Ground source heat pump loop ground temperature
Question:
A few weeks ago, I had a GSHP system installed. Along with the loops, I installed temperature sensors so that I could monitor the temperature of the ground at the loops. In this way, I figured that I could monitor system efficiency during the course of the seasons. This is a four ton, direct expansion system with four, 100ft vertical loops. The ground here is solid granite for hundreds of feet down and around. Little to no water movement. The measurements I’m making are as follows – Run the system normally for some period, i.e. a couple of days of operation. While the system is running, I’m monitoring the vent, room, and outside temperatures throughout the house. I’m also monitoring the temperature of the refrigerant lines to/from the compressor unit and have measured the current draw at the breaker panel. Next, I turn the system off for a period, typically the better portion of a day. During this period, I monitor the loop temperature sensors to see ground temperature recovery. Finally, I have a monitor buried at roughly the same depth (these are all vertical loops) so as to provide a ground control temperature. The results I’m getting are illuminating (and disappointing!) Control ground temperature started at 51F on 12/8/04. It is now 49.9F on 12/28/04. Total system run-time during this period has been 150 hours. The ground around the loops is freezing within <24 hours of system operation. Along with the loop cooling goes the vent temperatures. When the system is fresh (i.e. loop temperature recovered to the mid 40’s), the best vent temperature is around 93F. (My ductwork is tight and well insulated). Maximum vent temperature is within 2F of the temperature measured right at the air handler. Ground loop recovery is getting progressively worse as the usage increases. I assume this is because the ground around the loops is getting colder. Currently, it takes about 6 hours for the loops to "unfreeze" from about 30F to 32+F. Then, the loops rise at 1.1F/hour until they reach 38F. Then they slowly rise a couple degrees above that. Measurements just a week ago showed loops rising up to a about 42F before leveling off to about 45F. My suspicion is that within a couple more weeks of use the loops won’t even rise above freezing. My computed BTU output for the system runs from 32000BTUs/hr to 42000BTUs/hr. This system is rated at 4 tons, and I was told that it typically puts out closer to 5 tons at the start of the season and it drops to about 40000BTU by the end of the season as the ground cools. The system was "professionally installed". The installer came highly recommended by the manufacturer. On the initial install, the loops were backfilled incorrectly resulting in the tubing making minimal contact with the surrounding ground. These measurments are from after I corrected this problem. The system was supposed to have six loops, 100ft but the drill rig broke during installation. However, the manufacturer states that four, 100ft loops should be sufficient. The loops were supposed to be installed at least 10ft apart but they are 6ft apart. After doing many hours of research, I feel that there are several issues here. First, the thermal diffusivity of the ground is such that much more ground loop is needed to balance the thermal load being placed on it. I have typically read that about 250ft/ton is required for conventional gshp systems and under good conditions, this can drop to 200ft/ton for a direct expansion sysetm. so from the getgo, I’m at least 50% of the desired loop size. Second, the loops are too close together. At 6ft apart, the BTU capacity of the surrounding ground is seriously compromised. Most of what I’ve read suggests that 225 sq. ft/loop is required for long term ground temperature stability. This is running at 16% of this. The manufacturer stated that even 8ft loop separation is within spec. Ok, now besides telling me that I’ve been ripped off, I’m interested in other peoples experiences with GSHPs. Has anybody besides me taken the time to actually quantify the performance of their GSHP system? I’ve read everything on the standard GSHP sites and none of what I’ve seen actually shows what expected ground loop temperature is in the immediate vicinity of the loops. I’ve read about the overall ground temperature of the loop field, but haven’t encountered any literature discussing what I’m measuring. I’ve also seen measurements of water loop systems, where the water temperature to/from the ground loop is measured. This would be a good indication of the loop field temperature. Anybody have numbers from their systems? I am now investigating remediation for the system. I am considering two options: 1) Install 6 additional vertical loops, spaced 15ft apart from one another. this would give the required 225 sq. ft/loop and would increase the system to 250ft/ton. 2) go to a horizontal loop field since drilling here is so damned hard and expensive. Any suggestions on an effective, long term remediation would be greatly appreciated. Spare me the snide remarks about installing a gas burner and throwing away the GSHP. I actually have a working baseboard hot water system that this system is supposed to replace. Now I’m alternating between the systems to save oil.
