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Community and Q&A

Geothermal quote after rebate coming back cheaper than air source

BuildingAHome | Posted in Mechanicals on


I am building a new house in CZ5/6 with a Manual J design temperature of -2F.  The house is a ranch style with a walkout basement below. The footprint is 2500 sqft, so it is around 5k of total sqft. I requested a quote for 2 Mitsubishi SVZ+SUZ hyperheat ducted units, one for each floor, as having multiple units makes me more comfortable with air source units in case one of them dies in the winter (I see them as less reliable overall than the geothermal system, but maybe this is a bad assumption). I also got a quote for a single Waterfurnace 7 series with dampers to zone the main floor and basement. After the 30% rebate, the quote for geothermal is coming in a few thousand less. Quote includes drilling vertical loops beneath the home. Based on several sources, I really would like to choose this contractor, so I haven’t requested quotes from others.

A manual J hasn’t been run yet, but the quote is for a total of 5 tons. Based on the BetterBuiltNW HVAC sizing tool, I likely need more like 4 tons total, but I don’t think that will make a big difference in the situation here. I have an R15 slab, R21+R8 walls, .18 U windows, and R60 attic. Shooting for < 1.0ACH50, so I have already put money into the envelope.

I had originally decided to go with air source heat pumps based largely on raving reviews of the comfort of the system and it is great to electrify and expected the quote to be lower than the geothermal. Best I can tell from estimating energy usage, the cost to run the air source systems would be similar to a natural gas furnace. However, if I can install the geothermal system for similar or less money, then that seems best since it should use a lot less energy than the air source system in the winter.

Does anyone have any insight as to the comfort of the fully modulating Waterfurnace 7 with a 2 floor damper zoning system vs separate Mitsubishi ducted units? Seems like a large difference in comfort would be the only reason to go with air source given this situation.

Thank you for any input!

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  1. mjhil | | #1

    Two air source heat pumps not so much from a redundancy perspective but from an efficient zoning approach seems to make sense to me. The particulars of the basement would really dictate what style indoor unit you go with, but maybe is there an opportunity to go with something cheaper than a SVZ (and associated ductwork) for the basement? Some basements can be covered by a single wall/ceiling cassette or a step up from there being a concealed duct unit. Without knowing your layout it's hard to say.
    It's worth considering an oversized geo unit might not be the best for comfort or energy savings - let alone that actual savings of such units might not be so great after all factors accounted for. I know several people with geothermal units (waterless) that are mostly happy, but at least one is well oversized with all downstream effects you would expect.

    1. BuildingAHome | | #4

      The basement is mostly finished and has multiple separate rooms, so I definitely think a ducted unit makes the most sense there too. Yeah what really has me still questioning things is I have seen so many people on forums say how much they love their Mitsubishi ducted units. There just aren't that many people out there with the Waterfurnace 7, so I haven't read a ton of real consumer reviews. It may just be with the Mitsubishi ducted units being modulating that makes people so happy with them, while most of the nation is using single or two stage furnaces. The Waterfurnace 7 modulates as low as the Mitsubishis, so it is promising in that respect.

  2. paul_wiedefeld | | #2

    I’d go with the geothermal, but both are fine options. The geothermal can also do DHW, which is a great value add.

    Geothermal doesn’t always result in higher efficiency, but it’s worth the chance.

    GSHP being ~30% more than ASHP seems reasonable. The rebate is quite the scam on a policy level but hey good for you.

    1. BuildingAHome | | #3

      Thank you for your input. Yeah the ASHP credit being capped at $2000 while the GSHP credit is completely uncapped is quite interesting... The biggest advantage to the government I see is preventing grid overload in the winter.

      1. paul_wiedefeld | | #5

        Perhaps, but it’s a clunky way to go about that. Utilities have been using demand charges for probably a century now.

  3. user-1112693606 | | #6

    I’d get your heat load dialed in. Even 4 tons sounds excessive to me. If air sealing is done well, I’d think maybe 2 tons? Two svz-18 with optional resistance coils would certainly do it, maybe even 12s.

    On a more general level, if your air sealing is done well, you are going to need mechanical ventilation. If your insulation is installed well and your windows are pretty good, the building is going to gain and lose heat slowly. If you combine a good ventilation system (zehnder for ex) with a good thermal envelope, then zoning is suddenly a moot point because the ventilation system tends to move btus around and no area of the house is more than a few degrees different no matter what it’s doing outside.

