Geo-thermal HVAC and wells question...

[Richard]

My understanding with a geothermal system is you can have an open loop or a closed loop. I suppose each has their pros/cons.

With the open system, one way to get rid of the water is to dump it into another well dug into the ground (or so I've read)

I've also read that this can cause issues with your main well as it might change the temperature of the water.

My well is about 300' down if I recall but, its flow rate is slightly over 100 gallons/minute. Seems we have a good flow of water.

So, if we did a geothermal system, dug a second "dump" well say, within 20/30 feet of my existing, would the flow rate of the well be enough to not be too worried about mixing of the two waters?

Second, is there any way to ascertain which direction the water flows down there such that we could perhaps drill a well "downstream"?

(my understanding is a well weeps water into the hole, it isn't like an underground river)

Thoughts?

 

[davesl708]

I have geo and use closed loop in three wells going down 175' each.
I am not familiar with the open loop. Seems it would not be that efficient.

Closed loop prevents contaminating your water table as the wells are back filled with sealing slurry to improve ground contact.

 

[LD1]

Actually, efficient isnt the concern. But open loop is more efficient.

I have horizontal loops, and 5-ton worth of them. That puts my system having ~100 gallons of water circulating through it. And in order to make its "heat exchange" with the ground it has to do so through HDPE plastic pipe.....and we all know plastic is an insulator.

An open loop with a pond or well is dealing with thousands of gallons of water, in DIRECT CONTACT with the ground.

As to the well....I think you are correct about it not being a river. While I dont know for sure cause I have never been 300' down at the bottom of a well, but based on what I have been told....think of it more like an underground lake instead of a river. Or better yet, a giant underground french drain.

Weather or not tying a geo in will change the temp of the water, I think has alot to do with just how big this "underground" lake is. You say it flows 100gpm.....but for how long? Odds are, its going to be hundreds of times bigger than what you would contain in loops on a closed system. So I'd say the concern is very minimal about having adverse effects on the ground water.

But with that said, I'd think long and hard before deciding to go open loop. IMO, the downsides far out weigh the initial cheaper cost. You have to deal with water quality, possibly have to run a water softener. So that adds to maintenance costs buying salt. Cupronickle coil is a must. And the cost to pump the water is going to be more. A closed loop system, where water is supplied and returned to the same point (flow center) there is no head requirement. Pumping water out of a 300' well with any kind of flow is gonna require more HP...thus more money just pumping water.

So, what is the cost difference between digging another 300' well, vs hiring/renting a excavator and putting in loops (if you have the land)

 

[Valveman]

Open loop geo works great and is more efficient than closed loop, if the depth to water isn't too great. It doesn't matter that the well is 300' deep. What matters is the depth to water while you are pumping. Even if the pump is set at 300', if the water level is 50', then the pump only sees the 50' of lift. It takes a lot of HP to pump from 300', but not much to pump from 50'.

You can do what is called a Standing Column Well. If the water level is at 50' and the pump is set at 300', you can dump back into the same well. By the time the dump water gets back down 300' to the pump, it has already cooled or heated as needed. This also keeps the pumping level of the well at 50', so it doesn't cost much to pump.

Pump and dump into a separate well also works. But if it lowers the water level in the pumping well, it will increase pumping cost. The further away the dump well from the supply well the better, unless you have a big flow down hole that moves the heat out.

Pumping water back down the well or into a separate well can also cause the hard water to clog up the perforations in the dump well. Over time this can cause the dump well to stop taking water and back up. You have to check the quality of the water to make sure this doesn't happen.

Also with open loop systems, it is important to not let the supply pump cycle itself to death. I hear from many people who say it doesn't matter how much energy a heat pump saves if they have to replace the well pump every 18 months or 3 years. A Cycle Stop Valve can solve that problem. And there are ways to lower the pressure from the well pump to make it more efficient by using a two pump system.

 

[GaryM]

My geothermal system is closed loop with four 180' wells. A neighbor has a closed loop system with the loop in the bottom of a pond. I went with closed loop for two reasons.

1. In Missouri a discharge well requires a permit from the Department of Natural Resources and I didn't want to mess with that.
2. A closed loop system lets you control the coolant's makeup and cleanliness. We have 25 grain hard water and I'd be somewhat concerned about that going through the pumps.

