Geothermal loop pump size?
I am getting ready to start a DIY geothermal project and I want to double check something with you guys here.
My system will be closed loop horizontal with SDR11 3/4" pipe. There will be 6 loops of 500' each for a total of 3000' plus my 40' of 1 1/4" header. I will have a mixture of 25% Dowfrost(glycol) to 75% water. As far as the ft/head, there will be basically no difference in vertical level as my pipes will exit via my basement walls. I want to size my circulation pump correctly. My system has a 2 stage compressor and at full load the capacity is 5.5 tons. Figuring for 3.0gpm per ton I need about 16.5 gpm. What size pump is recommended? I am looking at the Grundfos U26-99f. This might be a little oversized but I want to be certain.
Any thoughts or suggestions would be appreciated. ST.
Do your self a favor and hire a pro.
Thanks for the suggestion but I am an HVAC tech with many years experience.
I have very little experience with geothermal but I do have some. I helped install a 2 unit, 8 ton total system about 2 years ago and it seems to be doing just great.
The cost of paying someone to come in and install my system would be too much and would prohibit the installation.
Hopefully, someone else will have helpful suggestions.
I just wanted to confirm my numbers. Thanks, ST.
Well, first of all read the site rules. They prohibit this kind of thing in this forum. Second, a loop is not something to take lightly, it will be in the ground for a long time. I install geothermal systems, but I would not attempt the loop field. I hire a dedicated contractor for that, it's just not worth it. Good luck.
2 grundfos 26-116 will do you. but the initial flushing of the loop will need need more gpm's to purge the air with this loop design. check with the distributor of your equipment they will be able to tell you the info you need.
Go to the supplier of your unit and get a flow center. That will make it easier to flush plus get the correct pumps.
I was under the impression glycol based antifreezes were frouned upon for ground loops. My contractor used Environol instead of his usual methanol based antifreeze as my loop is basically burried in the aquafer close to a creek. He didn't want any liability should a leek occur.
I wish you luck in your endeavor
Propylene glycol is very environmentally friendly. Problem is increased friction in the flow. Ethylene glycol is cheaper but not as environmentally friendly and same increase in friction. Methanol has less friction but is not environmentally friendly and may be a bit harder on the coil.
DesMec is right on all accounts. Best bet for DITY is to call a pump manufacturer Rep and discuss. He will make you the best recommendation.
If in doubt, oversize your groundloop. Luck to you.
It's Not Rocket Science, But It is SCIENCE
with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE
Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities
I ran the numbers you gave and it does not look good. You have about 27 ft of head loss in the system. That is the good news. It takes a 2 pump flow center to hit 15 GPM at that head. Using 2 Grundfus 2633 pumps you will hit 18 GPM. The bad news is that the Reynolds number is only 1857, listed as too low to perform effectively. Result is that there is not enough turbulence in the lines resulting in poor heat transfer. Problem is that you will have only have 3 GPM per loop with a resulting laminar flow. This has poor heat transfer due to what is called boundary layer in the flow in the pipe. Basically the fluid near the pipes surface will not move much and act as an insulating layer. One solution is a flow of about 25 GPM, which is not practical with standard flow centers, or a change in the loop pipes. Using 5-3/4" pipes just makes it and 4 is even better. The other solution is to use methanol instead of glycol. The significantly lower viscosity only results in 19 GPM of flow but the Reynolds number goes up to 3834 which is a good place to be.
Perhaps this will help as an example.
Originally Posted by southerntester
My 5 + 3 ton system design is closed loop, vertical, with SDR11 1" pipe. There are 8 loops of 600' (down and back) for a total of 4800', plus about 300' of 2" pipe.
With just the 5 ton unit running:
Two Grundfos U26-116F pumps, in series (push-pull)
21 gpm, which is 44 ft. of head, which is 19 psi (fluid is water)
19 psi matches sum of friction losses of pipe, HE coax, and valves/fittings.