Open loop GPM requirements
Last Post 20 Sep 2015 08:32 AM by joe.ami. 9 Replies.
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clevtUser is Offline
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09 Sep 2015 09:50 PM
I am building a house and trying to get all of my ducks in a row right now.  I have a deep well that produces slightly over 6GPM.  I've had the house sized for a 2.5ton unit so I've got that part out of the way.  I've been looking at all of the different brands and reading what I can on the geo units and keep finding conflicting information.

Most everything I've read says that 1.5gpm/ton is fine for open loop systems.  I found a 2.5ton GeoCool unit at Ingrams Water and Air and emailed them just to be sure that I had enough flow before I ordered it.  The support guy emailed me back and said the rule of thumb is 3gpm/ton.  He then proceeded to tell me that all the brands they carry  (GeoCool, McQuay, and ClimateMaster) required 3gpm/ton.

So is he just wrong or is this something that varies (double in this case) by manufacturer?  Any help would be greatly appreciated.
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10 Sep 2015 01:17 PM
I am just curious, but how deep is the water standing in the well. Shouldn't that also help the well meet the gallons per minute needed? I do not know but just asking.
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10 Sep 2015 02:16 PM
For DIY technical information, you need to be getting that from the manufacturer's literature rather than the seller.

Climatemaster specifies 3.0 gpm/ton for Closed Loop systems and 1.5 gpm/ton for Open Loop systems. For GeoCool, I would go to their website and download the Installation and Operation Manual (IOM) for the unit you have selected and review the manufacturer recommended flow requirements.

Is this well also being used as a domestic water source? If so, a well with 6 gpm flow rate capability seems insufficient. 2.5 ton x 1.5 gpm/ton = 3.75 gpm. 6 gpm - 3.75 gpm = 2.25 gpm. That is less than a typical shower head flow rate.

How deep is the well? Diameter? How deep to the static water level? How long has this well be in-service? Have you tested the flow rate during extended dry periods? Have you had the water quality tested? pH? Calcium Hardness?
jonrUser is Offline
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10 Sep 2015 03:02 PM
You can add large pressure tank that will supply high GPM for awhile - for example, I can take a complete shower on the pressure tank alone (ie, 0 GPM of new supply). Plus the well itself buffers water (as Alton says). Plus the heat pump will work (just less efficiently) with short periods of somewhat less flow.
clevtUser is Offline
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10 Sep 2015 07:31 PM
Thanks for all of the responses. I really appreciate all of the input and suggestions. I'll try to answer everyone's questions in order.

The water is at about 80' so I have a column of water from 80 down to 220. This will give me a good 'reserve' but doesn't increase the GPM that the well will produce.

I looked on the specifications for the model I was interested in on Geocool's website and it didn't list the flow. I will try to find the IOM as you suggested and see what it recommends.
The well was dug Monday of this week, so I can't answer all of the questions you asked. The flow was measured by blowing the air out via the well drill, which usually gives a lower flow than the well actually produces (as some is blown back into the seams in the rock). So I'm expecting a slightly higher flow but not really 'counting' on it. The well is 220' deep and 5" wide. As for testing the water, I have two other properties nearby that both have good water so as of right now I'm banking on that. I'll get it tested before I go a whole lot further but wanted to be sure geothermal was going to be possible before going much further. Both of the other wells keep the same flow throughout the year so again I'm just hoping for consistency.

I've got a big pressure tank I'm planning on using.. I can't remember if it's an 85 or 100gal but it's a big one.
clevtUser is Offline
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10 Sep 2015 08:23 PM
Well I guess I'm going to have to call GeoCool tomorrow and just ask them. Their literature just says 3gpm/ton for closed, but doesn't specify anything for open-loop. Mcquay's website doesn't have any literature on their products... it seems all of the pdf links are broken.

arkie6 -- Do you have any suggestions on where to have the water tested to give me all of the info I'd need to make the decision of whether or not to go with an open loop?
chrsUser is Offline
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10 Sep 2015 09:50 PM
It's really the combination of temperature and flow rate that matters, not open or closed and flow rate. The design manual should have charts showing the impact of flow rate and temperature.

Are you returning the water to the same well--a standing column design--or are you dumping it elsewhere?
clevtUser is Offline
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10 Sep 2015 10:52 PM
The plan was to dump elsewhere, though I haven't read about the standing column design. I'll read up on it so I'll at least have an alternative if needed. Thanks!
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11 Sep 2015 01:06 PM
A standing column well with a bleed valve might be worth considering. In this design you would return all of your heat pump exiting water to the well with only a fraction being bleed off or wasted via small bleed valve. To further enhance the amount of water returned to the well, you could use a solenoid operated bleed valve that operates only when a set water temperature is met. For heating purposes, you might set the setpoint at 40F to open the bleed valve. For example, if the heat pump exiting water temperature is >40F, then all of the water goes back into the well. Once it falls to 40F, the solenoid valve opens and releases as waste water 1 gpm, 2 gpm, 3 gpm or whatever you set based on a downstream throttling valve. Whatever you release outside the well would be made up by inflow into the well at your local groundwater temperature and should raise the entering water temperature to the heat pump. You may have to experiment with the volume of water released external to the well to maintain desired entering water temperature.

You can get well water test kits online (just google it) or at local water well supply stores and big box stores like Lowes or Home Depot. Some local health departments will also test your well water. You might give them a call and see what is available. To get a quick check on the amount of dissolved solids in your water, put a quart or so of it in a sauce pan on the stove and boil it dry and examine the amount of residue left in the pot.
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20 Sep 2015 08:32 AM
Generally 1.5 to 2 GPM for open loop. If in a Northern climate you may need the higher flow in deep winter depending on entering water temp. The opposite of course can be true in Southern climates.
EWT has a lot to do with required flow as chrs said.
Standing column can increase the required GPM but that can be controlled with bleed as was mentioned. SCWs and return/injection wells are easily fouled in some areas where there is heavy mineral content in the water.
Many on-line DIY companies fail to look at the big picture when you call. In one instance they were going to sell a 4 ton unit to a client with a manufactured home that only had 5 or 6 by 18 duct. They tend to oversize as well and perpetuate wives tales i.e. slinkies don't work.
We sold the client a 3 ton and helped him design his new duct system.
Joe Hardin
www.amicontracting.com
We Dig Comfort!
www.doityourselfgeothermal.com
Dig Your Own Comfort!
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