I concur with remarks directed at the OP regarding our requests for specific information as well as seeming a bit thin skinned as to replies.

In a teacher-student relationship the teacher works the rudder but it is the student who must labor at the oars. In other words, the burden falls upon the seeker of knowledge, not its providers.

That said, there is a curious relationship between design conditions and degree days worthy of consideration since it impacts long term costs of ownership:

In the humid southeast we might reasonably design a system to meet 95 in summer and 30 in winter. Once up and running the system faces 80-110% of the summer design load for 100+ days each year. Conversely, it only sees 80-110% of winter design load for perhaps 20 days each year.

Up north the situation is reversed - design conditions might be 90 / 5. This past summer was an exception, but normally up north heat waves might total only 15-20 days per year, but 100+ days during winter might be within 5-10 degrees of design.

> What does one do during the winter? A sprinkler system does not work well below freezing.

Nothing if you design for the bleed being optional or use some aux heat on those few days. Not that a sprinkler doesn't work below freezing - as ski hills and orchards know. Or use a french drain if soil conditions are appropriate for < 1 gpm.

Posted By joe.ami on 01 Sep 2010 09:09 PM

What if I asked you whether I could get to Toledo from here with the gas in my truck?

Why? What's in Toledo?

You should be able to find someone local that can review some of the options - nat gas, electric aux, air source heat pump, open and closed loop geothermal, etc and then do a manual J, a quote and a ROI analysis if one of these sounds reasonable to you. They will know the soil, local regulations, prices, etc much better than almost anyone here.

I can see how you would reach your opinion on some of the other matters you mention.

Am I to understand now that when you asked about equipment size and loop design we were also to conduct a viability study for you?
Honestly, if you want help, find a local geo pro (try igshpa website for certified installers).
To hijack a phrase from engineer, any tech that can fog a mirror has heard of manual J. We're not asking for some off the wall info, we're asking for the cornerstone of any design.
When I am called upon for a geo estimate, ROI (viability) is part of the presentation. I also sell fossil systems, so I don't have "geo ego". If a furnace or ASHP works best, I sell those too and will recommend them.
Manual J loss/gain calcs and loop design are also part of this process.
This thread asked about equipment size and loop design. For that I need to know the requirements of the dwelling served.
Not sarcasm, just a fact.

My gas tank scenario was intended to gently illustrate the impossible nature of your size and design questions. More data is required.

If you wish to attack us for suggesting a productive approach to design, that's fine. Now what?
If you wish me to remotely conduct a viability study, design calc (to size equipment) and loop design while you attack me.......ummm....
not even if you hold your breath until you turn blue.
(now that was sarcasm)
I really do wish you well.
J

Manuals J, D, and often S apply to ALL HVAC systems, however powered or fired. While we are sipping alphabet soup, it is worth noting that at least a working understanding of Manual T is necessary to correctly position supply and return registers and grills. If the contractors you have met profess ignorance of this alphabet, seek others. In the past month I've run two such calcs for $300 each for folks who recently replaced their heat pumps for big bucks and are frustrated by both uneven heating and cooling and the replacing contractors' inability to determine a cause or suggest a remedy. The systems weren't particularly badly installed, but the owners are uncomfortable in one or more rooms. I and other pros here are in the comfort business; many installers are not.

Forgo these calculations at your peril.

Mechanically venting an attic by any means, electric or wind powered, may indeed backfire:

Venting the attic causes it to go slightly negative (internal air pressure below ambient air pressure), aggravating the stack effect by which conditioned air is sucked up into the attic via can light openings, plumbing vent line paths, holes drilled for power and communication cables, attic stair door joints, etc. Conditioned air leaving via the attic is replaced by outside air that works its way in via gaps in exterior walls, windows, doors, etc. That outside air must then be heated, cooled, humidified or dehumidified, depending on the season and climate. That process costs money - it increases the capacity of the required HVAC system and lengthens the operating time of that system

We here will work with and overcome ignorance - nobody is born knowing this stuff. Note well we won't long abide arrogance, scorn and disdain.

help-me: Roughly, with your figures, the attic fans will create a 1/4 of 1% change in your air infiltration rate or a dollar or two per year in energy costs - if they have no offsetting benefits. As you suspect - not something you should worry about.

