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Geothermal vs. air-source heat pump

user-1093260 | Posted in Energy Efficiency and Durability on

I am building a tight insulated home in zone 6 and deciding wether to choose between ground water heat pump and air heat pump. After the 30% tax incentive there is not much increse in cost for the geo system. I am being told differnet stories in regard to system performance and longevity of equipment ( depending on what side of the fence your on). I am hoping to be in this house for the duration and willl be the last home I live in until my kids have to make the choice of where to send me. Dana

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  1. GBA Editor
    Martin Holladay | | #1

    There's no easy answer to your question. One caveat: manufacturers of ground-source heat pumps (GSHPs) don't include the energy used by pumps when calculating efficiency or COP ratings. As a result, actual COPs of these systems vary widely; many experts have been called to troubleshoot GSHP systems with grossly oversized pumps. Such pumps can kill the efficiency of the system. Sadly, engineers who understand pump sizing are relatively rare.

    The advantage of ductless minisplit units (a type of air-source heat pump) is that they are packaged systems that are hard to screw up. In contrast, GSHP systems are site-built from a variety of components by local contractors with widely varying levels of skill and knowledge.

  2. user-1012653 | | #2

    having been down this path, we are selected to go the GSHP route. Reasons being is growing to be fairly common in our part and there are many installers who can be trusted to install a good system and do not inflate the prices. A 2 ton system installs for under 20k with vertical wells and full ductwork. While the pump reduces COP, it is still fairly higher then ASHP, especially when the temps start to drop as they will in mine and your zone 6. Air source you will still need some sort of backup heat. Most GSHP will be able to handle full load when the temps are well below 0 assuming they were sized correctly. While mini splits are getting below 0 for working temps, their efficiency and output drops as well. With the tax credits as well as the local utilitiy rebates, the net price of our Waterfurnace 5.0 COP (less with factoring pump) is the same cost as the mini split install and less then a typical 16 SEER ASHP with gas furnace backup. Obviously this is location dependent, so be sure to check with your local electric supplier as well as they may have additional rebates available. Ours offers ones for GSHP as well as efficient home construction (such as Energy Star 3.0).

  3. kevin_in_denver | | #3

    I think if you analyze this closely, you'll find that a minisplit or two will come out on top.
    The good ones maintain a high COP right down to 5F outside, which eliminates the old advantage of GSHPs.

    First cost should be less than half of GSHP, with easier repair/replacement in the future.

    When you start with low heating cost, there isn't much money to be saved with a higher investment in equipment.

  4. user-836662 | | #4

    Because you are building new, not retrofitting, you have a great deal of control over the answer to your question. If you follow a Passive House philosophy and emphasize passive solar, shell air tightness, and insulation during construction, you will find it hard to justify the cost of a GSHP. I agree with Kevin D.- look at the new family of variable compression high efficiency air source heat pumps from Mitsubishi and Daikin. Mitsubishi has 2 models that will provide full COP down to 5 deg F and continue to heat down to -13 deg F, all without any resistive heating element. I installed one in my last project (my own house) in climate zone 5, 7000' elevation and it worked extremely well on bitter cold days during construction. But more importantly, after house completion, it has never been turned on because of the Passive House features built into the design. Bottom line: it's far more cost effective to invest in passive efficiency and passive solar, than in expensive and complex HVAC equipment.

  5. jambo13 | | #5


    I agree with the other posts that the ASHP is the way to go. I installed Mitsubishi Mr. Slim units in my new home last year and they exceeded all expectations in zone 6 (outside Boston). 3000 sq ft, 2 story home, R33 walls, R55 ceiling. I have not performed a blower door test yet.

    In pricing both options out I found the ASHP to be about 30% the cost of a GSHP system. That being said I found it much more effective to invest in the homes envelope with a portion of the money saved by not choosing the GSHP, ensuring the effectiveness of the mini split units. I was consistently amazed at my energy costs this past winter and the comfort they provided.

  6. user-1091831 | | #6

    Kevin is right. ASHPs still seem to have the bad wrap that they are inefficient at lower temperatures by many. Technology has come along way over the past 20 years and manufacturers like Mitsubishi are producing units that still claim 95% efficiency at 0 degrees. As Martin stated, there is potential for a wide range of workmanship in site built systems. It doesn't seem worth the thousands of dollars for excavation and repair should something go wrong down the road.

  7. Brian Knight | | #7

    Just to even things out more in this thread, I think GSHP could be the right choice for your project. As long as its designed and installed well, costs the same as a typical ASHP then it could easily provide you with more control of your indoor conditioned air than the 2 or 3 minisplit route.

  8. wjrobinson | | #8

    Yaa, what Jon say... Gotta research that more.

  9. GBA Editor
    Martin Holladay | | #9

    What Jon say? I'm baffled by both of you...

