Geothermal Heat and A/C for 10,000 sqft.

[Boomer2]

Currently working on county approvals for my hangar/garage build. So, I have some lead time and just trying to get my ducks in a row. Building will be 80' by 130' with 18' walls. A considerable amount of floor space and volume. 90 degrees in the summer and 10 in the winter. Land is not a concern so my thought was to dig some large trenches and bury miles of pex before covering it back up. Alternately I have 2 acres of water I could put the pex in, that would require a half mile trench to reach the pond.

Geothermal seems like the best possible option for heating and cooling needs. water to water for infloor heating and water to air for a/c. The idea was to have a red iron building with super insulated panels for the roof and walls. Of course as the R factor increases the cost increases. Of course the better the R value the lower the cost of the geothermal will be. So the million dollar question is where is the sweet spot. Wickedly expensive R80 panels that allow for a "cheaper" system or less expensivs R30ish panels and a larger more expensive system.

I am wondering if anyone has used geothermal in a large garage/hangar? Any insights or warnings you might share?

 

[OldSchool500]

Believe it or not, Modern A/C units are WAAAAY more efficient than they used to be. This makes a very expensive geothermal system not having many advantages. Payback times is decades. This is why geothermal is so uncommon.

A high quality air to air heat pump has almost the same efficiency as a geothermal system. And they cost hundreds instead of thousands.

 

[cannuck]

I need to hear where you are getting R80 panels? I assume you are building with MIPs and when I look at that market everyone stops at 6" for exterior roof panels. I have only found indoor refrigeration panels at 8" and even those rated only R60. Suggest you seek out greenhouse developers in your area to get some perspective on energy management. They have to deal with solar gain that is literally through the roof. Imagine they can give some geotherm ideas. Not knowing about your soil and subsoil I can only think of what some ag builders use and suggest cutoff insulation subsoil walls to let the earth beneath you do most of your cooling needs

 

[fitter30]

HDPE polyethylene pipe is recommended. Geo system is the most efficient. 45-50° ground is is more efficient to pull heat out of the ground or tranfer it back into. Have tens of thousands ******* in this building spend the money for a manual j for the load and engineering.

 

[bob smith1]

Sounds like a half baked idea. I have and built geo in my commercial and large buildings. There are LOTS of things to learn and understand to do it well. You need to grasp the sensible vs latent loads, and understand that better insulation and air seal is ALWAYS a good idea!

I totally disagree with the post that air source somehow competes with geo, it flat out DOESN'T! Bring the math and I will destroy it. That does not mean air source won't work, it means when strictly talking efficiency, EER, etc, geo easily serves it hot or cold. It's getting to that point that is a *****.

When someone says "bury miles a pex", you know it's not planned because we don't use pex for the field.

 

[OldSchool500]

I totally disagree with the post that air source somehow competes with geo, it flat out DOESN'T! Bring the math and I will destroy it.

Wrong. Average COP for a standard air-air heat pump is 3-5, and they can go higher. Average for a geothermal system is 3-6 COP. Air to air heat pumps are becoming INCREDIBLY efficient, and so, so much cheaper than geothermal.

You will never, ever make up that cost difference.

 

[bob smith1]

Wrong. Average COP for a standard air-air heat pump is 3-5, and they can go higher. Average for a geothermal system is 3-6 COP. Air to air heat pumps are becoming INCREDIBLY efficient, and so, so much cheaper than geothermal.

You will never, ever make up that cost difference.

I didn't say geo was cheaper! But as I stated, bring your math. You are now quoting COP at what ambients and conditions? compare that to geo? Your biggest problem that you didn't bring up is a heat pump in the worse possible Winter day will give you next to nothing for heat. Throw on those aux strips and watch that meter zing! Then you consider where that compressor lives if you need to fix it with 24" of snow. Mine lives comfortably indoors. Air exchange is where?

I DO agree, there is a serious price difference, both for proper design, and build out.

 

[OldSchool500]

I totally disagree with the post that air source somehow competes with geo, it flat out DOESN'T! Bring the math and I will destroy it. That does not mean air source won't work, it means when strictly talking efficiency, EER, etc, geo easily serves it hot or cold. It's getting to that point that is a *****.

I brought the math. It destroys geothermal. Typical A/C system, $2,000. Geothermal $10,000 +++.

It's obvious to me that you have no idea what you are talking about.

 

[bob smith1]

I brought the math. It destroys geothermal. Typical A/C system, $2,000. Geothermal $10,000 +++.

