I have recently retired as a Automotive Master tech and am acquainted with how auto HVAC systems work but not with the limitations of GCHP. We will soon start planning to build an tight energy efficient house, ICF basement. SLIP walls, and raised knee roof truss. Would like to use a GCHP for forced air heating and AC in a 1800 sq ft single story house. I have been reading as much as possible but I learned long ago that second hand information will only get you so far. My concern is that I have read several times that in colder climates, a closed-loop geothermal system does not work as well once the outside air gets at or below twenty eight degrees F. for an extended period of time The authors did not reference horizontal or vertical loops in their statements. Are vertical loops a better strategy in cold climates ? Are horizontal loops efficient in the Midwest? Should a back up system be installed when the HVAC duct work is being built ? The more I learn the more I realize how much more I need to know. Still trying to find a architect in N. Indiana that understands energy efficient residential building systems. Have not found house plans that will fit our needs. This is a really great forum to learn from !

They all work in the midwest and can be "efficient". What is optimal depends on specifics. Also consider air source heat pumps and natural gas.

Federal income tax subsidy for GSHP systems did not get re-upped in the recent extension of subsidies for solar & wind. System designers/installers are having to sharpen there pencils, and the total cost may not be 30% higher than it was in 2015, but it probably won't be cheaper. The most expensive GSHP designs can be the lowest bid price, since much is riding on the competence of the contractor- be very careful about vetting who you use if you go that route.

But rather than focusing on the HVAC solution in advance of designing the house, concentrate first on lowering the load. $25K of envelope improvements can do a lot more for reducing the operating cost than another $25K of GSHP. I've never seen a bid for even a small GSHP system for under $25 in my area, but I'm in a fairly high cost area, and YMMV.

In northern IN (US climate zone 5A) there may be better bang/buck going with high efficiency ductless air source heat pumps + rooftop PV solar, but a lot depends on the competence and cost of the GSHP system designer. It's unlikely that you'd have to pay more than $10K for a ductless ASHP system even for code-min 1800' house. I was recently involved in specifying a 4 tons of ductless ductless system for a house roughly twice that size, which came in at about $15K before MA state & local subsidies about $10K all-in. The 99% outside design temps in N-IN are in positive single digits F( https://articles.extension.org/sites/default/files/7.%20Outdoor_Design_Conditions_508.pdf ) and there are many ductless and ducted mini-split options that have specified output at -10F or lower. It's possible to specify something that actually works, and works at reasonable efficiency. (And in a high-R house you don't even need to design to the 99th percentile temperature bin to stay warm, but you're better off not pushing with potentially skeptical inspectors.) The efficiency of ductless technology isn't as high as GSHP, but with PV at $3.50/watt net-metered and reduced to $2.50/watt by the Federal tax subsidy it's actually pretty cheap to get to Net Zero Energy on an 1800' single story house with ductless ASHP with a decent better than code house. (Cheaper than going GSHP in a code-min house in my area.)

If you don't even have a preliminary heating and cooling load calculations, start there first. There's no way to tell how much system you need without running the numbers. Since the house is still under design, start picking away at what might be reasonably cost-effective load reductions and see how they pencil out. A reasonably tight reasonably thought out code-min 1800' house with 1800' of ICF basement would come in with a heating load of about 22,000 BTU/hr, which can be served up comfortably with about 2 tons of ductless, which would also be adequate cooling for most 1800' houses. But it's not hard or expensive to get it down under 15,000 BTU/hr, at which point 1.5 tons of ductless (or a ducted mini-split mounted at the basement ceiling) and some rooftop PV start looking really good. I'm not sure what a couple tons of GSHP costs in your area.

BTW: What are "...SLIP walls..."

Whoops, fat fingers or senior moment---SIP walls
Dana1, thanks for the input on alternative HVAC. Now I need to learn about ductless ASHP. What strategy for ventilation would be needed with ductless ASHP ?

With any high-performance air tight house, heat recovery ventilation (HRV) is really the way to go, with it's own smaller diameter duct system. Using the heating/cooling system ducts for ventilation doesn't work all that well in low-load houses- the air volumes are high when the system is running, but it's duty cycle can be quite low due to the low heating & cooling loads. Even with a GSHP solution you'd still want a separate HRV system for managing the ventilation.

