Converting Geothermal System to Closed-Loop
Currently have open loop water based geo-thermal (about a 3 ton unit) drawing from private well and discharging to local stream. Local PFAS contamination is driving state environmental department to condemn all private wells in area of contamination if they can be served by city water.
Our well is targeted and a state grant is funding the extension of the nearest city water system to our neighborhood. We will be losing the well in 2024 at the latest
Started looking at converting the existing open loop to closed loop and now wondering best approach.
Property surrounding home is pretty heavily wooded and unsuited for trench approach, am guessing we will have to either bore vertically or use directional drilling to set the closed loop heat exchanger loops.
Have seen plastic “slinky” systems for the heat exchanger. Plastic does not seem like an ideal heat exchanger material. Working fluid in these seems to be either a dilute mix of Propylene Glycol or a dilute mix of Methanol. Both have proponents and opponents for varying reasons including environmental.
Recently saw another system using copper tubing for the heat exchangers, probably more expensive although less is needed as the tubing is smaller, 1/4 or 1/2 instead of 3/4 or 1 for plastic. Working fluid is advertised as R410a.
What factors influence (or should influence) selection of a closed loop geo-thermal approach?
What should I be looking at. Cost is not necessarily the top driving factor but is a consideration.
Our property does not have access to natural gas, currently we are all electric (no pun intended). Life does seem to be a consecutive set of problems to solve some days.
Thanks for any insight or comments anyone may have.
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What’s your climate zone /location? There’s an pretty compelling case to skip a new geothermal system all together in favor of air source. Gets you similar performance without digging a pricey hole.
We are zone 5, near the border of zone 6. Western Michigan.
Initial plan was simply to convert to closed loop since existing equipment seems to work reasonably well and is only 7 years old. If I can avoid replacing everything I would prefer to.
Probably going to re-evaluate everything and also consider running gas to property. None of the available solutions will be inexpensive
Air source seems challenging when the outside temp is in the teens.
House is about 30 years old but with recent improvements to insulation and windows.
Teens are no problem for an air source. Is the distribution forced air or forced water?
Would you be allowed to leave a vertical loop in the old well and fill the well with grout so you could use the old well as a heat exchanger for your geo equipment?
Plastic pipe loops seem to be the best option as they tend to have the longest life expectancy even if the loop needs to be much longer to transfer the heat.
People that do the math quickly find the cost of a ground source loop can almost never be recover in their lifetime compared to the cost of an air source heat pump.
Walta
The old well is only about 35 -40 feet deep and I suspect the bore is too small for a loop. The well head is only a 4" pipe.
I suspect the copper system I looked at is what they call a direct exchange, need to investigate further.
Not too worried about cost recovery. More concerned about being comfortable, keeping annual costs reasonable and stable. Still working and hoping to avoid major expenses once retired.
I find my air source heat pump be comfortable, with the backup electric locked out above 9°.
Seemed like the idea of direct burring copper tube with refrigerant was abandoned in the 1990s.
If you are set on a ground source HP you may find more positive opinions on the Geo exchange forum
http://www.geoexchange.org/forum/
Walta
The plastic pipe doesn't really inhibit heat transfer significantly compared to the resistance of the soil and rock you are pulling heat through. So it's fine from that perspective and it will last a really long time.
Slinky loops are a bad idea for other reasons: the pipe is long so you use more pumping energy, but you don't actually access as much volume of earth as you would for the same length of straight tubing. With the smaller volume of earth, you are more likely to deplete it before the end of the season, unless you make it very long, such that it's expensive and has high pumping energy.
Vertical loops are the best performance, but horizontal drilling can be cheaper, and can be very close to the same performance because most of it will still be deep enough to the isolated from surface temperature drop in the winter. Whichever is cheaper is the way to go, and which is cheaper depends mostly on what equipment the drillers in your area have.
With temperatures in the teens, air-source heat pumps work fine. The seasonal average might be lower energy use with air-source. The main advantage of the ground-source would be that it would put less strain on the grid on cold winter nights, but that's an advantage for the system, not for you.
Look into trench less horizontal loops. I was given a rough quote of $9200 for a 4 ton ground loop piped into the utility room complete.
A while back I got 3 tons of cold climate ASHP for under $4k. These put out nameplate capacity down to 0F and run down to -20F.
COP might be a bit less than a very well designed and installed GSHP. I can't see why I would ever spend that amount on just the loop. ROI is measured in centuries.