It's obvious to me that you have no idea what you are talking about.

"old school" is probably on point if you think you are buying ANY equipment for $2k. I asked about test conditions, and you are talking equipment price. Where are you getting your data? The internet? Or are you trying to compare tiny mini splits that will be dead in 3Y, to durable geo equipment? Because you won't touch quality inverter heat pumps in that price point, sorry.

 

[Boomer2]

I need to hear where you are getting R80 panels? I assume you are building with MIPs and when I look at that market everyone stops at 6" for exterior roof panels. I have only found indoor refrigeration panels at 8" and even those rated only R60. Suggest you seek out greenhouse developers in your area to get some perspective on energy management. They have to deal with solar gain that is literally through the roof. Imagine they can give some geotherm ideas. Not knowing about your soil and subsoil I can only think of what some ag builders use and suggest cutoff insulation subsoil walls to let the earth beneath you do most of your cooling needs

I was on an Instagram building bender and there were wall/roof panels that were R80. Seemed extreme but that’s why I used the extreme in my example. Let’s say R60 is the max.

 

[Boomer2]

Sounds like a half baked idea. I have and built geo in my commercial and large buildings. There are LOTS of things to learn and understand to do it well. You need to grasp the sensible vs latent loads, and understand that better insulation and air seal is ALWAYS a good idea!

I totally disagree with the post that air source somehow competes with geo, it flat out DOESN'T! Bring the math and I will destroy it. That does not mean air source won't work, it means when strictly talking efficiency, EER, etc, geo easily serves it hot or cold. It's getting to that point that is a *****.

When someone says "bury miles a pex", you know it's not planned because we don't use pex for the field.

Bob it is a half baked idea that is why I was looking for some input from random strangers on the internet. This is a DIY site and I’m a DIY guy. I am sure once the rubber hits the road I will need to hire some kid to do some calculations and from there either bring in a pro geothermal group if it’s beyond my comfort level or learn something new. “We don’t use pex for the field” okay. So what do you use? I believe the basic concept is to use the constant temperature of the ground to pull or dump heat. I am sure there are super advanced ways to do it - I’m just not sure I need that. Seems like I could hand the HVAC guys some ground loops and some pex in the slab and they could sort it out.

 

[Boomer2]

Believe it or not, Modern A/C units are WAAAAY more efficient than they used to be. This makes a very expensive geothermal system not having many advantages. Payback times is decades. This is why geothermal is so uncommon.

A high quality air to air heat pump has almost the same efficiency as a geothermal system. And they cost hundreds instead of thousands.

If I use air to heat the building and heat still rises I would need to heat 220,000 cubic feet of air. If I use the slab to heat the building I only have to heat 73,000 cubic feet of air. So your eletric air to air heat pump need to be 3 times as efficient to even be in the ballpark. I don’t pretend to know the specifics on efficiency of either system. The geothermal hydronic heating system seems like it would almost be passive in comparison to the lyrical demands of air handlers. The plan is to bring in 480v 3 phase so maybe that helps the air handler argument.

 

[Firebrick43]

If I use air to heat the building and heat still rises I would need to heat 220,000 cubic feet of air. If I use the slab to heat the building I only have to heat 73,000 cubic feet of air. So your eletric air to air heat pump need to be 3 times as efficient to even be in the ballpark. I don’t pretend to know the specifics on efficiency of either system. The geothermal hydronic heating system seems like it would almost be passive in comparison to the lyrical demands of air handlers. The plan is to bring in 480v 3 phase so maybe that helps the air handler argument.

First off, where do you live?

Corpus Christi Texas is going to be much different thanI International Falls Minnesota

Second, do you plan on cooling the slab for A/C?

And last, while a heated slab can be somewhat more efficient, mainly because one is more comfortable at a lower temp when their feet are warm, it is not 3 times more efficient vs a good heat pump with air handler.

 

[cannuck]

I was on an Instagram building bender and there were wall/roof panels that were R80. Seemed extreme but that’s why I used the extreme in my example. Let’s say R60 is the max.

If you run your calcs on R50 (about spot on for the 6" MIPs you can actually buy) you will be a lot closer to reality.

 

[Boomer2]

First off, where do you live?

Corpus Christi Texas is going to be much different thanI International Falls Minnesota

Second, do you plan on cooling the slab for A/C?

