I became familiar with geothermal heat pumps several years ago, and have been a fan of the idea ever since. However, I have no direct experience with these, of any kind, and would like the to make the best informed decision I possibly can when the right time comes to do so. The idea is to start building a home in roughly 2 years (early 2014). Radiant heating will be the primary heating method used, with radiant tubing placed in 4-inch thick concrete slab floors, with sub-slab insulation. The house is ~90% single story, so no second-story heating is involved. My house design is exactly 1800 sq ft. So for starters, I am having trouble choosing which method of geo to go with. Direct exchange (DX) seems promising - it appeals to me for it's smaller footprint and better efficiency (no need for water pumping), but lately I've been becoming vaguely aware of what may be shortcomings, such as freezing the ground around the pipes, and possible local rules disallowing large amounts of underground refrigerant. Additionally, I have not decided on a place to live just yet. I have been looking around for years, and have it narrowed down to a list of places that all have snow and winter temps averaging between 10 to 30 deg F (-12 - 0 deg C). Thus I need a more generalized approach at this point, with these criteria in mind. Another basic question - is the above ground geo system better inside, or outside? Does it depend? If so, what decides? How big of a footprint does it actually take up? Both the above ground system, as well as the excavation footprint(s). I'd also like to know how well a heat pump can work as a light AC in the summer, with my radiant slab / thermal mass setup in mind. I don't plan to live anywhere hot, (i.e. nowhere it might average above 80 deg F), but there will obviously be a few days every year I need a dependable cooling solution. In a nutshell, I feel like I'm in need of some personal discussion at this point. There seems to be a variety of ways to go about this, and I'm trying to figure out what kind of formula I can use to figure out the most pragmatic, efficient, cost effective way to go. Vertical systems vs horizontal, DX vs water-pump, short term costs vs long term costs, overall efficiency, issues with certain approches, etc. Any help guiding me along would be appreciated.

DGF, when designing for a new home in a heating dominated climate, always put your first money in he thermal shell of the building (eg. superinsulation). At build time, the extra cost is a small percentage of the total, typically like 5%. That give you a very comfortable house that also both loses heat slowly in winter and gains heat slowly in summer. Then the choice of heating system is secondary, as it will be very small whatever is chosen. The choice then is a matter of what is available. If you build in an area where natural gas is available and cheap, that may be your best bet, although summer A/C may have to be done with window units. If you build in a rural area where you will have a drilled water well anyway, and the ground water is of good quality, then GSHP using standing column well (SCW) design, with delivery of heat via warm air through ducts can be an excellent choice, as it gives you heat in winter and A/C in summer (this is what I have).

With superinsulation, you may want to rethink radiant heat and why you want that. If it's for "warm piggies," forget it with a superinsulated house. You'll have to keep the slab temperature in the low 70s, or the air will quickly become overheated and uncomfortable. Forced warm air (or air conditioned air in summer) will distribute itself quite nicely in a superinsulated house, eliminating a perceived need for radiant floor heating and saving some money on plumbing. You'd need ductwork for A/C anyway, unless you use minisplit heat pumps.

http://www.greenbuildingtalk.com/Forums/tabid/53/aff/13/aft/79832/afv/topic/Default.aspx


Discussion about DX system see above link.
You sound like my climatezone. No need to go vertical if you can put a horizontal system in. They work equally well if designed correctly. An 1800 sqf house built to new energy code would be about a 3 ton system. Footprint would be 3x100ft trenches, 6-8 feet deep, 10ft apart, so 30x100 ft. Check with local installers since these are only ballpark figures. You can use a hybrid/combo unit like the WF Synergy 3D for radiant heat and A/C. Or a water-water unit with a hydronic air handler for A/C. I personally prefer the combo modules. Then you should decide how to make your hot water, you can run an indirect tank off the same heatpump, making your hot water 24/7 via the geo system. The rest is easy, slightly higher upfront costs but significant operational savings.

doc....are you saying that with a very well insulated/tight house would need 1 ton/600sf?  From what I've read on here over the years it would be approx 1 ton/900-1100sf. It seems I keep getting conflicting info here about this. Maybe I'll start a thread about this.

