# Indiana Evaluation of system using HDD and electric consumption

Discussion in 'Maintenance and Troubleshooting' started by RetiredIN, Jan 20, 2015.

1. ### RetiredINNew Member

I am the second owner of a 3500 sq. ft. home with a ClimateMaster Tranquility 27 geothermal system and electric auxiliary heat. This is my fourth winter in the seven year old house, and I've noticed that my electric consumption during the heating season seems to be trending upward from year to year. I realize that my first winter in the house was unusually warm and that last winter was unusually cold, so I decided to try and compare with electric consumption with heating degree data.

My electric utility has a website where I can download daily consumption in an Excel spreadsheet and I can also download weather information (minimum temperature, maximum temperature, heating degree days, etc.) from the government website in another Excel spreadsheet. I was expecting to see a rather strong correlation between HDD and electric consumption in the "mild" weather (when the auxiliary heat was not running) and a different correlation in the very cold weather (when the auxiliary heat was running a material percentage of the time.) I was also expecting to be able to see the approximate temperature (either in HDD or minimum temperature) when the auxiliary heat was running for a material percentage of the time.

When I made my analysis I didn't find quite the correlation that I expected, and I found some recent data that didn't even look reasonable (especially when compared to prior year data.) I've listed four consecutive day's data from the last two years for comparison.

Date Minimum Maximum HDD kWh
1/6/14 -15 5 70 220
1/7/14 -14 8 68 185
1/8/14 6 25 49 113
1/9/14 11 31 44 78

1/6/15 9 19 51 67
1/7/15 -6 12 62 147
1/8/15 -7 20 58 363
1/9/15 1 21 54 198

We keep the thermostat in the mid 60's, the compressor seems to run nearly all the time when the outside temperature is below the low 20's, and the inside temperature rarely drops below the set point. The auxiliary heat (20 kw) obviously runs in the "cold" weather, but I don't have a way to track the run time.

Has anyone else tried to compare their electric consumption with degree days? If so, what did they find?

What does my 2015 pattern suggest? (My first thought was bad data from the utility----but they deny that as a possibility.) The reported usage on 1/8/15 suggest that my auxiliary heat ran for at least 15 hours. I find this hard to believe, but is it possible that the heat pump contributed almost zero heat even though it ran (almost?) all the time? Any other thoughts?

Have you had a qualified service company do a system check up? A heat of Extraction/Rejection will tell the tech a lot about the health of your system.

Bergy

3. ### pfer10Member

Retired I do track something similar but using an external device. I soon hope to be able to pull in my whole house power meter data also. Having a thermostat that does some run time tracking helps. I have a Radio Thermostat CT-30 (older model). If your Tranquility is a two stage the CT-80 would be for you with 3 stages of heat. I am only a single stage but run time will show hrs for the geo and hrs for the aux heat in their web interface. I would think with a 3 stage unit they would have to show stage 1, stage 2 and aux run times.

I know the Ecobee thermostat gets good nods from the pros here and also tracks run time data compared to HDD but I didn't know that so I started tracking another way and I don't know if Ecobee or Radio Thermostat tracks the time in different stages. Maybe a pro can chime in on that? See below for data from Radio's website for my use the past two days.

One thing to note that I don't like about Radio's tracking time is it is usually low. What I do is poll the thermostat after midnight to get yesterdays run time. My data shows 10.36 for yesterday and 11.5 for 2 days ago. Their website divides it in hours so it polls the thermostat many times a day but misses some as it is always lower than mine.

