Are ground-source heat pump systems a good choice for beachfront NW Florida new construction? A cost-effective choice?
varocketry
| Posted in Mechanicals on
I haven’t seen articles explicitly describing GSHP builds in sandy beach environments in temperature zones such as Panama City Beach area. Assuming a goal of a Passive House or net-zero with PV on roof, is it economical or not advised.
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Jim,
The only advantage in your environment is that the equipment can be installed indoors (in a basement or a mechanical room), thereby protecting the equipment from salty ocean air.
Other than that one advantage, a ground-source heat pump is not advised. It will cost much more than other alternatives, especially a ductless minisplit system or a ducted minisplit system. If you save $10,000 or $15,000 on the ductless minisplits compared to the ground-source heat pump, you can use the money to buy a photovoltaic (PV) system that will generate enough electricity to run your heating and air conditioning system for free.
Here is a link to a GBA article with more information on this topic: Are Affordable Ground-Source Heat Pumps On the Horizon?
If you are investing in a high performance envelope which has both lots of insulation of course, and in your case good solar control and especially great airtightness you can forgo the expensive cooling equipment.
The idea is that you are keeping the heat out so your cooling load is very low. The problem is humidity in this case so there are some clever tricks in Passive House to deal with it. The first is to use an ERV to keep the moisture in the air out of the building when doing air exchange. The key for any of this to work is the air tightness to control vapor intrusion so with a tight envelope this will be a very effective way to maintain humidity. To dehumidify a post coil at the ERV with a very small heat pump has been shown to "wring" the vapor out of the air internally.
There are some schemes which utilize an ERV and then an HRV to maximize the condensing potential. Then a mini-split can make up for any heat gains. Other schemes take advantage of the dryer air in the home by reversing the airflow across the heat exchange membrane so any condensing water will evaporate and cool down the fresh incoming dry air. These set ups use a fraction of the energy a ground source heat pump would and cost much less.
Jim,
I think that there is a simpler way to say what Andrew just said.
1. To reduce your cooling load and to reduce your need for dehumidification, make your house as airtight as possible, choose windows with a low SHGC, and make sure that your roof overhangs provide plenty of shade.
2. To limit moisture entry into your building during hot weather, don't operate your ventilation equipment any more than you have to.
3. If your air conditioner doesn't keep your indoor air dry enough, install a small dehumidifier.
That's it.
Martin,
Not sure I agree with point two at all. For point three the idea is to treat humidity separately from cooling, which goes back to point two, use your air exchange for that and control humidity at the point of entry. There are many options but we definitely need to work on optimizing systems and an ERV in a tight house in Florida is a must.
Andrew & others:
First, thanks for the quick and thoughtful response.
Second, at the risk of hijacking my own question your comment touched on another question I have that I've really not seen addressed. Maybe I now expose a desire that fundamentally conflicts with a passive house design ...
I'm considering a home design that incorporates a product like NanaWall where the ocean facing southern side is opened up for temperate breezes when desired.
Question: I guess the air handling system must be designed with this in mind and have adequate required HRV/dehumidifiers for operation in a closed state. Correct?
Is this totally at odds with Passive House design goals?
Jim,
Of course it's possible to live without air conditioning in Florida. People did that for hundreds of years.
When you decide to close up your house and turn on your air conditioner, though, it will take a while to lower the indoor relative humidity levels and cool the indoor air. Once you've done that, you don't want to open up your windows again right away.
Switching back and forth between "doors wide open" and "air conditioning" mode is expensive -- so it's best to choose which way you prefer to live, and live that way for a few weeks straight. Don't keep changing your mind every couple of hours.
Andrew,
Here's the thing about "point 2": an ERV won't lower your indoor humidity levels. The more you operate your ERV, the more moisture you bring into your house.
So it's best to determine the minimum amount of ventilation you can live with, and stick to that level. You don't want your ventilation fan to operate more than necessary.
In Florida, it usually isn't necessary to address dehumidification separately from cooling (but sometimes it is). In the vast majority of cases, a right-sized air conditioner is all you need. If you find that there are some seasons when you need a little bit of dehumidification without cooling, then a small dehumidifier is probably all you need.
Sometimes ground source heat pumps aren't really very expensive at all relative to ductless heat pumps in the gulf coast states due to the proximity of the high water tables with very good heat transfer of the very wet or even saturated subsoils. This was even more true when those systems qualified for a 30% Federal income tax credit (that wasn't included in December's extensions of tax credits for wind & solar, etc.) With GSHP there is always system design and implementation risks- the most expensive GSHP system may be the lowest price bid from a clueless hack, but it's worth getting some quotes from experienced installers with good reputations.
