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Community and Q&A

Air-source heat pump vs. geothermal heat

silento | Posted in Energy Efficiency and Durability on

We are planning to build a new home in a rural middle Tennessee where the climate is considered as moderate. The builder recommends foam insulation throughout and plans to make it very tightly insulated. I’ve read numerous articles on the pros and cons between air heat pumps, mini-splits and geothermal. I’ve had a couple estimates from some local heat & air companies for both. Geothermal was estimated with a base price of $34K, but with some recommended additional added items like a fresh air exchange handler, dehumidifier, etc. it was closer to $45K. The air heat pump was $16,800, but with the same add on’s it would be around $27,800. Even with the 30% tax credit geothermal would be $2000 more. I like what I’ve read on the mini-splits as being very efficient, but don’t like the aesthetics of wall mounted units. I think I am in agreement that geothermal (while it has it’s place) may not be the best option due to the payback. Nobody can tell me what the average month savings is between a geothermal unit and an air heat pump. Are there good efficient mini-splits that can be centrally ducted and would it make sense to have a dual fuel option like propane since we plan to have this for our cooking and hot water?

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    Yes there are good efficient centrally ducted mini-splits, some that even modulate. But there are also some really great mini-duct cassette versions that modulate that are both more efficient and have a wider modulation range than the big-duct versions.

    Fujitsu's xxRLFCD mini-splits are perhaps the industry leader for mini-duct cassettes, since they're more powerful than the competition's and can handle longer and more complex duct runs, even though they are still no match to old-school big air handlers for moving massive volumes of air longer distances.

    But most of the big air handler stuff installed out there are 2-3x oversized for the actual loads- most houses don't really need it. For many houses (even in cold climates) it's possible to install one mini-duct mini-split per floor with phenomenal efficiency, at about $6-7K per floor. When it's getting to be 2000' per floor that may not work but 800-1500' often/usually can be made to work in a zone 4A climate.

    Before deciding on any heating/cooling solution it's important to get the most accurate heating and cooling load calculations possible, such as a Manual-J using the most aggressive reasonable assumptions on air tigthness & R-valvues. Those calculations should be done by
    someone who makes a living & reputation on the accuracy of the numbers, such as certified engineer or RESNET rater, and NOT an HVAC contractor who makes a living installing and maintaining equipment. Even though HVAC contractors sometimes know how to run those numbers and offer that as a service, the track record is abyssmal. It's human nature for them to be conservative, but the Manual-J instructions require being aggressive, and even then there is usually a double-digit percentage margin over the real loads.

    But a 10-15% oversizing upsized to the equipment step that covers it is very different from starting out with a 50-70% exaggerated number and upsizing from there to something twice the size it needs to be. At the high up front cost of geothermal it's usually better to size the heat pump at ~95% of the design load and make up the difference with heat strips for the really cold hours.

    Since the house isn't built yet you have control over the load numbers. If it's being designed by an architect the basic Manual-J should be performed as early as possible in the process, then updated as the design evolves, with decisions being firmed up on insulation & windows, etc.. Simulating the house with the DOE free tool BeOpt is also an excellent way to track changes and adjust the cooling & heating loads downward. A lot of optimization can take place for very low money when it's in the design phase. Even at code min an optimized house can end up with half or 2/3 the cooling & heating loads of a loose-cannon design where no consideration is given to the energy consequences of different aspects of the design.

    All foam insulation isn't the cheapest way to make it air tight, nor is it guaranteed to be air tight. Foam insulation in framing cavities only air seals the cavity- it doesn't seal doubled up framing such as jack studs or top plates, window headers, or the bottom plates of studwalls to subfloors, etc. Foam is overrated IMHO- cellulose has more thermal mass, and does a fair amount of load-leveling (though not nearly as much as insulated concrete form construction), and it's protective of the structural wood since it wicks & shares the seasonal moisture burdens without damage or loss of function.

