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And so on. If he could tell me the pipe size, I could tell him the flow rate. Was I oversizing? Nope, I was sizing up to the maximum flow you could move through that pipe without having a problem. Did the job need that much? Can't tell for sure, but I was thinking that if I were the original design engineer I would size my pipe just this way. I'd use the smallest possible pipe to get the maximum flow needed, along with the best possible economy for the building owner. If the job needed 45 gpm, why would I use a main larger than 2 inches?
Figuring Pump Head
Direct-return is sort of like a ladder. Figure the left side of the ladder is the supply; the right side is the return, and the rungs are the circuits. If I put in a circulator with enough head to make it up the left side, through the highest rung on the ladder, and back down the right side, then the circulator would have no trouble at all flowing through the lower rungs while it was at it.
So once we figured out the circulator's flow needs (based on the size of the pipe it served), I'd ask the contractor what the longest circuit was. Then I'd allow 6 feet of pump head for every 100 feet of length in that circuit. Again, this is from Gil's System Syzer. When you're at the maximum normally used hydronic flow through a pipe of any given size, the head loss is going to be 4 feet per 100 feet. To this we have to add 50 percent to allow for fittings, valves and other components that create more friction loss than straight pipe. If I add 50 percent to 4 feet per 100 feet, I wind up with 6 feet per 100 feet. Easy.
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But pump head is another matter. I learned that, while I had to figure my total flow on the needs of all the circuits that the circulator would serve, I would only have to base the pump head on what the flow would see while traveling through the highest-pressure-drop circuit. If my circulator was serving a two-pipe, direct-return system with five circuits, it would only have to worry about the worst-case scenario, which was usually the longest of the five circuits.
It's sort of like saying that if you're able to drink five beers without falling down, then you'll also be able to drink less than five beers without falling down. That analogy work for you? Good.| Dan Holohan - [Intro] | [Email] | [Website] |
| The views expressed in this article are those of the individual author and do not necessarily reflect the views of the management or staff of MasterPlumbers.com |
