Hi all,

Been thinking about retrofitting our house for geothermal. Here are some of the specs. 2000sqft well insulated house in Virginia. I did a estimated load calc and the heat loss is somewhere in the 50,000-60,000 range. We have a well, it is 400 feet deep. It is 8 years old and when originally dug gave us 12gpm. Below is a water analysis. The well is about 150 feet away from the house and downhill from the house. From some online resources, it looks like our surface temps average 60F. I've never actually measured the temp of our water though.

I was thinking we could use an open loop system (pump and dump), but worry about getting rid of the water. We have had 2 droughts here in the last 5 years and the thought of dumping the water on the surface bugs me. I know it will eventually make it back to the water table, but maybe not my water table. Some of the options I have considered after reading this forum are to have a shallow (or at least shallower than 400ft) well dug for the dump. Not sure how much that would cost.

I also wonder if it would be possible to put the water back into the well? I know that typically doing so will the water temp will equilibrate so that heat or cold can no longer be extracted. I was wondering if the 150' run back to the well would temper the water enough? If not, can I do a few horizontal loops on the way back (fewer than a typical closed loop system though).

I also wondered about a DX system. I haven't found anything on this idea (which probably means it won't work), but can I put a DX system into my 400' well? The pH of the water is not too bad, and as far as I know, if the refrigerant leaks out, it should not mix with the water.

I don't know if either of these ideas would pass code, though.

And if anyone has a suggestion for a GT installer in the Charlottesville VA area, let me know.

Any thoughts?

BM99

[b]Water analysis:

________________________________________________________________________

Parameter MCL(mg/L) Result(mg/L)

________________________________________________________________________

[b]Total Coliform Bacteria[/b] <1/100ml 0/100ml

[b]Iron [/b] 0.30 <0.03

[b]Manganese[/b] 0.05 [b] 0.12[/b]

[b]Lead[/b] 0.015 <0.001

[b]Mercury[/b] 0.002 <0.0002

[b]Sodium[/b] 50.0 5.75

[b]Nitrate [/b] 10.0 <0.30

[b]Arsenic [/b]0.05 0.001

[b]Hardness(CaCo3)[/b] 250.0 57.60

[b]pH [/b] 6.5-8.5 6.51

________________________________________________________________________

[b]All testing performed in accordance with USEPA approved methods[/b]

[b]MCL[/b] = Maximum Contaminant Level > = greater than [b]<[/b] = less than

you also have to keep in mind that pumping costs for a 400 ft well could very easily eat up your savings.

Bartman,

 

How old is your home?  What type of heating system do you currently have?  If you take your heating bills for the last three months and look at the amount of fuel you used compared to outside temps do you get similar heat loss numbers as your calculation?  Your calculation seems a little high for the average to me.  

 

You maybe better off looking at a DX system.  I don’t think your well at the 12 gpm will cut-it if your calculations are right.

 

Well looping has a very high risk of not working.  I would look to something a little lower on the risk factor.

 

Please keep us posted on what you end up doing.

 

Eric D

Our house was built in 2000. It is well insulated with 2x6" construction and blown cellulose insulation. The ceilings are cathedral (2x10) with blown cellulose, rigid foam and a radiant barrier. We have a wall of South facing windows, for passive solar gain and a lot of thermal mass, which may have affected the calculation. We heat with radiant floor heat. The "boiler" is a high efficiency propane Polaris water heater. This water heater also provides DHW. Propane is also used for the clothes dryer, for cooking and for grilling. It is therefore very difficult to tease out how much we use for heating. Last year we used 715 gallons of propane at ~$3/gal. Electricity here is 6-10cents/kwh. Why are well loops risky? Can we put a DX system into the well? Thanks, Bartman

The electricity consumed by the "pump" part of pump-and-dump can easily gobble up any energy savings gained by substituting water source (geo) HVAC in lieu of air source (what 99% of your neighbors have, at least for AC).

Why?

Two reasons:

1) Well pumps are quite inefficient, 40-50% at best

2) Well pumps delivering domestic water operate at 5+ times the pressure needed in a typical geo heat exchanger - 30-50 psig vs 10 or so feet water column. All that extra pressure is unneeded but costs a lotta Watts.

Closed loop geo systems operate a very small (~1/10 HP or so) circulating pump designed and sized to move fairly high GPM but against very low differential (~10 ft WC)

Using a regular well pump to supply an open loop geo system will in all likelihood be an exercise in futility. The geo compressor + well pump amps will exceed that of an air sourced heat pump's amps. The net result will be a system costing much more to install AND much more to operate.

Do something else...

So are there more energy efficient well pumps available? Not for geothermal, but just to make my pumping of my well more energy efficient. BM

You keep harping on about how inefficient well water pumps are. There should be a pump that is able to supply water to the geothermal system without having to have the pressure so high. My next thought is the well pump simply pumps water into a tank and shuts off when it reaches a certain pressure and on again when the pressure goes below a certain point. Surely this range can be adjusted to a lower pressure setting. There must be something you can do to maxize the effeciceny of an existing pump, even if it requires replacing the pressure tank with a larger one. 

