Have any of the professionals on this board considered a hybrid system, a combination of Geo and air source? In particular, maybe a one ton or so Geo in combination with a two ton air source for a typical 3 ton cooling load? If both units were water to water, they could work in combination or alone. The Geo would essentially be the first stage of cooling, the stage that is used the most. Ground loop expense would be minimized. At least for the cooling loads we get on the Gulf Coast, a 1:2 sizing ratio would probably work out pretty good, especially with chilled water.

Hmmm...A first stage system sized to meet just 1/3 of the design load (1 ton out of 3) would be overwhelmed during any but the mildest 'shoulder season' weather. On the Gulf Coast it would provide useful cooling in May and October, and useful heating in November, early December and also March and April. The peak months of January, February, June, July, August, and September would require long hours of operation of the unit providing the remaining 2/3 of the load.

Suppose we flip the capacities, that is provide 2 tons of geo to meet a design 3 ton load, with an ASHP option making up the extra ton, that could be much more useful. 2 tons meets a 3 ton load much more of the time, with excellent humidity control, a significant Gulf Coast consideration. The peaking unit would only be needed during hot afternoons from June through September and cold mornings December through February; far fewer hours

I'm much more likely to lean toward 2 stage system to meet southeast US loads owing to the need to control humidity. That said, if I for some reason wanted to spec a hybrid system I would not make both water to water. Instead I think I might suggest a mini or multisplit ductless ASHP as the peaking unit with its evaporator(s) located in the rooms (possibly family, living, dining or kitchen) likely to experience the heaviest loads (heating or cooling) during design or design+ weather.

Just my $0.02, take it or leave it.

I sort of lean towards an Asian style multi split because of the VFD driven compressor, which has a hidden bonus in my area: you can start it with a typical 5000 watt portable generator because the VFD limits inrush.  We have power outages due to weather down here.

I agree with you on the sizing, one ton is just a number off the top of my head.  To be scientific, you'd have to calculate loads and determine peak sensible and latent loads, and probably size the smaller stage for something like 125% of peak latent load, or some similar methodology.  However, if you added a couple of hundred gallons of storage to the chilled water loop, you could load the loop higher during peaks, though you could only exceed capacity for a limited amount of time before you drew down all the excess capacity.

Of course, I think the real limitation is probably that it is unlikely that a manufacturer makes a one ton water to water geothermal heat pump, which makes the whole exercise academic, I was just curious as to others opinions on this subject as most of the discussion on Geothermal seems to be limited to applications in heating dominated climates.  It seems like a good solution in my area could be a smaller geothermal for base line cooling/dehumidification plus DHW coupled with a less efficient back up for peak cooling loads, similar to back up resistance coils on heat pumps for heating.

You can certainly use tanks of chilled water to store off peak cooling capacity. In some cases, lower electricity rates at night make it very attractive. Tanks are somewhat bigger than you are thinking though.

most off peak cooling is done with brine or ice and on a large scale as in 1000's of tons for process or district cooling....but technology will come around to the point were it will be adaptable to smaller projects. 1st ton of field is way more costly than the second as I need to pay for estimating, travel, layout, coring, trenching, pumping station, and electrical no matter how big the job is. Also a/c equipment costs very little between 1, 1.5, 2 and 2.5 tons... Eric

There is no off peak residential rate in my state right now but there could be in the future.  There used to be a good rate but I think the power companies managed to eat enough natural gas capacity to drive the price up on their biggest competition.   A really intelligent person would design his building envelope so that the thermal lag transfered as much of the load to off peak as possible through the use of thermal mass and insulation (I'm thinking of building with ICF).  Then, the ASHP would be operating under more favorable conditions.  Just from empirical evidence, I would estimate most wood framed residences here to have a peak heat gain at around 5:00 PM CST during the Summer.  If you could shift it to, say, 9:00 PM, you'd pick up some efficiency in your AC.

