What is a reasonable delta between deep soil temp and entering water temp? On the one hand 10 miles of bore per ton would get it near zero but  that's not feasible.

On the other hand, a 20 F delta here in Florida results in EWT of 90+. At that point EER / SEER is not much better than air-source, so why bother with the $10k+ worth of drilling?

For the purposes of this question please leave DX aside - 2 stage equipment is highly desireable here owing to its ability to handle zoning as well as stretch out run times for better dehumidification.

I have a slinky system with 1250' of pipe per ton. My loop delta is about 7° Right now my loop temp into my heat pump is 63°

How many feet of bore hole per ton did you get?

 

But isn't loop delta highly dependent on water flow and size of the heat exchanger?

I.e., my loop delta is only 4.2°F at the moment.  But I'm pushing 14 gpm through my 3-ton heat exchanger that's actually only running 1st stage cooling at 70% capacity - 6.7 gpm/ton.

Best regards

Bill

Yes. 

I was hoping you would let us know how your loop is doing.  If engineer has a loop delta of 20°,  I think he has a problem.

Didn't get any - mine's open loop - In at 70, out at 83, driven by artesian pressure so no pump. Could run more GPM but hate to waste the water and dropping the leaving temp with extra flow diminishes the desuperheater's effectiveness. Hourmeters suggest I'm better off with an extra 1/2 amp or so into the unit in exchange for reduced time on the water heater element (wife and 3 kids - much hot water)

At any rate reason I've asked is am both debugging another house (beach front, 6 tons, 7 3/4" bores @ 250', EWT 103 LWT 113 / 116 low / high stage) and trying to determine if my builder's prior practice of accepting 90+ EWT is the best we can do going forward - I hope not.


The beach house likely needs another pump - 3 psig across machine suggests barely 14 GPM across 7 bores - one or more may be in laminar flow. That it is on the beach causes a local driller to say that deep soil temp is 80 rater than 70, and that tidal action flushes loop field soil twice daily. Interesting, but I'm not totally on board with either idea.

WaterFurnace Envision NDV038 (3 ton), running 1st stage: 79° EWT, 4.2° DeltaT, 14 GPM, 30 KBTU/h HR into vertical loop field, 8 holes, 300' deep each.

Inside avg RH: 40%

Outside temps: 85° at the moment, 103° high for today, 77° low for today, 24 degree-days today, 87° avg temp for last 30 days

Location: Dallas area

Best regards,

Bill

Let me get / keep this straight - 20 F delta EWT / LWT (unit delta) would mean not enough flow through unit. EWT 20 F above ground temp suggests not enough loop, laminar flow problem or some other earthside issue...or perfectly acceptable...that's what I'm after

A six ton unit should have a minimum of 18 GPM on a closed loop. The tidal action flushing the loop is suspect. If this were truly happening the loop temp would not be 103°.

The other possibility is that the loop has not been properly purged of air. If air is trapped in any of the the loops, they can not be flowing at all.

My first course of action would be to flush the loop booth directions several times.

Do you have a pump as a backup? Can the Artesian pressure drop to a point where it now longer works if too much water is pumped out of it, such are development in the area where there are thousands of small wells or several larger municipal wells? Also how does one control the GPM with no pump?

During my debug trip I asked if I could get at the manifold, figuring if any loops were not flowing at all they'd be at or near ambient. Unfortunately the manifold is buried 2+ feet below grade, I was told.

I agree that 18 GPM would be better, both for delta T as well as turbulent flow. System now has a single Grundfos 26 pump with a spare space in the flow center for another. Another pump seems a no-brainer.

It was flushed when the unit was upsized from 4 tons to 6. I didn't witness that.

Thanks for the thoughts and tips.

Y'all oughta hear what the air sounds like in low stage when only the bottom floor (~4 6" registers) zone is calling. 1100 CFM through those 4 registers makes conversation difficult. Actually it probably isn't 1100 CFM as I'm quite certain (don't yet have a manometer) the static is well above blower rating.

