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Loop size
Last Post 20 Sep 2010 01:43 PM by docjenser. 32 Replies. |
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Gopher-Buckeye
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Posts:7
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| 14 Sep 2010 10:22 PM |
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Per Joe’s suggestion I am starting a new thread. We have moved beyond the “What to ask before you buy geothermal“ stage and need to make a quick decision as to loop size. WF recommended 4 holes 150’ deep with ¾” diameter PE34308, but another contractor recommended 4 holes 200’ deep. We will be installing a 4 ton unit for a house 2,300 square feet in size and decent insulation. The additional 200’ of hole depth (400’ of pipe) would cost an additional $2,400, but would presumably give us better efficiency. The question is how much additional efficiency would it give us and what would be the payback time. My gut tells me not to undersize the loop, but then again money doesn’t grow on trees. We live in SW Ohio where in January the average low is 21 and the average high 37. In July the average low is 65 with the average high 86. Soil condition is sandy/rocky and average temp of between 52 and 54. Any sage advice? Thank you.
P.S. Just out of curiosity, would there be a difference between 4 holes 150' deep and 3 holes 200' deep? |
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Palace Geothermal
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Posts:1609
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| 14 Sep 2010 10:29 PM |
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Posted By Gopher-Buckeye on 14 Sep 2010 10:22 PM
P.S. Just out of curiosity, would there be a difference between 4 holes 150' deep and 3 holes 200' deep?
None as far as how they perform. Deeper might cost more. Fewer holes might cost less. |
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Dewayne Dean
<br>www.PalaceGeothermal.com<br>Why settle for 90% when you can have 400%<br>We heat and cool with dirt!<br>visit- http://welserver.com/WEL0114/- to see my system |
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Palace Geothermal
Veteran Member
Posts:1609
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| 14 Sep 2010 10:31 PM |
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Posted By Gopher-Buckeye on 14 Sep 2010 10:22 PM
The question is how much additional efficiency would it give us and what would be the payback time. My gut tells me not to undersize the loop, but then again money doesn’t grow on trees.
Each bidder should be able to show you a computer print out with the info you are wanting. If they don't have the print out, then they are probably guessing about the size. |
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Dewayne Dean
<br>www.PalaceGeothermal.com<br>Why settle for 90% when you can have 400%<br>We heat and cool with dirt!<br>visit- http://welserver.com/WEL0114/- to see my system |
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jonr
Senior Member
Posts:5341
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| 14 Sep 2010 11:12 PM |
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> Just out of curiosity, would there be a difference between 4 holes 150' deep and 3 holes 200' deep?
If you hit water before 200', the 3 holes will perform better - better thermal conductivity where there is water. Also compare the grout used, # and size of tubes, etc - they make some difference. |
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joe.ami
Veteran Member
Posts:4377
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| 15 Sep 2010 09:42 AM |
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Posted By Gopher-Buckeye on 14 Sep 2010 10:22 PM
1) WF recommended 4 holes 150’ deep with ¾” diameter PE34308, but another contractor recommended 4 holes 200’ deep. We will be installing a 4 ton unit for a house 2,300 square feet in size and decent insulation. The additional 200’ of hole depth (400’ of pipe) would cost an additional $2,400, but would presumably give us better efficiency. The question is how much additional efficiency would it give us and what would be the payback time.