  10. user-1127834 | | #10

    Editing :
    bldr AJ man w/ tools and uderstandable to me....
    look for sentences in english to helps...

  11. user-1127834 | | #11

    GT loop circulation power:
    Today it is about ~200 watts for 5 ton installations (20 years+ ) under 4400 total 5-5.1/2 ton GT watts being less than 5% of the COP (compare 4.00 then to 3.8+ SPF-GT COP's).
    Martin mentioned all written needs to be more readable, too, and so here it is again.

    It has been possible to have over 10 compressor tons and over 12 "rated-size-tons" per B&G PL 55 2.4 amp 230 1ph -500 watt pump or about 700 watts open well 70- ft lifts of water, since the EARLY 1980's. (just so we all can put the pencil to the numbers)

    Several tangent points:

    Like airHP there are a few "name brand" popular GT systems.
    Both manufactured HP system styles can show that with either the outdoor section fan/ecm,variable- refrigeration or not, GT has more to offer WITH THE PUMP IN COP's today b/c of the greater use of low pressure drop water coils on loops.
    Will they ?

    Heat exchangers like ss plate with appropriate y-strainers were proven practical and durable since the turn of the century for closed loop. Designs through 6 tons with a 200w pump that have been just fine where applicable. So a COP may-be-somewhat afftected by less than 4% - 5% of a systemic KWH comparison today.

    Zone 6?
    or any zone is not just HVAC, unless one gets free hw, and a 30% break on the water systems elsewhere.

    Better written than I try to achieve, K,J,J,M would be in all ways be situationally correct unless what Mr. Holiday pointed out: "Few Engineers..." (do exist to assist your decision).

    NY's ASHRAE 2008 about commercial school goals of savings were shocked at finding a GT 4th level of application almost 12% less costly to operate than the next closest GT installation.
    About HVAC+HW as the bundled heating(s). I always ask since 1996, have you seen the simultaneous HW On-Demand by temperature control as any "instant HW Heating" while in a call for Cooling? Did you know that is in one unit?

    For over 30 years GT 4th-level systems synergistically shut off the well or loop pumps for to allow thermal energy to move from warm air in a space being cooled - through the GT Ht-P system and into Domestic Water Heating (double vented HW coils).
    This really is like no Mini is yet doing, unless you can point me to that.
    ?? var.rfg.IQ/ or ?? Dual STANDARD SINGLE SPEED, which over any problems is just less expensive to replace? Variable compared to Dual, 3 staging, has no more higher end COP by ROI that I have seen among many motoring devices.
    More combos with one being a double-stage compressor can be thought of.

    But MiniAirHP 4-zones only begins to appeal to the comforts in 1400 sq.ft and larger homes.
    Consider better 2 zone ducting appropriately sealed AND CONTROLLED then 3-Staging "SIZE 4+ GT" dual compressor can be priced against and , too, that also can have OEM 4 zoning.
    NOW ADD a WATER SYSTEM x 30% + credits to those who qualify:
    +The HW 100% in and out of cooling.
    (tech's : Here an oversized desuperheater in heating is allowed to a point to work on the HW in heating until all the space comfort is needed more, and not in a Priority call, then).
    +A DEMAND PRIORITY HW heating that is "INSTANT" and finishes HW heating in usually 20-minutes of recovery depending on compressor sizing.

    Appropriate Apples:Apples or App's to App's:
    4-6 zone AirHP will USUALLY be even highr first cost/ 4-ton or even a Mini-just an Air HP...
    or esp. in the like of a AirHP IQ/Variable refrigeration inverter system that does need to be 4- 4.1/2 tons rated to compare to a non-rated GT-COMPRESSOR-tons of 3.1/2 (smallest dual)
    4th level Synergistics HW+HVAC+Simultaneous Heat-Reclaim
    turning OFF the loop or well (ground) source mechanics
    :WHILE in cooling simultaneously heating HW tank(s) "instantly" and saving 50% more than a desuperheater alone for usually in the same 4 year ROI, but mega-savings in 10 years running.

    Some (more than half) currently uninformed but finely intelligent GT designer/installers will, as 'Engineers' and "Experienced" and with many replies to their record, who really are mostly great to talk with, have not been relating the 4th level GT Systemic layouts
    for maximizing all ROI's: Tight house, shorter duct zoning, not all to walls, good return-Aire's !
    from a single unit that just has an oversized DeSuperheating coil. That's it.

    $expense using additional Wtr:Wtr unit to do precisely what of many thousands have reliably installed since the late 1980's (one manufacturer's 1981 patent is not Hydro-Delta's "hydro-heat")- not referred to, here.

    Larger loop of 3/4 ton more suggested for Priority, is WHY larger savings in 10 years!
    or 200ft compared to 175/ton designs X the LOADS (not just looking at the unit) is for this GT having a larger DeSuperheating and Priority HW coil HX; and this is double vented for codes.
    Some are OEM internal 4-zone boarded for inexpensive additional zoning by , say Belimo or Arzel (Clev,OH) or even DuroZone motorized and pneumatic dampers.

