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Old 01-12-2020, 04:02 PM
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I'd bet that a 9000 BTU mini split system with a 28 SEER rating (about 322 running watts) will consume no more than the best geothermal system with a similar 9000 BTU capacity.

One problem with many geothermal systems is that the ground gets heat saturated over time, leading to a loss of effectiveness. Even here in Florida, an efficient AC system will cost less to operate than a underground based system. Many users have found that they need an above ground condenser to take over when the ground won't absorb any more heat. (net savings is only the consumption of the condenser fan)

I'd suggest considering an efficient AC and a small grid tie solar system to offset costs.
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Old 01-12-2020, 11:02 PM
fordtrucksforever fordtrucksforever is offline
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The operating cost of system from original post will only be the fan running. I dont see how any a/c unit with a compressor can be less to operate, no matter how efficient.
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Old 01-12-2020, 11:15 PM
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Read this:

https://news.stanford.edu/news/2014/...or-112614.html

And

https://news.stanford.edu/2017/09/04...cess-heat-sky/

It can be combined with a chilled water loop to also provide a chiller system that only requires a pump and fan/blower.

Also do your night temps fall below 65F? You could run cold air economization to chill your ground tanks over night again using only fans and pumps to avoid the heat buildup mentioned above.

I've been looking into this for quite some time.

Also any chance at re designing your cabin? If you berm over it with about 5' of earth you will drop your temps dramatically.

I currently have a bunker that has a negative skin load in Florida just due to the thickness of concrete and earth berming over the structure. If you are wearing leather soled shoes, the floor literally sucks the heat out of your body and that is in the dead of summer with no ac running
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Old 01-13-2020, 06:14 AM
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I might have missed this, but what is the landscaping like around your house? A few well placed trees (5-10 years later) can reduce the indoor temps of your house 10+ degrees. Pine trees between your house and the prevalent wind in the winter will make your house easier to heat, deciduous planted to the south will give you good shade in the summer and open views in the winter, when the sun is in the south part of the sky. For year round hvac needs you also need to consider the effect of the wind. Are the cabin's windows situated to take advantage of breezes in the summer? Mounting a solar hot water heating system on the roof reduces the temperature of the house underneath it. This last point, to me makes common sense. About 8 years ago I found a study that found that flat panel solar hot water heaters can reduce the temperature of the house underneath it by 15+. A few months later I tried to find the article but could not. Here is a discussion of the temperature reduction that happens with pv cells. If the temperature under your house is much cooler, you could put ducting from your hvac system under the house (I assume it is a crawl space?) to precool the air that is taken in my your ac system.
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Old 01-13-2020, 06:38 AM
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Another cheap solution is to paint the roof white.
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Old 01-13-2020, 07:23 AM
fistfulladirt fistfulladirt is offline
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I’ve been running geo-thermal since 1999, but my furnace is open loop so not much help.
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Old 01-13-2020, 12:14 PM
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The operating cost of system from original post will only be the fan running. I dont see how any a/c unit with a compressor can be less to operate, no matter how efficient.
Maybe so, but a 9k split system can be run off a solar system.

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Old 01-14-2020, 12:20 AM
fordtrucksforever fordtrucksforever is offline
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During the hottest part of summer when air temps are over 100, the ground underneath cabin and in deep shade is only 86 degrees. Inside of cabin rarely gets over 92. But when the humidity outside is frequently and extremely high, that makes inside not really comfortable.

With closed loop chilled water circulating thru something comparable to a radiator, I hope to decrease humidity along with the air temp. The cabin is very well insulated, but without something to decrease the inside air temp, it will not be livable.