I was thinking of a system with veritcal loops, with a heat pump sized to use just one vertical loop (I realize that depends on site conditions), plus a DSH for hot water, and maybe a pool in the loop, and maybe about 300 gallons of chilled water storage, which would allow you to draw down 1/2 ton extra an hour for four hours.  You could kick in the second stage based on return water temperature and compressor duty cycle. 

Some professionals on other sites claim that a conventional two stage AC (i.e. Copeland Scroll) does not get a low enough coil temp in low stage to adequately dehumidify.  Does anyone have an opinion on that?

I can attest to the success of ICF in transferring load to off peak times in my own house: 3400 SF near JAX FL, heated and cooled with an 038 Envision locked in low stage by a handswitch on zone board. That works out to just over 2 tons in a house with plenty of windows and kids who leave doors open, etc.

Humidity stays in the low 40s even without selecting dehumidification (lower air flows). The thermal mass of the house 'carries' hot afternoon loads deep into the night, providing for system cycling all night even when outdoor temp drops into 70s and there is no solar gain. This really helps with latent load.

I get the same effect in winter - that same 2.25 tons holds 72 inside even when it is low 20s outside, though I saw some days last January with 20+ hour runtimes.

I also noticed the thermal mass after returning from summer vacation - I had the system shut off for a week and returned on a hot afternoon. I allowed the system to run in high stage for recovery but it was 6-8 hours and deep into night before it caught up.

Engineer,
What kind of roof do you have on your house?

I am thinking foamed roof deck + cool roof shingles or metal.  Code minimum for attic is R-19 here.  I think air sealing plus cool/reflective roof is a higher priority, then more foam if I still have room in the budget.  We are at the same latitude, so solar gain characteristics should be similar.

Grey asphalt shingles atop OSB with 3" closed-cell sprayfoam applied beneath sheathing. The sprayfoam is effective, but on frosty mornings I can see the locations of where the truss chords touch the sheathing.

Here is some right-now real time data: It is 90+ outside, 78 in my 2nd floor office, 89 in the attic and 171 under one of my shingles on the west side in a 3:12 section. I'll bet the upper floor's 6:12 section facing west is hotter still but I'm not motivated enough to climb up and check.

I will shortly be doing a geo top out on a beach house with a flat concrete roof topped with "cool deck" I plan to measure roof temp out there. I'm betting (and hoping, for the sake of my load calc) that it is no more than 10-20 above ambient...

I agree that staged is the way to go. I jumped on Tranes' 2 stage ashp and ac when they came out 15 y ago with dual compressors. They were around 25% of sales after 2y.I dont think the two stage compressors work as well as a dual compressor set up as far as coil temp but 410a has as much to do with that. My point is I don't think a hybrid system will save much if you look at installed costs for your avg 3 ton geo system.
Is cool deck construction just a vented space between 2 decks. would a radiant barrier help in this app....Eric

I agree on the cost, Ilgeo.  Geothermal for me would be a luxury and wouldn't cost justify, but might not be such a terrible cost premium if I could get the total heat load on the house to 2 tons cooling, which, considering Engineers experience with similar construction in a very similar climate, might be feasible.

As far as radiant barriers and cool roof, the cool roof stuff is just materials added to shingles to increase reflectance and increase emissivity in a manner that more solar radiation is reflected away from the surface of the roof, so it is like a radiant barrier except the roofing material serves this purpose.  Exeperience with light colored shingles through the years on a minimally insulated house tells me this approach has merit.

A few years ago, a zero energy house was built in Lakeland, FL with fairly mild specs (it had a PV array).  It was only R-10 on the walls (exterior insulation over masonry), r-30 vented attic, and a white tile roof, 2 ton SEER 16 AC on 2100 or so sq. ft house, high performance windows.   This suggests controling solar gain to be the biggest bang for the buck, though we probably have slightly less solar gains, higher latent loads, and slightly colder Winters here.

So my thinking is ICF walls, cool roof plus code minimum (r-19) foamed attic as a start, then possibly geo if budget allows, rather than more foam.  Alternative to geo is mini-split multi zone because of the VFD driven compressor.

