talking about speed right now, and that brings us to pipe size. If the pipe is, say, 3/4-inch, the GPM will be a certain number, but if the pipe is two-inch, the GPM will slow down. It's still the same amount of water and the same amount of time. It's just going slower because it's on a wider road. That's velocity.

    Here's what I meant: Let's say we have a controlled flow of 4 GPM in a 3/4" copper pipe. It moves at 2.5 feet per second. That same controlled flow rate in a 1" pipe will move at 1.5 feet per second. It's going slower now, but it's still 4 GPM. I suppose I should have used a different term than GPM. I'm thinking in pictures. I'm seeing a wide, slow-moving river funneling into a narrow, fast-moving rapids.

    Your turn. Thanks.
    Retired and loving it.