    At that point you can get rid of the complexity and expense of a ducted system (or a radiant system, or a geo system…) and put in a couple of high walls—one upstairs and one down. The downstairs handles most of the heat load because the stack effect pushes heat up, and the upstairs one handles cooling load. This is how most two story passive house are conditioned.

    Any well designed and competently constructed new house shouldn’t need the complexity and expense and risk of a ground source heat pump. Put that money toward windows, air sealing, ventilation. My 2 cents anyway. That _pretty good house_ book might be helpful in thinking through things.

    1. Patrick_OSullivan | | #8

      > At that point you can get rid of the complexity and expense of a ducted system (or a radiant system, or a geo system…) and put in a couple of high walls—one upstairs and one down.

      No ducting in bedrooms with closed doors will result in comfort issues. This is a hill I am willing to die on.

    2. Expert Member
      Michael Maines | | #10

      I agree--unless the house is full of windows, 2.5-3 tons sounds more likely to me.

      I've always been leery of GSHPs and this recent article supports my skepticism: It's only a single project but it highlights one of the many unknowns with GSHPs.

      And thanks for recommending the PGH book!

      1. user-1112693606 | | #13

        Thanks, you are right about the heat load. I misread—not 2500 ft2, but 5000 ft2. That’s… big.

        The book is great, so another thank you! I bought three. Saves me all kinds of time. Now when somebody asks for advice on the house they are building or remodeling I just loan em a book. I might be saving the world more carbon by loaning books than by doing the actual work!

        1. Expert Member
          Michael Maines | | #23

          I misread the square footage as well; I though it was 2500. I've never designed a 5000 sq.ft. house and don't have a sense for what a reasonable heat load would be.

      2. BuildingAHome | | #16

        There is a good amount of glazing. I know it is inefficient, but windows sure make me happy. Based on the builtbetternw hvac sizing tool, I will end up closer to 40k btus at design temp. Due to the efficiency drop in both the geo unit and ashp at design temp, I have planned to need a 4 ton unit in either option as they come close to that 40k mark. Doesn't seem like I can cost-reasonably make up for heat loss with extra insulation to get myself down to where a 3 ton unit at design temp would work. Obviously I need to get a proper manual J done still, just wanted to get the process started to get high level costs figured out.

        I do really wish there was a similar study to that linked article with a vertical closed loop system like is done around me most typically.

        And thank you for PGH, it was well worth the purchase.

        1. user-1112693606 | | #17

          Your sweet spot might be two svz units sized to less than design load with the optional strip heaters to handle a couple really cold days. The resistance heat kits are available in 3,5,and 8 kw on the svz 12 and 18. That translates to 10,000 or 17,000 or 27,000 btus. So your heat pump is sized for maximum efficiency, and if you spin the meter with resistance heat a couple days a year, that’s offset by the fact that the heat pump is running most efficiently for the rest of the year. Pay attention to duct design if you go down this road—static pressures need to be kept low. Also if possible I’d suggest putting each svz on its own compressor—the big multi heads suffer from short cycling.

          If you’ve got a lot of windows make sure you look at cooling load too—south and west can bite you.

          And it may be too late but if you haven’t broken ground I’d suggest a smaller house. No offense.

  4. Patrick_OSullivan | | #7

    > A manual J hasn’t been run yet, but the quote is for a total of 5 tons.

    Stop. Do not pass Go. Do not collect $200.

    This is backwards. The process starts with a load calculation. You don't back your way into it.

  5. Expert Member
    DCcontrarian | | #9

    I'd jump on a geothermal for the same money as a air-to-air, but that's just me.

    But I would make my acceptance conditional on a thorough Manual J, and sizing confirmed by the manufacturer.

    Looking at the WaterFurnace website, the 7 series comes in 3 sizes from 3 to 5 tons, and the 5 series comes in a whopping 14 sizes from 1 to 6 tons. No reason not to have exact sizing.

  6. pnw_guy | | #11

    One thing to consider is complexity, likelihood of needing repairs, and AVAILABILITY of both parts and skilled labor to perform the repairs.

    The parts and knowledgeable labor to repair a Mitsubishi is probably fine.

