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Calculating head loss
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Elliott Ray
Member Posts: 3
I am working on a house that is 3000 sq feet. It is a staple up project with 4500 feet of 1/2" pipe coming out of two connected FN5 8 port manifolds. My circuits are all around 300 feet and I have 16 of them. I don't know what kind of pump I will need though or how to calculate head loss for this kind of system. Mostly I want to know why I will need whatever pump I need, and how I can calculate that for future projects. I've attached a pdf of the house I'm working on for reference, any help would be greatly appreciated, thank you for your time.
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head loss
You might purchase a copy of John Siegenthaler's Hydronic Design Studio. It will let you lay out the system in schematic form and will tell you all you need to know as far as component sizing.
The basic method is this:
First of all, it is important that the loops are very close in length, perhaps 10% or so.
You have 280' loops @ 8" on center. Grundfos has an HVAC design Booklet that has tables in the back telling you head loss per foor at certain flow rates in certain diameters of tube at certain glycol concentrations. Each loop covers 187 square feet of floor area. If the heat load in that room is 25 btu/sf, that loop has 187 * 5 = 4675 btu/h heat load at design. The flow rate required by that loop is given by the equation GPM=(heat load)/(500*tempdrop), so the flow rate required by that loop is 4675/(500*20), or 0.47GPM.
Find that flow rate for 1/2" pex in the chart and take the number in the chart and multiply it by the length of the loop. 0.0099*280=2.8ft of head loss.
The flow rates of the loops add, and the head loss should be close for each loop. If it isn't, you might need circuit setters.
You must also add the head loss of the distribution piping to and from the manifold.
Hopefully that makes some sense.1 
aaaaah
Yes that makes perfect sense. I've been trying to find anything on head loss all day but all the information I get is in the form of vertical head loss. The .47 GPM would be the only thing that stacks for the pump right, i.e. 8 circuits would be about 4 GPM?0 
Yep
Yes, the flow rates add. There should also be Cv values published by the manufacturer for any valves in the system. valve head loss (feet) = (1.52*GPM/Cv)^2
There is also head loss due to fittings in the system.0
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