GPM flow rate 1/2 or 3/4 what to use ??? explanation plzz
Last Post 18 Sep 2014 09:26 PM by sailawayrb. 6 Replies.
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nightlightUser is Offline
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18 Sep 2014 12:46 AM
can some one explain the flow rate through tubing ? I read 1/2pex = 0.25flow rate per 100 feet - so max 300ft loop should see 0.75 gpm flow right? 3/4 = 0.5 per 100 feet and can be as high as 500ft per loop 2.5gpm thats on a single zone single loop now what if i have a manifold after the pump for 3 loops each at 300ft 1/2 pex thats a total of 900ft so i would take 9*0.25 = 2.25GPM i should see on the pump ? right ? and if i use 3/4pex thats 9*0.5 = 4.5 GPM so i should see 4.5 GPM on my pump ? or do u take 4.5GPM and divide by number of loops (3) so that would be 1.5GPM at the pump??? for 3/4 and if 1/2 pex 2.25divide 3 = .83gpm can some one clarify this for me
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18 Sep 2014 01:18 PM
The higher the flow rate for a given diameter pipe/tube, the higher will be the resulting hydraulic friction head loss (typically measured in feet). The smaller the pipe/tube diameter for a given flow rate, the higher will be the hydraulic friction head loss.  The higher the hydraulic friction head loss, the higher will be the required head supplied by the pump (also typically measured in feet) to achieve the required flow rate. So there really isn’t a maximum flow rate for a given pipe/tube diameter. The problem is selecting a pump that will provide BOTH the required flow rate and required head for the PEX hydronic network.

Typically, you don’t want to exceed a circuit length of 300, 450, 600, and 750 feet for 1/2, 5/8, 3/4 and 1 inch diameter PEX, respectively for typical residential serpentine pattern PEX layouts. You should really consider using hydronic radiant floor heating design software to determine the PEX diameter, PEX spacing, PEX circuit lengths, manifold balance valve settings, circuit flow rates, circuit friction head loss, circuit supply/return temps, and select pump(s) that will provide the required hydronic radiant heat gain (which should first be determined by performing a proper room-by-room heat loss analysis). We have free DIY heat loss analysis and hydronic radiant floor heating design software on our website that you might want exercise after first reading the associated instructions:

Borst Heat Loss Analysis Software

Borst Hydronic Radiant Floor Heating Design Software

1/2 inch diameter PEX is likely the most common size used for residential hydronic radiant floor heating applications.

If you want to explore and better understand pipe hydraulic friction, perhaps exercise our pump & pipe design software:

Borst Pump & Pipe Design Software (Hazen-Williams)

Borst Pump & Pipe Design Software (Darcy-Weisbach)
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18 Sep 2014 06:21 PM
the reason i ask is I already have 2 units runing radiant heating basicly i have 2 houses side by side and i experiment in each


house 1


I have 3/4 pex 2 loops at 300 each but i think 2nd look is a little less maybe 200 ? for a total of 500-600sqf living room


than i have 2 bedrooms in each i ran 1/2pex each zone has its own pump so 3 pumps this is for the living floor -




when i run it i push 140deg in and it comes back 125? 110 heater kicks in and stop when it reaches 125 and so on and it works fine




now i ran 3/4 in slab i have 2 zones - 1zone 1 pump 1 loop 300ft done when i turn it on i see 1.4GPM 125in 80 out and i ran for 3 hours and the return keept going up 90



2nd lzone i have3/4 3 - 300ft loops when i turn it on i see 2.1GPM 125 in comes back as 60-70 deg MAD COLD huge drop



thats why i ask how do u calculate the FLOW rate do i need a stronger pump to be able to push 4.5GPM on the 3/4 3 -300ft loops or do u take 4.5GPM and divide by 3 ?????? and because its a slab it will take a while to heat up and as it will heat up the return temp will increase


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18 Sep 2014 06:31 PM
house 2

I have 2 - 300ft loops of 1/2 pex with an Alpha pump stainless on level 3 I see 2 GPM on the pump and on the heater it says 1.8GPM flow and of course if i set to level 2 or 1 this number would drop


140 in comes back as 115 i think???? or maybe 125? i forget but who cares it works fine it heats up after 3-4 hours temp in house goes up 65 to 72 and i stopped the test



how to properly calculate the flow?


I would take 6 (for 600 feet of pex) times * 0.25GPM = 1.5GPM and i see 1.8 at level 3 or would i take 1.5 and divide by the number of loops which is 2 so that would be 0.75GPM what should the pump see to support this loop

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18 Sep 2014 07:32 PM
I already suggested how one would properly determine the required circuit flow rates…one exercises the design software…

When one designs a hydronic radiant system, one selects the allowed circuit temp drop (typically 15F or less for residential) and one runs the design software to get all the aforementioned design parameters BEFORE installing the system. For example, if you have manifolds with balance valves, the balance valve settings significantly influence the circuit hydraulic friction head loss and flow rates. As such, the balance valve settings need to be determined and accounted for in the design when selecting the pump(s). All the actual installed balance valve settings, flow rates, supply/return temps, and differential pressure across the pump(s) should precisely match the design values. You can’t just place PEX in a slab and not even know if your circuit lengths are 300 or 200 feet. There shouldn’t be any guesswork or experimentation at all. Sorry, but I can’t really sort out what you have or even understand what you are talking about without a system schematic.

Who designed and installed your systems, and why aren’t they resolving your issues?
Borst Engineering & Construction LLC - Competence, Integrity and Professionalism are integral to all that we do!
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18 Sep 2014 07:59 PM
I designed it i built it and there is no issues i am testing all variations and just asking for advice, planing to build a 3rd home and want it to be perfect



that software asks for way to much usless info you make WAY TO BIG OF A DEAL out of NOTHING this is simple radiant not rocket science ive built assembled and disassembled my heating system dozens of times EXPERIMENTING you learn with trial I actually get to SEE THE RESULT from what i have built and how it performs


and I asked to clarify a simple FLOW rate needed to support a loop or how it is calculated to steer me in the right direction



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18 Sep 2014 09:26 PM
It's certainly true that practice makes perfection and the third time is often the charm...  Good luck with your project!
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