Response:
- Hide quoted text — Show quoted text – >A few weeks ago, I had a GSHP system installed. Along with the loops, I >installed temperature sensors so that I could monitor the temperature of >the ground at the loops. In this way, I figured that I could monitor >system efficiency during the course of the seasons. This is a four ton, >direct expansion system with four, 100ft vertical loops. The ground here >is solid granite for hundreds of feet down and around. Little to no water >movement. >Control ground temperature started at 51F on 12/8/04. It is now 49.9F on >12/28/04. >Total system run-time during this period has been 150 hours. >The ground around the loops is freezing within <24 hours of system >operation. Along with the loop cooling goes the vent temperatures. When >the system is fresh (i.e. loop temperature recovered to the mid 40’s), the >best vent temperature is around 93F. (My ductwork is tight and well >insulated). Maximum vent temperature is within 2F of the temperature >measured right at the air handler. >Ground loop recovery is getting progressively worse as the usage >increases.
snip.. >Any suggestions on an effective, long term remediation would be greatly >appreciated. Spare me the snide remarks about installing a gas burner and >throwing away the GSHP. I actually have a working baseboard hot water >system that this system is supposed to replace. Now I’m alternating >between the systems to save oil.
A couple of items to think about.. 1. You may have to wait a season to get full benefit.. During the summer the GSHP will build up the stored heat, which you can then tap in the winter. 2. If this is a closed loop system.. Try adding some tap water to each well casing. Water is excellent conductor of heat. 3. Consider adding some solar hot water panels to your setup to boost well temps in winter. Probably a lot cheaper than drilling additional wells. Exact configuration depends on your site.. A self draining system which dumps hot water into the well casings has the advantage of simplicity.
Response:
– Hide quoted text — Show quoted text ->A few weeks ago, I had a GSHP system installed. Along with the loops, I >installed temperature sensors so that I could monitor the temperature of >the ground at the loops. In this way, I figured that I could monitor >system efficiency during the course of the seasons. This is a four ton, >direct expansion system with four, 100ft vertical loops. The ground here >is solid granite for hundreds of feet down and around. Little to no water >movement. > The measurements I’m making are as follows – > Run the system normally for some period, i.e. a couple of days of > operation. While the system is running, I’m monitoring the vent, room, and > outside temperatures throughout the house. I’m also monitoring the > temperature of the refrigerant lines to/from the compressor unit and have > measured the current draw at the breaker panel. > Next, I turn the system off for a period, typically the better portion of > a day. During this period, I monitor the loop temperature sensors to see > ground temperature recovery. > Finally, I have a monitor buried at roughly the same depth (these are all > vertical loops) so as to provide a ground control temperature. > The results I’m getting are illuminating (and disappointing!) > Control ground temperature started at 51F on 12/8/04. It is now 49.9F on > 12/28/04. > Total system run-time during this period has been 150 hours. > The ground around the loops is freezing within <24 hours of system > operation. Along with the loop cooling goes the vent temperatures. When > the system is fresh (i.e. loop temperature recovered to the mid 40’s), the > best vent temperature is around 93F. (My ductwork is tight and well > insulated). Maximum vent temperature is within 2F of the temperature > measured right at the air handler.
Sorry to hear of your troubles, I have been looking forward to installing a GSHP here in update NY. But your experience is giving me second thoughts. In my area, there is a lot less granite, and a lot more ground water, so maybe your experience is unique to your situation? Anyway, I’m planning a horizontal loop system. I’m curious, how are you measuring the ground loop temperatures when the system is off? Are these temperatures near the surface or deep within the ‘wells’? With 100 ft deep wells, I would have expected the bottom to be much warmer than the surface. daestrom
Response:
(snipped out original long posting) – Hide quoted text — Show quoted text -> Sorry to hear of your troubles, I have been looking forward to > installing a > GSHP here in update NY. But your experience is giving me second > thoughts. > In my area, there is a lot less granite, and a lot more ground water, so > maybe your experience is unique to your situation? Anyway, I’m planning > a > horizontal loop system. > I’m curious, how are you measuring the ground loop temperatures when the > system is off? Are these temperatures near the surface or deep within > the > ‘wells’? With 100 ft deep wells, I would have expected the bottom to be > much warmer than the surface. > daestrom
My guess is that in an area with plenty of water movement, the problem will be greatly reduced as water in motion is great at carrying heat. My bores are pure granite. I live on a "mountain" which is a local diabase intrusion. I have two wells on the property. The backup well which the previous owner drilled in 1990 is 950 ft. deep and has a recovery of 1.25gpm! As for the measurements – I embedded temperature sensors in epoxy then lowered these 50ft into the boreholes along with the loops. I believe that you have to go considerably deeper to start seeing warming. I have read that the geothermal gradient is on the order of 75F/mile. When the loops were first installed, the baseline measurements showed a temperature of 51F at all loops. After all the research I’ve done now, I would suggest that anybody considering a geothermal system pay for a thermal diffusivity analysis to see what the thermal gradient will be around the loops during actual use. For example, I would like to see the thermal gradient based on a 100% runtime of a 50k BTU load. Can the heat from the surrounding strata come in as fast as it’s being pumped out? How fast would it come in? Where’s the break point – i.e., if you run the system X% of the time, it will be pumping out the same amount of heat as can diffuse back in. I think that is the magic number for any loop configuration. Without this analysis, I wouldn’t trust any GSHP installation. As for Tim’s comments: > 1. You may have to wait a season to get full benefit.. During the > summer the GSHP will build up the stored heat, which you > can then tap in the winter.