2) My gut tells me not to undersize the loop, but then again money doesn’t grow on trees. We live in SW Ohio \
3) P.S. Just out of curiosity, would there be a difference between 4 holes 150' deep and 3 holes 200' deep? 1) I went through this with a client yesterday and the results are interesting indeed. We tend to run 150'/ton in mid MI (unless uncommon soil conditions are discovered). Client had noticed correlation between EWT and COP so we explored different loop lengths. 20% more loop in his case raised min. EWT 3* and raised COP.01 (yes 1-100th). The savings (electricity is cheap here) $16 on heating (plunged from $432 to $416) or less than 4%. Pay back only about 65 years (the tax credit brings horizon under 100 years). In this case, not good advice. BTW raising the min. EWT 10* took twice the pipe raised COP about .31 and saved $43/yr in operating cost. Additional loop cost nearly $5,000. So why would people add extra loop? Couple reasons.... - Soil conditions. Unusual soil conditions may take loop out of our design parameters without extra footprint. - Sales distinction. As awareness about geo loops increases so does the emphasis on loop design. Buyer ignorance to extremely meager ROI on "a little extra loop" (or even an extra ton of loop which was offered to you) allows sales- person to suggest "value added" in their design. While a "little extra" never hurts it often has little value. - Designer ignorance. Among the worst reasons some geo installers "pad" your design at your expense to protect themselves from their mistakes. - Sometimes loop is added at installers expense because they are not sure conditions support design or because they are doing their own system and pay very little for "a little more". - Some folks will want extra loop capacity after some systems performed poorly in the "100 year" heat wave. It is never bad to have "extra loop" but it is often not worth the price. 2) When you say your gut tells you not to undersize loop, I agree, but "right sizing is ok and over sizing...... We use 150' ton in mid MI. It is reasonable to expect that works in southern OH. Even if you have less ground water your conditions are more mild. "Right-sizing loops has been the subject of much study over the years as loop expense is one of the biggest costs of geo. 3) Usually different depths are selected based on geological conditions. Guys in my area heat water under Under 150' and bedrock between 120 and 150. Much easier and cheaper to drill above the bedrock. One job going in near the Detroit River has bedrock at about 50-80' In this case driller will be mostly in rock anyway and suggested 3 holes vs 6. Another factor is in driller's equipment. Guys who always found water under 150' down that add geo drilling to their services, may not have a 300' reach with equipment available to them. Good Luck, Joe |
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Joe Hardin
www.amicontracting.com
We Dig Comfort!
www.doityourselfgeothermal.com
Dig Your Own Comfort! |
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jonr
Senior Member
Posts:5341
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| 15 Sep 2010 11:51 AM |
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In a cool climate, an accidentally undersized loop (ie, the soil isn't quite what was expected) means you rely on aux heat a little more. In a hot climate, it can mean that your heat pump shuts down. |
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docjenser
Veteran Member
Posts:1400
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| 15 Sep 2010 12:53 PM |
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The whole point about loop design is to bring the EWT into an acceptable range, in heating dominated climates during the peak heating season, to ensure the heatpump is always supplied with EWT water above 30 degrees.
So 3 degrees can make a large difference in ensuring that your EWT stays at 30 degrees instead of 27 degrees F. The COP for me is secondary in this intent.
With horizontal applications, we always slightly oversize, go down to 8" instead of 6" (that itself adds 3.5 degrees!). It does not cost us very much to go the extra mile and put some extra pipe in, nor do we charge the customer for it.
With vertical, yes, we pay a lot by the foot, and we make sure we are right on target.
It gives us a peace of mind, and we do not have account for every tiny difference in soil conductivity.
Most design software only allows you to pick 3-4 different soil types, if you are off it changes the layout dramatically. I have seen loops behave significantly different, nothing the design software with fixed algorithms accounts for.
3 degrees difference is simply put an extra safety margin I do not want to give up! |
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| www.buffalogeothermalheating.com |
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joe.ami
Veteran Member
Posts:4377
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| 16 Sep 2010 10:42 AM |
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Posted By docjenser on 15 Sep 2010 12:53 PM
1) The whole point about loop design is to bring the EWT into an acceptable range, in heating dominated climates during the peak heating season, to ensure the heatpump is always supplied with EWT water above 30 degrees. So 3 degrees can make a large difference in ensuring that your EWT stays at 30 degrees instead of 27 degrees F.
2) The COP for me is secondary in this intent.
3) With horizontal applications, we always slightly oversize, go down to 8" instead of 6" (that itself adds 3.5 degrees!). It does not cost us very much to go the extra mile and put some extra pipe in,
4) nor do we charge the customer for it.