Like ICFs, SIPS are easy to air seal, but that doesn't mean the installers always get it right. With SIPs or any other higher performance house, if the contractor can't tell you the cfm/50 or ACH/50 numbers for the last three houses they built it's not a good sign. (That's cubic feet per minute or air exchanges per hour at 50 pascals pressure, which are the standard metrics in the biz for air tightness.)

From a $ / performance point of view they can make sense at mid-performance levels in markets where "time is money" is a driving factor. In Zone 5 to hit Net Zero Energy with a system that fits on the house usually takes a whole-wall R (all thermal bridging accounted for, not center-cavity R) of about R30. With EPS core SIPs that's about an 9" thick SIP (no matter what their marketing says it's R-value is.) A 2x4 / R15 studwall with 4" of rigid polyisocyanurate foam on the exterior of the sheathing would also get you there (in 8"), usually at a lower cost if the contractors are up to speed on how to do that. Getting there with polyurethane core SIPs takes about 6.5", but it's more money, and comes with a hefty environmental hit from the blowing agents used. (HFC245fa is going to be phased out eventually, but that hasn't happened yet. In the US the only vendor I'm aware of who has switched to a very low impact HFO1234 variant blowing agent is LaPolla.)

Wow, even more good information. The three building techniques I have found for R30 wall are SIP, double 2x4 wall and the wall you described. You know so much, what is your profession ? Do you believe that HRV is better than energy recovery ventilation ?

A double 2x4 wall has to be about 10" thick to hit R30 whole wall, and is more complicated to build than a foam-clad 2x4 wall:

http://www.greenbuildingadvisor.com/sites/default/files/Rigid%20foam%20wall%20sheathing%20-%20FHB%20-%20cropped.jpg

You may want to absorb this document:

http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/high-r_value_walls_case_study_2011.pdf

Compare the details & discussion around Case 4 and Case 10 (double studwalls) vs. Case 2b.

Case 2b is a 2x6 wall with 4" of exterior foam which only slightly outperforms a 2x4 wall with 4" of exterior foam.

A base-case 2x4 16"o.c. studwall like Case 1-bii comes in at about R10 whole-wall, and adding 4" of polyiso on the exterior of that brings it up to a bit over R30 (R34 if using the labeled-R for the polyiso rather than a more realistic temperature derated performance.)

An ERV makes more sense at high ventilation rates in warmer more humid places than yours, say the US climate zone 2A/ gulf-coast region. ERV cores are more prone to frost damage than HRV cores and doesn't really buy you much in zone 5A. During the sticky dog-days of summer it's fine to just dial back the ventilation rates to keep from pulling in too much latent load, or to run a room dehumidifier when the outdoor dew points are in the mid-70s, which are the peaks, not the average in IN. In the gulf coast region mid summer dewpoints can average well north of 70F, and an ERV can buy you a lot.

I'm just an engineer (with a building performance & HVAC hobby. :-) )

A bit off topic. How is the siding secured to the exterior wall sheathing through 4" of foam ?

Bob,

I live in Cincinnati, OH, which is a bit warmer than northern Indiana.

I'm not sure if you're planning on being full electirc, or having some sort of natural gas heating as well. However, if you're planning on all electric, I think my electric usage history might be useful to you. I've been keeping track of my electric usage for years, with both an air-source and a geo unit installed. Keep in mind that the numbers I present on my blog are for my whole house, but they should give you a general idea of what savings you could get w/ a geo unit. Since you are in a slightly colder area than I am, and because the majority of the savings I'm getting is in the winter months, I would expect you to have slightly better results than I did.

https://bloodforge.com/post/Detailed-Geothermal-Analysis-in-Cincinnati-OH

Posted By Jubal on 26 Feb 2016 06:02 PM
A bit off topic. How is the siding secured to the exterior wall sheathing through 4" of foam ?

The typical method is to fasten the foam to the house with 1x4 strapping through-screwed to the studs 24" o.c. with 7" pancake head timber screws (they have to penetrate the studs by a minimum of 1.5")then hang the siding on the furring.



The air gap is an excellent capillary break, an back-ventilates the siding for quick drying.