And last, while a heated slab can be somewhat more efficient, mainly because one is more comfortable at a lower temp when their feet are warm, it is not 3 times more efficient vs a good heat pump with air handler.

Live in Unfunafornia. This garage will see summer temps in the 90’s and winter temps in the teens with about 2’ of snow a year.

I think cooling the slab leads to moisture issues so the plan is to use liquid to air for cooling.

I’m not suggesting the heated slab is 3x more efficient, just that it would have 3x less work it has to do.

 

[My Old Tools]

Wrong. Average COP for a standard air-air heat pump is 3-5, and they can go higher. Average for a geothermal system is 3-6 COP. Air to air heat pumps are becoming INCREDIBLY efficient, and so, so much cheaper than geothermal.

You will never, ever make up that cost difference.

Ideal COP of 3-5 at ambient temps of 50 degrees. Take it to ambient of 10 degrees and the COP is 2 or less. The geo is still producing COP 5 because the ground source is still 50. Same in summer. 90 degree ambient vs 50 degree ground source. Geo is more efficient once you leave ideal ambient temps.

 

[My Old Tools]

My geo is a Waterfurnace Series 7 connected to our 50 acre lake with Slim Jim lake plate. It services a 4000 sf home with a huge amount of glass. My payback is 7-10 years vs new standard high efficiency heat pumps. Using the lake vs drilling wells saved a lot.

 

[bob smith1]

Ideal COP of 3-5 at ambient temps of 50 degrees. Take it to ambient of 10 degrees and the COP is 2 or less. The geo is still producing COP 5 because the ground source is still 50. Same in summer. 90 degree ambient vs 50 degree ground source. Geo is more efficient once you leave ideal ambient temps.

He is banned. Went way left of center the other day.....nuts really.

Anyway, another thing worth remembering is the delta T of ambient vs set point dictates the amount of BTUs needed so on the worst Winter day, you need the MOST BTUs, yet with air source, that is when your COP suffers.

But absolutely, costs are a factor! All just needs carefully calculated. Mine was just a no brainer because I got both The Tranquility 27s with desuperheaters for free, and I have the experience and equipment, so the cost was really just down to the geo loops, which still is not trivial but worth it. 30T excavator and dozer so took no time and if I need more, I will do it.

Your lake feed is a DREAM for me!

 

[Boomer2]

My geo is a Waterfurnace Series 7 connected to our 50 acre lake with Slim Jim lake plate. It services a 4000 sf home with a huge amount of glass. My payback is 7-10 years vs new standard high efficiency heat pumps. Using the lake vs drilling wells saved a lot.

50 acre lake. Hello! I’m less concerned with the ROI. I want to do geothermal because I’ve never done one. On a daily basis eletric splits will annoy me. In the back of my mind I’ll be thinking about an eletric bill. If it’s geothermal I won’t think about it.

 

[pcmeiners]

Same in summer. 90 degree ambient vs 50 degree ground source

User may not be here anymore but the average ground source temperature in Texas is not 50°, posters around here fail to recognize it varies greatly from one region to the next.

"The average ground (soil) temperature in Texas ranges broadly by season, averaging 46°F to 54°F in the winter and 84°F to 90°F in the summer at shallow depths (2 to 4 inches). Deep underground (shallow geothermal constant depth, about 10–20 feet), the steady year-round ground temperature averages around 67°F to 72°F across the state. [1, 2, 3, 4, 5]"

Average Texas air temperature....
"The overall average annual ambient temperature in Texas is about 67.7°F (19.8°C), varying widely by region from the cooler northern panhandle to the warm southern valley."

The average air/ground temperatures are the same which makes it more difficult to claim geo is worth the added cost.

 

[My Old Tools]

Wrong by a bit. When the average ground temp in Texas is 67-72, the average air temp is 90-105, 30 degree delta T. Reverse in winter, when the soil is 50 degrees in winter, air temps can be 0-90. And averages in Texas are ridiculous to use. Amarillo may see -20 on the same day Houston is 85. In general, my system sees 85 degree lake water in August while the air temps are 100. The other factor is water has a huge heat capacity that air can't match. Heat a piece of metal. Had you rather blow 95 degree air over it, or dunk it in 85 degree water to cool it? Our lake never drops below 45 or so, even when we had the deep freeze a few years ago and 2/3 of it froze over. Geo is always more efficient with a correct install.