Gotta agree with Robin and Dick on this one - 600 SF per ton is way old school anywhere other than Death Valley or Fairbanks, AK

A well designed and executed 1800 SF house in moderate climate should be able operate with a 2 ton system if some attention is paid to building envelope details as construction unfolds. In particular, have a blower door test done before drywall goes up so as to easily identify and fix gross air leaks.

Thanks everyone so far.
My exterior walls will be AAC (autoclaved aerated concrete). No drywall involved. I am currently working out how thick the walls are going to be, but it looks like they will be between 10 - 12 inches thick. The roof should be R36 - R40 throughout.

I am not planning on using a standard central air system. Instead, I plan to vent somewhat from the outside, albeit through a ground dampening PBC earth tube system, with 3 inch diameter tubes traveling underground over a hundred feet, then around the warm foundation walls a few times to soak up the air from the radiant heat system, before entering the home. No moving parts involved. I'm not a fan of stuffy, sealed off home interiors (the norm), so rather then focus on heating the air in the home, I'm focused on heating the home structure itself (6 inch interior concrete walls and 4 inch thick radiant floor slabs), which in turn keeps the air warm inside, indirectly, as a result. Also, with tubes, I don't have to worry about grime and dust accumulating in hard to reach places, like in traditional boxy ventilation system; I can just pull a damp towel with a cleaning agent through with some cord when I need to. Wallah. Not my idea, mind you, but when I heard of it it made perfect sense.

So in other words, I want the heating/cooling system and the ventilation system to be somewhat distinct from one one other. Not independent, but not one and the same.

So far I do plan on getting my hot water directly from the geo, whichever way I go.

The SF per ton info is very useful, and gives me an idea of what to expect. I'll have to see if modern system are closer to the 1000 SF per ton limit.

My underground PBC earth tubes will likely be placed opposite side of property from where the geo system is installed.

A small house uses proportionally more BTUs per sqf than a larger house. Also a single story house looses more BTUs per sqf than a 2 story, roof area is larger, 1st does not help heating the 2nd floor. It also depends on the Bin data, you might have a 100% manual J design temp of below 0 degrees, if the average is between 10-30F and the summer is not warmer than 80F it is pretty far up north.
Then with radiant, as stated in the sentence before, you don't usually have a strip heat element to bail you out so you need to design for 100% of the load. Now you are stealing some heat to potentially run your DHW tank as a zone to get 365/24/7 hot water, and the heatpump would have to have some extra capacity for this. One size smaller is a 2 ton with less than 20000 kbtu capacity. Now count in some domestic hot water generation (non DSH) and you are looking at 15000 BTU/H for space heating. Tough to get a 1800 sqf rambler house down to that (not impossible). Plus you want some reasonable response time with high mass radiant floors.

Has nothing to do with old school, but simply with the fact that concrete radiant floors change the dynamics.
I do agree that some thoughts have to be reserved for the fact that the concrete floors are slow responding, might make the house slow responding and might create issue with floors still heating, much solar gain coming in and making it uncomfortable. Sow reacting floors with high thermal mass have drawbacks in very efficient houses.

But even forced air we are at around 600 sqf/ton.
Here is a good example. 2100 sqf new built to newest NY state energy code. 3 ton. 700 sqf/ton.

http://welserver.com/WEL0603/

It is online since end of January. Unusual warm February here. 80% running in first stage, 20% second stage,a couple hours aux heat. Dipped down to 22-23 F last night, was running in second stage for a continuous 6 hours. 20F seems to be the balance point for aux heat. What Curt calls old fashioned from Florida I call right on target in Buffalo, NY.

http://welserver.com/WEL0337/

Another example. 1804 sqf, 4 ton, 450sqf/ton, new built 2010 modular home, dipped down to 15 F last night, was cycling in 2nd stage. 90% first stage, 10% 2nd stage, 55 hours aux heat this year so far. warm wintern so far, balance point is about 10F for aux heat. Again, I call it perfect on target. 3 ton unit would have been 600 sqf/ton, but pretty much undersized. Not an old fashioned opinion, simply data.

