Earth Tubes and the mini split anyone ever done it

[nokiasixteth]

Ive seen there is no one out there that makes geothermal mini splits . They are all air to air . Know that the constant temp is what makes geothermal so efficient being that theres no mini split geothermal .
Was wondering if any one has done any dealins with earth tubes. And wonderin if you ran a extra set of tubes dedicated just for the outside air handler. If it would boost efficiency. Or would it be just a big fat waist of time ?
Design notes on earth tube cooling

[jeff5may]

Ground-source and mini don't really make sense from a financial perspective. Most consumers are not going to dig or install their own ground loop or source well, they are going to contract the project out. The savings using a mini system would take forever to pay for, especially in mild or temperate climates. A variable speed "inverter" unit comes close to the efficiency of a ground source unit when the weather is not harsh, and can be installed in one day, so it makes much better "common cents". No matter how you slice it, a $2000 investment will have a faster ROI than a $20000 investment.

From a DIY perspective, the situation changes drastically. The more work you can do on your own, the less you have to spend. A handful of ecorenovators have published their success stories and resulting savings. Whether or not it's worth your time depends mostly on you.

Running a set of earth tubes for an air source outdoor unit would only improve efficiency during extreme weather conditions where the heat transferred into or out of the ground made up for the "drag" on the air source. On the one hand, if you let the unit "suck" the air through the tubes on its own power, the loss of airflow would be substantial. On the other, if you use a blower to pump air through the tubes, the pumping power would reduce overall energy efficiency. At some point, there would be a balance point where the earth tubes would move more energy than they drug down the system.

[MN Renovator]

I'm having trouble thinking how you could provide the required CFM that a mini-split would need through the earth tubes without consuming enough energy to blow your savings of putting warmer air into the unit.

Earth tubes used to provide the [usually] two digit CFM usually needed for air exchange is far less than the incredible air volume used by an air-source heat pump of any type. You'd need a huge very long run of very large pipe to effectively get that CFM and exchange the air without using massive amounts of electricity to do it. ..once that is said and done with that much air moving through the pipe, it won't have much temperature gain from the soil. This is why fluid is used underground with a ground source heat pump, easier to move with less power and water has higher energy density than air.

WaterFurnace makes a heat pump with an inverter compressor now. You just don't have the mini-split head option but if you went with a hydronic system of some kind in the house, you'd lose the energy requirement of a large central blower in exchange for a pump to move the water to whatever your radiating body is. ...of course if you need cooling, you need to move air so having a mini-split head would be nice. Either that or when you really look at it, the ECM motors in air handlers use far less energy than older shaded pole and PSC motors do.

I've never been able to make the math work for geothermal. A 4" wrap of XPS or polyiso over the outside of a house, a layer or two of properly applied insulating plastic to the windows, air sealing, and summer shade on two sunniest sides through either heavy deciduous trees or 90% shade cloth should make most homes that aren't a McMansion into a house that can operate with less than a 40,000BTUhr heat load and under 1.5 tons of cooling load as long as there isn't a huge amount of glass area. This winter I've measured my heat load down to 16906BTUhr at -11f outside and 68f inside with only 3/4" foam on the outside with R13 batts in the walls and 7" cellulose in the attic. An additional 4" of wrap on the outside and R60+ in the attic is upcoming along with two layer plastic sheet window inserts too. Summer cooling capability measurements with my current setup suggest 1.5 tons of cooling would have me at 75 degrees inside when its 91 outside without any added window shade(future plan), MN design temp is 88.

Spending even 10 grand on a geothermal setup is hard math to do with potentially under 10k loads on heating and cooling peaks being in my future, I'm just not seeing the potential savings adding up when a DIY job to get to that point will be possible for under $3k although my numbers on paper suggest I might have the whole job done for closer to $2k.

[AC_Hacker]

Originally Posted by nokiasixteth

Ive seen there is no one out there that makes geothermal mini splits...

I spent two lovely years living in Ocean Springs, Mississippi.

As I recall, the spring flowers began to bloom in mid to late January.

Realizing this and also knowing the calculated values of your Heating Degree Days (HDD) and Cooling Degree Days (CDD) would be very helpful to those who are attempting to give you useful advice.

You could provide this information by going RIGHT_HERE and filling out some forms.

-AC

[nokiasixteth]

Th links not workin out. It brings me to a http 404 page.

I didnt think that it would be worth the time of day to do the earth tubes and mini split

[jeff5may]

Check here.

Main idea: Your heating and cooling degree days are almost equal.

From an incredibly simplified (business) viewpoint, mini-split units are the next step up from a window or portable unit. Easy to install, not highly complex, relatively low purchase price. Much like other "major home appliances" in the price range, they are built to fulfill the needs of the widest audience possible with the least amount of installation skill.

As such, a geo system is outside of this audience. Just as the average customer is not looking for a 120" HDTV, he is not looking for a geo system. Nor will he likely ever be. There are "niche products" built to supply this small customer base, but the big dogs aren't interested.

Considering your climate data, an air-source unit would do an awesome job about 90% of the time. It would do better at heating than resistance heating 99% of the season (maybe the whole season, depending on the unit).

[nokiasixteth]

I had thought that it would be better than a radiator . I plan on puttin in a solar grid tie later on so im tryin to drop my power consumption and make the place as efficient as i can.
We usually dont get many days that are really cold . This year has been a oddball year .Supposed to snow here tomar . Here the schools shutdown for snow . Unlike up north

[AC_Hacker]

Originally Posted by nokiasixteth

Ive seen there is no one out there that makes geothermal mini splits... They are all air to air . Know that the constant temp is what makes geothermal so efficient being that theres no mini split geothermal . Was wondering if any one has done any dealins with earth tubes. And wonderin if you ran a extra set of tubes dedicated just for the outside air handler. If it would boost efficiency. Or would it be just a big fat waist of time ?

In spite of the other comments, I think you're onto a great idea!!

The beauty of geothermal is the temperature stability of the earth (the deeper, the more stable).

In spite of the other comments, it could be made to work. A mini-split does draw in enormous volumes of air, so your earth tubes would need to be able to pass enormous volumes of air with minimum friction losses. Air friction decreases with greater diameter, and air friction increases with greater length.

So your earth tubes would need to be long (maybe a couple hundred feet), to pick up sufficient warming or cooling from the earth, and they would need to be large in diameter, and/or many tubes. You'd have to do quite a lot of earthwork to install the large diameter tubes deep in the ground, but once installed, you'd have a wonderful thermal advantage with no moving parts.

So nokiasixteth, I'm all for your idea, I think it is absolutely terrific!

If you have the property and can get the tubes and can do the earth work, I'll do what I can to crunch some numbers to help you out.

But you'd have to promise to take loads of photographs of your project.

-AC

[nokiasixteth]

Ive been givin it some thought . But have thought about a dc water pump on a solar panel on one of the wells i have . Build a box with a large enough radiator so it would get the constant temp that way prob a bit cheaper