IMO, some glue between foam and studs and under the strapping (in addition to the screws) makes it significantly stronger - foam can transfer a lot of force as long as there is no slippage. But perhaps the screws alone maintain sufficient friction between the foam and wood.

The whole purpose of ground source heat pumps are that their performance is independent of the outdoor climate, which is in contrast to air sourced mini splits.

They work great with closed loop ground heat exchangers in heating dominated climate.

Some forward thinking builders have gotten to a very simple formula which gives you the biggest bang for the buck:

1) Reduce the building load by modestly increase the insulation /air infiltration beyond code minimum. Foam is your friend.
2) Pop in a geo system to significantly reduce your building load
3) Put on a solar roof or wind turbine to make up for the electrical consumption of the house, including the geo system.

Now you have your net zero house. Usually at less than 5% cost premium using the current subsidies.

Keep in mind that us humans have to breath oxygen, and you get at a point of no return with added insulation/air tightness, where you now must mechanically ventilate air into your house and heat up the make up air.

1. Even at IRC 2012 & 2015 code-maximum air leakage the calculated natural air exchanges are below ASHRAE 62.2 levels by quite a bit

A leaky house is no substitute for ventilation air, since the infiltration volumes & locations aren't necessarily going where it does the most good, and may even be entering via a less-than-healthy path. The notion that a house can be "too tight" is simply wrong. Even leaky homes are often under-ventilated, paying both an energy and indoor air quality cost.

Bottom line: ALL houses need ventilation systems to ensure good indoor air quality, not just tight houses. Heat recovery ventilation makes sense at high ventilation rates, but there are many reasonable alternatives.

Foam isn't always your friend, especially in the hands of incompetent installers who can't get the mix or temperatures right. There are many lawsuits over outgassing of sprayed polyurethane foams

2. It's not clear how a geo system reduces the load. It serves the load, maybe, if the system designer did a good job, but the load is still the load. Geo is only rarely the cheapest way to serve the load on a lifecycle basis, but done right it is both efficient and reliable. (Emphasis on "done right".) Geo being part of "...the biggest bang for the buck..." can sometimes be the case, but it's far from certain, especially in better-than-code new houses.

In a tight better than code houses the domestic hot water energy use is often higher than the total heating & cooling energy use, and spending the money on hot-water heating efficiency will often have a better ROI and save more energy than going from air-source to ground source heat pumps. Heat pump water heaters and drainwater heat exchangers can do a lot. (Heat pump water heaters do double duty turning latent cooling loads into hot water in more humid climates.)

3. An extremely minute fraction of homes in the US would have sufficient wind resources near the ground to hit Net Zero. In new construction it's usually possible to orient the roof lines optimally and specify the amount of solar it would take to hit net zero with heat pumps of any type. The size of the array necessary to hit Net Zero is usually bigger with air source heat pumps than with GSHP, but the difference is surprisingly small, and getting cheaper by the minute.

The 40 year learning curve of PV has shown more than 20% cost reduction every time the volumes double. The volumes have been doubling about every two years for the past decade, but both the doubling time and the learning curve exponent have been increasing. At the utility scale PV in 2015 was at levelized cost parity with combined cycle natural gas power plants. Before 2025 (and maybe before 2020) small scale rooftop PV will be at cost parity or cheaper than combined cycle gas powerplants.

An investment bank's assessment of levelized cost in 2015:

https://www.lazard.com/media/2390/lazards-levelized-cost-of-energy-analysis-90.pdf

Small scale solar is averaging $3.39/watt in the US so far this year:

https://openpv.nrel.gov/

That was the average cost of utility scale PV in the US as recently as Q4 2011:

https://g.foolcdn.com/editorial/images/170846/solar-cost-chart_large.png

http://www.fool.com/investing/general/2015/06/27/solar-energy-revolution-past-point-of-no-return.aspx

What do you think rooftop PV will cost in 2020? About a buck fifty, just like utility scale is in Q1 2016? Maybe a buck-ten after applying the tax subsidy? The cost of making up any difference in efficiency with a few extra panels on a better than code house is already pretty cheap, and headed relentlessly lower (and that's no kidding!). At this point any financial rationale for PassiveHouse has long since flown out the window. As long as there's enough space to accommodate the array for Net Zero (including charging up the electric car), paying a premium for the potentially higher efficiency of ground source heat pumps is also losing some lustre. Solar electricity at a buck-fifty/watt costs less than the wholesale price of electricity, let alone grid-retail.