    1. silento | | #4

      Dana- Thanks for the quick reply and information on Fujitsu's xxRLFCD mini-splits. I'll have to do some more research on this. To better give you the size home, it is 2753 sq ft. main floor (on a crawl space) with an 800 sq.ft unfinished bonus room over the main living area. The builder I am working with told me foam is more costly, however he has built some homes that were 5000+ sq ft and he indicated the monthly cost was under $200 a month. I'm not sure if he was going to use foam exclusively or with some combination of traditional fiberglass or cellouse. The upper roof area was going to be open cell foam and closed cell under the crawl space. I also have another question on insulation. Does it make sense to insulate between the main flow and upper since we will probably just use the upstairs for storage for the time being? I thought if we ever did finish the upper floor we could install a mini-split.

      1. Expert Member
        Dana Dorsett | | #7

        >"... 2753 sq ft. main floor (on a crawl space) with an 800 sq.ft unfinished bonus room over the main living area."

        The 800' bonus room can get it's own half-ton or 3/4 ton wall coil type minisplit when the time comes.

        If the crawl space has ample room for routing ducts, that's probably where it should go. A room by room load calculation and the layout would be needed to know of the whole 2753 square feet can be handled as a single zone by a single mini-ducted minisplit. It will most likely need two. A pair of 1.5 ton -18RLFCD Fujitsus can deliver roughly (a combined) 43,000 BTU/hr at central TN type 99% outside design temperatures. It's unlikely that your heat load would be that high or that your cooling load would be as high as 3 tons, so there's some optimizing that can be done. You may be able to swing this with pair of 1 tonners, or a 3/4 tonner + 1.5 ton.

        But without some preliminary load calculations it's a WAG. But I could heat my own sub-code 2400' antique 2x4 framed 1.5 story bungalow + 1600' of insulated basement with a pair of the 1.5 ton Fujitsus at my local design temp of +5F with margin to spare. If your house can't end up with a heat load lower than mine it needs some serious design revision.

        >"Does it make sense to insulate between the main flow and upper since we will probably just use the upstairs for storage for the time being?"

        No, there is no advantage (and some disadvantges) to insulate between two conditioned spaces, even if they are on separate floors.

        If the roof deck is being insulated to R49 (the IRC 2012 and later code minimum for zone 4A) with open cell foam there's next to nothing to be gained energy-cost-wise by insulating between the unfinished attic space and the main living area. Insulating between floors would mean that the attic would run seasonally warmer/colder than with no insulation, but won't affect the whole house load by very much.

        1. silento | | #8

          Thanks Dana for the information. This keeps me from over analyzing the design. The builder told me Bedford county, Tn just updated their building codes to 2009 standards (whatever that means) I think the ceiling needs to be at least an R-38 for this area. I'm going to research on who has experience around here with Fujitsus in Tennessee.

          1. Expert Member
            Dana Dorsett | | #9

            Just because the county doesn't make you install more than R38 doesn't mean that's anything like the optimal, or most financially rational amount of insulation.

            A house only built to code minimums can be viewed as "the crummiest house that is still legal to build".

            For half the budgetary price of $34-45K for the geothermal system you can buy a LOT of building envelope performance above & beyond code-minimums!

            For a starting point on what might be financially rational on a longer term basis (or rational compared to an up-sized geo system) see the zone 4 row of Table 2, page 10 of this document:

            https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf

            Note, those are "whole-assembly R" values, not center-cavity. An R38 cathedralized ceiling is typically about R32-R33 whole-assembly, after factoring in the thermal bridging of the rafters, the R-value of the roof deck & shingles, ceiling gypsum, interior & exterior air films, etc.

            Some examples of the R45 "Compact Roof" in Table 2 would be R38 fluff or open cell foam under the roof with 2" of exterior polyiso above the roof deck, or R49 under the roof deck with 1" of exterior polyiso, or R30 rock wool between 2x8 rafters with 3" of exterior polyiso.

          2. Expert Member
            Zephyr7 | | #10

            “updated their building codes to 2009 standards” means they are now using the 2009 revision of the code book. Many municipalities lag behind on codes, for various reasons. Codes are periodically updated (the National electrical code, for example, is revised every 3 years and we’re currently on the 2017 version) to reflect new ideas, new products, and new expierence.

            All the county meant was that you are supposed to build to the 2009 code requirements as a minimum. You can build better than code requires, but not less. You probably don’t want your building built to the minimums allowed by code if you’re reading this site.