I have similar data to you on the 1/6 - 1/9 (Central Indiana)
Code:
```Date  Hrs   HDD    Ratio
1/6  17.23  52.76  .3265
1/7  21.08  62.98  .3347
1/8  24.00  63.20  .3794
1.9  21.38  55.89  .3825```
Note my HDD days are calculated from 68 degrees since that is where I set my thermostat. I think the norm is to use a couple degrees below the set point.

I have noticed other users post Ecobee data that tracks run time hours/HDD. That is the ratio I posted above. Notice on the 1/6 and 1/7 my ratio is lower. Those days were sunny. On the 8th my unit ran 24 hours and had to use aux heat the next morning as it lost the temp of the set point in the house. I am sure my kWh usage that day was up but I don't have a way to log it yet. The weather was also mostly cloudy and it was full clouds on the 9th so that dictates what my ratio does as I have a big south facing window for heat gain on sunny days.

Below is the graph that I did before I knew how Ecobee or some standard did it as I was just trying to track data on my house.

That shows my run time vs Degree-Days every day since 11/8/2014. I took the slope of the line from the equation at 2.657 to get an average of the data. I have typed in my log sunny and cloudy days and any point that falls above the line is pretty much a sunny day. Any point that falls below the line is overcast as my run time hours is greater than the average for a lower HDD. For my house to convert to the Ecobee way I just need to take 1/2.657 which is .3763 (which I think is kinda bad from the few posts that I have seen?). I plan to make the ratio smaller with insulation and getting the house tighter but I wanted data before I begin. Since I am single stage and I know how much heat my unit puts out and how much run time I have per HDD I can get a decent idea of my houses heat loss.

I have determined from getting the numbers the guys are asking you for that my system is running right about where it should be so I am looking at other areas to help control heat loss in my house and pump loss in my open system. I have also adjusted my thermostat to minimize aux heat and get some longer cycle times. I would like to upgrade before the tax credit goes away since my unit is over 10 years old and would like to downsize a little if I could. Two stages would be nice also. Sometimes I also think don't fix what isn't broken as I might get more troubles. lol

Get the numbers the guys ask for so they can help you otherwise call a local shop in to see if the unit is running right. There are all kinds of things that could cause high aux heat on time.

4. ### pfer10Member

I just looked at Radio Thermostats web interface and they have upgraded it some. I am able to pull data in Excel form and also look at any day I want. Below is a screen shot from the 1/8 when it ran aux heat in the morning. So far that is the only day I have had to run aux heat. I was home that whole day and I know the unit ran 24 hours like my data shows but their data shows it shutting down some in the afternoon and evening. Now if I could just figure out how to poll aux heat from the thermostat directly like I do run time.

5. ### RetiredINNew Member

I've paid for regular service from the company that installed the system. The service hasn't included providing me with any documentation of pressures, flows, current draw, or much of anything beyond a statement that the system is working properly. I have been trusting the company to provide me with the service that I need. I'm beginning to realize that I may need to start over with a new company.

It's interesting to see someone else's analysis of run time versus degree days---especially with comments regarding other factors such as sunshine. I am using Honeywell TH8320 thermostats (three zones) with a Jackson 300HPS zone controller. It's a dual stage heat pump with the third heating stage being electric resistance. I don't have any information regarding run time(in either stage) other than the fact that I can hear the compressor noise (the system is in my attic) nearly constantly when the outside temperature drops to around 20. The heat pump is a ClimateMaster TTV049 (four ton unit) with 20kWh resistance auxiliary. Without any good data on the operation of my system I can only assume that the heat output is about 33,000 btu/hr in the first state, 50,000 in the second stage, and auxiliary will add about 68,000 when it kick in.

I would like my system to be a comfortable and efficient as practical, but the practical part rather limits my options. The house has some inherent inefficiencies (10' ceilings, ducts in attic, lots of recessed lights in the ceilings, limitations on the placement of ductwork because a portion of the attic is floored, etc.) and my total electric bill only averages about \$115 per month.

6. ### Mark CustisNot soon.Industry ProfessionalForum Leader

retieredIN:

I have been working with geothermal systems since Jimmy Carter was president. He and I can not stop doing in retirement. I think it was George Bernard Shaw that said, "Those that can do and those that can not teach".

Here is what happens in the HVAC business in our climate. Bigger than me out fits need to keep the crew busy in the shoulder seasons. So they sell service contracts. It allows them to keep folks on the payroll until they are needed in Polar Vortex or heat wave mode. So who ever shows up at your home does not need to know what is going on with your system. They just go to the basement and make noise.

They do not know what to really test on a geo system so they do not write down what they tested.