Basically a very substandard system that will slowly remove heat from inside cabin. But hoping it may work well enough to lessen the reliance on air conditioner during off peak hours of use.
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Old 01-14-2020, 08:37 AM
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cujet cujet is offline
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Quote:
Originally Posted by fordtrucksforever View Post
The operating cost of system from original post will only be the fan running. I dont see how any a/c unit with a compressor can be less to operate, no matter how efficient.
No pump? Remember, pumps use plenty of power.
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Old 01-14-2020, 10:27 AM
thegoat thegoat is offline
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Earth coupling is a very effective method but does require planning and maintenence.
At the simpleist level, have you considered using an attic vent to pull in air from below the cabin. I would then put an a second vent/fan on the roof allowing air to escape in equal CFM. You could make it active or passive depending on solar or available electricity. Thermodynamics will at least moderate the inside of the cabin to the outside temperature and without active cooling or dehumidification it would be the same as sitting in the shade which is more comfortable than sitting in a closed box with no airflow baking in the sun (hence why your house inside temp is 92 when under the cabin is 86). You would just want to make sure you screen and filter the air (for insects/animals/debris) and moderate the vents (even just using magnetic covers) for colder times of the year.

Just a thought. But if you move forward on the first idea, earth coupling cooling (ground water loops) can be very effective.
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Old 01-14-2020, 10:36 AM
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Maybe so, but a 9k split system can be run off a solar system.

Air Conditioning On Off Grid Solar... Can It Be Done? - YouTube
If completely off grid, running a DX AC on solar will require a lot of maintenance and cost. Hard start kit, invertor, regulator, UPS/battery array, etc and a lot of solar panels to get consitant power to keep it from killing the AC. I guess a marine/RV DC based DX HVAC system could cut out some of that but still adds additional cost. Its always possible but an expensive option especially for a primary use like DX HVAC which is totally inefficient.
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Old 01-16-2020, 04:24 PM
fordtrucksforever fordtrucksforever is offline
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No pump? Remember, pumps use plenty of power.
For the original system I had posted about, there needs to be a small pump just to circulate the water. Having two electric motors running is a complete waste. Especially in this case neither one of them pull much load. Eliminating one motor drops that amount of current draw out of the equation. So a very small pump will be powered from the fan motor.
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Old 01-18-2020, 10:05 PM
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So a very small pump will be powered from the fan motor.
I get that you are trying to save energy. But the work you need to accomplish remains unchanged. Air must be moved, and water must be pumped. A circulation fan with enough "uumph" to move air through a heat exchanger (in your case, a water radiator type device) will be at least 100 watts.

The pump(s) will be about 200 watts minimum. Yes, I know you can get 25 watt pumps, but with only 4 to 5 feet of "head" (at 0 flow) they won't be able to lift water from an underground heat exchanger unless it's extremely close coupled. There is a reason even the smallest well pump that puts out a paltry flow uses 3/4 HP worth of electrical power (500 watts)

I don't believe you can do it using less power. I certainly couldn't.

And even if you drive a pump from a fan-motor, the additional load of the pump will increase the power drawn by the motor, by exactly the amount of work you need done. Any way you slice it, moving a working fluid as a thermal mass takes a bunch of power. As only a certain amount of heat can be carried in each fluid.

What I think you are not considering is the "rate" of water flow and the distance/height necessary. Remember, your water is going to be just a few degrees cooler than the air. That means to carry any significant BTU with a small "Delta T" (difference in temperature) will require a high flow rate.

Consider: for a given BTU (amount of heat or cooling) the flow rate reduces when the working fluid's temperature becomes more extreme (higher temperature differential)
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Old 01-20-2020, 04:33 AM
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For my geothermal greenhouse at 1970 sqft it is 7 pipes 7 feet deep 245 feet looping from above ground in a dog house(it snows and no dog with chicken wire cover) beneath the ground and it comes up in 2 locations in the greenhouse.

It would work the same based on calculations divide by 2 except the distance traveled and depth. 3-4 pipes (based on 800-900 sqft) need 245 feet to fully change temperature. I’d do it in a heartbeat on my house, but I have loads of stuff in the ground in the way.
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