5) With vertical, yes, we pay a lot by the foot, and we make sure we are right on target. It gives us a peace of mind, and we do not have account for every tiny difference in soil conductivity. Most design software only allows you to pick 3-4 different soil types, if you are off it changes the layout dramatically. I have seen loops behave significantly different, nothing the design software with fixed algorithms accounts for. 3 degrees difference is simply put an extra safety margin I do not want to give up! I don't think we are as far apart on design as these posts will suggest doc, but I do have strong feelings contrary to a few of the old school assumptions....so with much respect; 1) While I agree that the design norm is 30 degrees (min. in heat dom. clims.) I don't percieve it as the holy grail. Installers who employ slinkies for instance tend to buy lengths of 600-800'. Therefore "more" is significant (20-25% on the average system). With that in mind I'm not going to shorten a system if it happens to deliver 33* or lengthen a loop field that happens to come in at 28*. There is no major meltdown if we hit 27* EWT and as pointed out op cost difference is nominal. Breakdowns of this are provided to the customers who invariably choose to avoid the "50 year ROI" features. 2) COP secondary? I actually agree, but I think ROI is primary, so to me while COP and EWT are involved, an insistance on 30* EWT may not be. 3) 8' vs 6' excavation in my state costs quite a bit more as it kicks in many OSHA requirements (i.e. shoring and such). It would be difficult to convince my excavator to do that for free. How much "extra pipe" are we talking about? As I pointed out previously, a significant amount is required to impact EWT by even 1*. 4) Companies charge more when work costs more or become insolvent. I think it is disingenuous to suggest that there is no cost to your customers for your design practices. I'm not saying they won't benefit, I'm suggesting you don't give it away. That said, if you work with slinkies and 550' instead of 600' will do, it would not impact cost much to throw away 50' of pipe and dig trench 10' shorter. Therefore customers will pay full price for the 600' slinky. 5) Loops do behave differently based on soil, ground water, antifreeze, length, load, size etc. An 8' deep loop in dry sand may perform 1/2 as well as a 5' deep loop in saturated sand. These are things that make it difficult to design loops remotely. One thing for sure is that if an installer's refrences and reputation are good, that will go further to ensuring loops will be fine than dissecting design. J |
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Joe Hardin
www.amicontracting.com
We Dig Comfort!
www.doityourselfgeothermal.com
Dig Your Own Comfort! |
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docjenser
Veteran Member
Posts:1400
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| 16 Sep 2010 12:16 PM |
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I think we are on the same page! But you got to shoot for something when you do your design.
For me it is all about added value. For the customer, who gets a more efficient system which keeps going even during extreme temperatures, and for me, who will have happy customers and no callbacks.
The problem is that most people do not monitor their systems. When you actually do you see what makes a difference and what not. I was referring more to the extra safety margin I have when something out of the ordinary happens, like an unusual long cold period. No problem when a loop then hits 25-30 degrees F, but if other factors come in, you are grateful for every degree. For example we found that loops buried at 8ft run 3.5 degrees warmer in January than buried at 5 ft.
I have a 10 ton loopfield in my back yard where each loop is different, one slinky, one straight, one at 5 ft, one at 6ft...and so on, with sensors on each one of them. So you learn what works better, and then you decide if it is worth the effort, or ROI.
The point is that with mobilization time and costs it takes our crew a day to install a 2-3 ton loopfield, but also a day (although a long one) to install a 6 ton loopfield. On the niagara escarpment in we can have the best clay in the world and 10 miles down the road sandy loom. So it does not costs so much more to throw one more loop in, pumping power is the same, and I have a wonderful piece of mind.
This must be especially said for old retrofit homes where the manual J is many times a guestimate, and the heatpump runs the loop much more down than you thought at 10 degrees lower than outside design temperature with a 40 mph wind....
But also we pay twice as much for electricity here in New York, so the impact on operating costs is twice as high.
Apart from that, I very much agree with everything you have said.
It is a balance point, a loopfield too large can cost more to operate and more to install (extra pumps and pipe, more excavation) and thus does not add value at all. |
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| www.buffalogeothermalheating.com |
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jonr
Senior Member
Posts:5341
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| 16 Sep 2010 05:47 PM |
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I agree that the number the design software comes up with is an estimate - and the likely error in that estimate is more than 10' of trench or a few degrees. Maybe someone will produce an inexpensive soil thermal probe that will reduce the error.