Run time and electric bills will not tell you what is going on at your home. If you are looking at those clues you are uncomfortable and do not want to pay the price to get that way. You need someone who knows heat transfer and how the equipment is supposed to work. A geothermal heat pump faces a moving target. By the time one gets enough run time to look at water temps coming and going the home is 78*, (in heating).

I just had an idea for a way around the moving target issue, but I will keep it to myself this time.

There are ways to "trick" systems to be more comfortable at less cost. Look at JR, he changed the zoning weights and is now comfortable and happy with his run times. We will know more when the electric bill comes in the mail. He has yet to get me what type of zoning he has, when he does we will go on to fine tuning his machines.

If you heat your home and do all other thing electrical for \$115/mo., count your blessings.

Mark

johnny1720 likes this.
7. ### dgbairJust a hobbyForum Leader

I did a real quick graph.. hopefully I didn't have any mistakes.

File size:
67.5 KB
Views:
500
8. ### RetiredINNew Member

Mark,
I think that I understand at least part of your post. There are certain things that only a qualified professional examination of the geothermal system can tell me.

You have had decades of geothermal experience, and I have had decades of internal auditing experience. The internal auditor (sometimes) needs to have a different perspective on the situation. There is no way that I can dismiss the value of a qualified professional examination of my geothermal system. However, my real goal is to stay comfortable at a reasonable cost (as opposed to having a perfectly performing system which may or may not keep me comfortable and may or may not be inexpensive to operate. My system is keeping me comfortable during the heating season (although it may be the result of auxiliary heat rather than a geothermal heat pump.) I was trying to look at a comparison of HDD with kWh to get some idea as to whether the system (including auxiliary heat) was operating at a reasonable cost. A geothermal system should produce more btu per kWh than resistance heat. I have a lot to learn about geothermal systems, but I feel safe in saying that an improperly operating system will be less efficient than a properly operating system. Resistance heating pretty much operates at peak efficiency or it doesn't operate at all. While a comparison of HDD with kWh may not tell me whether my geothermal equipment is operating properly, I think that it still should provide some relative idea of how much the auxiliary heat is operating. If the auxiliary heat runs very little during very cold weather, it should be an indication that either the geothermal equipment is operating in a reasonable fashion or the system is considerably oversized. If the auxiliary heat runs a large percentage of the time, it should be an indication that the geothermal equipment is operating in a less than ideal fashion or the system is considerably undersized.

pfer's comparison of run time and HDD is interesting to me. I agree that it doesn't tell much about the efficiency of his equipment. Rather, it demonstrates that HDD is not a very precise indication of the actual heat load ---at least when viewed on a daily basis.

dgbair provided a comparison of HDD and kWh on a monthly basis. It does demonstrate that more kWh of power per HDD are required in colder months than are required in warmer months. I suspect that the data would be even more meaningful if there was a way to eliminate the non-heating electric consumption, and to ensure that the base of HDD (65 degrees?) was appropriate for his building. For my purposes, I don't think that looking at monthly summary information is nearly as valuable as daily information. (Hourly would be better, but I doubt that many people have access to it. I know that I don't.)

I'm not sure I quite understand your point about a "moving target". Any heating equipment faces a moving target in some sense. A heat pump has the disadvantage (compared to a gas furnace) that it's risky to cycle the compressor rapidly and heat pumps are sized smaller so they can't "catch up" as quickly.

You suggested that I "look at JR". How or where can I do that? I don't see a forum member with the handle of JR, although several names does start with the letters JR.

I am a cheap SOB, but I mentioned my average electric bill just as a reference point. A (non-believer) heating contractor once told me "Geothermal is for somebody who wants to save money regardless of how much it costs them". I suspect that my system is undersized and/or the attic location and ductwork loses too much heat. However, spending many thousands of dollars in order to save a couple of hundred dollars a year doesn't make sense

9. ### Mark CustisNot soon.Industry ProfessionalForum Leader

RetiredIN:

I like the way you think. Bean Counter? Cheap SOB? I do barter, I can trade you three years of shoe box records and my business checking account on line and you tell me if I make any money doing this and I will tune your heating system.

Let me try to better explain what I mean about a moving target measuring a geothermal system. What it is doing and how well. It is not impossible to do or understand.

To look at how a system is running we need more than just a screen shot of what is going on. HDD and KWH will give a good mean or average. Which can generate very useful numbers on long haul operating efficiency.

Most HVAC folks want to see a system "under a load", real hot or real cold. The reason is they want to look at operation numbers and go home. When I say moving target let's look at what happens on both sides of the heat transfer.

Since it is January lets look at heating.