TG - I've lived here 13 years and the artesian well has done its thing all that time. The head is presently 2-3 times what the WF needs (low delta P across heat exchanger was one of the reasons I chose WF). I valve and restrict flow at the output as advised in install guidelines.

If I were you, I would purge the system myself. IGSPHA recommends two feet per second flow to insure proper purging. For 3/4" pipe this means 4.3 gpm.

If you have 7 loops in parallel ,then you need at least 30 gpm through the system for purging. It is also important to flush both ways through the loop.

With the 14 gpm you have now divided 7 ways you have 2 gpm through each loop. The WF loop calculator returns a Reynolds Number of 2351 at that flow. Reynolds Number of 2500 is recommended to achieve turbulent flow. If you increase the system flow to 18 gpm then you would have a Reynolds Number of 3022.

Hope this helps

It does - thank you.

I last studied Reynolds numbers in 1987. It is on my to-do list to reacquaint myself with them soon.

A flush cart is on the list as well. The flusher in this case was the regional WF distributor who has a vested interest in the success and proliferation of geo systems, so I'm confident he put forth a good effort, but I wasn't there.

I have a suspicion that the marginal-for-7-loops flow now present is not evenly divided between loops simply because nothing theoretical works perfectly in the field, so some loops are quite likely at least a bit starved of flow.

You may find the regional distributor also has an interest in helping you flush the system now.
Great thread.

Funny you should say that - he's the guy that did it when they upped it from 4 to 6 tons. I'm told he ran a 3/4 HP pump for 30 minutes or so. Does 3/4 HP sound like enough for a single flush cart on a 7 x 3/4" loop field?

Builder also said he used several coolers full of ice to chill the loop water - neither of us knows why?

A 3/4 HP pump is pretty weak. If he only ran the pump one direction he could go for 24 hours and there could still be air in the loops. When you think about having to push air down 250' you can see how easy it would be with a small pump to not purge the air out.

Our purge cart has a 2 HP pump. I would say that the chances are pretty good that there is still air in the loop.

Is a pool pump (high volume, relatively low pressure) appropriate for this use?

This is the purge pump that we use.

It is high volume high pressure.

I think this is a fascinating question.  I wish I understood the science of heat transfer from water in plastic pipe to the earth.

I've been thinking about this for a while.  Here's a summary of my thoughts.

It would seem you can get whatever delta (EWT vs deep soil temp) you want, based on depth of bore holes, size of pipe, configuration of the pipe, type of soil, and grouting material.  And, I think there are significant s/w programs that allow you to model the borehole heat exchanger field's entering and leaving temps.  I know, for example, WaterFurnace's authorized and trained installers have access to (or have to purchase) loop field design software.

In your case, I looked up your 'down deep absent a borehole field' constant year-long temperature approx for JAX: 71°F.  So I think the question is, what kind of vertical loop field design is required to keep EWT lower than some number, like 80°, or 85°, during periods of prolonged heat dumping to the ground, taking into account all of the thermodynamic properties of the borehole field.

In my case, my 'down deep absent a borehole field' temp constant is 68°.  Since I've been keeping track, my EWT has ranged from 59° (27 weeks ago) to 84° yesterday - see http://welserver.com/WEL0043/EnteringWaterTemperatureDailyAvg.gif .

While most of the dynamics I don't understand, one thing that's obvious is that my borehole field cannot absorb the amount of heat at the moment I'd ideally like it to (which for example yesterday was 820 KBTU for the 24 hour day, a high for the year so far - see http://welserver.com/WEL0043/DailyGroundLoopHeatTransfer.gif ).

So a key observation is, I think, it looks like I'm gradually heating up the soil in my borehole field, over a period of a few months, resulting in higher and higher EWTs, as long as I'm trying to dump so much heat into the ground.

Even on a minute by minute basis, my EWT changes upward of 5° depending upon time of day - it was at a low of 78° this morning, it was at a high of 83° a couple of hours ago (4 PM Dallas time), and it's now gradually coming down.  It's like the ground needs more time to absorb the heat than the pace my house is generating heat, and thus my system becomes less efficient (lower EER) in the afternoon than in the morning.  All with a steady 68° 'down deep' soil temp absent a borehole field.