The coils on the outdoor units of mini-splits and other split-systems is weak point that can see more rapid deteriorate the warm wet salt air of the Gulf as compared to the desert southwest or any of the US interior. That hasn't stopped most people from running air conditioning with air source units, but the shorter lifecycle is still a factor to consider. A well designed well implement ground source heat pump should last longer and run at higher efficiency, but "should" doesn't always mean "will". There's nothing wrong with the "system in a can" approach of mini-splits.
It's pretty easy to build a Net Zero worthy building envelope in a US zone 2 climate. Using BA-1005 as a crib sheet ( http://buildingscience.com/documents/bareports/ba-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones/view ) , in Panama City you should be able to hit Net Zero with:
R15 whole-wall R (=2x6 24" o.c w/ R23, a 2x4/R16 walls with 1.5" ZIP-R sheathing, or a minimalist 2" EPS + 2" EPS insulated concrete form, for hurricane resistance)
R50 attic (= 15" of blown cellulose in an energy-heel trussed roof)
R10 crawlspace walls (1.5" of continuos polyiso on the interior, or 2.5" of EPS on the exterior
....or R20 whole-assembly floor if pier foundation (R30 in 2x10 joists with 1" of air-tight foil-faced polyiso nailed to the underside of the joists)....
.... or R8 slab edge down to 24" below grade, no sub-slab insulation
U0.30 low solar gain (windows)
Limit the amount of east & west facing glass to a minimum limit the high gain of low-angle sun in summer, keep it a simple gabled roof with the ridge running east-west for optimal uptake with the solar array. Build taller walls and a ceiling level duct chase if slab-on grade (or for 2- story building top floors) NEVER put the ducts & air handlers above the insulation. For 1-story buildings above historical flood levels it's fine to build a conditioned crawlspace to accommodate ducts, but most builders in your area are more comfortable with slab-on-grade.
Florida has some of the least expensive solar in the US, at $2.50/watt (before tax credits) for the average installed price in a recent survey. This compares to about $3.50/ watt for the US average. The politics of solar is pretty hot in Florida right now, with competing referendum movements, one supported by everybody from the Green Party to the Green Tea Party, the other driven by the utilities and the fossil-fuel interests. The issue dividing them is the third-party ownership issue, where in other states solar companies can own the solar and cut a power purchase agreement with the homeowner, which runs afoul of Florida's monopoly utility model regulatory climate, where only utilities are allowed to sell electricity. But with the extension of the tax credits and $2.50/watt solar it's almost a moot-point, unless the utilities and fossil-purveyors can successfully attack net metering.
By contrast to Net Zero, getting to PassiveHouse levels of energy use in Florida is extremely complicated, and arguably not "worth it". The biggest fly in that ointment is the peak and avearge dehumidification & cooling energy use.
Jim,
There is no conflict between Passive House and opening windows and doors for ventilation. In fact there is a work sheet in the energy model PHPP just for that. A sizable part of the standard is for comfort, so it is at the user's discretion how to operate the building. It becomes more complicated with building programs like dorms where you don't want the occupant to open windows and run the H/AC at the same time.
There is also no reason to believe that you cannot use a NanaWall system in your climate. The way the energy model will work is that you have give and takes to see how your overall design and discrete elements work. There are only a few must do thresholds that you have to meet- like fresh air which is obvious in tight houses for durability, health and comfort. Its important to note that a good design will achieve cascading benefits as you achieve better performance. Decoupling the humidity control from cooling to a certain extent is not required but makes for a more comfortable home which saves energy.
Importantly in my mind is finding synergies with low energy equipment coupled with a high performing envelope is how we further advances in low carbon building design. Solar is not very grid friendly in electrically dependent buildings as California is struggling with and does not absolve us of developing comfort with small electrical loads. I hope to see that a more nuanced conversation on Net Zero Energy can take place on these pages and in our larger community in the US.
Related to Dana and Andrew's comments, perhaps straying a bit from the original question, as you insulate the walls better and use better windows, the percentage of the A/C load that is dehumidification increases. Although non-electric solar cooling hasn't proved to be very successful, using solar thermal to run desiccant dehumidification works out much better. There's one residential scale commercial product for that that I know of:
https://www.greenbuildingadvisor.com/product-guide/prod/novelaire-comfortdry-250-dehumidifiers
Setting one of those up involves extra complexity that is unlikely to be worth the trouble in building one residence, but if you are doing this in part to pilot what you think is better technology, I think that approach has a lot going for it.
Dana and everyone - thanks.
I appreciated the nuanced comments about differences in gulf coast building and living. I am planning for storm resistant ICF on piling foundation as it will be V/VE Floodzone lot. Water table won't be very deep.
I'll research all of those suggestions. Thanks again.