Jon - depending on the attic fans, you can create a heck of a lot more than a fraction of a percent change in infiltration. Ignore the whirlybirds for a moment and think about the typical 1,000 CFM fans people install, often in attics that have minimal air intakes. You lose several times.
First, you pay money to run the fan. During the summer, that can easily add to several kwh's per day. Around here, that's about $0.50/day or $50/summer.
Next, you're drawing, easily, 100-200 CFM from the house which, as Engineer notes, is like pumping those CFMs of outside air into the house. Conditioning that much air isn't cheap.
Worse, the same effects are in action in the winter even when the fans aren't running. I've seen too many roofs rotting or in the process of rotting due to interior moisture moving into attics. Roof fans have no place in most homes. They simply do more harm than good. People absolutely should worry about these things. They're real.

Of course, if you pick entirely different circumstances, you get different numbers. If someone did want big powered attic ventilation (and I don't know why they would), they would be well advised to use a push pull configuration to keep the attic pressure neutral.

Please understand that I do want to get Manual A - Z done.  I am not planning on forgoing this step.  I have to be there to let the contractor into my house.  I cannot just tell him or her that the spare key is under the planter and the alarm code is 54321.

Have a nice weekend.

FWIW: Those whirlybird vents pull far more air than 50cfm EACH, even in fairly light wind (even the teeny-tiny ones):

http://www.empirevent.com/vents_turbine-ventilators.html

If it were 50cfm total their value & function would be "in the noise" from both a ventilation and energy consumption POV in an otherwise well-ventilated attic. If you have three 6-inchers and an average wind speed 5mph you're looking at over 1000cfm, and yes, it will depressurize the attic (by how much depends on the ventilation cross section.) A handful of years ago I measured the effect of adding a 4" whirlybird added to a passive sub-slab radon remediation system with a manometer- it was real enough, and the subsequent followup radon tests were consistent with that depressurization- it was doing the job. (Tests done with a venturi-type vent cap were a bit harder to measure.)

Whether it turns out to have energy consequences (in either direction) for you depends a lot on your average wind speed, how air-tight the attic floor is, and your insulation level in the attic, etc. With a fairly tight attic floor and low R-values they can reap a net savings on the air-conditioning front, but add a little to the heating load. If fairly leaky it'll add to both.

Both the Florida Solar Energy Center and Texas A & M have studied powered active-ventilation of attics going back to the 1970s, and the results are pretty dismal- typically it's a net increase in cooling load, the only question being "how much". But in one of the more recent Florida experiments an off-grid self-powered solar attic fan resulted in single-digit percentage savings on cooling power. A "right sized" whirlybird could probably do as well (or better) in some installations.

The rationale for attic ventilation holds little water in cooling dominated climates though, and houses with ventilated attics (even without whirlybirds or fans) with leaky attic floors can have significant infiltration drives that houses with sealed attics do not. The primary benefit of a vented attic is to purge moisture to avoid condensation issues in colder climates, and even there it can be a solution-problem- creating a problem by pulling more humid conditioned air into the attic while giving it a path to escape (but with a net energy cost.) In very humid cooling dominated climates putting the attic within the pressure boundary of the conditioned space results in drier attic conditions, if slightly warmer attic temps (unless insulated at the roof deck.) The solar reflectivity & emissivity of the roofing materials has a larger effect on peak attic temps than sealed vs. ventilated. See: http://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1496-05.pdf

Posted By Dana1 on 03 Sep 2010 04:24 PM
FWIW: Those whirlybird vents pull far more air than 50cfm EACH, even in fairly light wind (even the teeny-tiny ones):

http://www.empirevent.com/vents_turbine-ventilators.html 

If you have three 6-inchers and an average wind speed 5mph you're looking at over 1000cfm, and yes, it will depressurize the attic (by ow much depends on the ventilation cross section.) 

1000 cfm in a 5mph wind? 