DGF,

Do you plan to insulate the AAC external walls.  If so, then with what and where will the insulation be?

Alton - AAC is the insulation
http://en.wikipedia.org/wiki/Autoclaved_aerated_concrete

docjenser - Look at the subject line. I barely have any idea what your latest post means. The real reason I'm here is to obtain an understanding and have that stuff explained to me. I don't comprehend all the geo-talk; not yet anyway. Might have to explain these percentages here and what they mean. I'm sure in a month or more I'll be able to sling the same stuff around, myself.

As for your previous post, I understand enough of it, counting out the last paragraph, which you may need to explain.

"Now you are stealing some heat to potentially run your DHW tank as a zone to get 365/24/7 hot water, and the heatpump would have to have some extra capacity for this."

Absolutely. I am planning 2 full bathrooms; whats a modest estimate for shower capacity, tonnage-wise, that I can slap on top of whatever the house's heating requirement turns out to be?

One thing I am familiar with is thermal mass. It responds slower... but that's also the whole point - it resists temperature fluctuations. When completed, the internal temperature will be adjusted to whatever I find comfortable at the time; from there it will only require mild, consistant nudges. The mass keeps everything from fluctuating wildly. The more mass, the longer the resistance to outside temperature swings. With enough mass, the home interior can easily resist temperature fluctuations on a weekly and even monthly basis. This concept is actually quite different then what typically goes on in [most] homes. In the mass-oriented approach, the heat pump doesn't have to vary its output significantly, it remains fairly steady and consistant, corresponding with weekly/monthly average rates of heating, rather then daily outside swings.

The floor slabs will be approximately 4 inches thick, with sub-slab insulation. Before pouring, I plan on raising the radiant heat pipes up a bit by placing them on a raised cage, so they aren't resting at the bottom of the slab; instead, they'll be more centered top-to-bottom, which should warm the mass much more evenly throughout.

I've already worked out some strait-forward ways to control solar-gain, which if I've done my homework properly (currently working with a solar specialist, haven't over-glazed), shouldn't be an issue.

Frankly, I'm mostly concerned with finding the best approach to my geothermal heat pump itself. Which, as stated before, is what I'm not quite as familiar with, at the moment. I'd like to say otherwise in the relatively near future.

That DX thread link was somewhat useful, but my understanding of the subject still could use some enlightenment. For the sake of staying on one topic at a time, I'll focus on this. There was a lot of focus on "DX efficiency claims", mostly stemming from copper's greater thermal conductivity, but I think that's an insignificant argument. The ground itself can only conduct heat so fast, which becomes the bottleneck to any greater efficiency copper might add.

No, what appealed to me (and this is where I might need some perspective) is the simpler system of a DX, which eliminates the water pump. I assumed that one less mechanical mechanism, meant less energy used, which in turn is where I assumed the bulk of the "greater efficiency claims" came from. If that's off, please explain.

What also appealed to me is that one less mechanical system, in and of itself, should mean one less potential point of failure down the road. Or in other words, 2 mechanical systems (a water pump and a compresser) ought to be more prone to fail, and lead to additional maintenance, then just a compresser alone. Again, correct me if you think that's off.

The first and only technical issue with DX that I'm so far aware of, is the ground water freezing issue. It seems as if the smaller volume and more concentrated heat exchange area of a DX system could potentially draw enough heat away from the surrounding ground to freeze it, which I assume would be all kinds of bad - both in terms of stifling further heat gain and causing heaves which could potentially warp or damage the pipes. Perhaps this could be alleviated by increasing the number of pipes and coverage area. But, perhaps that results in diminishing returns.. I don't know. Thoughts?

I appreciate the feedback.

I know you're asking about geothermal, but I think you should reconsider your decision to go AAC, especially since you haven't chosen a location or builder yet.