            I’ve always liked the saying that “building to code is the minimum passing grade, which is like a D-“

            Bill

  2. Jon R | | #2

    If $2K is the accurate difference, then it might be worth it on indoor and/or outdoor aesthetics alone. I'd expect geo to be slightly more efficient, but the real answer is "it depends".

    Also consider the longer expected life of geo and the ability to heat domestic hot water.

    1. silento | | #5

      Hi Jon- Based on all the articles I've been reading on this site it is becoming clearer to me that for the size home we are building (2753 sq. ft.) heated with an 800 sq. ft. unfinished bonus room, the min-splits look promising. I know three people that have geothermal or ground source heat pumps installed and so far they are happy with them. That being said, all of these homes range between 4000-5000 sq. ft .I'm starting to think that maybe geo makes more sense on larger buildings and if we super insulate the home and have geothermal, it might be overkill. I mentioned $2000 as the variance, but that is only if the tax credits can be applied on the upgrade options such as an evaporate humidifier, electrode steam humidifier, 65 pint dehumidifier for the crawl space, an ERV and air purifier lamp. The base cost for the geothermal system (Bosch, 4ton) was $34K and a Trane 16 seer dual fuel system was $18.6K. If I just took the difference without the additional add ons, the difference is more like $5200 after the 30% tax credit. If I include the additional options the difference is really $1900 after the 30% tax credit. Of course now I would be spending $45K upfront ($34k for the unit +$11K) for the options. My thinking after reading numerous articles on this site is to put the money in the insulation? Also, I don't think the builder has a favorable impression on geothermal heat and air because of the cost.

  3. GBA Editor
    Martin Holladay | | #3

    Silento,
    In almost all single-family residential applications, an air-source heat pump makes more sense than a so-called "geothermal" system (more accurately called a ground-source heat pump).

    One of the biggest risks with a ground-source heat pump is the risk associated with the fact that it's a one-off design -- a site-built system put together by more than one contractor. Researchers who test installed ground-source heat pumps often find design errors, pump specification errors, and plumbing errors resulting in disappointing performance and lower-that-expected efficiency.

    With a ductless minisplit, you don't have this type of error.

    For more information, see "Are Affordable Ground-Source Heat Pumps On the Horizon?"

    1. silento | | #6

      Thanks Martin- You are bringing up a good point. One thing I have noticed here in middle Tennessee is that TVA promotes geothermal heating on their website. There are numerous heat and air companies that offer both around here. Mostly when I ask them which is more efficient and the better way to go, they want to steer me to geothermal or as you say ground source heat pumps. That raises my curiosity in that maybe they are more profitable for them. As I mentioned to Jon, I know several people who have had them installed and they seem to like them, but they have much larger homes to heat and cool.

      1. Steve Grinwis | | #15

        When installed proficiently, geothermal will definitely work. But that's what the industry is geared to work towards. Functionality, not efficiency. It's a lot easier to put a bigger pump than you need on your coolant loop, which guarantees functionality, but costs you efficiency. This kind of thing is rampant in the industry, because these systems are super hard to design really really well, and the homeowner isn't going to have the technical chops to tell if they've designed the system to be efficient or not. It's a complete gamble on how efficient the final product is going to be, because it's going to vary considerably from installation to installation.

        Conversely, in an air source heat pump, everything is designed and implemented at the factory. The correct fans and compressors are all mated together at the factory, and shipped as a complete optimized system. I've actually seen a lot of really expensive geothermal systems underperform good mini-splits in the real world.

        In a study in Ontario, the stand-alone efficiencies of a GSHP and ASHP were within striking distance of each other, and this is where it's quite cold which should really favor the GSHP...

        http://www.mitsubishieng.ca/en/hvac/PDF/zuba-central/ASHP-GSHP-Technical-Brief.pdf

        1. silento | | #23

          Thanks Steve- this is a great analysis on the comparisons between geothermal and the improvemens being made by the Air heat pump manufacturers.

  4. Expert Member
    Peter Engle | | #11

    You mention "options such as an evaporate humidifier, electrode steam humidifier, 65 pint dehumidifier for the crawl space, an ERV and air purifier lamp." You won't need most of those options if you build a well-insulated and tight house.