Winter is here and November was tough, December was OK and January is OK so far.

I get to your home and the homes is at 70* +/- and you are comfortable. The EWT are still at 38*, (wild ass guess) and LAT is 100* out of the def-users. Life is good.

I am with you to see how well the system runs. In side is good. Loop is good. Nothing is running and all are comfortable.

I need to see what happens when we have a long run time on a call for heat. So we bump the tstat 3 degrees. The out door ambient is what ever it is today. I want to see the system run/work. I do not want the back-up on which is why only a 3* call. Since heat pumps are slow and steady, numbers start to move. After XX mins, loop is down to 33* and LAT is up to 105*F.

Both sides change as one is trying to see how well a heat pump works. Few in my trade can work on heat pumps. They do not get heat transfer or comfort, so they should not be working on geo units.

I guess I am saying one needs to understand the micro numbers to make the macro numbers mean anything.

Mark

10. ### RetiredINNew Member

Mark,
I think we both agree on the definition of Cheap SOB. I'm not so sure about auditor. When I got out of college I was looking for a computer programming job. One company that I interviewed told me that they already had enough computer programmers, but were hunting for an auditor. I asked " What the ---- is an auditor?" Their answer was that an auditor was an employee with a company car and an expense account. That sounded good, so I became one.

Definition of jobs change over time. The current definition for internal auditing is : "Internal auditing is an independent, objective assurance and consulting activity designed to add value and improve an organization's operations. It helps an organization accomplish its objectives by bringing a systematic, disciplined approach to evaluate and improve the effectiveness of risk management, control, and governance processes." Talk about a mouth full!!!!

I can count beans, but don't really enjoy it these days. You know that you have made money when you are able to take more (personal) money out of the business account than you put into it (or when the balance of the business account grows if you don't take money out.) If you are into bartering, it won't show in your shoe box records or bank account. (Best not to discuss it where the IRS is listening.) Much beyond that is just massaging your ego or the IRS. Those invoices that you sent out five years ago and which have never been paid aren't really income. That fancy truck you bought is definitely a business expense if the IRS is listening-----and less so if it is pulling your fishing boat.

I understand what you are saying about the system "moving". I still don't have a clue about the "target" part. The math doesn't sound like rocket science----when and if you can get accurate numbers to play with. A btu is the amount of energy needed to raise one pound of pure water one degree Fahrenheit at its maximum density, so I should be able to make a swag estimate of the amount of energy my system is extracting from the ground loop if I know the delta T of the EWT and LWT and the flow rate. (I know that my calculations would be more precise if I adjusted for the fact that I'm running something other than pure water through my loop, and that the EWT is probably not at the maximum density. Thinking about these adjustments seems a little premature since I don't have a way to measure the flow rate and don't have a clue as to what kind of solution is running through the loop.)

If I knew how much energy I was extracting from the loop, I would think that I should be able to compare that number to the manufacturer's propaganda to get some idea of how that part of the system was functioning. Obviously I would need to go through a similar process on the air output side of the system. Checking static pressures and the amperage drawn by the various electric devices (motors) sounds like a good measure as well.

If I would see my EWT dropping like a rock during very cold weather, I think that it would be time to try reducing my flow rate and/or start having negative thoughts about the folks that designed/installed the loops.

I suppose that I should do a manual J calculation of the heat load in my house, but I wasn't there when it was built and am not very sure about a number of factors. What "concerns" me is the 60+ recessed lights in the ceilings, the seven vent fans, and just looking around the attic. The HVAC is in the attic, and part of the attic is floored (which limits room for insulation and placement of ductwork). The duct system consists of 12". 14", and 16" flexible insulated main ducts that lead to ductboard distribution boxes for the three zones. Flexible insulated ducts go from the distribution boxes to the ceiling registers. I'm guessing that a substantial portion of my system output is being left in the attic.

11. ### Mark CustisNot soon.Industry ProfessionalForum Leader

If NSA does not know all about me I am sure Google for a fee will tell them the rest.

So I guess we need to get you some tools. Measurement tools. Then we need to get you the tables so that what you learn testing allows you to make an evaluation.

I have been interrupted the last two times I tried to explain how these system are a FAST moving target. Wives suffering from TBI take priority over this BB>

Lets do this without a chart or table, yet. These numbers are made up.

We start today and want to know what is going on.

We need to know:

GPM of flow through the system

Entering water temperature, (ETW)

LWT

EAT, taken as close as possible, to the coil in the center of the return air duct. With a new clean air filter or the filter removed for the testing.