    But if a ground source goes down, will you easily be able to get it fixed? Will parts be available? Will somebody with the knowledge to fix it be available? What if the company that installs it goes under? Is there another local company that you would trust?

    As I've gotten older and dealt with more home issues, I've come to realize that convenience and reliability are amongst the most important considerations in any home improvement decision.

    1. BuildingAHome | | #14

      There are about 5 waterfurnace contractors near me. Unfortunately, in my area, geothermal is much more popular than ASHPs for whole home hvac as far as I can tell. I actually know of people with geothermal systems, and I haven't heard of anyone in a single family residence with a ducted air source unit. So if I ask your questions today, I think it may actually favor geo here. However, in 30 years down the road when I have this expensive and complex system break, will everyone have moved onto ASHP as the tech continues to advance? Maybe, and that is a concern of mine.

      1. Expert Member
        DCcontrarian | | #15

        Interesting. That's my impression of ground-source, it's highly dependent on the soil structure to work well.

        1. walta100 | | #18

          “Interesting. That's my impression of ground-source, it's highly dependent on the soil structure to work well.”

          If you are using vertical wells it seems like the soil would be rock after the first 50 feet or so and almost no ground water since they fill the wells with grout.


          1. Expert Member
            DCcontrarian | | #20

            It really depends on the area. I did a bit of googling, there seem to be a fair number of places in the US where bedrock is 1000 feet down or more. Apparently in Los Angeles there are spots where it's six miles down to bedrock.

            I would think a spot with deep layer highly conductive soil that is cheap to drill would be ideal.

          2. Expert Member
            MALCOLM TAYLOR | | #21


            I had no idea those types of areas existed. Here if you don't hit rock within 25 feet, it's a huge surprise. On most sites there is some visible.

          3. Expert Member
            DCcontrarian | | #22

            I've never lived any place where there wasn't bedrock jutting out. At my current house I have 12' of soil until bedrock according to the geotech report, and 3' of that is fill.

            But they exist.

        2. Expert Member
          Michael Maines | | #24

          I'm surprised as well at that depth to bedrock! My lot has 0
          -30" of soil cover over bedrock, and I rarely work on a project with more than 10' to bedrock.

      2. Longstory | | #26

        I am amazed at the range of information floating about on GSHPs. In my area, Zone 4 middle TN, there are only a few residential GSHP installs. I did my first for my 4500 sq ft home in 2012 with 4 vertical wells 300ft each. Florida Heat Pump units now Bosch and I sold the home last month with 11 years of trouble free performance and significant energy savings over the propane and AC units they replaced. Downsizing now in the same area with a 2000 sq ft home and recently connected a 3 ton ClimateMaster GSHP with 2 300 ft vertical wells that has been turned on for 2 weeks now.

        As far as expensive and complex, the wells are anything if not simple plastic pipe loops inserted into a well with grout. Pipes connect to unit that now has built in pumps. The HVAC company works on these in commercial settings all the time (schools and municipal install) and tells me they are not much different from ASHPs. Parts are not weird and obscure by their telling.

        I know we all have our anecdotes but my system is also going to be near the cost of the ASHP option, so with my 11 year usage history I felt comfortable with GSHP performance.

        And BTW with vertical loops my loop temps vary about 10 degrees by season from the 55 degree baseline at my latitude

        Good luck with your decision

  7. walta100 | | #12

    All too often HVAC contractors use min splits as part of a bait and switch game.

    You ask for a mini split bid they give you a bid for an oversized system with a head in every room and when you are shocked at the high price tag, they just happen to have bid for a system you did not ask for that seems much more reasonable.

    Make sure to get two more bids for air source HPs from people not bidding ground source and at least one other ground source bid.


  8. walta100 | | #19

    When I see people on the geo exchange forum post their loop temps by late winter, they almost always seem to be near and even below 32°


  9. Mauro_Zammarano | | #25

    Geothermal is very popular in the DC area as well. For example, there are 20 waterfurnace installers within 23 miles and most of the people I know that care about energy efficiency go for it in this area. Of course, incentives play a key role. Definitely it is a great option if you want to use a single HVAC unit in 2 stories houses. Mitsubishi air source do not allow for damper systems for zoning so a two units is required if you want to put 1st and 2nd floor on a different zone. At that point geothermal with incentives might be cheaper.

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