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Gopher-Buckeye
New Member
Posts:7
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| 17 Sep 2010 01:20 AM |
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Sorry for a stupid question, but what is EWT? |
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docjenser
Veteran Member
Posts:1400
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| 17 Sep 2010 05:05 AM |
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Entering water Temperature= The temperature the water returns at from the ground |
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| www.buffalogeothermalheating.com |
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Down2Earth Geothermal
New Member
Posts:59
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| 17 Sep 2010 12:05 PM |
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A valid reason to oversize a loop would be to allow for the future replacement of the heat pump which are invariably replaced with more efficient units capable of extracting additional BTU's from the ground. The ground loops will outlast the heat pumps by decades and it's often difficult/expensive to add loop in the future due to access issues, locating manifolds underground etc.
Not many people plan out this far at 25 or 30 years, however we are seeing the issue today in the replacement of the first generation of U.S. heat pumps. The existing ground loops are sometimes not adequate for the new, significantly more efficient units.
-Adam |
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geome
Advanced Member
Posts:987
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| 17 Sep 2010 12:45 PM |
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Posted By Down2Earth Geothermal on 17 Sep 2010 12:05 PM
A valid reason to oversize a loop would be to allow for the future replacement of the heat pump which are invariably replaced with more efficient units capable of extracting additional BTU's from the ground. The ground loops will outlast the heat pumps by decades and it's often difficult/expensive to add loop in the future due to access issues, locating manifolds underground etc.
Not many people plan out this far at 25 or 30 years, however we are seeing the issue today in the replacement of the first generation of U.S. heat pumps. The existing ground loops are sometimes not adequate for the new, significantly more efficient units.
-Adam
Would you please explain this in more detail? If a unit is sized correctly (now), wouldn't the BTU demand be roughly the same in 25-30 years assuming weather data is roughly the same and assuming no significant changes to the house effecting heat loss or gain? I don't understand why a more efficient future unit would need a larger loop. Wouldn't the more efficient unit just use less power to extract the same BTU's from the ground? |
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| Homeowner with WF Envision NDV038 (packaged) & NDZ026 (split), one 3000' 4 pipe closed horizontal ground loop, Prestige thermostats, desuperheaters, 85 gal. Marathon. |
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Looby
Basic Member
Posts:401
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| 17 Sep 2010 01:29 PM |
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Posted By Down2Earth Geothermal on 17 Sep 2010 12:05 PM
... invariably replaced with more efficient units capable of extracting additional BTU's from the ground.
I share geome's curiosity on this matter.
I fail to see how efficiency or BTU capacity are directly related
to loop length; and the only obvious (to me) indirect relationship
(kW versus brine delta-T) would lead me to expect loop length
requirements to be reduced as efficiency increases.
The laws of thermodynamics tell us that it is ALWAYS possible
to increase efficiency by designing for a smaller delta-T across
a larger heat exchanger. Unfortunately, infinite heat exchangers
tend to be nearly as spendy as frictionless/weightless pistons.
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| One measurement is worth a thousand expert opinions. |
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jonr
Senior Member
Posts:5341
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| 17 Sep 2010 01:32 PM |
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I would say that the capacity (not efficiency) of units is going down as people improve insulation. On the other hand, a COP=2 heating unit takes only 1/2 the btu delivered from the ground and a COP=4 unit takes 3/4 of the btu from the ground. So yes, loop need can increase. |
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docjenser
Veteran Member
Posts:1400
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| 17 Sep 2010 02:48 PM |
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Posted By Looby on 17 Sep 2010 01:29 PM
Posted By Down2Earth Geothermal on 17 Sep 2010 12:05 PM
... invariably replaced with more efficient units capable of extracting additional BTU's from the ground.
I share geome's curiosity on this matter.
I fail to see how efficiency or BTU capacity are directly related
to loop length; and the only obvious (to me) indirect relationship
(kW versus brine delta-T) would lead me to expect loop length
requirements to be reduced as efficiency increases.
The laws of thermodynamics tell us that it is ALWAYS possible
to increase efficiency by designing for a smaller delta-T across
a larger heat exchanger. Unfortunately, infinite heat exchangers
tend to be nearly as spendy as frictionless/weightless pistons.
