Converting to natural gas would be cheaper than oil.

I've heard of a commercial building in the area that was constructed with a geothermal system about 10 years ago. It sounded like they weren't too happy with results since it didn't really save them very much compared to their other facilities. I'm not sure if there were any long-term maintenance issues since I lost touch with them about 5 years ago.

It will depend a lot on what the cost of electricity is in your area, would you use a loop field or a pump and dump, how much heating vs. cooling will you need to do, how well insulated is your house, etc.

I have a geo system w/ a pump and dump. Put it in 9 yrs ago. At the time it looked as if NG prices were going to go through the roof. Since then, with the advent of fracking, the supply of NG has increased thus lowering the price. If my geo were to crap out on me now, I would replace w/ a NG boiler and forced air unit since my heating days significantly exceed my cooling days.

You really should talk with a reputable HVAC contractor as there are a lot of factors that go into which system if the most efficient.

You may also want to look at an air source heat pump. That may make more sense, especially where you're located.

I've had an open loop WSHP for more than 20 years. I use a Nat Gas forced air back-up for heat when the WSHP cannot keep up. You can't beat the efficiency of the HP using 55 degrees water year round. I save a ton on AC in the summer, and heat in the Spring/Fall. In the winter it keeps my gas bill less than my neighbors. I have a large house (4800 SQFT) that is well insulated.
I have replaced the WSHP twice. Both times it was because of a leak developing in the water coil since we use open loop well water that is high in sulfur. I then found out that the contractor should have specified a CUNI (CooperNickle) coil for well water applications to avoid corrosion. I think they got it right this time.

If you already have a well, and a place to dump the exhaust water, it is THE cheapest way to heat/cool.

Mac

I was on fuel oil at about $4500 per year. I switched to closed loop geothermal in 2011 for $28k. After the 30% tax CREDIT, cost was about 20k. Heating bill is about $150 a month now in the cold months and air conditioning is really cheap too. So my 4.5k heating bill dropped to about $750. System paid for itself pretty fast not even considering the air conditioning savings. Highly recommended if natural gas is not an option like for me. I just bought a new house with two propane furnaces and a wood burner. Cost $1500 for the first tank fill up. Geothermal is first on my list of things to get done this spring. At least in Michigan, the electric companies have discounted electric rates for geothermal systems. I should also mention that I had to upgrade the house electric service coming in to 300 amp buried line. That's included in the cost above.

I agree with the post above about open loop being the cheapest option. I went with closed loop only because I didn't like the idea of pulling 300k gallons of water out of the ground a year. I didn't want to flood my/my neighbor's property or lower the water table (cone of depression). That 300k is the number I remember being told by the Geo installers I had quotes from, but my memory might be wrong on that.

Unless you have a soil situation the does not allow the water to soak back into the ground, the water will find its way back down to the water table (just as rainwater does), and not lower the water table. 300k may actually be low, depending on your BTU needs. You do need a deep reliable well as a source for open loop.

I have a closed loop system. I wouldn’t do it again. First there was settling of the loop trenches for 3 years (now they bore the coils). Cost a lot to fill and reseed. Then, last summer the evaporator coil froze up. The HVAC people thought it was a Freon leak, thankfully they were wrong. A new coil, that comes with virtually no warranty, was 4K, and a new geothermal unit was 8k...at least this would come with a warranty. I was pissed. The extra money I spent 11 years ago for this system was supposed to pay for itself with time, and now I was already looking at replacing it?!? My house is well insulated, but 36 feet of the 44 feet the back of the house has floor to ceiling windows...so electric is not cheap anyways. I didn’t want gas in the house...between the risk of explosion and mainly CO worry...plus, in my old house I had a duplicate system. A hot water boiler that was gas, and I installed electric forced air heat (this was for air conditioning mainly...old house with no ducts that I installed for this). Anyways, I had the unique ability to compare the cost of heating with natural gas vs electricity in an environment that was identical...everyone says natural gas is way cheaper. It wasn’t...almost the same cost...thus, no gas in this house. Sorry, long winded...I’d get a good air-to-air heat pump...they are almost as efficient as a geothermal, at a fraction of the cost. One good thing about a geothermal is no outside unit...it’s all contained indoors, so no loud fan along one side of the house.

I put one in 10 years ago. 4 ton unit, meaning 4 loops. I took out a heat pump. Electric bills are half what they used to be. I can count the times on one hand where it had to go to aux heat. It’s 0 degrees right now and no aux heat. Runs great. Would do again.
Back then I got s huge tax credit also. I don’t think these are still available today.
I also get my hot water from this system. Basically unplugged my electric feed to the water heater and just use the tank. Very nice set up.

We have 5 heat pumps, 2 are gas back up . We recently removed all blown in fiberglass insulation along with fiberglass
bats in walls. Changed to 8.5 inches of open cell spray foam on all roof . We have a massive roof, results were a 38%
reduction in HVAC cost.

I have a closed system with five drilled well loops. I have had no problems with the system excepting that I had to replace the heatpump and exchanger at a cost that was really surprising after 14 years.

When buying check on the expected life of your heatpump and exchanger unit and weigh that in your calculations

Slightly off topic since the OP isn't building, but still might be pertinent.

When we built 8 years ago, I thought seriously thought about Geo. Decide to take the difference Geo vs. NG and sink it into making the house more efficient (6" walls, a skim coat of foam, more insulation in the attic, etc.). By shear luck, given the increases in electric vs the stable (or decreasing) cost NG, combined with a better R factor (insulation) - that was likely a better choice. I did spring for a heat pump tho that we still still use down to 35 or 40F. Honestly don't know the crossover point w/respect to heat vs. cost (at some point NG makes more sense). Side benefit is we have a backup system if the other goes out, depending on the temp tho. Another issue is we have a Generac whole house gen...the Geo might really tax that system if it went into aux electric heat.

If I would build again, I'd have between 1.5-2" foam on the outside and go with a 2x4" exterior walls.

Quick Geo question and Shep pretty much answered it already...I always wondered about an open loop system. Does contamination in the open loop (hard) water cause problems over the years? Seems like closed loop would a better option long term.