It's not just ground source heat pumps- many in the efficiency biz are starting to feel the competitive heat of ever cheaper PV. It's a disruptive technology to the entire energy business, including oil, in the face of the impending electrification of the transportation sector. China (the primary driver of oil demand increase in the past three decades) has increased policy support for electric cars/buses/trains. Norway (tiny, but an oil exporting country) has drafted (but not yet signed) legislation that bars sales of internal combustion cars & light trucks beginning in 2025. India's energy minister last week stated a policy goal of 100% electric cars & trucks by 2030. Behind that decision has been their massive solar PV mission to install more solar (both large and small scale) in India by 2030 than the entire installed base of solar today.

India's national solar policy alone will cause the installed price of solar to collapse measurably faster. There is no question (except in the minds of a few oil company execs in deep deep denial) that solar will be by far the cheapest source of energy of any type by 2025, and that worldwide demand for oil will be flat or falling before 2030.

The Saudis are not crazy for pumping it as fast as they can while there's still a market for the stuff, since the market is pretty much guaranteed to disappear before they have to resort to exotic extraction methods. In June 2000 former Saudi oil minister Sheikh Zaki Yamani stated:

"Thirty years from now there will be a huge amount of oil—and no buyers. Oil will be left in the ground. The Stone Age came to an end, not because we had a lack of stones, and the Oil Age will come to an end not because we have a lack of oil."

History seems to be proving Sheikh Yamani right in some ways, and oil isn't the only industry going down the tubes on that schedule. (At the time he was predicting a precipitous oil price crash from ~$25/bbl to something much lower by 2005, so not all his details were right, but the 2030 end point to the oil age is looking pretty likely.) PV is going to eat many peoples' lunch.

1) The point was that there is a point of no return for added insulation. Better then code minimum is certainly good, but passive house standard (whatever that is) is kind of dead, as you said. I was not referring to keep houses leaky, just to not go overboard with all the insulation.
2) You are correct, the load does not change, I meant to say the energy use to satisfy the load. While people underestimate the energy needed for hot water, (15 gal per day x 8.33 x 70 delta T = 8750 BTUs / person/day) it really is not close to the energy needed for space conditioning (A/C and Heating).
3) Solar is great, but unfortunately the sun does not shine in the evening when we need it most. Thus the duck graph. Until we have better and more means to store the energy, we also need the wind to a certain degree to power things at night, or at least provide some capacity.

I really do not see a long term alternative to electrically driven heat pumps for space conditioning and hot water production on the horizon, since no other technology reduces the energy demand of a US household as much, and no other space conditioning equipment can be powered as efficient as GSHPs with renewable energy sources, wether it is wind, solar, or other things.

Certainly once solar gets even cheaper, lesser efficient systems might be an alternative, but geo is unsurpassed as combining energy storage and on demand extraction of thermal energy.

We have 2 needs from an energy perspective:

1) Reduce overall energy consumption. Geo cuts down the energy consumption of a site/house like no other technology.
2) Shave peak demand. Pretty much the whole peak demand problem is traceable back to A/C needs. Again, no other system cuts down peak demand better than GSHPs.

The oil price is more politically motivated than anything else. Nothing got the Russian economy down to its knees as quickly as a few more barrels of oil on the world market. No cash no war. When was the last time you heard about the war in Ukraine? Modern warfare! Strike 1.

Side effect #1: Iran does not have the cash to be come threatening to the Saudis. Strike 2.

Side Effect # 2: The current lack of oil exploration due to low oil prices will render the oil market even more sensitive to changes in oil production. We will be more at their mercy (OPECs) down the road. Strike 3.


While leveling out in the US and slightly reducing in Europe, World petroleum demand is still climbing steadily.

https://www.eia.gov/cfapps/ipdbproject/IEDIndex3.cfm?tid=5&pid=5&aid=2

In the long run, oil will always cost as much as it costs to produce it, plus a nice profit.