If BTUs for a given house are constant (e.g. you do not want an addition in the future), a heatpump with a COP of 2 (30 years ago) of lets say 4 tons extracted 2 tons out of the ground and got 2 tons from the grid (mostly compressor heat). A new heatpump with a COP of lets say 4 now takes 3 tons from the ground and 1 ton from the grid. 50% more load on the ground loop than before. BTU output is the same, however higher efficiency means more heat extraction from the ground. |
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| www.buffalogeothermalheating.com |
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geome
Advanced Member
Posts:987
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| 17 Sep 2010 03:13 PM |
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Posted By docjenser on 17 Sep 2010 02:48 PM
If BTUs for a given house are constant (e.g. you do not want an addition in the future), a heatpump with a COP of 2 (30 years ago) of lets say 4 tons extracted 2 tons out of the ground and got 2 tons from the grid (mostly compressor heat). A new heatpump with a COP of lets say 4 now takes 3 tons from the ground and 1 ton from the grid. 50% more load on the ground loop than before. BTU output is the same, however higher efficiency means more heat extraction from the ground.
Does COP include BTU's from compressor heat that migrates out of the unit to the surrounding air (an unfinished basement for example), or does COP include only the compressor heat that goes to the supply air? Let's not have the discussion again on the heat being as usable in my unfinished basement as in my living area, ok?  |
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| Homeowner with WF Envision NDV038 (packaged) & NDZ026 (split), one 3000' 4 pipe closed horizontal ground loop, Prestige thermostats, desuperheaters, 85 gal. Marathon. |
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docjenser
Veteran Member
Posts:1400
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| 17 Sep 2010 03:40 PM |
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OK...after Joe brought up an interesting point I ran some scenarios with the software, 55KBTU/H, Buffalo (NY), 4 ton system with desuperheat, going from 600 ft total borehole lenght to 720 ft borehole (20% larger) changes COP from 3.69 to 3.80, and saves $62 per year (at 16 cents/KWH). Min EWT changes from 29 F to 31 F.
Going the other route (500ft borehole) drops min EWT to 26F and costs you $151 more per year.
Interestingly, making a horizontal loopfield bigger by 20% only increases COP from 3.71 to 3.77, and increased the min EWT from 29 to 30 F.
The main problem I see is that all these programs assume an average and constant heating degree days or BIN days. It is all great in theory. Then came winter. I had a monitor system on a fresh loop (new install), ground was not fully settled yet, old farm house, retrofit, pump was correctly sized by the book, loop was 20% oversized and buried at 8 ft (horizontal slinky), and at around 29 degrees at the middle of February, where it should be. Outside design temperature for Buffalo is 5 degrees. Then we had a cold front for 2 weeks, temperature sometimes at 10 below, with up to 40 mph winds. Heatpump was running at 2nd stage constantly, with supplement heat kicking in. After 11 days EWT was down at 25 degrees F......now what? System performed nicely, happy customer, happy me! But boy was I glad about the 1-2 degrees higher EWT thanks to the 20% oversize, and maybe another 2 degrees more thanks to the 8 ft versus 6 ft depth. Probably much better now that the soil is settled.
This is when you throw the software, ROI and COP out of the window and make sure the thing works.... no matter what. |
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docjenser
Veteran Member
Posts:1400
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| 17 Sep 2010 03:48 PM |
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Posted By geome on 17 Sep 2010 03:13 PM
Posted By docjenser on 17 Sep 2010 02:48 PM
If BTUs for a given house are constant (e.g. you do not want an addition in the future), a heatpump with a COP of 2 (30 years ago) of lets say 4 tons extracted 2 tons out of the ground and got 2 tons from the grid (mostly compressor heat). A new heatpump with a COP of lets say 4 now takes 3 tons from the ground and 1 ton from the grid. 50% more load on the ground loop than before. BTU output is the same, however higher efficiency means more heat extraction from the ground.
Does COP include BTU's from compressor heat that migrates out of the unit to the surrounding air (an unfinished basement for example), or does COP include only the compressor heat that goes to the supply air? Let's not have the discussion again on the heat being as usable in my unfinished basement as in my living area, ok?
Good question, I think AHRI rating is supply air only. But since heat raises, you know..... |
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