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Old 06-09-2009, 09:58 AM   #1
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How to best filter water for open loop geothermal


I am preparing to install a 2.5T geothermal heat pump and would like to use water from a 3 acre pond adjacent to the house. The pond water is quite clear but I have not yet had it analyzed for mineral content, etc but will soon do so. I would prefer to use an open loop, bringing in water from near the deepest point, about 8 feet.

What is the best way to filter the water from organic matter and debris? I am thinking of a fine screen at the inlet then a container near the bank, say something like a 55 gal drum, filled with clean sand to filter out the remaining contaminants. I'm thinking I could have inlet and discharge configured identically so I could occasionally reverse the flow and backflush the sand filters. The two inlet / discharge points would be far enough from each other to prevent thermal crossflow.

I am not too worried about eventual corrosion of the heat exchanger (mine have copper vs the cupronickel exchanger) because I have 6 identical working FHP heat pumps recently removed from service during an apartment complex demolition. I can afford to swap out units a couple times if heat exchanger corrosion or sedimentation is not too rapid. Fortunately, I have access to the technician who maintained these heat pumps and who is very familiar with their performance and service records - both excellent.

I would prefer an open loop if possible, but will consider a closed loop sunk in the pond if necessary.

I have monitored the water temperature at depth for the past year and it stays well within the recommended inlet water temperature range for my heat pump.

Any recommendations based on experience with open loops would be appreciated.

Thank you,
Jim

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Old 06-09-2009, 07:38 PM   #2
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How to best filter water for open loop geothermal


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Originally Posted by Jim S. View Post
I am preparing to install a 2.5T geothermal heat pump and would like to use water from a 3 acre pond adjacent to the house. The pond water is quite clear but I have not yet had it analyzed for mineral content, etc but will soon do so. I would prefer to use an open loop, bringing in water from near the deepest point, about 8 feet.

What is the best way to filter the water from organic matter and debris? I am thinking of a fine screen at the inlet then a container near the bank, say something like a 55 gal drum, filled with clean sand to filter out the remaining contaminants. I'm thinking I could have inlet and discharge configured identically so I could occasionally reverse the flow and backflush the sand filters. The two inlet / discharge points would be far enough from each other to prevent thermal crossflow.

I am not too worried about eventual corrosion of the heat exchanger (mine have copper vs the cupronickel exchanger) because I have 6 identical working FHP heat pumps recently removed from service during an apartment complex demolition. I can afford to swap out units a couple times if heat exchanger corrosion or sedimentation is not too rapid. Fortunately, I have access to the technician who maintained these heat pumps and who is very familiar with their performance and service records - both excellent.

I would prefer an open loop if possible, but will consider a closed loop sunk in the pond if necessary.

I have monitored the water temperature at depth for the past year and it stays well within the recommended inlet water temperature range for my heat pump.

Any recommendations based on experience with open loops would be appreciated.

Thank you,
Jim
What flow rate do you need?

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Old 06-09-2009, 08:38 PM   #3
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How to best filter water for open loop geothermal


Wouldn't out the intake too close to teh bottom.
The fine screen will clog very quickly.
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Old 06-10-2009, 11:39 AM   #4
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How to best filter water for open loop geothermal


IMO, I think the benefits of a closed loop circuit far outweight that of an open loop system. That way you could program controllers to let you know if the system is operating optimally. For example, if your filter gets too clogged it can make your pump overwork itself and burn out faster ($$$). If you had a feedback loop that measured flow rate periodically it could let you know in advance if there is a problem and possibly turn itself off until you fix it. That way you could change the filter or know where to start troubleshooting from if its something else.

I'll admit that I'm not familiar with "clear sand" but it sounds like using sand as a filter for contaminants would solve one problem but create 2 more. Sucking water through a medium like sand would probably cause an immense load on your pump rendering it inefficient and expensive to run; and preventing the sand from getting near the pump would require fine meshed filters which are probably expensive enough to begin with without having to take into account the frequency with which you will have to change them out. If my concept of a sand filter is way off just disregard this comment.


Just out of curiosity, have you given any thought as to what temperature your return line will be dumping back into the pond? If you don't allow the return line sufficient cooling time before it gets back to the pond it will raise the temperature of the entire pond. This could ruin your efficiency and destroy the entire ecosystem, which is bad for fishing!

Last edited by the_money_Pit; 06-10-2009 at 11:41 AM. Reason: added more info
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Old 06-10-2009, 03:30 PM   #5
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How to best filter water for open loop geothermal


The manufacturer of my heat pumps recommends about 1.5 GPM water flow for each ton capacity and publishes a chart with water change temperatures for my model at both 4 and 8 GPM. Assuming 6 GPM, I can expect about 16 degree F temp rise during cooling and about 6 degree temp drop during heating. Given those flow rates and the large size of my pond, I believe the thermal impact on the pond and acquatic life will be minimal.

Money Pit: I don't have much direct experience with sand filters except to know that they are (were) frequently used to filter swimming pool water at several times the flow rate I will be using. I'll give greater consideration to a closed loop.

Thanks again,
Jim
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Old 06-10-2009, 05:02 PM   #6
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How to best filter water for open loop geothermal


At a 6 temp difference in heating, thats only 17,755 BTUs of heat.
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Old 07-05-2012, 04:12 PM   #7
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How to best filter water for open loop geothermal


My 2 ton FHP Geo open loop is coming from the 80 acrea lake in the back yard. i build a well head and use a 1.5 hp supmersible pump to get the watet to the unit. The pump is ~100 off the shore line in 8 ft of water

I at first installed a whole house filter but switched to a pool sand filter so I could just back wash it as needed turns out to be about every two weeks

The sand filter is a 22" unit I bought off ebay.

Had some issues with leaks untill I lowered the pump pressure to 15-20psi

I am planning on reworking the plumbing this fall so I can mount the filter and badder tank inside a plastic yard container (freeze protection and general appearance I have a Pressure reducing valve to install so the filter never sees more than 25psi

my coil is nickel cooper.


I went this way after getting sticker shock from a local well driller.

All told I have maybe $500 in the loop side of the system and the effecicty is out of this world power bill is ~80 to 150 a month. we have had 100degree heat for the last few weeks and the units purring

Moncks Corner SC

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