My guess is that air to water heat pumps + thermal storage (water tanks) plus more PV solar (than one might use with a GSHP) will win.

Of course there's a limit to insulation. (PassiveHouse has lost it's rationale.)

At the year 2009 levels of 15% PV efficiency and mini-splits with an HSPF of 10-ish, getting to Net Zero is possible with an array that fits on the house if it's built with "whole assembly R" levels found in Table 2 p10 of this document:

http://buildingscience.com/file/5806/download?token=GouEIX9Y

At those levels hot water is the biggest slice of the energy use pie, not space heating & cooling. Heat pump water heaters and drainwater heat exchangers are both cheaper on a lifecycle basis than the amount of GSHP necessary to hit comparable efficiencies.

Today's typical rooftop PV is 20% (33% more power per square foot), and typical mini-split efficiencies are in the 12s or higher (20% more BTU per kwh.)

At $3.50/watt PV, mini-splits usually pencils favorably against the higher efficiency GSHP + PV. When PV hits below $2/watt (it already has in Europe and Australia)

The whole "sun doesn't shine at night" thing is a red herring. Smarter loads are cheap and becoming readily available, and the overlap of fixed-tilt PV with air conditioning loads is high, which both lowers the peak and pushes the lower peak needed to be covered by storage or other sources later into the day. Nissan is already marketing it's car to grid system in Europe, and where EV batteries can bid into the wholesale markets it can be a very cheap and very local way to flatten overall grid demand needed to be covered by other dispatchable resources. Right now that's difficult or impossible in the US due to the utility regulatory environments, but those environments are changing (fast), particularly in NY. The wind blows at night, often picking up at sundown, particularly but not exclusively near coastal areas on high air-conditioning load days. This hasn't gone unnoticed in Texas, where they are now installing more wind near the gulf coast to reap peak-demand revenues, wind power that might otherwise had been installed in western Texas where the average wind resource is much higher. Utility scale wind is currently cheaper than combined cycle gas and utility scale PV, but PV will likely beat it hands-down within a couple of years. Both are getting cheaper year-on-year, but PV is getting cheaper faster.

NREL just released a technical potential analysis showing that 40% of all US grid power could be delivered by rooftop (not utilty scale) arrays:

http://www.nrel.gov/docs/fy16osti/65586.pdf

Of course, if all that were implemented peak output would exceed peak load- it won't actually all get built even when rooftop hits a buck-a-watt (unless electric vehicle roll outs exceeds even the most optimistic expectations), but it does highlight the size of the resource. A LOT of it will get built before 2020.

Oil demand growth is slowing, but growth is still growth. The major oil guzzilng nations have had flat or declining demand, and that will continue (due as much to carbon policies as price/cost. ) The policy drivers that are going to make the biggest difference in the next decade are happening in China & India, collectively more than 1/3 of the world population, and the largest drivers of oil demand growth. Both have turned sharply toward electrification of the transport section which means peak oil demand is likely to happen prior to 2025.

At current expenditure levels Saudi Arabia will go bankrupt in 2 years if oil prices stay at this level. The royal familiy is between a rock and a hard place- Saudi citizens aren't likely to sign up for austerity quietly. But they can't really afford to not pump, since cheap production elsewhere can still pick up the slack in the short term, and in the longer term demand is going to crash. The Russians can't really afford to stop pumping either. None of them can really control the market, OPEC nothwithstanding. Price volatility is not their friend, but inevitable. Oil price volatility drives the world ever faster to the more stable pricing of electric transportation. In the case of both India and China, not having to waste hard-currency reserves on imported fossil fuels (both oil and coal) are serious factors driving policy even at $25/bbl, let alone $50 or $100.

Rather than being priced at cost-plus, in the long run oil will be worthless. Electric transportation is already competitive at $25/bbl, and the technologies behind EVs are enjoying a steep classic learning curve- it'l be much cheaper in 2020 than it is today, and cheaper still by 2025. It'll be illegal to sell a car or small truck with an internal combustion engine in (oil exporting) Norway in 2025, and the same will be true in India by 2030, based on recent legislation and stated policy goals. Even at a subsidized $8/bbl utility scale PV beat oil fired power generation in open bidding last year in Saudi Arabia. Who is going to want to buy that stuff? Oil is poised to become relegated to a shrinking number of applications, and I suspect the Saudis have a better handle on that reality than the EIA (who have consistently missed the mark on renewables development by huge margins for decades.)