I need to see the data?

I have three 12'' whirlybirds. 

please show me at the very least a link that has cfm rate of whirlybirds by size and wind speed.
something perferably from the manufacture and not backyard science guy.

Posted By help-me-conserve on 03 Sep 2010 08:58 PM

please show me at the very least a link that has cfm rate of whirlybirds by size and wind speed.
something perferably from the manufacture and not backyard science guy.

Did you look at the link?  Scroll down to see the table.  This appears to be the manufacturer, unless I missed something.

Maybe it's just me, but you don't appear to be serious about learning here considering your comments.  Sarcasm is bad, but insults are ok?

I guess he needs to see the whole curve to extrapolate the 5mph rather than accept that it's actually higher than the 4mph performance datapoints provided by Empire Vent?

Beats me?

I haven't worked as a scientist for decades now, but as an engineer design & verify stuff that actually works, and I don't just make numbers up. Even when they're just guesstimates they're not just WAGs (but I'll tell ya when it is.) Call my manometer measurements "backyard science" if you like- I'm not insulted. I tend to trust people who actually measure stuff than those who throw out unsubstantiated cfm numbers (apparently) pulled from their backsides, more than an order of magnitude different from what the manufacturer's spec says, eh? ;-)

According to Empire Vent a single 12" whirlybird moves 631cfm in a 4mph wind, assuming very low head (a lot of cross-sectional vent opening), which means 3 of 'em will pull ~2000cfm if you have sufficient cross sectional area to your attic ventilation (and have enough power to depressurize the attic if you don't.) Go see for yourself(again):

http://www.empirevent.com/vents_turbine-ventilators.html

The third column from the left is the vent size, the 6th column over is the cfm moved with a 4mph wind.

2000cfm isn't just a little bit more than 25 or 50cfm.

At 5mph it'll be considerably more than 2000cfm- count on it (or measure it yourself- tell us your methods, and maybe I'll trust the number, or at least tell you why I don't.)

They are not precisely linear, but it's not a wild curve either. A competitor's 12" product pulls

345cfm @ 5mph (69cfm per mph)

544cfm @ 8mph (68cfm per mph)

969cfm @ 15mph (65cfm per mph)

(see: http://www.acehardwareoutlet.com/ProductDetails.aspx?SKU=50585)

...which is linear enough, within a reasonable range of windspeeds, and though only about half what Empire specs, is still an order of magnitude bigger than a help-me-conserve WAG. I'll stick by my 5mph estimate of the three six-inchers from Empire, even if I HAD scrolled down to the wrong size, and over estimated by only 2x. ;-) (I must have been looking at the 255cfm-per @ 4mph for the 8-inchers when I did the quick linear guesstimation of 320cfm-per @ 5mph in my head, and rounded up. Mea culpa! Friday afternoon numbers are not to be trusted! :-) )

In the radon-system I measured it was orders of magnitude lower volume than the free-air spec due to the restrictive nature of the system, but there was an easily detectible depressurization in light winds using simple measurement tools. The point of that exercise wasn't to fully characterize it's performance across wind speed & duct head, only to verify that it was providing at least some slab depressurization. I didn't file that data away, and I didn't measure the windspeed under which the pressure differential was measured.

The number would be completely irrelevant in this application anyway, but was only mentioned to point out that the depressurization potential in a restricted cross sectional area situation is significant- enough to suck the radon from beneath the slab, anyway. Which is more than enough reason to believe it could drive a lot of air-infiltration from conditioned space should the attic floor be less than perfectly air tight.

Or, you could just make it up as you go along, rather than do the 5 minutes of web research (or more the more substantial backyard science measurements) to figure it all out- it's your house. If turbine vents didn't suck (and suck pretty hard in a high wind) there would be little benefit to putting them up in the first place. Experience would indicate that they work pretty well.

But in the real world the aerodynamics of the roof shape and other venting location interact as well. Free-air cfm performance isn't the half of it. How much it actually depressurizes the place depends on many factors:

http://digitalcommons.fiu.edu/cgi/viewcontent.cgi?article=1322&context=etd