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Old 04-10-18, 11:40 AM   #1
Blake.80486
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Default 3Ton Ground Source

I don't have the room for a horizontal loop.
So I'm debating between an open loop (well to well).
Or vertical closed loop.

I don't know for sure, but I suspect the area has pretty hard water, so an open loop is high maintenance.

I believe there's about 100' before I hit bedrock.
Normally you'd need 600' of borehole for a 3 ton unit.
I think I'd get better efficiency if I don't use the top 20' (sins the soil will cool so much in the winter). So 80' per bole hole. So 8 holes.


My idea for was to drive 1' black iron pipe down with the end capped off and sealed. The end would have an auger like bit, that should let me wind it in like a screw, and wind it out if I hit a rock.

Once done, insert a 1/2" pipe all the way down, that will give me a feed and return for each.

My reasoning is that 1" iron pipe should have the same conductivity as a plastic pipe loop. A 1" pipe has a circumference of 4"
a 1/2" pipe has a circumference of 2.6"
being metal should make up the difference.

I should be able to do that for $2500

Black iron pipe should last 50 years, the only other option is stainless (any coatings would scrape off). And that would triple the cost.


What do you think?

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Old 04-13-18, 08:09 AM   #2
jeff5may
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Ok your armchair calculations are way off. A 1 inch pipe has a circumference of pi inches, plus a little bit for pipe thickness. Half inch pipe has about half the circumference. Iron is not that much better at conducting heat than poly tubing. The limiting factor is the Earth. More holes spaced far enough apart equals more capacity, don't short circuit yourself trying to get more cold into the Earth (don't make a big underground popsicle). The first two feet might not transfer much heat, the rest will. Try one first, then decide if you are willing to do more. The process sounds difficult, but actually performing the process depends on details. It can be impossible without the proper mindset and equipment.

Read the many examples of what you are trying to do both in AC Hacker's manifesto thread and the individual sagas of others on this forum. With a pump and dump system, you will have all the capacity you need, if your source well will produce enough water and you go big enough on the well pump. A coaxial heat exchanger is highly recommended over a plate exchanger for that configuration. Both a borehole loop and a pump and dump system need a water pump, but the pump and dump system will require a lot more powerful pump to overcome the well head.
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Old 04-13-18, 10:40 AM   #3
Blake.80486
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No need to be condescending, here are my "armchair calculations"

Spacing of holes gives you thermal capacity, surface area gives you thermal conductivity.

1" pipe has an outside diameter of 1.3"
1/2" pipe 0.84", so a 1/2" pipe loop has 29% greater surface area.

Cast iron has a thermal conductivity of 55 W/m K
Polyethylene pipe 0.5
Modelling conductivity of dirt is vary difficult, a static number for wet clay soil is about 1.5

Either way, the 200' per ton is a vary inaccurate estimate.
Keep in mind, I was thinking 640' of pipe for 3 ton would work, but it's pretty easy to measure inlet and outlet temp on the heat pump and decide if you need more.
A pump and dump system is definitely more efficient, cheaper, and easier to install. Unfortunately the property doesn't already have a well (municipal water).


We haven't yet decided if we're going to live here long term, or sell it in a few years (if say I need a bigger shop). So I wanted the system to be more idiot proof, needing to descale the heat exchange every fall isn't something that helps sell a house. But I don't mind doing it for myself.

But a better option might be to just convert the heat pump to air source as an AC only system before selling. I would be keeping the natural gas furnace as the auxiliary anyway.
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Old 04-13-18, 11:59 PM   #4
mejunkhound
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re"drive 1' black iron pipe down with the end capped off and sealed. The end would have an auger like bit, that should let me wind it in like a screw, and wind it out if I hit a rock.
"

A long long time ago when I was 20 YO I though to drive a pipe 20 or 30 ft deep.
Hah! Found out about sol friction real fast!

You will be lucky to drive (or screw) 10 ft down unless you live on a swamp or quicksand.

Go online and read some texts on driving pilings or similar technology.

I am NOT being condescending, just have been there and tried that.

PS: do not forget Reynolds numbers, boundary layers, distance integrals, etc in thermal exchange calculations. (slightly condescending<G>)

There is also a thread on DIY well drilling. Not simple if you have rocks and no heavy equipment.
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Old 04-14-18, 12:17 AM   #5
Blake.80486
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Well, people have had no issue with 20-30ft depths with sand point wells by hand (look those up).

I was not intending to do it by hand.
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Old 04-14-18, 04:19 PM   #6
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If you have any doubt, go with an air source outdoor unit. They are boasting heavenly seer and heat gathering performance numbers, and the good ones can beat a pump and dump system if you have deep well water. The further away from the equator you are, the better a deep borehole system will satisfy you in the winter. But the Earth is still the limit here. How much worse will a one foot diameter popsicle gather heat with a one inch core than a half a inch? The world may never know...
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Old 04-14-18, 07:21 PM   #7
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Smile

Will have to remember that popsicle analogy!

Like th guy who froze his swimming pool solid
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Old 07-06-18, 12:02 AM   #8
phreich
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If you using 1 inch black pipe, then i believe it is made of steel -- not cast iron. It will likely have better heat transfer than cast iron too, because its crystalline structure will likely be much more orderly. Interesting idea of putting a smaller diameter pipe down inside the larger one as the supply, and using the remaining area in the larger pipe as the return.

Since the only real differences between black pipe and galvanized is that black pipe is painted black and galvanized is, well, galvanized, i wonder if it might be better to use galvanized pipe because the galvanization would help slow down the rusting of the pipe. The cost difference isn't that much, if i recall correctly.

I am thinking that another possible option would be to possibly put in a larger diameter steel pipe, and then insert a close fitting thin-walled plastic pipe as a non-rusting inner "sleeve", and then make the smaller "supply" pipe non-rustable -- either plastic or copper. I wouldn't be surprised to discover that there is already some sort of plastic sleeved or inside coated steel pipe manufactured already to deal with corrosive chemicals that might be useable for this.

One of these days I am hoping to leverage off of everyone's experience on this forum and do my own closed loop vertical pipe ground source heat pump.

Thanks for sharing your ideas and experiences everyone!

Philip
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Old 07-23-18, 09:12 AM   #9
Elcam84
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Yeah Black pipe is made of steel. It just carries the name iron pipe from way way back... Like Tin cans... It is painted black and not for direct burial. There is epoxy coated steel pipe made to be buried. And as mentioned there is also galvanized which in my experience the new stuff seems to rust as fast as bare steel.

Also remember that the fittings you buy are made of iron and extremely weak. You would need to use couplers like for a sand point well.

As to getting a 1" pipe driven/drilled down 80'. Not realistically going to happen. Yes there are guys that have driven sandpoints down to 25' but even in good soil its extremely difficult. I have sand here and sand is often hard as concrete when it has fine silt in it...

Also the size of the pipe is a big issue like mentioned. You really need a minimum 4" well to be feasible. However the issue there is cost. Unless you have your own drill rig its going to be extremely expensive. Here a well runs a minimum of $25 a foot. Thats minimum 15k just for one hole in the ground. If you want multiple shallow wells the price goes up even more because they have to move the rig.

I would love a ground source heat pump system myself as well but the cost of wells far outweighs the cost of electricity and with solar getting so cheap it makes more sense just to have more solar panels. As for ground loops they don't work here as the ground temp is too high in the summer unless you go really really deep. Our drinking water is 88* right now... If you live on a lake or have a pond then its the way to go as its a cheap install then.

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