LTA, in the center of the supply plenum with the strip heaters off.

A great additional bit of knowledge would be CFM of the air flow.

Another plus would be to know the static air pressure over the system.

Let us amp all motors in the system so we can "see" if they are doing any work.

I am sure I have forgotten stuff I look at out of subconscious habit. Anyone please feel free to note what I may have missed.

In the time it takes to run through the list using my good, but needing to read tools, when I get to the bottom of the list the top item may have changed 5 to 10%. Maybe more. So we start over from top to bottom and we get another 5% swing in the first item, ETC. Now the space is 78* and folks are taking off their clothes and the loops are below freezing.

Not a real test of how the system works. We did not include any refrigeration system measurements, and that is a whole new list of items.

It would have shut down when the tstat hit set point. The loop would recover and so on.

So to really test the operational function of a system one needs to be able to interpolate the readings with the system running or have more test equipment than Bill Gates could buy and tag them all to Windows 10.

Even then the system performance is a moving target.

Mark

12. ### engineerWell-Known MemberIndustry ProfessionalForum Leader

If the system has "Pete's ports" it is a relatively simple matter to measure and calculate water flow and heat transfer from / to loopfield.

A competently done Man J, room by room, is important to understand the load side. While you are at it, have a blower door test done as well. The BD test helps dial in the Man J and identifies low hanging opportunities to reduce air leaks and the cost of heating all outdoors.

I was in my attic last weekend adding insulation. As I slid on my back under the distribution plenum attached to my air handler, I noticed that the ductwork installer didn't tape the seam between the two on the bottom. He did however seal the top and front, which you can see from a standing position (go figure!). No big deal I thought, I'll fix it up. As I got behind the air handler, I was greeted with a nice warm stream of air exiting the seam on the back side, strong enough to make me wince. Trust (almost) no one.

14. ### RetiredINNew Member

I think that I am started to get educated. Let me paraphrase the last couple of posts to see if I am getting it right.

Curt is saying that "Pete's ports" can be used to measure and calculate water flow and heat transfer from / to loopfield. I interpret this to mean that the flow rate would be estimated by using the pressure drop through the heat pump as an orifice flow meter. (One would use tables supplied by the manufacturer to translate pressure drop into flow rate.)

Mark is saying that one first calculates the heat transfer from the loop by using EWT, LWT, and flow rate. The next step is to try and calculate heat output from the system using EAT, LAT, and air flow rate expressed in CFM. The comments related to the filter and static pressure suggest that the CFM numbers are from tables supplied by the manufacturer-----which would not be appropriate if the flow or air was obstructed by a dirty filter or if the static pressure was significantly different that what was used to develop the tables.

I can't think of another way to say "amp all motors in the system so we can "see" if they are doing any work."

Once this is done, you use try and evaluate the system efficiency by comparing output from the loopfield with the heat being pumped into the ductwork. The booger is that the output from the loopfield has probably dropped by the time that you measure the heat being pumped into the ductwork.

Curt as saying that a competently done Manual J calculation and a blower door test will help me understand the load side.

Let me play "devil's advocate" and go back to my original thoughts about comparing HDD and kWh . My really high usage "issues" seem to occur when the weather is darn cold. Mark's examples do a great job of explaining how a geothermal system works in the heating cycle in moderate weather. (I'm guessing that the cooling side gets to be more of an art form when you need to account for the energy used to dehumidify the air.) If Mark were to come and evaluate my system on a zero degree day, I imagine that (without using auxiliary heat) the EWT and LAT would both drop as the system output failed to keep up with the home's heat loss. I think that there was something in ClimateMaster's propaganda that specified a minimum (20 degree?) EWT. What happens with my system when the EWT drops really low? I avoid climbing into my attic where the system is located----especially in sub zero weather. I don't recall hearing that compressor shutting off during the sub zero weather, but my electric usage suggests that almost all of my heat must have been from the auxiliary heat. Is there some sort of switch that shuts off the compressor if the EWT drops too low or if the loop circulation stops (because the loop freezes or a pump fails?) I'm generally comfortable with Mark's evaluation of a system on a moderate heating day, how relevant is the amount of heat extracted from the loopfield if system is pumping a reasonable amount of heat into the ductwork? i.e., if the system is getting good results, it must be doing things right.

15. ### AMI ContractingA nice Van Morrison songIndustry ProfessionalForum Leader

Yikes I'm not retired in MI. Brevity will sometimes find more answers than pages of detail.

16. ### Mark CustisNot soon.Industry ProfessionalForum Leader

RetiredIN:

