your unit should run more efficient in first stage, so I would try to make sure it ran in first stage as much as possible. I would think your unit would start in first stage and only switch to second stage if the program finds that its taking to long or not staying close enough to set point. I'm not aware of units switching to first stage rite before they shut off but I'm not an expert on all equipment.

Is your single a single zone or is your house zoned off?

John, good point. I forgot these units can run with loop temps as low as 20f. I see that in the manual now. I'll stop worrying about this and move on to something else

The installer wanted to protect the loops to just 30f. Glad I insisted on 20f for a safety factor. I understand that too much methanol can be bad for performance, but so is my loosing sleeping over the ground loop freezing.

HHCI, I believe the thermostat is downstaging to 1st stage when the temperature is close to the set point. From there, it's just a matter of whether 1st stage can finish the job of satisfying the thermostat, or if 2nd stage needs to be engaged again if the temperature drops instead of rising and satisfying the thermostat.

heimdm, if I understand your question correctly, we have 2 geothermal units (one for each floor) with 2 thermostats that do not "talk" to each other. The only thing the units share is a common loop and flow center. I'm not really sure what to call this setup with 2 separate thermostats and 2 units. If anyone knows, please enlighten me.

GM, need to thank you, after years here and a thousand of posts, new questions keep it entertaining. Fundementals are often the same, but new angles amuse....

I have to assume you did your due diligence and trust your contractor....
So regarding your "If a person waits until their loop temp is too low" question, I ask how was it designed, and how has it been operated?
If system has been "right sized" (IMHO) and auxiliary designed in, disabling aux. (as many here think is appropriate) risks exactly what you are worried about.

RE what happens at 32*, noting other than a reduction in COP. There is heat in an ice cube.

In other words, no one should interefere in the operation of your system without consulting installing contractor. They promised performance and operating cost that should be fairly accurate without our tampering.

While many of us here don't agree on design, we do agree that geo works and will save you money if left alone.
j

Ok Joe, you asked for it (a new angle that is).    This is a serious question though.

Let's assume our system can maintain 66f running in 2nd stage 100% of the time with no aux heat even though aux can freely turn on if called for by the thermostat (no aux lockout, aux breakers are on, etc.)

If we have our thermostat set at 66f, the geothermal system runs 100% of the time and is continually pulling heat out of the ground.

But if we have the thermostat set at 71f, the system will run until the inside temperature drops roughly 3 degrees to 68f (remember the system can only maintain 66f), and then aux heat will come on.  If aux is sized correctly, it will be sufficient to raise the temperature (with the geothermal running) back to 71 and the system will turn off and not pull any more heat out of the ground until the system turns on again.

So, which thermostat setting will maintain more heat in the ground for use later?  I assume not turning the thermostat down will retain more ground heat in this example.  This seems counter intuitive on the surface, but makes sense to me in this example.  So, is it better for a homeowner to set back during a particularly cold spell or not given standard design practices?

Anyone can feel free to comment on this question.

Is the answer to use the set point in the original design and not vary from that? Or, does it not make a difference?

Geome,

By your definition, the first scenario subjects the ground loop to maximum heat extraction, i.e., at continuous second stage rate for the period.

The second scenario, with the aux. help, allows for breaks. There is idle time when temp drops from 71˚ to 68˚, no heat is extracted from the ground. The heat extraction is intermittent, and possibly starts at the lower rate of first stage compressor operation. All this will result in a lower level of heat extraction for the same period under case two assumptions vs case one’s.

A couple of subtle points: Higher EAT in case two, I think, will result in slightly higher LWT, ceteris paribus. Higher air flow (stage 3) will have the opposite affect.

Regards,

Masoud

I guess I'm missing the reason for wanting to save heat in the ground for "later". A properly sized loop field shouldn't ever "fail" should it? My understanding is that the loop temps will hit their minimum temps at peak load periods but as soon as the weather moderates to "average" winter temps the loop temps will recover.

Am I missing something?

I would agree with what you have said.

That's been my opinion all along, but there is a competing school of thought that holds that a loop overtaxed in January remains hosed for the rest of the winter, leading to excessive aux heat use. Since that school of thought is attended by respected members here, I don't know what to think. I don't have meaningful winter closed loop experience sufficient to reach an independent conclusion. Some systems with 'Welserver' data gatherers might provide data to decide.

Perhaps it varies by soil composition - systems in dry soil may take longer to recover

There is yet another school of thought claiming that heat added by summer cooling is stored for use in winter. I don't buy that one since I'm quite certain any residual effect of winter or summer dissipates during spring / fall.

I have hard data that shows that my slinky system does in fact store heat in the summer that is still there for use in the winter.

When the heating season started, my EWT was 68°.  The ground temp away from my loop was 63°.  I have monitors on two systems and both show similar results.

I don't yet have a monitor of a vertical loop so I am not sure  if any heat storage will occur.

Attached is a graph from another slinky system. 

The green line is EWT.  You can see that it has stayed above the ground temp for the first part  of the year.

Graph


It is my opinion based on systems that I have monitors on that slinky loops can recover some when load is reduced.
I have no reason to think that vertical loops would be any different.

gregj,  engineer's quote above is what I was thinking when I expressed a desire to save the heat for later in the winter.

Since that time our outdoor temperatures have become more moderate and our loop temperature has increased from the low 38's to 39.5f.  In the last 10 days we had 6 days with highs ranging from 54f to 61f.  The warmer temperatures have also melted snow, we had some rain, and the ground is no longer frozen, but saturated.  Obviously, our systems aren't running as much either.  I imagine all of this is helping the EWT (it sure isn't hurting it!)

In the printouts from our installer, I see something called "Ground Lag Time" 33 days, "Soil Conductivity" 0.75, and "Soil Diffusivity" 0.6.  These printouts were generated before any digging occurred, so I assume these are just standard figures for our area.  But what do they mean?

DeWayne, good info.

I wonder if that effect results from normal thermal lag or represents a temperature change imposed by heat transfer from the geo loop.

I am pretty sure that we are seeing heat stored from the heat pump.

When we installed the loop we buried sensors about 15' away from the loop at 5' and 9' depths.

The loops are buried at 9'.   If the EWT is higher than the ground temp as measured 10' from the loop can't we imply that we are using stored heat?


Do you have any pics of the snow that you made using your heat pump?

Bad photo link

I'll try again later

This is so funny!  Nice Job.
You may want to send this in to some TV program and possibly win $1,000.

the link worked for....good job.   Looks the kids were having a blast.

Try again

http://picasaweb.google.com/snowman32043/2010SnowPix#