 

[pcmeiners]

What is about averages that user not comprehend it use. For a specific location with known temperatures and other variables it does not make sense, but for the average person without knowing all the variable it does.
I
Was not referring to lake water average temperatures as I specified in my post, which I should have in your case, apologize That said most users are referring to ground temperatures which vary considerably but most poster using around 50°F, good for mid atlantic/central states but not most of the states.

"
Northern and Upper Midwest States: Ground temperatures drop lower, averaging 37°F to 46°F (3°C to 8°C).

• Central and Mid-Atlantic States: Ground temperatures sit right in the middle, averaging 50°F to 55°F.

• Southern States: Ground temperatures are warmer, pushing toward 65°F to 72°F (18°C to 22°C). [1]"

Agree, ground based geo is more efficient than air heat pumps, but particularly in the southern states it it hard to justify the cost difference. On the other hand if you have a lake heat exchange that raises the efficiency quite a bit,

 

[bob smith1]

What is about averages that user not comprehend it use. For a specific location with known temperatures and other variables it does not make sense, but for the average person without knowing all the variable it does.
I
Was not referring to lake water average temperatures as I specified in my post, which I should have in your case, apologize That said most users are referring to ground temperatures which vary considerably but most poster using around 50°F, good for mid atlantic/central states but not most of the states.

"
Northern and Upper Midwest States: Ground temperatures drop lower, averaging 37°F to 46°F (3°C to 8°C).

• Central and Mid-Atlantic States: Ground temperatures sit right in the middle, averaging 50°F to 55°F.
• Southern States: Ground temperatures are warmer, pushing toward 65°F to 72°F (18°C to 22°C). [1]"

Agree, ground based geo is more efficient than air heat pumps, but particularly in the southern states it it hard to justify the cost difference. On the other hand if you have a lake heat exchange that raises the efficiency quite a bit,

Not sure where you are pulling data, and even I'm sort of skeptical on lake temps, like that lake is shallow because deeper water is usually way more stable.

In any case, geo has ways through thermal mass, specific heat, and delta T, to hug a very good COP or EER, year round. Whether that pencils out for YOU, depends largely on your location and needs. A larger predictor or possibility is your ground water temp throughout the year. many soil studies are not deep enough to be useful.

 

[theoldwizard1]

Land is not a concern so my thought was to dig some large trenches and bury miles of pex before covering it back up. Alternately I have 2 acres of water I could put the pex in, that would require a half mile trench to reach the pond.

I am not a fan of "trenched" geothermal installations. If the calculations on piping are wrong, it is difficult/expensive to correct. Well(s) cost more money to install, but you can easily add another one if you are not getting enough heat transfer.

Make sure there is a temperature gauge near where your water leaves the building and where it comes back in !

 

[theoldwizard1]

Wrong by a bit. When the average ground temp in Texas is 67-72, the average air temp is 90-105, 30 degree delta T. Reverse in winter, when the soil is 50 degrees in winter, air temps can be 0-90.

I am no soils expert, but a well at least 50' deep is going to stay about 45°F to 55°F all year.

Commercial installation always do wells for the reason I posted before.

 

[bob smith1]

I am no soils expert, but a well at least 50' deep is going to stay about 45°F to 55°F all year.

Commercial installation always do wells for the reason I posted before.

By your terminology, you are not experienced in geo or the involved math. Primarily there are 3 main categories, horizontal closed, vertical closed, and open loop. there is exactly NOTHING wrong with any of them if they are designed and built right, and typically closed loop has less operating costs and lasts longer. The ol "pump and dump' certainly works, but you get to eat the pumping costs.

in more extreme climates, vertical loops become more ideal than horizontal, but temp/depth studies matter because as as you start moving BTUs, the soil does NOT stay at that temp!

 

[mike93lx]

By your terminology, you are not experienced in geo or the involved math. Primarily there are 3 main categories, horizontal closed, vertical closed, and open loop. there is exactly NOTHING wrong with any of them if they are designed and built right, and typically closed loop has less operating costs and lasts longer. The ol "pump and dump' certainly works, but you get to eat the pumping costs.

in more extreme climates, vertical loops become more ideal than horizontal, but temp/depth studies matter because as as you start moving BTUs, the soil does NOT stay at that temp!

Knowledge on a subject doesn't hold him back from posting. Ever

 

[bob smith1]

Knowledge on a subject doesn't hold him back from posting. Ever

Thanks dad.

 

[mike93lx]

Thanks dad.

No problem, son.