To see the minute-by-minute changes of EWT across an 8 hour time span, I created a chart this morning for it - see http://welserver.com/WEL0043/TstatTemps-3TonUnit-Zoned.gif .  It's the red and pink lines (ignore the labeling).

And furthermore, as each very hot day goes by, the ground does not have enough time to absorb the previous day's load, and thus is gradually heating up.  So I would expect my EWT to keep slowly rising until the amount of daily heat I'm dumping starts to decline.

My guess is the sophisticated s/w that loop designers use can model all of this, produce the details of a borehole field design, that then enables estimation of cost of drilling (# of holes and depth) and cost of materials (type of pipe, actual lengths of pipe, grout, etc.).

It would be interesting to see if your situation can be modeled to something other than 90° EWT, what the resulting borehole field design would thus have to be, and how different it might be from what you / your builder have been installing.

My guess is an investment into loop design s/w, along with copper tube brazing equipment, is going to be needed soon for your future plans.

Best regards,

Bill

It is not unreasonable that a borefield gains temperature as the day goes on. By late afternoon on a design day I imagine both your heat pumps have run several hours or may even be running nearly continuously.

I have also read that it is normal that a borefield warms as the cooling season progresses. It will be interesting to see if yours keeps rising until late August or September and then begins to fall back toward 68, and then below 69 when you begin to heat.

It would be nice if borefields stored heat from the summer for use in the winter, and conversely, but I gather it doesn't work that way.

At what depth did you hit water and what proportion of your tubes are grouted, if any?

Interesting subject - wish I was better versed on the thermal dynamics of heat movement in soil so I could understand what's going on.

The chart I created yesterday, showing minute-by-minute changes for EWT for an 8 hour period (pink/red lines on http://welserver.com/WEL0043/TstatTemps-3TonUnit-Zoned.gif ) continues to illustrate the dynamics, but unfortunately I don't understand the reasons for what it's showing.

EWT gradually dropped through the night down to a low of 76° at 5:30 AM this morning.  All that was running last night was my 3 ton unit running at 2.1 tons (1st stage only).  The amount of heat needed to be rejected into the borehole field obviously was small enough to not only be completely absorbed, but also small enough that the earth had time to dissipate heat from the previous day.  This is my layman observation for why EWT continued to drop throughout the night.

Then at 5:30 my 5 ton unit, running at 3.5 tons (1st stage only), started up.  It's rejecting enough heat into the soil such that the earth cannot instantaneously accept it, but, its 'duty cycle' is short enough that over this morning's hours EWT is drifting back down to 76°.

My guess is that once both units start generating x amount of btu/h, that's greater than what the borehole field can instantaneously absorb, then my EWT will start rising today.

Getting back to the original question, the design of my borehole field will govern how high my EWT will go today.  And the design will govern how the EWT high will gradually keep getting warmer, day after day, while I'm dumping between 600 - 800 KBTU / day into my borehole field (yesterday was about 740 KBTU).

Specific to your comments above, engineer:

My guess is a borehole field gains temperature as the day goes on only if x or more amount of KBTU heat is dumped into the field.  My guess is there's a certain KBTU number for which the field neither gains or loses temperature (which for my field is certainly less than the 600 - 800 KBTU / day I'm currently dumping in the field).

And in the Fall, the borehole field temperature falls toward the 'down deep' constant temperature because less than x KBTU / day is being dumped into the field.

My two GSHP units were sized such that they never get out of 1st stage, even in this 100+° weather we're now having.  The 5 ton unit is running on avg 7.8 hrs/day and the 3 ton unit 9.1 hrs/day.  Virtually none of the run time is in 2nd stage.  And the 3 ton's run time is high because of zoning (good illustration of how zoning is primarily for comfort, not for efficiency).

I think the earth's 'thermal conductivity' is too high for it to be a good heat storage facility from Summer to Winter.

The drillers hit water at about 30 feet - it was a mess! 

All of my holes were grouted, and, all of them were grouted from the bottom to the top.

Best regards,

Bill