Qoute:
Using a regular well pump to supply an open loop geo system will in all likelihood be an exercise in futility. The geo compressor + well pump amps will exceed that of an air sourced heat pump's amps. The net result will be a system costing much more to install AND much more to operate.

I have to disagree with you here, while it's true a close loop or even a DX system is more effiecent than an open loop, open loops are much more effeicent than oil or gas. This winter I paid about the same to heat my 3500+ Sq Ft house as some co-workers townhouses that use natural gas. Given that the wells already drilled, the costs of installing an open loop system are minimal when compared to a closed loop system. ideally we all should have a DX system, but not all of us have 30k or more to spend on a new system. I think even an open loop system is a step in the right direction. While it may be true that some areas of the country it might be more feasiable to install a air source heat pump, I'm sure the same is not true for all parts of the country. 
 

Ok, I wasn’t going to chime in on this topic because I felt it was an individual’s opinion and everyone is entitled to one, but the notion that an open loop system is inferior to other heat pump systems, including air to air is very misleading to people coming to this site to learn.  

 

To have a better understanding of an open loop system lets compare some real numbers.  As an example, I have a half horse power submerged pump.  It draws 4.8 amps while pumping.  It produces 30 gallons per minute at 35 psi, and at 45 psi the flow rate drops to 26 gallons per minute.  The pump operates in a range of 35 to 50 psi.  My 3.5 ton open loop geothermal unit uses about 6 gallon per minute.  So, the geo system uses roughly 23% of the pumps capacity.  If the geothermal is running at 100% duty cycle the well pump draw is about 1.1 amps, or .2kW/h (23% of the 26 gal per min @ 4.8 amp).  This compared to a closed loop system that has two pumps for push pull at .75amps each.  They run 100% of the time the geothermal is on.  So, .75 + .75 = 1.5 amps or .3kW/h.  Bottom-line, taking into account the well water will in most cases have higher inlet temps then a closed loop system; one could argue that the open loop is more efficient then closed.  The fact of the matter they are both close and it becomes more of an installation dependent issue then the type of system. 

DX has some advantage over both closed and open loop in transfer losses due to the exchangers and a few other variables.  Installation cost might offset this efficiency savings.  

 

To the question of improving the pump efficiency, you can on some systems.  Depending on make of well pump, some of them have a controller that can save about 25% on current draw.  Franklin pumps are an example.  They make two styles of controllers.  The most common used is a capacitor start, inertia run.  This controller uses more current then the second controller that is a capacitor start, capacitor run.

 

There is a third option, but the cost maybe more then what might be recovered over time.  It is called a soft-start system.  The pump controller regulates its motor speed to control the volume of water needed to hold a set pressure.  No pressure storage tank is needed.  This is a relatively new system that works well, but the cost seems to be too high yet in my opinion.  Advantages that are claimed are longer pump life due to the soft start, more constant pressure supply and higher efficiency overall.

 

Hey, this is my opinion and I’m sticking to it until someone can show why I should change it.  

 

Regards,

The pump arrangment is quite different in a pump and dump vs. a closed loop. Think of a teeter toter. Closed loop ex: The load is balanced on both ends and it takes very little effort to "lift" the load. Pump and dump ex; only one load is placed on the teeter toter, the force to "lift" the load is equal to the load itself. In a 400' well, 1-1/2" pipe there is 5 cubic feet of water, over 200 lbs?

A pump used in a closed loop system could not pump from a well. I'm not going to mess with your amp calcs because that is a more complex method with many unknowns. If you just stick with Work=F*D, the above examples will clearly show much less energy is use the circulate a closed loop. A siphon shows that as long as the exit is below the intake water will move. As you raise the exit the water slows until stoping. When even, it's ballanced and takes very little enegry to move it in any direction.

Patrick T

Patrick,

 

You make some valid points.  However, in most if not all residential closed loop systems, they are far from being “balanced” during operation.  If they were you wouldn’t need both a push and pull pump system.  The primary reason for two pumps is to overcome restrictions from the tubing, exchanger, along with areas of air entrapment in the system.  The circulation pumps have to be sized to overcome these inherent restrictions and flow enough fluid to supply the needs of the system.   For residential use I’ve only seen fixed rpm centrifugal style pumps for loop systems. No matter what the demand, they operate at full rate.  In an ideal world you would want variable speed pumps that draw only what is needed to circulate the loop fluid at a rate, the system needs, nothing more.

 

The best way to evaluate the goodness or badness of a fluid supply system for comparison to a different system would be current draw for the equivalent btu supply.

 

The question of which system is more efficient takes second place to what the installation location can support.  In other words, if drilling was out of the question because of bad supply or not enough available water, then I might choose a closed loop system, be it fluid closed loop or DX.  This being the case, I personally would choose DX over a fluid loop because no other pumps are needed other then the compressor.  

 

In my opinion the only way a fluid filled close loop might be more efficient is if some company comes out with a variable speed close loop pump system.  Even then I doubt it would be more efficient then the now available variable speed well pumps.  If all else was equal on current draw between an open loop to closed, the delta T of the open loop supply would still win on efficiency.  Again, just my opinion and I’m still open minded that there maybe something better out there then what I seen and worked with.