I had a system last week, where the customer and his want to be HVAC tech, electrician changed a thermostat. They even called WF tech help to make sure they had the installer set up menu correct. In the end they set the thermostat to duel fuel instead of four stages of heat. The result was for a year if the system saw a call for third stage heat third stage was engaged, but the stat turned off the compressor. I got a "not enough heat" service call.

The thinking went since stage three is provided by the high end domestic hot water tank via a flat plate heat ex-changer and a custom designed hot water coil in the duct work, down stream down stream from the two stage WF W2A unit, gas fired hot water and geo meant dual fuel.

When I designed the water feed aux heat system I was unable to get a coil that would fit in the duct work and cover and handle the entire heat load on that system. It only does about 75% of the load on a design day. So our trade off was to use the coil that fit rather than redo the duct work. I got this system as a rescue mission when the original contractor could not handle finishing the project.

We installed the water coil because the customer did not want to buy a big enough back up generator to handle the strip heaters. The high end A. O. Smith 50 gal. finishing tank is fed from a 119 gal buffer tank. The buffer tank is fed by the de-super-heater loop. The system ran like a Swiss watch until the boys changed the thermostat a year ago. Always had enough heat and DHW even with two teenagers in the house hold during the great polar vortex.

Stephanie, my personal low voltage wiring expert and IT person, redid the installer set up menu, now the system runs as it was designed to run. Heat pump, DHW and the aux coil all at once.

Moral of the story is the little things count and be sure you know how the system was designed to work.

Mark

17. ### pfer10Member

"If Mark were to come and evaluate my system on a zero degree day, I imagine that (without using auxiliary heat) the EWT and LAT would both drop as the system output failed to keep up with the home's heat loss. I think that there was something in ClimateMaster's propaganda that specified a minimum (20 degree?) EWT. What happens with my system when the EWT drops really low?"

That is all simplified of what is happening. The ground temp also starts to drop real slow so at the end of the heating season your EWT will be lower than at the start. If you look at the table below you can see how EWT and entering air temp changes performance. Notice this 3 ton unit doesn't put out 3 tons of heat unless you feed it 60F water at 68F entering air temp.

As you can see there are plenty of moving targets as your outside temp is never that steady and your unit usually doesn't run 100% of the time. The pros factor in aux heat by your heat loss of the house and what area of the country you live in. They save you money by not "over doing" the loop and let the aux heat handles those few times that the house looses more than the unit can put out. If they don't hit the numbers right then the aux heat is being used more and there goes all your saving. As the guys say the installer is more important than the equipment.

Palace GeoThermal likes this.
18. ### RetiredINNew Member

I think that my general understanding of a geothermal system is pretty much in line with what Mark and pfer 10 have been trying to tell me. My first recollection of geothermal heating was the hype several decades ago that praised geothermal because "the ground temperature is a steady 5x degrees". I know a little bit more now, but am still curious just how fast a ground loop will cool down, and when happens at the lower temperatures.

Mark's example have the EWT dropping a half dozen degrees during his visit. Pfer says "The ground temp also starts to drop real slow so at the end of the heating season your EWT will be lower than at the start". This seems to be a bit of a contradiction unless Mark liked my cooking (or he was being paid by the hour) and the example service call lasted a month or two. I finally found some ClimateMaster propaganda that suggested the heat extraction would be about 16,000 btu with an EWT of 20 degrees at 11GPM flow rate (based on a 15% methanol antifreeze solution.) Has anybody had an actually seen the EWT drop to 20 degrees (or below?) Does the system just continue to run until the loop eventually freezes up?

19. ### AMI ContractingA nice Van Morrison songIndustry ProfessionalForum Leader

"the ground temperature is a steady 5x degrees"
A bunch of us geo heads have discussed this on a Linkedin group. There often is talk from geoenthusiasts of the "steady ground temperature" when in reality it is in reference to undisturbed ground temperature. The introduction of geo loops disturbs it and changes that temperature.

"Has anybody had an actually seen the EWT drop to 20 degrees (or below?) Does the system just continue to run until the loop eventually freezes up?"

I've certainly seen loop temps in the 20's but most stay around 30 and up. Once the soil around the loops is frozen it often stabilizes EWTs as the frozen soil conveys btus more quickly. Ultimately These temps to vary in individual cycles and over the course of a season and are impacted by loop size/design, moisture in soil, solar impact, btu demand from conditioned space etc.

20. ### pfer10Member

Retired you might look at this page below. As you know I have been collecting data on how my house performs thermally and always looking for ways to analyze it as I am in learning mode. This page gave me some ideas at how to look at the data but also looks at loop temps.

http://groundenergysupport.com/wp/performance-monitoring-ground-loop/