I've got a vertical loop that I'm wondering if it's performing properly.  I'd value some comment, please.

A 50 hour chart of my loop's performance is shown below - it's also at http://welserver.com/WEL0043/EnteringWaterTemps.gif .  On this chart is Entering Water Temp (EWT) and Heat of Rejection (HR).  (Note that you have to add 70° to the chart's EWT numbers, and multiply by 10 to the KBUT/hr numbers.)

(The EWT numbers are 'sample & hold' numbers, to eliminate values when unit(s) are not running - 3 minute sample frequency.)

Here's where I'm confused.  My deep earth temp is 68°F (Dallas).  And I've got almost 1 mile (4800') of 1" HDPE U-Tube pipe in the ground.  Yet still my EWT gets as high as 84°.  (Over the course of a full 24 hr day, I'm rejecting into the earth about 750 KBTU, at a rate of about 30 - 82 KBTU/hr depending on which GSHP units are on.)

And I'm also confused because during the course of a day, I'm seeing an EWT swing ranging from 76° to about 84°.  It surprises me to see an 8° EWT swing during a 24 hour period.

And lastly, I don't understand why, long term, my average EWT seems to track the average ambient outside temp.  I thought that the deep earth temp stays constant, at 68°.  Yet, this chart here, http://welserver.com/WEL0043/EnteringWaterTemperatureDailyAvg.gif , shows clearly that my average EWT tracks very nicely to average outside temp, which doesn't make sense to me since I have a vertical loop.

I'd appreciate hearing your thoughts.

Here are some footnotes with technical details:
8 boreholes, each 300' deep, each spaced at least 20' apart
Don't know how each hole is interconnected
Damp silt/clay soil, holes grouted (but not with anything special)
No hot water generation
Don't know my home's cooling load
Heat Rejection (HR) rates (30 - 82 KBTU/hr) are real time and accurately measured
Water flow 14 - 21 gpm, dependinging on which units are running.
Two units - WaterFurnace Envision 2-stage variable speed - 3 and 5 tons

Many thanks!

Best regards,

Bill

What was your EWT at the very beginning of the cooling season? Fairly close to 68, maybe lower if during heating season you had "stored some cold"

Do you have auto-changeover between heating and cooling such that in the early AM in late winter or spring you were heating and then later that same day cooling? What were your EWTs then?

My based-on-zero-actual design-info hunch is that your loops are working relatively well - your peak daily EWT will continue to slowly climb for the next several weeks and then begin to tail off just a bit during early fall.

I'm not suprised at the daily swing as well - ground within several feet of your tubes warms during the afternoon and cools a bit during the night

All those tens of millions of Btus you have rejected are building up underground. I guess much depends on both your ground's conductivity and any lateral flow in your aquifer(s)

Engineer,
Would adding another 300' deep well lower the increase in EWT?  Disregard the additional cost.

Probably, but not but much more than a degree or so.

A couple other issues obtain:

1) location of new well in relation to rest of field - the further away the better

2) Another borehole could require more flow - Bill has 1" lines - standard for residential is 3/4". While extra diameter provides more surface area for heat transfer, it also increases minimum flow for turbulent flow, which is even more important for heat transfer. Bill is already 'off the chart for water flow through his Envisions - if another borehole in parallel with the existing ones increases required flow for turbulence, he would both be further off the chart as well as possibly need more pumping power. Would the slight reduction in het pump wattage owing to marginally lower EWT be partially or totally offset by increased pump wattage - I'd say there is a good chance of that being so.

Bill,
Your loop is working fine. I hope nobody told you that you could pump all that heat into the ground and not see a rise in ground temp :) What is the maximum EWT for your unit? If your loop gets just below that temp before starting to decline then you are right where you want to be. Certainly more pipe in the ground would give you lower loop temps but the cost would have been substantial.

Most home owners are not at all aware of how their loop temps go, you are the exception. As has been stated above, the ground can absorb heat , just not as fast as you are pumping the heat in.

If the EWT gets to the point where your unit won't run, then you will be having a serious talk with your designer / installer. Let's hope it never gets that high.

Whoa!

WF Envision can take EWT of up to 120, but I can't imagine designing a loop field to deliver an EWT of 119 Deg F at summer's end

A 5 ton Envision at 120 EWT delivers less than 5.5 tons of cooling with an EER in the low 9s, which probably translates to a SEER of 7 or so. Welcome back to 1980...disco anyone? I can't imagine that operating point is a desireable design goal...

Am I missing something here?

I very much appreciate the comments.

I think I'm learning - some key understandings now:

1.)  There's an 'economic balance point' between performance (EWT and thus EER) and vertical loop size.