That will certainly help some. However the balance here is definitely weighted towards heating degree days, so my guess is that, if I’m lucky, I’ll get really good Air Conditioning performance early in the season, then as the ground warms up, it will warm back to somewhat above the baseline temperature. The problem I see is that the current loop field is so small that in a matter of a couple weeks of use, the heating system will draw out all the useful heat. But time will tell. Thanks for the thought. > 2. If this is a closed loop system.. Try adding some tap water to > each well casing. Water is excellent conductor of heat.
Another good idea. Actually, since they botched the backfill on installation, I did that myself using hot water (we have a direct hot water hose connection), so the bores got filled with 120+ water. In practice, the heat in the water was sucked out PDQ by the ground thermal mass. > 3. Consider adding some solar hot water panels to your setup to > boost well temps in winter. Probably a lot cheaper than drilling > additional wells. Exact configuration depends on your site.. A > self draining system which dumps hot water into the well casings has > the advantage of simplicity.
Funny you should mention that one. I was running an analysis on that option as well and may yet go that route. The problem, and please correct me if I’m wrong, my references show about 25,000BTU/day per panel is pretty good. So if I put 8 panels on my roof, then I’d be pumping in, at best, about 200,000 BTUs/day into the system. Now, when the system is running to spec, it should be pumping out about 50,000 BTUs/hr. Some of this will come directly from the compressor power, and the rest from the ground. So I was figuring on some 35,000BTU/hr being pumped out. During my test runs, I found the system to be running between 16+ hours/day at 15F. So I’d still be pumping out 16*35000 = 560,000BTU/day which is 360,000BTU/day more than I’d get from the solar panels. I still think solar could be a good addition as that would really decrease the thermal load on the wells. One other option – if I install solar, then would it not be more efficient to just add radiant heating wherever I can to my house? Then I’d be pumping the heat right to where it’s being used. as such, all those BTUs would go into the house and reduce the load on the ground thermal mass. Lots of thoughts. Please keep em coming. Thanks!
Response:
First, having installed FHP geothermal systems for a number of years, I have never heard of less than 200′ per ton in any installation. Second, there were a number of "geoexchange", (dx) systems installed in this area and the contractor may yet go broke. The soil here has high thermal conductivity and high water tables, but the DX loops corroded and lost a good bit of heat transfer capability. Then there were the leaks… Third,I would hold the contractors feet to the fire. By your explanation, the system was not installed as designed. You mention backfill, but these loops should have been grouted to insure uniform bonding with the borehole walls. Fourth, even in correctly installed and functioning loop systems I have never documented a very large energy savings over a high efficiency heat pump due to the shift in loop temperatures. You are correct that you would have close to 5 tons at times and closer to 3 tons at others. The 5 ton figure is a very short period during changeover. The 3 ton figure is considerably longer. Finally, if I were in your situation, I would investigate open loop water sytems. http://www.fhp-mfg.com http://www.energy-miser.com – Hide quoted text — Show quoted text –
Response:
– Hide quoted text — Show quoted text -> First, having installed FHP geothermal systems for a number of years, I > have > never heard of less than 200′ per ton in any installation. > Second, there were a number of "geoexchange", (dx) systems installed in > this area and the contractor may yet go broke. The soil here has high > thermal conductivity and high water tables, but the DX loops corroded > and lost a good bit of heat transfer capability. Then there were the > leaks… > Third,I would hold the contractors feet to the fire. By your > explanation, the system was not installed as designed. You mention > backfill, but these loops should have been grouted to insure uniform > bonding with the borehole walls. > Fourth, even in correctly installed and functioning loop systems I have > never documented a very large energy savings over a high efficiency heat > pump due to the shift in loop temperatures. You are correct that you > would have close to 5 tons at times and closer to 3 tons at others. The > 5 ton figure is a very short period during changeover. The 3 ton figure > is considerably longer. > Finally, if I were in your situation, I would investigate open loop > water sytems. > http://www.fhp-mfg.com > http://www.energy-miser.com