    I'm not sure what you mena by an "evaporate humidifier." Evaporative humidifiers are also called "swamp coolers" and used to be be common. They are still somewhat common in desert areas. They are worthless in TN with the high summer humidity. But you might be talking about something else.

    You do mention a steam humidifier. Houses run dry in winter because of excess air infiltration. The low-moisture air warms up inside the house and gets very dry. If your house is tight enough to need an ERV, it won't need any humidification. You might need to dehumidify, even in winter, though bumping up the airflow in the ERV usually works in winter.

    The ERV is the only one of those options that almost always makes sense in a tight house, but it should be used regardless of your HVAC solution, so it shouldn't be in the cost tradeoff mix.

    The crawlspace dehumidifier may also be unnecessary if you build a properly insulated and sealed crawlspace. With proper moisture barriers, insulation, and air conditioning, the crawl space should act like the rest of your house without any active dehumidification. However, there have been a few threads here where very tight houses need whole-house dehumidification in summer, because the latent humidity loads exceed the sensible cooling loads.

    Skip the geo and all of the add-ons and apply those funds to a better building envelope. The result is a house that is more comfortable, more resilient in cases of power failure and the like, more durable, and cheaper to maintain.

    1. silento | | #13

      Thanks Peter! My guess is the heat& air quote for the various be add-ons was to make some additional sales. I'm not sure what a evaporate humidifier is either. I like your suggestion on focusing on the envelope. That seems to be the concesus in the replies.

  5. Walter Ahlgrim | | #12

    The first thing in your question that caught my eye is dehumidifier and Tennessee. If you were in Houston I could be convinced it is necessary. But in Tennessee a well sized AC will be able to control humidity. What scares me is if someone installs 3x oversized systems adding a dehumidifier will make the system work as long as you do not care about the electric bills.

    One of the reasons no one want to talk about operating costs is the loops / wells vary so much from one site to the next. Another thing is all the sales literature talks about the earth’s 55°temp but almost every loop is close to 32° by late February.

    Also the pump that moves the water thru the loop is often made by a different manufacture so any numbers you do see are very unlikely to include any of the pumping costs.

    Dual fuel sound like a good idea on paper but in reality a propane furnace blows much warmer air than a heat pump so the occupants will almost certainly turn off the heat pump in favor of the propane. People will get use to a heat pump but it takes time.

    Please make sure any and all duct work will be installed inside the conditioned space and not in the attic.

    Before you decide propane is a must have please do some cooking in an induction range they heat and cool faster than gas and do not pollute your indoor air. In most places a propane water heater is no cheaper to operate than electric.

    Walta

    1. silento | | #14

      Hi Walter- I'm not sure why I would need a dehumidifier either. The heat & air contractor was specifing this for both Geo and dual fuel. We don't plan on having any duct work in the attic. It will be under in the crawlspace as seems to be the norm in this area. I'm going to investigate the induction range idea. We have electric currently, but we had gas years ago and liked it. I'm not a big fan of propane, but we at least wanted a gas fireplace. We currently have natural gas where we are now, so you're right...the heat would be warmer and quicker, but I would only want to use it when the heat pump would be running hard in the cold snaps. We only have about two months (Jan and Feb) when the temperatures dip below the 30's. I thought having propane for water would be more practical in the event of a power outage.

      1. Jon R | | #16

        You need a dehumidifier for comfort because there will be times with little to no sensible load and significant latent load. This will occur, but it's your choice to either remove the humidity or live with it. I'd get one (portables are most cost effective).

        The need for a humidifier is also a comfort issue and depends greatly on how much water you put into the air, HRV vs ERV, ventilation rate and climate (tight houses CAN get too dry for optimal comfort). You may not need one.

        An example: at 30F, 75% relative humidity and 100 CFM of ventilation (HRV+wind), you need 21 pints/day to maintain a recommended 30% inside.