2.)  The earth can't absorb heat at the rate of 80 KBTU/hr very quickly, even w/ a vertical loop, using traditional techniques appropriate for small, residential-style projects.

To answer questions from above:

My WaterFurnace units don't have a max EWT per se, but, I think EER defines a 'practical max.'

I.e., EER is only 9.3 at 120° EWT (for my 5 ton unit).  But if I assume a 'lowest desired EER' of say 15, then I need my EWT to be no higher than about 94°.  Hence, perhaps 94° EWT is my 'practical max.'

Turning this around, if I assume my EWT won't go higher than 85° this year, then my 5 ton unit will have a worst case EER of about 18 during the the afternoon / evening hours (and about 21 in early morning hours w/ 76° EWT).  And this is probably just for the month of Aug.

Hence, I think I better understand the comment that my loop is performing well.  It would be extraordinaly expensive just to add one more well.  And probably if I had 7 instead of 8 wells I'd still stay below 90° EWT to maintain a minimum, during-the-heat-of-the-day 15 EER.

I further see how helpful geothermal loop design s/w can be, and that it's probably a 'must' to ensure a loop performs 'economically.'  Put in the geographic location, earth conditions, type/size of tubing, hole spacing, total tons, desired max EWT (based on desired min EER), flow, etc., and out comes the amount of borehole length.  I'm sure it's more complicated than this, but I'm voicing 'out loud' what I think is a good summary.

Other comments:

engineer, you're right!  EWT at beginning of cooling season was 67°F - "fairly close to 68°."  EWT wasn't much colder than 68° because here in Dallas we don't "store much cold."

You can see this on the chart below - at about week minus 18, about beginning of Apr.  Chart is also at http://welserver.com/WEL0043/EnteringWaterTemperatureDailyAvg.gif .  We flipped over from heating to cooling - no autochangeover.  You can see the sudden rise in EWT at this point.

Don't have data on how much EWT 'swing' was during a 24 hr day back at this point.  Probably minimal since I'm sure my HR was in the range of 60 KBTU/day, not 10X like it is at the moment.

Many thanks!

Best regards,

Bill

A remaining question, getting back to past week's discussion on whether or not my water flow is too much:

Is it possible that my 'off the chart' water flow is not giving the earth optimum opportunity to absorb the rejected heat?  Is it possible I'm 'pushing through' some portion of the heat right back to the EWT because of my high volume?

And that if I were to somehow slow down my water flow, my 'earth heat exchanger' performance would improve?  By a little / lot?

And is it obviously so, or, do I need to invest into an inexpensive loop design software package to enable accurately doing the 'what if' analysis?  (Or are only the good design packages the expensive ones?)

I can't / won't change number of loop holes, loop piping, etc, as it's not economical to do so.  But I can adjust water flow via changing pumps in  perhaps in an econmic manner.

Thanks,

Bill

No, that's a comon fallacy. A heat exchanger doesn't lose efficiency with too much flow. Higher flow means higher efficiency, though you certainly get to a point of rapidly diminishing returns. In your case I think you could see efficiency gains from reducing flow, but only from reducing pumping work. And without changing pumps you might not drop the power that much by just throttling the flow. It depends on the pump efficiency curve.

So are you saying that there's not a relationship between EWT and flow rate (within reason, assuming all paratemeters are within normal operating ranges, like turbulence, Reynolds number, etc.)?

And thus EER cannot be optimized by adjusting flow rate?

I think what you're saying is that, for my situation, I can improve performance (EER) via a slower flow rate, but, the performance improvement will be limited to the lower power consumption necessary to move the water at the slower rate.

This is very helpful, and much appreciated.

Best regards,

Bill

I think that's what I was trying to say... There is definitely a relationship between EWT and flow rate (EWT will approach the temperature of the earth near the pipes as the flow increases). As you say, though as long as you maintain turbulence in the pipe there is little benefit to increasing flow beyond a certain point. There is definitely an optimum flow for EER or COP as long as these numbers include the loop pump power. If you reduce your flow, the compressor power increase will since the EWT will rise slightly. If the pumping power is reduced by more than the increase in compressor power you'll have a net gain. Just throttling the flow might not realize any gain depending on the pump efficiency, though it would show you the relationship between EWT and flow. If the reduced flow makes the pump operate in a less efficient area you might not save much power at all. Unfortunately efficiency curves for the typical residential circ pumps used in geo systems are hard to come by. The manufacturers are probably embarrased by how low the efficiencies are and I don't blame them. I wouldn't publish an efficiency curve if it topped out at 20% either!