Thanks for the practical background info. We’ll see about corrosion. I hope not. The groundwater pH here isn’t to acidic and they installed a galvanic protection system, so at least that portion (hopefully) won’t be an issue, but time will tell. The DX systems don’t grout because of the high temperature swings. Research I’ve seen indicates that grouting is a mistake with DX as the cement tends to shrink away from the tubing and crack. In this installation, they use sand, which can shift and fill and maintain a good thermal contact. So goes the theory. I had asked for a good Bentonite grouting and this is what they told me, as did an independant reference at one of the colleges that specializes in GSHP. I checked the possibility of open loop first, since that’s typically the most efficient system available. Unfortunately, I’ve got extremely little ground water here. The two wells on my property together can only supply a couple gallons a minute. Thanks again for your input.
Response:
get away from the air handler, and use your baseboard. the needed temps are less, and your system will run more efficiently. I have no experience with vertical loop systems, only horizontal slinky coils. Steve Spence Dir., Green Trust http://www.green-trust.org – Hide quoted text — Show quoted text – > A few weeks ago, I had a GSHP system installed. Along with the loops, I > installed temperature sensors so that I could monitor the temperature > of the ground at the loops. In this way, I figured that I could > monitor system efficiency during the course of the seasons. This is a > four ton, direct expansion system with four, 100ft vertical loops. The > ground here is solid granite for hundreds of feet down and around. > Little to no water movement. > The measurements I’m making are as follows – > Run the system normally for some period, i.e. a couple of days of > operation. While the system is running, I’m monitoring the vent, room, > and outside temperatures throughout the house. I’m also monitoring the > temperature of the refrigerant lines to/from the compressor unit and > have measured the current draw at the breaker panel. > Next, I turn the system off for a period, typically the better portion > of a day. During this period, I monitor the loop temperature sensors to > see ground temperature recovery. > Finally, I have a monitor buried at roughly the same depth (these are > all vertical loops) so as to provide a ground control temperature. > The results I’m getting are illuminating (and disappointing!) > Control ground temperature started at 51F on 12/8/04. It is now 49.9F > on 12/28/04. > Total system run-time during this period has been 150 hours. > The ground around the loops is freezing within <24 hours of system > operation. Along with the loop cooling goes the vent temperatures. When > the system is fresh (i.e. loop temperature recovered to the mid 40’s), > the best vent temperature is around 93F. (My ductwork is tight and > well insulated). Maximum vent temperature is within 2F of the > temperature measured right at the air handler. > Ground loop recovery is getting progressively worse as the usage > increases. I assume this is because the ground around the loops is > getting colder. Currently, it takes about 6 hours for the loops to > "unfreeze" from about 30F to 32+F. Then, the loops rise at 1.1F/hour > until they reach 38F. Then they slowly rise a couple degrees above > that. Measurements just a week ago showed loops rising up to a about > 42F before leveling off to about 45F. My suspicion is that within a > couple more weeks of use the loops won’t even rise above freezing. > My computed BTU output for the system runs from 32000BTUs/hr to > 42000BTUs/hr. This system is rated at 4 tons, and I was told that it > typically puts out closer to 5 tons at the start of the season and it > drops to about 40000BTU by the end of the season as the ground cools. > The system was "professionally installed". The installer came highly > recommended by the manufacturer. > On the initial install, the loops were backfilled incorrectly resulting > in the tubing making minimal contact with the surrounding ground. > These measurments are from after I corrected this problem. > The system was supposed to have six loops, 100ft but the drill rig > broke during installation. However, the manufacturer states that four, > 100ft loops should be sufficient. > The loops were supposed to be installed at least 10ft apart but they > are 6ft apart. > After doing many hours of research, I feel that there are several > issues here. > First, the thermal diffusivity of the ground is such that much more > ground loop is needed to balance the thermal load being placed on it. I > have typically read that about 250ft/ton is required for conventional > gshp systems and under good conditions, this can drop to 200ft/ton for > a direct expansion sysetm. so from the getgo, I’m at least 50% of the > desired loop size. > Second, the loops are too close together. At 6ft apart, the BTU > capacity of the surrounding ground is seriously compromised. Most of > what I’ve read