    Please ask your Q's for detailed qualified comparing, and you may find better than any form of AirHP for many applications and commfort and superior ROI's.

    ASHRAE 2008 in NY's Spring meeting had the experience of getting to know how this system, like today, still, is getting savings goals improved beyond 12+% of conventional GT app's.
    --- meaning there are a "few who understand" how to select and design the very highest PRACTICAL ROI, I have reason to believe.

    It made the difference at Fort Knox, Trane company calling me, and the facility manager of the GT Engineering, changing by freely advising to drop from 110 TDH to a max of 40TDH-Antifreeze with the penalty of glycol 22%, meaning not over 25 ft TDH water (11 psi) including all fittings, HX Water coils, pumps, loop - everything. -to get to 24+ Rated-Equipment-Tons per circulation 1hp.

    ? Like a mini? The same can have the blower shut off and an ordered internal OEM 'Zone-Coil' can be plumbed to a remote fan coil for another extension of HVAC on insulated PEX/ O2Barrier lines. Blower varies by programmed set plenum temps for affixed comfort/sharing.

  12. user-1127834 | | #12

    Summary Questions to ask:
    What all can legitimately be tax credit inclusive?
    What utility rebates and incentives are included?
    What about "scratch and dent" GT or Air HtP from a reliable source? (this is a simple reality, situationally)

    What if I can only save 300 or 400 a year over AirHP with GT? -Will the resale values in the next ten years be there with ? -for each comparable accounting (censoring) of the different HtP system proposed ?

    How will I heat HW and afford it within an HVAC contract(s) Total(s)?
    Maintenance expected to cost ? - in 3 years, 6 years/ 10 years- 12 years and replacement(s) in 10-15-25-30 years?

    Do I need a 10-yr warranty or is 1 year all and 5yr compressor enough?- etc.

    'Controller' mentality applies to overall peace of mind and distributed comforts which may profitably produce a lack of stress and higher morale among any compromise - that - really may lead to a better functioning immune system and long life, scientifically. If peaceful ROI's/Comforts are lubricating the bones of efficiency to avoid 'arthritic' endeavors of comparing just the billing dollars, then some 'Leather' option may be the best return of all (and more biodegradable).

  13. wjrobinson | | #13

    "uderstandable" couldn't of said it better.

    Me fav new word for sure. Whut?

  14. wjrobinson | | #14

    Uuderstandreichk- my German translation of "uderstandable ".

    Dana, with minor adjustments to your plan for the long and short term psychopathic state of mind of near feline life you got it knockin you do.

    Carry on, feel the wind building, set sails...

  15. wjrobinson | | #15

    Oh and forget not to pull, the anchor my man.

  16. user-1127834 | | #16

    What was the real heat load at say -15 and 0'f?
    Living in zone6- to 5.8, with that 1994 winter of -22 for 2 nights- , -area, there is nothing that compares to a GeoThermal HOTWATER recovery and HVAC Priority on Demand HW heating inclusive.
    Note, sizing may come towards a selection just for a desired HW heating size of the GTHP-HW. Since 1980 (an AR manufacturer) and now Carrier/Climatemaster cloning the Priority HW Hydro-Temp of 1981 patents has replaced the w:w needs plus GT-justHVAC, but few have so figured. Since it is reliable and not problematic, following manufacturers' (both) details, this is ideal zone 6 to 4000 sq ft (modern insulating and ERV etc) with just a "size 4 " even, but also on a "size-5" ground loop to MATCH THE BUILDING LOAD -to-loop- - - and not sizing a loop to the GT-Box. HW on-demand specs BEGIN at 185-200 vertical ft/Building-Ton and a 3/4 pipe, or 3/4 x per 500 ft in a 245 long ditch, x 6ft deep trench in damp soil/clay, not dryer sand.

    So if your at 2-tons you may find the 3.1/4 tons of DUAL compressors for 3-staging is perfect with a "size 1.5 + size 2+" in the GT-Box (3.1/4 compressor tons is like "SIZE 3.8-4tons" ) but on a least a minimal 4ton ground loop if depending on Demand HW Production.
    When EER's of units with desuperheater-little HW generators hit the 20's , this Priority On-Demand full Heat recovery in Cooling modes (loop pumps not running) had COP's over 5.0 and EER's as high as Climatemaster is now reporting, but since 1981... for 40 to 35,000 btuh HW heating like a gas HW recovery, staging for a smaller loaded home, - all too easily for over 3 decades.

    See 1993 National studies did not include the Priority on Demand... but it was the only one in Cooling that had 100% heat reclaim. True Priority, in Cooling.

  17. wjrobinson | | #17

    whaat he said... yaaa

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