I’m trying to compare Open to Closed loop for my home. I’ve searched and searched the forum for my answers and have found bits and pieces but not all. If this has already been talked about please accept my apology and kindly direct me to the correct thread. We’re looking at a 3 ton two stage variable speed heat pump. I do not have a ready source of surface water so Open Loop would require using my existing 1hp well pump and discharging into a new well specifically drilled for that purpose. I suspect there could be other ways and I’m open to suggestions. I don’t want to discharge into my septic tank and my smallish 1/3 acre pond is over a 1000’ from my house and it almost dries completely out in the summer. I’m told that an Open loop is cheaper to install and is more efficient than Closed Loop. I believe that it would be more accurate to say the heat pump itself would be more efficient but the system as a whole might not be because of the increased pumping energy. I can see that using the "virgin" water at a near steady ~60f increases the efficiency of the unit. Especially as the heating/cooling season stretches as since the closed loop continues to saturate the earth with cold/heat and affecting the incoming water temperature and reducing the efficiency of the unit. If I’m seeing this right closed loop efficiencies are going to be the highest at the beginning of the season then gradually diminish. In my case (if I use the existing well pump) my pumping wattage would be 5-10 times higher with the open loop. I just checked my well pump and it draws 10 amps @ 240v (2400 watts). I estimate my current static water level to be a 150' down right now. I’m pretty sure if I was to do an open loop and start pumping all that water my static level will drop causing the pumping wattage to go up. Also as the aquifer drops when summer dries everything out my pumping power will only go up because it's pulling the water from deeper and deeper. Well is 505’ deep and the pump is set at 420’. Of course wattage (power) is only part of the equation; kWh’s (energy) is what really matters. I realize it’s not a straight calculation because even though the circ pump(s) wattage is much lower they will run longer (a lot?) than the higher wattage well pump since it will cycle of/on to meet the flow rate. I assume the pumps in an open loop are running anytime the indoor unit is. Can I assume that a one of those Grundfos type circ pumps in draws around 300 watts or less? Will a 3 ton unit require one or two circ pumps? I did catch a thread where that was discussed but I’m not sure I quite followed it. There seemed to be some disagreement about 3 Tons always requiring two pumps. I like the idea of saving money on the install and increasing the efficiency but what happens if my aquifer drops below my well pump after a particularly dry two or three summers in row? Also, I can change a Grundfos type Circ pump myself but pumping all that extra water will certainly reduce the life of my existing well pump which I can’t change that pump out myself. Then there’s issue of the heat exchanger and lines fouling. We have a fair amount of iron and our water is a little hard. Oh yeah…and I’m pumping 10,000 gallons of water a day out of one hole and putting into another…which could have a whole other set of issues that one can’t readily put a $ value on. Sorry for the long winded post. I think I’ve might have answered my questions. Unless the open loop efficiencies are substantially higher (25%+?) than closed loop I suspect it’s life cycle cost will be very close or even higher than an closed loop for me. That and the other reasons seem to really favor closed loop for me. Are my assumptions in the neighborhood of correct? Thanks in advance! This is a great resource and it's very nice to have so many pros take time out of thier day and help us newbies out. Travis
Travis you summed it up pretty well. I'm gonna leave size/design questions out for the moment that'll be a different conversation. Generally in my neck of the woods a 3 ton system would use a 1 pump flow center (~1 amp). There is no free lunch in geo. Open loop costs less to install than closed, but not much (a couple thousand generally). For that you might have higher maintenance (depending on water quality) and certainly need more well pumps (about a grand each every 8 or so years) operating cost difference is modest in my area where people get electricity for 7 cents a kwh, however where electricity is expensive, open loop may cost much more to operate. The main reasons we look at open loop in my area is 1) DIY friendly 2) sometimes it fits better on smaller lots. Good Luck, J
We really like to see the water level during drawdown in the well to be < 70 ft below ground surface to consider it for open loop use as this is where the pumping requirements start to get large enough to cancel out the more stable EWT. Consider horizontal slinky loops--as space permits--as your next most cost-efficient installation. -Adam Hydrogeologist
Thoughts We considered changing our 2 year old open loop system which uses very pure water to a closed loop. It is doable but we could not justify the $7,000.00 - $9,000.00 additional cost at this time. There is a geo company "Briggs Air" who operate out of Sumpter in our state who work very closely with a competent well driller out of Myrtle Beach. What he had proposed for my specific area for the cheapest cost was drilling 3 vertical bore holes fairly close to each other and running the heat exchange lines directly into the water table 120 feet below us so that much of the lineset would have rested permanently in the water or close to the bottom. That would have given us @ 58 degrees winter, 70 degrees summer, or something like that. Note that with this idea, you don't have to worry about grouting everything with the right grout and whether there is enough heat transfer going on in the ground itself. Your primary heat exchange is through the water, not the ground Note that in this example you are not changing the water table. It would have been separate from my existing well. Circulating pumps always draw a tiny faction of what well motors draw. Depends on where you live and what you are willing to pay.
Please, are you serious? I just finished writing a 400 level course entitled "grouting fundamentals" If you are going to drill you need to grout. Period. Drilling and not grouting flys in the face of responsible stewardship, and the national industry standard per IGSHPA. Any heat exchanger will work if properly designed and implemented. Eric
That installer better check the regulations in South Carolina that require the top 20' to be grouted with a tremie pipe and very specific regulations below the water table and in crossing aquifers for geothermal boreholes: http://www.scdhec.gov/environment/water/regs/r61-71.pdf -Adam Hydrogeologist
Apologies I stand corrected. I can fix hvac. I do not do drilling. I am not claiming to be a driller. If that is the way it is done, then that is the way it is done. Let each speciality do what they do best.
I looked into both but went with a closed loop because of water quality. My ground water is loaded with iron, calcium and manganese which I suspected eventually would foul a coil. Since then I've been hearing stories of people in my area having trouble with their units after their open loops left heavy mineral deposits in them. I am glad I spent the extra money to get a closed loop, even though it was a hard financial decision to make. My unit works beautifully.