First, AACs have far lower R value than ICF, SIP or an advanced stick framing with spray foam.
Second, the thermal mass thing is not helpful if you're in a climate with limited temperature swings - humid south, or the northern US/Canada.
Third, if thermal mass is helpful, you'll find ICF has more of it.
Fourth, AAC is a pretty rare construction method, and you'll end up paying a premium for installers.

We looked at AAC for about a week in 2005 before we realized the products limitations and that the installer base was almost non-existent. We ended up going ICF/geothermal/radiant. The ICF allowed us to keep our 2200 sq ft house with a 2 ton system, supporting radiant and domestic hot water. We've never gone into emergency heat in Ohio.

On new construction you get the choice of what to spend the efficiency-money on, and when you look at the difference in system costs between a 3-4 ton geo compared to what it takes to get the design condtion loads under 2-tons and use inverter-drive air-source (hydronic or split-system "mini-split") that uses the same or LESS kwh/annum the results are not a given. Average winter outdoor temps affect the equation considerably when looking at air source, though, since ASHP output at the outdoor design temp may not always cut it.

But in places with -5C <==>0C average temps and 99th percentile outside design temps of -20C or higher there are hydronic air-source systems that can fill the bill at half the cost of geo, and nearly the same efficiency (notably, the Daikin Altherma series: http://www.daikinac.com/residential/altherma.asp?sec=products&page=53 ) In places where the average winter temps are 0C or higher, with a low-temp slab the system efficiencies of best-in-class air source can exceed that of geo when the full system power (including all pumping) of the geo is factored in. As "canned systems" there is far less system design cost & risk as compared to geo. So if another $20K of building envelope upgrades gets you there...

Building envelope efficieny is downright cheap on new construction if one takes the time to do iterative designs and apply the cost savings of the smaller/cheaper/less-risky mechanical systems to the building envelope. By contrast, retrofitting an exisiting building to a high-performance structure is usually cost-prohibitive, which sometimes tips the balance strongly toward geo. On new construction it pays to not get locked into your thinking on ANY of it too early.

For ergnut, without going on a really long tangent here:

AAC is a poor thermal mass provider, and it will not be used whatsoever in this regard. It is strictly for my exterior walls, where it ought to perform well. The interior, on the other hand, will sport 6-inch standard-density concrete walls, as well as 4-inch standard concrete floor slabs, which will provide the thermal mass I need.

I'm choosing AAC on the exterior primarily because of its fire-proof nature (i.e. not just fire resistance). One of my many ambitious goals is to obtain a frugal, custom insurance policy, covering only liability and theft; which avoids costlier insurance policies covering disasters I can avoid entirely through clever construction and meticulous site selection. That's actually a whole other, much larger subject.

AAC, as a system, also does better as an insulator then you might think. An important concept that many people are not aware of is how well it performs at slowing heat transfer, which, combined with significant thickness, magnifies the insulating property beyond what it's R-value conveys. 12-inch-thick walls are pretty darn good from what I understand. They've been using the stuff in Scandinavian countries quite successfully since the dawn of the 1900s.

ICF and SIP are both solutions I've seriously considered before; both are terrific in many meaningful respects, but both involve outward-facing foam that ultimately fails the "smile back at a potential fire" test that I value so highly. I don't discourage anyone from using these, I just have my own set of values that I've formulated for myself, that needn't apply to everyone.

In any case, I appreciate the suggestion, but I've gone through this stuff enough since ~2007 to practically talk anyone's head off.

I could further discuss this, but in this thread, for now, stick to the geothermal stuff. See my previous post for my main questions and concerns.

You should talk to an insurance agent before you design to that objective. If you plan to have a mortgage, you'll need replacement value insurance. An ICF house actually is a bit more expensive to insure because of replacement cost, which added more than than was offset by savings from reduced risk of damage by wind, water & fire damage (relative to stick). AAC is probably similar to ICF, as insurance companies don't create special risk pools for highly niche clients. In order to be insured, you might be lumped in with people who have stick built houses covered with brick, ICF houses and other masonary products.

Well in any case, I'm trying to keep this thread on the geothermal stuff. Thanks though. Maybe I'll start up another thread explaining my strategy for insurance.