suggests that 225 sq. ft/loop is required for long term > ground temperature stability. This is running at 16% of this. > The manufacturer stated that even 8ft loop separation is within spec. > Ok, now besides telling me that I’ve been ripped off, I’m interested in > other peoples experiences with GSHPs. Has anybody besides me taken the > time to actually quantify the performance of their GSHP system? I’ve > read everything on the standard GSHP sites and none of what I’ve seen > actually shows what expected ground loop temperature is in the > immediate vicinity of the loops. I’ve read about the overall ground > temperature of the loop field, but haven’t encountered any literature > discussing what I’m measuring. > I’ve also seen measurements of water loop systems, where the water > temperature to/from the ground loop is measured. This would be a good > indication of the loop field temperature. Anybody have numbers from > their systems? > I am now investigating remediation for the system. I am considering two > options: > 1) Install 6 additional vertical loops, spaced 15ft apart from one > another. this would give the required 225 sq. ft/loop and would > increase the system to 250ft/ton. > 2) go to a horizontal loop field since drilling here is so damned hard > and expensive. > Any suggestions on an effective, long term remediation would be greatly > appreciated. Spare me the snide remarks about installing a gas burner > and throwing away the GSHP. I actually have a working baseboard hot > water system that this system is supposed to replace. Now I’m > alternating between the systems to save oil.
Response:
> The system was supposed to have six loops, 100ft but the drill rig broke > during installation. However, the manufacturer states that four, 100ft > loops should be sufficient. > The loops were supposed to be installed at least 10ft apart but they are > 6ft apart.
When I talked to one DX system manufacturer, I was told that the wells would be drilled at a 45 degree angle, so that the well heads would be in a 15 foot diameter circle, but that the majority of the wells would be far enough apart for adequate thermal separation. — George Eberhardt (732)224-8988
Response:
> After all the research I’ve done now, I would suggest that anybody > considering a geothermal system pay for a thermal diffusivity analysis to
Interesting idea. Is that a fairly regular test or analysis? Eg, for well drilling they can test flow/recovery to know if it is sufficient. I wonder if they could do something similar for a GSHP… A neighbor has a GSHP and really likes it. However his is open loop, and the outflow maintains a nice pond (1/2acre?). – Hide quoted text — Show quoted text -> As for Tim’s comments: > 1. You may have to wait a season to get full benefit.. During the > summer the GSHP will build up the stored heat, which you > can then tap in the winter. > That will certainly help some. However the balance here is definitely > weighted towards heating degree days, so my guess is that, if I’m lucky, > I’ll get really good Air Conditioning performance early in the season, > then as the ground warms up, it will warm back to somewhat above the > baseline temperature. The problem I see is that the current loop field is > so small that in a matter of a couple weeks of use, the heating system > will draw out all the useful heat. But time will tell. Thanks for the > thought. > 2. If this is a closed loop system.. Try adding some tap water to > each well casing. Water is excellent conductor of heat. > Another good idea. Actually, since they botched the backfill on > installation, I did that myself using hot water (we have a direct hot > water hose connection), so the bores got filled with 120+ water. In > practice, the heat in the water was sucked out PDQ by the ground thermal > mass.
The fact that the ground sucked the heat out of the water so quickly is a good sign for summer heat storage. After all, that heat did not just disappear, it was dispersed via conduction. > One other option – if I install solar, then would it not be more efficient > to just add radiant heating wherever I can to my house? Then I’d be
Probably. One interesting point with solar though, is that lower operating temperature is more efficient/effective at harvesting and transporting that solar energy. For radiant heating, you probably want water at least 100F at the panel output. However to dump heat into the ground you could probably get significant benefit at only 80F or even colder output temperatures. In the summer you could easily get temperatures high enough to provide all your domestic hot water needs. That may even be a better design starting point — add solar water heating sufficient to provide all domestic needs, and if it works out well, add capacity for space heating. Lower initial investment for a trial. sdb — Wanted: Omnibook 800 & accessories, cheap, working or not sdbuse1 on mailhost bigfoot.com
Response:
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