        See below to estimate sources. You may well not get close to 21 pints and so experience periods of drier than optimal air.
        https://www.greenbuildingadvisor.com/article/moisture-sources-relative-humidity-and-mold

      2. Expert Member
        Dana Dorsett | | #17

        Dual fuel is almost always a bad idea- the furnaces are as a rule ridiculously oversized, way too oversized for comfort. The oversized furnace results in the hot-flash followed by the extended chill type of operation when it's actually cold outside. A conventional 1-2 stage heat pump + heat strip sized correctly (including not oversizing the heat strips) are as-cheap to run delivering more stable room temperatures and long run cycles during cold weather. A right sized mini-split solution is almost always cheaper to run than propane, and runs nearly continuously during the colder weather, modulating with load.

        A propane or diesel or gasoline fueled backup generator makes more sense than a propane water heater. Last I heard TN still had first-world type grid reliability- how often do you actually lose power at that location, and for how long?

        In a tight house it's unlikey you'd ever need to add humidity to the air to stay comfortable in winter- throttling back on ventilation rates during extremely cold weather is usually enough. In a low solar gain tight house some mechanical dehumidification might be necessary, but in most houses the air conditioning would handle it. With a mini-split it can be operated in "DRY" mode instead of normal cooling or several hours at a time if the indoor humidity is creeping up. There is always some sensible cooling in DRY mode, but the sensible heat ratio (SHR) is shifted heavily toward maximum latent cooling when operating in that mode. (This is something fairly easy to do with modulating compressors and variable speed blowers that just isn't possible with 1-2 stage AC.)

        1. silento | | #24

          Thanks Dana- your explanation makes sense to me.I obviously need to educate myself more on the new mini-split technology. You
          are correct, Tennessee has some good stable electric grids. It's underground where are lot is. I need to ask the neighbors on power outages. I am not a big fan of propane. All electric also concerned me based on heat bill discussios at the office. I'm trying to find the best low cost option since NG is not an option in that area.

        2. silento | | #26

          Great points Dana! After reading all the posts I'm really leaning towards mini-splits and maybe the electric heat strips. I don't like propane due to it's volatile prices. I think the current electric rates are around 10 cents per kwh and for the most part.Power outages are rare except when there are ice storms. I've heard so many horror stories from people I work with on their electric bills in the winter it concerned me. Of course the houses are older and probably not insulated well. I do have another question for you....what would be the efficiency differences between the full house ducted vs the ductless? Is it minimal assuming the duct work is insulated properly or is there longer run cycles required? My wife is warming up (no pun) to the ductless type. As I mentioned to some others in these posts, our two coldest months are Jan/Feb. We can get below 0, but that is a very rare event. The summers are more challenging due to the high humidity, but then I'm a transplanted Yankee from Michigan. LOL!

          1. Expert Member
            Dana Dorsett | | #32

            A typical 2-stage ducted standard heat pump w/heat strip backup delivers a seasonal average of about 8000-9000 BTU/kwh (HSPF rating of 8.0-9.0), if properly sized for the heat load. It will cycle on/off except during the coldest weather when the heat pump is running nearly 100% duty cycle at it's high stage, with the heat strips cycling on/off.

            Modulating ducted heat pumps w/ heat strips such as the Bryant Evolution Extreme or Carrier Infinity Greenspeed have only about a 2.5:1 turn down ratio, but will run very long cycles during colder weather, and deliver a 10-13,000 BTU/kwh, depending on the air handler option.

            A mini-ducted Fujitsu mini-split delivers about 11,000 BTU/kwh (HSPF 11-ish) and with much larger turn down ratios will run nearly 100% duty cycle all winter long, modulating it's compressor and blower speed up/d0wn with the load. They have the output signals for controlling an auxiliary heater, but do not have internal heat strip options.

            A best-in class modulating cold climate ductless mini-split delivers 13-14,000 BTU/kwh when properly sized to the load, with no heat strip options. A best in class multi-zone ductless doesn't modulate the heads, but delivers about 10,000 BTU/kwh.

      3. Expert Member
        Zephyr7 | | #22

        Many water heaters require electricity to run. I think all of the really high efficiency ones do. To be able to run the water heater with only propane during a power outage, you’ll need to make sure it has no electronic controls and no inducer blower.

        What’s the point of having hot water during a power outage? Hot water won’t keep your pipes from freezing. I’d be more worried about keeping the house warm than running the water heater. Don’t design your HVAC system around power outages. Design your HVAC system to keep you comfy and to be efficient while operating. If power outages become a concern, then think about adding a backup generator.