Posted By DGF on 07 Mar 2012 01:40 PM
docjenser - Look at the subject line. I barely have any idea what your latest post means. The real reason I'm here is to obtain an understanding and have that stuff explained to me. I don't comprehend all the geo-talk; not yet anyway. Might have to explain these percentages here and what they mean. I'm sure in a month or more I'll be able to sling the same stuff around, myself.

As for your previous post, I understand enough of it, counting out the last paragraph, which you may need to explain.

"Now you are stealing some heat to potentially run your DHW tank as a zone to get 365/24/7 hot water, and the heatpump would have to have some extra capacity for this."

Absolutely. I am planning 2 full bathrooms; whats a modest estimate for shower capacity, tonnage-wise, that I can slap on top of whatever the house's heating requirement turns out to be?

One thing I am familiar with is thermal mass. It responds slower... but that's also the whole point - it resists temperature fluctuations. When completed, the internal temperature will be adjusted to whatever I find comfortable at the time; from there it will only require mild, consistant nudges. The mass keeps everything from fluctuating wildly. The more mass, the longer the resistance to outside temperature swings. With enough mass, the home interior can easily resist temperature fluctuations on a weekly and even monthly basis. This concept is actually quite different then what typically goes on in [most] homes. In the mass-oriented approach, the heat pump doesn't have to vary its output significantly, it remains fairly steady and consistant, corresponding with weekly/monthly average rates of heating, rather then daily outside swings.

The floor slabs will be approximately 4 inches thick, with sub-slab insulation. Before pouring, I plan on raising the radiant heat pipes up a bit by placing them on a raised cage, so they aren't resting at the bottom of the slab; instead, they'll be more centered top-to-bottom, which should warm the mass much more evenly throughout.

I've already worked out some strait-forward ways to control solar-gain, which if I've done my homework properly (currently working with a solar specialist, haven't over-glazed), shouldn't be an issue.

Frankly, I'm mostly concerned with finding the best approach to my geothermal heat pump itself. Which, as stated before, is what I'm not quite as familiar with, at the moment. I'd like to say otherwise in the relatively near future.

That DX thread link was somewhat useful, but my understanding of the subject still could use some enlightenment. For the sake of staying on one topic at a time, I'll focus on this. There was a lot of focus on "DX efficiency claims", mostly stemming from copper's greater thermal conductivity, but I think that's an insignificant argument. The ground itself can only conduct heat so fast, which becomes the bottleneck to any greater efficiency copper might add.

No, what appealed to me (and this is where I might need some perspective) is the simpler system of a DX, which eliminates the water pump. I assumed that one less mechanical mechanism, meant less energy used, which in turn is where I assumed the bulk of the "greater efficiency claims" came from. If that's off, please explain.

What also appealed to me is that one less mechanical system, in and of itself, should mean one less potential point of failure down the road. Or in other words, 2 mechanical systems (a water pump and a compresser) ought to be more prone to fail, and lead to additional maintenance, then just a compresser alone. Again, correct me if you think that's off.

The first and only technical issue with DX that I'm so far aware of, is the ground water freezing issue. It seems as if the smaller volume and more concentrated heat exchange area of a DX system could potentially draw enough heat away from the surrounding ground to freeze it, which I assume would be all kinds of bad - both in terms of stifling further heat gain and causing heaves which could potentially warp or damage the pipes. Perhaps this could be alleviated by increasing the number of pipes and coverage area. But, perhaps that results in diminishing returns.. I don't know. Thoughts?