As always, your mileage may vary.

 

[Richard]

Wow, a lot to digest!

Some general comments (don't know if they will be relevant)

Been trying to find my fact sheet on my well....can't find it right now but I know it's here.

The well is either 400 feet deep, pump 10/15 feet off the bottom and 90 feet of water above the pump or, the well is 300 feet deep, pump 10/15 feet off the bottom and 90 feet of water above the pump.

Flow rate inside the well is 103 or maybe 107 GPM. The driller said he's only seen a couple gushers like this his entire life. There IS an underground spring nearby so my guess is we're pretty close to it. Aside from that, I am right next to a lake so maybe it's just lake water osmosing its way towards my well. I don't know.

We're on a TVA lake (Tennessee Valley Authority). I"ve been told but not verified that we could do a closed loop and bury the loops in the lake. Personally, I have a bit of issue with that because of getting snagged by fishermen. (I picture it simply being lowered into the water, not buried)

I have enough land where I could bury (what I recall being told) 1,200 feet of pipe underground. I don't recall if it's 1,200 feet out, 1,200 feet back or 1,200 feet total (been several years)

I have a gravel driveway that is my right of way. I could use my deeded "driveway" which goes through the woods and bury it on that. I also have a backhoe so am able to dig whatever trench needed myself.

Actually, that even makes me wonder if I could install most of it myself and hire out the final assembly...

Then again, the wife might put the nibs on this and just buy a new heat pump.

 

[LD1]

How many ton unit are you looking at?

1200' of pipe would be a 2-ton. 1200' out +1200' back would be 4-ton. If your area to dig is long and narrow, straight run would be an option.

But a manifold and slinkey loops are good if you lack room but have a little width. The total area of my field loops is only 60' x 80' for my 4-ton (which is actually 5-ton of loops. 4 runs of 800').

Sninkey allows you to put approximately 10' of pipe per foot of trench. But you need a 3' wide trench, and trenches about 15' apart. So 4 trenches for 4-ton, each 60' long and 15' apart is less than a 60 x 60 area.

Given that, which ain't a whole lot of digging, unless you lived right on the lake, a lake loop may not be best. My house is only 200' from my 1-acre pond, and I opted for trenches so I never have to worry about them if I go fishing, or want to dig out cattails

 

[techman]

There are a number of factors to consider:

- Open loop well is the most efficient, due to relatively constant water temp. Down side is water quality, and in your case pumping a 200'-300' lift will add cost to pump, and as mentioned pump lifespan issues.

- Closed loop has somewhat lower efficiency, but water quality is not an issue. You need approx 500' of pipe per ton. A long trench with a single pipe has issues with high pumping pressure and low turbulence (low Reynolds number) which can lower heat transfer. So it is common to put 2 or 4 pipes in a shorter trench, separated by a couple of feet. This lowers efficiency somewhat due to cooling/heating the area around the trench. A slinky loop is the easiest to dig, generally a shorter, wide excavation. A slinky has lower efficiency than a trench since the pipe per sq foot is higher, and again localized cooling/heating becomes an issue.

- Don't believe comments about the plastic of the pipe being an thermal insulator and affecting overall efficiency. The heat flow through the 3/16" of pipe wall is a magnitude better than the heat flow through soil. An average soil has about 1 BTU per foot per deg F. So with a 10 deg differential, you can only extract 10 BTUs per foot. The result is you either need a long trench, or the temp in the trench, compared to the surrounding 10 to 20 feet will get hotter or cooler.

paul

 

[davesl708]

One thing to keep in mind if you consider the horizontal loop. In my area during the hot summer time the ground temp at 6ft. down can get to 70 deg. during the prolonged heat spells.

You need to check your area.

 

[LD1]

One thing to keep in mind if you consider the horizontal loop. In my area during the hot summer time the ground temp at 6ft. down can get to 70 deg. during the prolonged heat spells.

You need to check your area.

70 degree ground water shouldnt effect the ability to cool the house. Its better than an air-to-air unit that is trying to do the heat exchange with 90 degree + outside air.

We can take 40 degree ground water and keep a house heated to 70 in the winter. Using 70 degree air to keep it cooled to 70 should be no problem. Geo's work in florida, and I am sure their ground water gets hotter than in KS