        If you think you might want a backup generator in the future, you may want to consider having an ATS (automatic transfer switch) installed as part of your intial electrical installation in your new house. You don’t have to install a generator right away — the ATS will just sit there with nothing to do until you add a generator, but you’ll save on electrical installation costs by installing it at the same time as your main panel and service entrance (the ATS installs between those two things).

        Bill

        1. silento | | #27

          Thanks Bill! We'll....we actually did experience a long power outage in the area back in the 90's due to an ice storm. No power for Four days with
          temps in the low 30's. Having gas for hot water made it a little more tolerable. That said, I am not as worried. I was thinking propane for the fireplace at a minimum to warm up if needed. I also like gas for hot water. It seems to be a little more efficient but, maybe it's a wash with the new tankless options. I'll have to investigate on how many times the electric has failed in the area. It's more rural, but the subdivision has underground electric.

  6. Walter Ahlgrim | | #18

    Please make sure your crawlspace is fully conditioned. In that the walls of the crawlspace are air sealed and insulated in the same way the walls of the main floor are done, both supply and return registers should be installed in the crawlspace, the ground needs to be covered by a vapor barrier and covering it with a thin concrete slab (rat slab)is a good idea.

    I do not think your weather is very different than what I see in Missouri we only need dehumidifiers in basements that are very damp.

    We like our GE induction cook top and installed a propane fireplace as the only gas appliance.
    I have been without electric and hot water for 7 days and hot water was not on the list of things I was thinking about.

    My wild guess is your HVAC contractor is looking to pad this job with every bell and whistle he can think of. I would get 2 more bids, preferably from people who do not think there is no budget on this house.

    Walta

    1. silento | | #19

      Thanks Walta! I'm keeping your suggestions and advice when we meet with the builder. He has mentioned in the past about vapor barriers as a necessity in the crawlspace. I can bring our current dehumidifier with us since we currently have a basement home. Also checking in to induction cook tops. BTW- is your propane fireplace vented or ventless? That seems to be another controversy in articles I've read

      1. Expert Member
        Malcolm Taylor | | #21

        Silento,

        A frequent contributor here, Stephen Sheehey, had a good suggestion for those considering an induction cooktop, which was to buy a small portable one to try. I took his suggestion and picked up one for $70. I'm sold on them now.

        1. silento | | #29

          Thanks Malcolm! Everything I've read so far looks good on induction cook tops. We will probably go with this. My wife and I forgot about this option.

    2. Deleted | | #20

      Deleted

  7. Walter Ahlgrim | | #25

    Defiantly vented I think the idea of vent free fireplace in a tight house is a dumb idea.

    I put my fireplace in the center of the house air sealing becomes a challenge on an exterior wall.

    I moved my dehumidifier but after we dried out the construction moisture I have had no need to run them.

    Walt

    1. silento | | #28

      Thanks Walt! I was coming to the same conclusion in reading some concerns on ventless fireplaces in tightly insulated homes. The architect spec'd a ventless. I need to talk to the builder about a vented fp. It's on an interior wall, so I am thinking a flue pipe could be added in the wall just like gas hot water heaters.

  8. Expert Member
    Peter Engle | | #30

    The architect should be slapped for suggesting a ventless fireplace in a tight home. You never want the products of combustion in your breathing space. The vents for a direct-vent sealed combustion fireplace will take up some room in the wall, but that' can be accounted for in the design phase.

    You don't mention looking at heat pump water heaters as an option. You should take a look. They are the best current alternative for an all-electric house.

    +1 on the recommendation for a sealed, conditioned crawl space. There are some very good articles on the subject on this site, along with construction details. Send some to your architect to include in the plans. Better yet - make the architect sign up for a subscription. maybe he/she will learn something here!

  9. Walter Ahlgrim | | #31

    You should be able to vent thru the wall above the fireplace the instructions do not say it but the fireplace will work better if the flu is vertical for a foot or two before any bends.

    Before the fireplace is set in place make sure the exterior wall get insulated. Drywall tape and mud the chase behind the fireplace so ruff framing needs to leave room for the drywall.

    Walta

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