I appreciate the feedback.

the write up was in response to some comments in regard to the sqf/ton we usually see. While I guessed that you might be looking at 600sqf/ton, others thought that this would be oversized and cite more 900sqf/ton. So I posted a link to (2) new build houses here in the Western New York area which have an online monitoring system installed. The rule of thumb is that we hit a sweet spot in terms of operating efficiency versus total capacity (upfront cost) when the units run 80% in first stage only, 20% in second stage and about 2-3% of total run time with aux heat on. A Balance point where the aux heat kicks in and supplements the 2 stage geo operation, we target around 15F. The actual monitoring demonstrates that one house has 450 sqf/ton, the other house has 600 sqf/ton and appears to be right on traget with the best balance between total system capacity, upfront cost and operational efficiency in our climate. Now, those houses were not ICF walls, closed cell foam etc. Just R-21 walls, R-40 roof, etc.
In regards to the DX, you hit the nailon the head that the ground seems to be the bottleneck, especially with much lesser ground around the pipes to extract heat from. While simpler in design, which is intriguing, the COP is actually worse than more modern water to air heatpumps. The problem is that other data to support any higher efficiency claims missing, no one is able to post anything or find anything, and the ones who are making the claims do not have anything either, which make you wonder what they based their claim is on.
It really is not a big deal going through another heat exchanger and to have a circulation pump on it. Yes, there is an efficiency penalty, but again, the W-A heatpumps seem to run at a higher COP despite the addition of the second heat exchange. I don't think the ice is an issue. Water sourced systems freeze the ground all the time, too. The issue is that the ground cannot keep up and is not 25-30F but 15F or less, killing efficiency and possibly compressors.

I'm leery as to whether your liability-only strategy will save enough money to justify the risk. I am building an AAC house in a town that has a sprinkler requirement. My agent is quoting $200/yr, and I'm covered on the stuff you couldn't dream up, like a deer in the house or a runaway uninsured car, six-inch balls of hail, the neighborhood teen rampage, frozen pipes....

Gotta agree with Dana. Geo in a well-done 1800 sf house is overkill in most climates.

Posted By jim on 07 Mar 2012 02:02 PM
I know you're asking about geothermal, but I think you should reconsider your decision to go AAC, especially since you haven't chosen a location or builder yet.

First, AACs have far lower R value than ICF, SIP or an advanced stick framing with spray foam.
Second, the thermal mass thing is not helpful if you're in a climate with limited temperature swings - humid south, or the northern US/Canada.
Third, if thermal mass is helpful, you'll find ICF has more of it.
Fourth, AAC is a pretty rare construction method, and you'll end up paying a premium for installers.

We looked at AAC for about a week in 2005 before we realized the products limitations and that the installer base was almost non-existent. We ended up going ICF/geothermal/radiant. The ICF allowed us to keep our 2200 sq ft house with a 2 ton system, supporting radiant and domestic hot water. We've never gone into emergency heat in Ohio.

I disagree with point "Second"; that thermal mass is not useful in humid southern climates. Thermal mass provides two useful benefits in hot humid sunny Florida:

1) It delays and carries some sensible cooling load into the home long after dark, giving the HVAC "something to do" late at night. A few extra on cycles in the late night hours go a long way toward managing humidity long after the sun sets

2) It shaves the edge off late afternoon peak high temperatures all summer as well as occasional cold snaps. I have a 3400 SF ICF house near Jacksonville that heats and cools on less than 2.5 tons. It has 50 windows and good but not noteworthy infiltration, 1500CFM50.

ORNL disagrees with point "Third." Encasing the mass in foam limits ICF's ability to respond on a diurnal basis, and thus its abiility to buffer 24-hour temperature extremes. Here is fla compliance report for Aercon, an AAC manufacturer based near Orlando. http://www.aerconaac.com/TECHNCAL%20MANUAL%5CThermal%20Efficiency.pdf
Show me a similar mass effect claim by an ICF manufacturer. (Won't find one because the facts don't live up to the perception.)

Posted By robinnc on 06 Mar 2012 10:22 PM
doc....are you saying that with a very well insulated/tight house would need 1 ton/600sf?  From what I've read on here over the years it would be approx 1 ton/900-1100sf. It seems I keep getting conflicting info here about this. Maybe I'll start a thread about this.

You can not nutshell sizing that easily. The actual comment is with current energy code specs and in an area that sounds like his climate......
That also is impacted by price/kwh....
You keep looking for a Rosetta Stone and fail to acknowledge geographical and envelope disparities.
In my AO, climate is slightly milder and electricity cheaper, so we tend to load a little lighter.
j