A couple things to consider:

1) There is price to using the grid as a needed buffer. Without the storage problem solved, the costs per KW will go up with the need to have the backup capacity ready. People are paying over 30 cents/kwh in Germany.
2) The Saudis caused the current oil price with a snip of a finger, initiated and supported by the US. They have the financial reserves to hold out longer, and the lowest production costs world wide. It is easy for them to get us back to $100/bbl, but they do not want to right now.
3) This is an aircraft carrier, it does not turn around on a dime. When they (and the UAE, and other gulf countries) decide to reduce production, a lack of infrastructure investment in the last couple years will create a supply problem. Lets see where oil prices are then. Fact is that the world uses more oil than ever before, and a record number of drill rigs parked at idle. Current wells are diminishing capacity, and new once are not drilled. Taking 10% of the oil off the market will double to quadruple prices soon.

Partly because of confirmation bias, as the owner of a GSHP system, I like to think that peak load charges or similar rate structures will eventually make GSHPs start to look better than the down now compared to ASHPs.

But jonr makes a good point that the right comparison would be an air-to-water heat pump plus a big tank. How well that competes depends in part on what time scale the storage is needed for.

A 1000 gallon tank with a 20 F temperature swing stores 160 kBTU, or 16 kBTU/h for 10 hours. For getting through a cold night that could do the job, perhaps with letting the house temperature sag some if needed. So if the grid is in need of help overnight sometimes, that storage can do the job. It will be interesting to how the market for 1000 gallon tanks does compared to the market for batteries. 160 kBTU is about what you can get out of two 7 kWh Tesla Powerwalls, if you use a COP=3 heat pump. You can get an 1000 gallon uninsulated, unpressurized plastic tank for as little as $529. Even after insulating that beats the two Powerwalls at $3000 each, or the upcharge for a GSHP over an air source heat pump, at $30k+

But if the issue is a cold week with low renewable energy yield for whatever reason, and the combined load of heat pumps is high, having more GSHPs on the grid and fewer ASHPs could help the system get through that week, whereas 1000 gallon tanks and powerwalls aren't really big enough, although 1000 gallons of powerwalls would probably get you through a week, and you would start burying tanks outside, and probably still be cheaper than powerwalls.

Partly because of confirmation bias, as the owner of a GSHP system, I like to think that peak load charges or similar rate structures will eventually make GSHPs start to look better than the down now compared to ASHPs.

But jonr makes a good point that the right comparison would be an air-to-water heat pump plus a big tank. How well that competes depends in part on what time scale the storage is needed for.

A 1000 gallon tank with a 20 F temperature swing stores 160 kBTU, or 16 kBTU/h for 10 hours. For getting through a cold night that could do the job, perhaps with letting the house temperature sag some if needed. So if the grid is in need of help overnight sometimes, that storage can do the job. It will be interesting to how the market for 1000 gallon tanks does compared to the market for batteries. 160 kBTU is about what you can get out of two 7 kWh Tesla Powerwalls, if you use a COP=3 heat pump. You can get an 1000 gallon uninsulated, unpressurized plastic tank for as little as $529. Even after insulating that beats the two Powerwalls at $3000 each, or the upcharge for a GSHP over an air source heat pump, at $30k+

But if the issue is a cold week with low renewable energy yield for whatever reason, and the combined load of heat pumps is high, having more GSHPs on the grid and fewer ASHPs could help the system get through that week, whereas 1000 gallon tanks and powerwalls aren't really big enough, although 1000 gallons of powerwalls would probably get you through a week, and you would start burying tanks outside, and probably still be cheaper than powerwalls.

Perhaps 3500 gallons (8x8x8 = only 64 sq ft) is more realistic. Preferably with an outdoor reset feature such than in non-design-day weather, it's not overheating/cooling the tank by the full 20F. And in heating mode, most of that efficiency loss would be offset since it would primarily operate the oversized air source HP during the day (when it is warmer).

Of course water tanks can work with geo too.