My point is why add the complexity of a hybrid system if there is no benefit to doing so. I am all for 2 stage hvac. I think is is necessary for humidity control. I have built equipment before it was readily available 15 years ago for an open loop 2 stage with dx cooling and hot water converter for radiant heating. I believe that hybrid would offer no real value in most applications unless say it would allow for horizontal field to be installed instead of going vertical. then it might have merit. And if cost is not an issue why would you want to install an ashp...Eric
ps watch chilled water in high humidity areas as coil temp is warmer than dx systems.

On the gulf coast it's probably cheaper to boost the insulation and air tightness levels of new construction to the point where the sensible cooling load is near-zero than the cost of the most minimal geothermal system. With active ERV ventilation running at minimum duty cycle during the cooling season a dehumidifier would keep up with the latent loads.

The PassiveHouse approach works, and R25 walls/R50 roofs just aren't all that expensive compared to the R60/R75 or more you'd need to go that route in colder climes. With favorable sub-soil temps you probably won't need much in the way of foundation insulation- just sufficient slab-edge insulation to earth-couple the place to the thermal mass of the subsoil. And insulation has far lower maintenance & operating costs than compressors, as well as a longer lifecycle.

Going high-R isn't usually cost effective with foam, but it can be with a combination of spray foam + blown/sprayed fiber. The major benefit of foam is air-tightness, but once you have that, boosting the total R with cheaper goods usually a better value than ever more foam. On houses with mostly simple flat surfaces rigid EPS can sometimes be a reasonable cost "total solution" at high-R though.

The high efficiency of geo makes sense if you have a substantial load, but in new construction it's often just as easy (or easier) to shrink the load by nearly an order of magnitude, at which point the efficiency of the mechanical systems necessary to handle the load are irrelevant.

Posted By ilgeo on 02 Aug 2010 02:40 PM
1st ton of field is way more costly than the second as I need to pay for estimating, travel, layout, coring, trenching, pumping station, and electrical no matter how big the job is.

Can the same be said for a larger geothermal unit itself - the cost per ton goes down the larger the unit?  I can see this being the case unless duct work or electrical requirements would require additional capacity with a larger system, but that may be needed with a hybrid system anyway.

What are the benefits to going with a hybrid system?  If they are independent systems, one benefit may be that one system may partially back up the other system in the event it has a problem.  The drawbacks that I see are the expenses associated with purchasing and installing an additional unit, maintenance for 2 systems and repairs for 2 systems.

Yes it does as in 20k for a 2 ton does not make 30k for a 3 ton. But lets say we have a house with a 48k heat loss and 36k of heat gain with a yard that will just fit a 2 ton horizontal field not any more. This would be a good candidate for hybrid system lets say it also has a great room with 200 sq ft of south facing glass and the home owner likes to entertain in the summer to the point of having 30 or so people and wants to maintain 74* at 4pm on a sat afternoon. This would be a perfct situation for an inverter ashp say 2 ton to meet the occupancy load...Eric

Dana1,
This is off topic for the Geo board, but what kind of slab edge insulation approach would you suggest in a termite prone area that requires visible concrete for inspection?  Are there any non-foam based products, like maybe a vermiculite or perlite based product that would be better in these situations?

Most houses that I have noticed around here have no visible termite barrier (doesn't mean it's not there, I just haven't noticed) and all have the slab edge exposed, probably for termite inspection.

There are vermiculite and perlite based insulating concretes, maybe that could be used to coat the edge of the slab.  Have you ever heard of something like that?

Dow's Blueguard termite proof foam might also be of some use.

tigerfan I'm in so. Louisiana, the exposed edge of the slab is as much about not paying attention to details as it is about termite inspection. If you use a direct applied finish on the edge of the slab you can meet the termite inspection requirement.

Biggest thing that occurs to me at first blush is that the first ton of geo is the most expensive.
Jumping from 1 ton to 3 might cost little more than a good ASHP (sat 4 k for a horiz. loop system). Certainly less than a high end unit.
joe

Joe, Thats what I'm trying to say generally the 1st ton is by far the most costly until you run into extenuating circumstances that cause an escalation in price.