 

[theoldwizard1]

there is exactly NOTHING wrong with any of them if they are designed and built right, and typically closed loop has less operating costs and lasts longer. The ol "pump and dump' certainly works, but you get to eat the pumping costs.

I was not advocating"pump and dump" ! Vertical wells can be closed loop also.

I have heard of too many cases where horizontal loop were improperly sized. Big problem in cold regions. As I said, it is easier to add a vertical well than expand a horizontal field.

 

[bob smith1]

I was not advocating"pump and dump" ! Vertical wells can be closed loop also.

I have heard of too many cases where horizontal loop were improperly sized. Big problem in cold regions. As I said, it is easier to add a vertical well than expand a horizontal field.

Yes, I mentioned "closed vertical". With any of them, there is drama if designed wrong! And again, there is nothing wrong with horizontal if designed right, and with any of them, you better have an expansion plan! You don't dig up a horizontal loop and extend it! You add another one, but what you are describing is telling of someone that is a contractor, not a design engineer. It's typical of the industry that people consult premade charts rather than real calculations.

 

[brewchief]

As an HVAC guy that doesn't do geo I'll give my advice, you need proper engineering done for whatever system you decide on, start with a heat loss calculation. Have it done with the varying r values in the panels you're looking at using. Price out the cost of infloor heat, tubing, manifolds and underslab insulation add up, it wouldn't shock me if it cost 2$ a square foot just to put it in the concrete without a heat source or associated piping.

Are other fuel sources available other then electricity? Natural gas not an option?
Do you need back up if the power is out? Geo may require a larger generator to keep operating.

Are there local, state or federal incentives to do a geo system? If so will your planned system qualify?

DIY on a system like this is possible but I would highly recommend having it fully engineered, don't expect an HVAC contractor to show up to a slab with some tube in it and a bunch of buried pipe and make an efficient system out of it.



In my area geo was popular for a short time, as soon as federal and state incentives died off it basically died off.

 

[theoldwizard1]

Are other fuel sources available other then electricity? Natural gas not an option?
Do you need back up if the power is out? Geo may require a larger generator to keep operating.

When designing any heating system in a region where sustained cold weather exists and whose primary energy source is electricity, a back up system is very important ! In laws installed propane fireplaces. Never used them, but they looked good ! Bonus, you can get away with a much smaller generator !

Another bonus with having propane as a back up. Other "heating" appliances (stove, dryer, water heater) are less expensive.

 

[Fav Onefour]

I think it's hard to give a blanket assessment on Geo.

In our area heating is the highest load. Utility providers have different programs. I'd want to know what program is used by my provider before making any cost based decision.

At our house the utility wants us to have dual fuel systems so they can switch off the heat pump.
Heating is done by forced air. We have a high efficiency gas furnace sized large enough to supply all needed heat if the heat pump is switched off. These systems typically use the furnace fan and plenum with an expanded coil. The heating and cooling is done through direct demand on the heat pump with a traditional compressor and coil in the plenum. Because of the full backup heat, Geo systems are sized to run roughly 90% of max heating capacity needed. The Geo is on it's own meter and rates are roughly 1/2 of regular. Overhead costs on Geo are slightly reduced because of sizing but there is the added cost of a backup heat system.

In the area where we have the farm, the local Co-op provider incentive is based on leveling demand. The incentives are similar but the systems are set up differently.
In this area, the systems are designed with to have storage capacity in the form of water. The tanks look like glorified water heaters. Most of the homes have hybrid heating with radiant floor and forced air. With a zone call for heating, the stored water is pumped through floor loops or an air exchanger depending on the zone. These systems are quite efficient on heating because of a couple reasons. The heat pump only starts when the storage tank needs more heat. Additionally, water can hold a lot of heat. That is advantageous because simply heating water is a relatively low consumption process. The storage tank temps can be topped and stored between 90 and 140F. The storage temps offer a lot of performance when the water needs to heat to indoor comfort temps. Properly managed systems can often handle multiple calls for heat without kicking on the heat pump.
The disadvantage to this style is cooling. Water has less range to cool before freezing and doesn't exchange quite as efficiently. The storage tank is generally around 45 to 55F. The floor heat radiant loops are not used so all the cooling is done with the air exchanger. This type of system also pulls less moisture from the air because of the higher exchanger temps vs. refrigerant type coils. Typically, you need to plan on a good dehumidifier system in addition to cooling.

