I've heard that measured performance of actual, installed geoexchange systems can be much lower than advertized COP numbers.

Is anyone aware of any accessible, reliable reports about real-world performance of these systems?

If not an academic-level paper, anyone have any good anecdotal information?

Thanks in advance,


Tags: geothermal

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There was an excellent whit paper from Steven Winter associates a read a couple of years ago that actually did field studies of installed systems in the North East. Bottom line is that the systems performed 20-30% below rated or anticipated efficiencies. 

The problem I have with geothermal systems is that there is still too much variability in terms of the design and installation capabilities of the installers. I have audited a number of systems that were operating horribly and had ridiculous full size oil boiler back up systems. Don't the installers believe the equipment will actually work?

I am advising all my clients to install inverter air source heat pump systems which in my opinion clearly outperform their rated efficiencies. They have none of the down sides that geo has.

Hi Joseph,

I have a client w/ three GeoX systems in a 4000+ residence that clearly do not function properly & have ruled out envelope deficiencies that would explain discomfort and high use.  I want to recommend retro-commissioning and ask if you use an RCx protocol.  I've checked PECI site which at one time had a GeoX commissioning protocol, and have contacted Geothermal Association and several other spots.  I'll share what I find here, and hope you might provide some direction.  Thanks! 

The customers who own Geo systems have either been very happy with them (installed right) or horribly disappointed with reliability issues (installed wrong). On customer I had last week said her electric bills dropped in 1/2 when she went Geo.

The following snippet is from my study, "Geothermal Home Heating & Cooling Economy -- Marvelous or Mythical?".

"But why fret, I told myself. If this system was truly as economical as it's touted, not to worry. But I did worry. Even with a government rebate incentive, that $18,480 appeared to be a large chunk to recoup. In order to pay for itself I would somehow need to justify those dollars by comparing its operation costs with those of a more conventional system. But how? It's not as if you can try one before you buy one."

I made our decision not to go with geothermal on our new home only a few days ago. I decided that if my study helped us it may help others in their decision as well. You may check it out at http://goo.gl/3PWWQ

Len Miller

Hi Leonard. 

May I recomend a solar thermal combi system.  It heats the domestic hot water and uses the excess heat stored in that water to heat the home at night time.  I have one system on a heavily insulated home (R 70 walls/ roof, R 40 under slab) and I am beating the passivhaus standard by 31%.  the passivehaus standard is 15KWHA/M2.  The solar thermal home is 10.39 Killowatts per square meter per year for space heating in Western Montana (7700 HDD).  Don't forget to use the sun!  This system costs seventeen grand including the radiant floors.

What is the name of the system? Any info on the home?

Hi Richard.

Radiant engineering in Bozeman MT designed the closed loop glycol system using Caleffi colectors and a crown 120 gal stainless water storage tank.   The house is 1400 ft2.  r 70 walls and roof.  R 40 slab perrimeter  with R 30 underslab.  ACH50 0.49    Tripane Gienow windows.   electric water heater backup heat.  Broan ERV.   1351 KWH used per year for heating.  At seven cents per killowatt hour is $95 dollars per year for heating.  House costs about $225,000 to build with septic but not the well.

Leonard's study is a good read for anyone that is considering geothermal(btw, I had to sign into my Google Docs to open it).

I did a similar study for a client here in the Northeast that showed similar results with geothermal higher install and operating costs when comparing it to a high efficiency gas boiler, even thought we have many times more heating degree days. A comparative analysis is a necessary step that must be done to make a valid decision, and this is best done by an independent energy consultant.


In another case, my client built a large home (8500sf) to low loss specifications with a geo system, and has an operating cost half the cost of the oil alternative, with the geo install cost about the same as the oil boiler after the federal tax credits for a new installation. Here the client is saved $1700/year in 2011 dollars and will save more in the future as the cost of oil escalates faster than electricity costs.

Geothermal systems when properly designed and installed are far superior to an inverter air source heat pump especially in the Northeast region. When properly designed and installed with quality equipment and components the system operates flawlessly without needing any service. A closed loop system doesn't warrant the need for any service, all you have is a few small magnetically driven pumps along with the heat pump. 

I installed my two WaterFurnace systems (3.5 tons each) 5 years ago and the systems have operated flawlessly since. I have no back up boiler to my systems and don't know why someone would pay for a complete boiler back up.

Geothermal properly designed and installed is a no brainer   

There are several types of geothermal (aka geoexchange) systems, and they will all operate differently depending on the application.  Closed loops, standing column wells, pump and dump, direct exchange... they all have their pluses and minuses. 

Published COP numbers are "ideal", which means they are  the best the unit can do. These are not applicable in the real world.  In reality, ground temperatures drop as heat is removed, so the efficiency drops as the season progresses.  Standing column wells mitigate this effect, but require pumping water out ("bleeding") the well to get the heat back up.  Standing column wells also require water with a low mineral content, or there will be maintenance issues with the well pumps.  

There is also the issue of sizing.  If the heat pump is sized for 100% of the load, it will frequently cycle on and off during most of the heating season.  This reduces efficiency.  In Europe, heat pumps are usually sized for a portion of the heat load, with the balance of the heat load coming from a supplemental heat source.  This can bring the cost of the installation down (less drilling and smaller equipment). 

The best case scenario is low temperature output, such as radiant floors, low heat loads, and low electricity costs.  Any heat pump operates more efficiently the closer the output temperature is to the input temp. 

If your annual oil cost is $4000 and switching to Geo makes your cost $2000, is that a win?  

Yes or No? 

Next question: When you factor that at COP of 4 the cost should be $800, is it still a win?  

Yes or No? 

Next question:  When you consider Air Source technology is likely to deliver $1200 annual cost in this example, and significantly reduced cost and risk,  Geo still a win?  

Yes or No?  

IMO the only way to justify Ground Source is with:


1 - very large load

2 - very rigorous design

3 - very rigorous installation

4 - monitoring efficiency.  

If you don't close the loop (so to speak) with M&V, pretty hard to say selling ground source cost justified with any "Energy Savings" promises is ethical.  

At FSEC, we tested two geothermal systems in Florida, one vertical well and another open loop system. The systems were instrumented in detail in several very low energy homes. Performance of both was well below expectations as documented in this report. Moreover, in one case, a similar vintage building next door with an air source heat pump fared much better in energy use and with a much lower cost for the installation.

Pump power of such systems is not part of the rating and yet, we found their energy use to be extremely important to realized performance.

Link for the report:


In particular, see the discussions on page 13 and 14 of the HVAC systems.  While I would not argue that the performance of these systems could be good, they are engineering and installation sensitive and pump power and loop heat exchange performance is critical. Generally, air source systems may provide better performance in mild climates and at a much lower installed cost.


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