 

Regards,

There are really several distinct issues here. One is water temp - open loop systems will generally run closer to the mean ground temp all year and will have an advantage in efficiency for this reason. Also factoring into the efficiency is the pumping work as you all have noted. This really comes down to a case-by-case issue, but in general closed loop systems will require less pumping energy. The energy cost of pumping comes down to:

required (GPM X Head Loss) / Pump Efficiency

For an open loop system the GPM required may be slightly lower due to the higher temperatures available.

The head loss is the difference in height between the inlet and outlet plus frictional losses in the pipe and heat exchanger. For open loop systems this will generally be dominated by the actual distance the water is lifted, though depending on the design pipe losses might be significant too. Closed loop systems by definition have no elevation change, so the head loss is 100% pipe and heat exchanger friction loss. Again, the total head loss varies for each individual setup. My 6-ton closed loop system has a head loss at 15gpm of about 32 feet (and that's too high in my opinion due to a marginally sized heat exchanger). So an open loop system pumping from a 100 foot depth would require more than 3 times the energy to move the same amount of water.

The other factor in the pumping work is the efficiency of the pump. As Eric correctly pointed out, the small "wet-rotor" circulators like the Grundfos typically used are horribly inefficient. The generally recommended pump for systems in my size range is the 26-116, which is only 22% efficient at 15GPM! (I don't mean to pick on Grundfos, they are no worse than other circs of the same type and seem to have a very good reliability record). A good variable speed well pump might easily get into the 50-75% range at the same flows. So yes, in some cases a closed loop system will use a similar or even greater amount of pumping energy as a shallow well system.

To put these losses in perspective, the average compressor power over the course of a season for my unit is about 5kW. If I was using typical wet rotor pumps my pump power draw would be about 410 Watts, so my COP would take an 8% hit. Too high in my opinion, but not all that bad in the grand scheme of things. As it is, I used an Armstrong E9 pump which is nearly twice as efficient at the same cost, so my actual pump power is about 210 Watts, less than 5% of the annual energy use of the heat pump, or $50-60 per year.

Also, there is no inherent reason that a closed loop needs a push-pull configuration. It tends to work out well since larger heat pumps usually end up needing 2 pumps so you can split the major pressure drops in the system (heat exchanger and loop field), but a single larger pump works just as well. From what I've seen, a lot of the closed loop systems out there are overpumped and wasting energy, the result of a limited selection of fixed speed pumps that have to cover a wide range of pressures and flows.

Hi,

OP here.  Getting back to my original question.  Assuming I am willing to eat the potential energy loss with an inefficient well pump or am willing to install a new higher efficiency (soft start) pump, is it possible to use a well water in an open loop system such that the well water returns to the well?  It was mentioned earlier that this is risky?  Why?  Is it because the temps. may equilibrate and no longer be able to heat exchange? Some other reason?

A few points to ponder:
Our well is 400 ft deep.  12gpm.
The well is ~150' from the house

Is there a way to calculate whether the heat exchange with the water going back to the well and the fact that it is 400' deep will provide enough heat exchange so that it never reaches equilibrium?  Would adding a few short horizontal loops also help?

Thanks,

BM

That is the risk.  If  temp does not change much, then this should work great.


Running the return through a horizontal loop probably won't do much since the ground will not be as warm as your well water.


If you want to test your well,  pump water out, run it through a water heater so that it is 10° warmer and send it back down the well.

Monitor the temp coming out of the well,  if it doesn't change much over several days then you might be in business.


You need to understand that what you  are asking is not the norm.  Probably no one on this board has done or seen what you are asking so we can't tell you if it will work.

You certainly have a few things working in your favor.

I hope it works out for you.  Let us know.

Thanks for the input.  After digging around some on the internet, it sounds like what I am proposing is similar to the "Standing Column Well" design.  Does anyone have expereince with this type of system design?

BM

Hi BM,

I was just reading the following article today of a commercial GSHP installation using 1500' Standing Column Well. This SCW design locates the pump near the top of the well to ease pump maintenance and reduce pumping head pressure.

http://www.philly.com/philly/health_and_science/20080416_Energy_from_the_ground_up.html

Searching further into this I believe the well designer is 'ARB GeoWell Systems' (American Refining Biochemical)
located in Conshohocken, PA. Check out http://www.arbgeowell.com/technology_products.php.

Regarding "estimated load calc and the heat loss is somewhere in the 50,000-60,000 range." This seems to be very for a well insulated 2000 sf house. I am located just above you in Madison, VA and have a 2500 sf home. Using HVAC-Calc, my heat loss/gain came out to be 38,000/24,000 btu's. How did you come up with the load calculations? I just installed a 4-ton GSHP this year and it handled the high teen cold weather we had in February.

Bob G.

BM,

I had contact four contractors. Never heard from two of them, played phone tag with one and one came out but could not get a quote from him. So I did the work myself. I sub out the duct work and trench excavation. I had some problems when I ran it rock. System is working as design.

Thanks,

Bob