Looking at a new build? I'd personnaly price a hybrid system in our area. Radiant heat off storage and cooling done by coolant exchanger.
It would also be nice to run a desuperheater on hot water in summer. Winter is a tough call. Maybe a backup on demand?

Loop fields are another whole subject. There are pump and dump systems still around but I'm not aware of anyone doing them in new installations. Exchange capacity is good but the other issues outweigh benefits.
In the city, vertical is the main game. I have five 160' deep wells and buried manifold at the city house.
Horizontal is the main game in farm country. We dig a trench out in the field and throw coiled loops in hole. Cost of installation obviously varies. Operating efficiency is basically the same if the systems are sized correctly and done in good soils. (Dry sandy soils will destroy efficiency and capacity.)
If you can do a lake plate, it's by far the preferred choice. Simple, cheap, servicable?, and excellent capacity.

Overall, I'll go back to the original big factor. Local utility programs with preferred rates are a big factor. I'd look at those numbers before deciding.

 

[HoosierBuddy]

As a guy who sells natural gas for a living, my experience with geothermal is limited to several commercial customers, over the years, who have called me begging to have a natural gas service installed in the middle of the winter as:

1. There geothermal loop has failed.
2. Contaminated loop has destroyed their system.
3. They are running on backup resistance heaters which can't keep up and are costing a fortune.
4. They have called the guy that installed the system for them when the building was built and they can't be there until spring and suggest they call the gas company.

So, I only hear about the blow ups not the success stories. It has occured to me that 3 or 4 months of running on resistance heaters could wipe out years of the energy cost differential of geo vs air source heat pumps. Again, not a statistically valid way to look at this, because I only get called when the system bites the dust.

A couple of these have been real issues for me though, because a retrofit gas service into an existing commercial building sometimes involves trenching through the parking lot to get a service line to an exterior building wall...and the asphalt plant is closed for the winter too.

I would be in the "Air source heat pumps" are pretty great camp, unless I had one of those lakes right next to my house. Then I'd be all about geo....assuming I couldn't get NG of course.

 

[Fav Onefour]

@HoosierBuddy , brings up an often overlooked point. A poor installation can be a real deal breaker. I'd definitely want to work with someone that knows the systems.
There is no excuse for a failed loop. It's basic stuff but you still have to do it correctly.
Contaminated systems can be a number of issues. Main one I've seen is **** water used in the loop mix.
I'm not sure why they would run a system long term on backup. That's crazy! If it's a lack of people to service the system, I guess that goes back to the first point.

I'd also caution someone looking at Air source pumps to be diligent about installers etc. The resistance backup on those is the same thing. In our area it's a problem if people don't monitor their systems. I know of examples where they didn't know until they saw the bill.

 

[HoosierBuddy]

@HoosierBuddy , brings up an often overlooked point. A poor installation can be a real deal breaker. I'd definitely want to work with someone that knows the systems.
There is no excuse for a failed loop. It's basic stuff but you still have to do it correctly.
Contaminated systems can be a number of issues. Main one I've seen is **** water used in the loop mix.
I'm not sure why they would run a system long term on backup. That's crazy! If it's a lack of people to service the system, I guess that goes back to the first point.

I'd also caution someone looking at Air source pumps to be diligent about installers etc. The resistance backup on those is the same thing. In our area it's a problem if people don't monitor their systems. I know of examples where they didn't know until they saw the bill.

In this area (rural) Geo only gets installed when specified by the architect. During building construction the GC has to find someone from out of the area to come in and do the install and/or use someone that doesn't do alot of them. Then there's no local service.

The one I was talking about where we trenched the parking lot? The loop was under the parking lot too. The entire lot was pretty much building or pavement.

 

[fitter30]

Geo loops are like anything else they need to checked at least twice a year for ph and protection. With any closed looped system using glycol 30%* is minimum strength max is 50% for the protection package. Boiler/ chilled water glycol will have two temperature ratings slush or freeze. Slush will protect from a freeze and not break fittings or pipe. Don't recommend leaving a auto fill open to a closed system. If there were to be a leak it would dilute the glycol protection without you knowing about it.
* Under 30% propylene can turn to acid very quickly.

 

[theoldwizard1]

Local utility programs with preferred rates are a big factor. I'd look at those numbers before deciding.

The only "preferred rate" we get is a discount on interruptible A/C service and a discount for overnight.

If you have gas forced air with A/C, I wonder what the upgrade to full heat pump would cost on a new system. If you are going with forced air AND a heat pump, the cost of geothermal is probably not worth it.