Take for instance in heating our pool. If I can raise the water temp by two degrees at 40 gallons per minute or .5 degree at 160 gallons a minute, which is better. I choose 2 degrees at 40 gpm because it uses 1/4 the wattage at the pump I am getting the same amount of heat out per hour.

I think this is the same situation you would have slowing down the water in your field. Instead of 80F return you might see 70F, but if it's at 1/2 the flow rate the same overall BTU's will gained (or lost), so it won't really change anything with the exception of lower pump electrical consumption. I don't think the COP would really change since again the same BTU's are being transferred.

My guess is the ground immediately adjacent to the pipe is warming up as you dump the heat and it takes a bit for the heat to radiate out and cool back down.

I am not sure if it would help or not, but what if you could run a circulation pump 24/7 and see what the temps are without the system dumping heat? It would be interesting to see the results.

Out of curiosity, do you set back the temps during the day or anytime? Or the loading we see in the charts due to sun/outside temps? Another way may be to take advantage of the lower demand time and drop the house temp a degree or two so you even out the loading on the loop? Or wouldn't that matter in the end? (I think I am learning more then anyone else here).

Cyngeo - BINGO!. I'm so tired of the fallacy that if fluid flows too fast through a heat exchanger total heat transfer drops since the fluid isn't in the heat exchanger long enough. People often confuse the reduced delta-T that results from higher flows with reduced heat transfer.

BINGO again for the comment that increased efficiency is possible via reduced flow but only from reduced pump work.

Bill, consider two scenarios in which I pick somewhat random numbers to illustrate - Say you run a 5 ton geo heat pump with 15 GPM and it has EWT of 85 and LWT (Leaving water temp) of 95. Suppose you can choose to pump it much much harder - say 30 GPM. With EWT of 85 LWT would be 90 (flow up by x2, so delta T down by x2). Of course the borehole field experiences the same 30 GPM so it takes the 90 F water and cools it back down only to 85.

In the 30 GPM scenario heat pump EER might be slightly increased by the LWT being 5 Deg F less, but that gain would be more than offset by all the extra pump power needed to go from 15 to 30 GPM. Note that in both scenarios EWT would be 85. Actually all the extra pump work would hit the loop field as extra heat, so EWT might even rise a couple tenths of a degree at 30 GPM.

It is important to differentiate between heat pump EER and system EER. The published EER includes certain amounts of energy for blowers and pumps, but overpumping will reduce EER

Your system may be slightly overpumped, based on actual verses suggested flows through your Envision units. However, you have larger 1" rather than 3/4" loop tubes which may well demand the extra flow to stay turbulent.

Other threads have mentioned that you are over pumping.  If you have two units that equal 8 tons and you need 3 GPM / ton then 24 GPM would be just right. 

The above thread says you are pumping 21 when both units are running.  We did the notion of overpumping come from?


On some of the commercial loop jobs that we do,  they buy as much loop as they can justify and then run a fluid cooler at night to keep the loop temp down when the demand is high.

Bill is overpumping individual units by a few GPM when only one unit is running - I think he calculated 21 GPM through his 5 Ton and 17 GPM through his 3 ton

Bill,

I have attached  three hypothetical loop designs for your house.  I used two cooling load values.  One at 60,000 BTUH  and one at 82,000 BTUH.

The 60,000 I pulled out of the air,  the 82,000 comes from your original  post.

As you will see,  for 60,000  your loop is sized properly,  for 82,000 you are short  800' of bore hole.  This is assuming we want to keep you EWT below 90°.

If we let your EWT go to 100°,  then your 2400' of bore hole can handle the 82,000 load.  This is shown in the third attachement



 Since your units never run on second stage,  I   think your cooling load is closer to 60,000 than 82,000.

Hope this helps

New Topic dovetailed w/ "GeoExchange"

Has anyone used "GeoExchange" for radiant hydronic cooling?

Cost return? Do they function well enough to forego a/c units? There is a lot of confusing (experts suggestions at home shows on barriers over existing slab before laying radiant. One stated no metallic the other stated foam boards under fiberglass gypsum Thanks

Radiant cooling is tricky and not for the faint of heart. If you can't find a local contractor within proven expertise and experience in it I'd stay away.

Yes, we've done it with a radiant ceiling application. It'd need a good HRV / ERV in a more humid climate than here.

It's pretty efficient. In fact, the best way to do it would be to use the product at www.bekausa.com
These capillary tubes work wonderfully for heating, too. Usually the required supply water temp for heating is only about 90 deg. F. They are pricey, but they'd eliminate any need for duct. And, there'd be no need for the radiant floor with its complications.