Consider a solar heated PassivHaus class super insulated house (Philadelphia mixed/humid climate) where all the forced air returns go to the center top of the house. At the center top is
1. whole house exhaust fan
2. downward path to basement, e.g. duct or stairs.
3. a chilled coil (water cooled to ground temperatures)

In the basement is the PassivHaus style ventilation system and a inlet panel that has small intake and large intake options.

When conditions are moderate (e.g. night time), a computer opens louvers in the attic and the large intake in the basement and allow heat to convect out. More severe, and the attic fan would activate to coax more flow and ventilation. Still more severe you close up louvers, Activate the ERV small intake ventilation system (also chilled to ground temperatures), and activate the attic chilled coil to boost the return flow.

It might make no sense to have the chilled coil in the attic, it would be easier to just cool the incoming ventilation air, but I was imagining it could help boost the air flow as well as help cool. I think this should be modeled.

Is there anyone here would could point me towards a modeling program that could have a shot at predicting how successful this might be? I have PHPP, but I'm interested in the air flow dynamics of the proposed ducts in a 4K sf 2 story+basement house. The long bent ducts will have a fair amount of static pressure, so its definitely not a classic solar chimney. I have no idea if the relatively weak thermosiphon effect could overcome this.

I'm also interested if anyone thinks this would have lower energy use than a straight up PassivHaus with a small heat pump. Place your bets now and I'll let you know when I figure out how to model it. Is it worth messing with the ducts and roof envelope for the sake of passive cooling? My proposed system ends up with slightly more conditioned space above the second floor ceiling to keep the ducts in conditioned space and accommodate the extra hvac equipment. I'm using free solar heat, so that's not an issue.

FWIW - superinsulation trumps passive solar space heating. According to reports about the Saskatchewan Conservation House (1970s), the waste heat from appliances and occupants were sufficient to keep warm. And that's a cold climate.

I think you're chasing the solution to a problem that doesn't really exist. In a superinsulated house of any significant thermal mass you never need mechanical cooling for sensible loads in a climate such as Philly unless it were mis-designed with too much unshaded south & west facing glazing, incurring huge solar gains. You could coast through 3 days of heat-wave on only ERV venting, and during normal summer weather passive nighttime ventilation (as in "open a window" should be sufficient. The interior of the house will coast along at about the daily average temperature + some amount from interior energy use, but if you're following PassiveHouse parameters that energy use should be quite small. (I s'pose the other way you could screw it up is to have 3 Tivos, 4 refrigerators and a 60" plasma TV going, in which case you'd need passive cooling in mid-winter, active cooling in the summer.)

Nighttime ventilation alone (passive or active) or passive dehumidification from earth-coupled vent ducts + ERV (the PassiveHouse standard) might not be sufficient for controlling Philadelphia's considerable latent loads though. There may be weeks where mechanical dehumidification will be necessary to keep the interior humidity under 60% RH (the knee point over which which mold growth potential starts to creep up and human comfort suffers.) The ducts+ ERV will have a better chance of keeping up with latent loads if the house has substantial hygric buffering (12-14" thick cellulose in the exterior walls, and interior walls/floors/ceilings/attic filled with cellulose for thermal mass & hygric buffer). In less humid regions passive dehumidication via the PassiveHouse standard earth-coupled ventilation air ducting seems to suffice. It might keep up in Philadelphia too. If it doesn't, an ultra-tiny solar driven absorption-chiller or dessicant-wheel might be a good solution, if one existed, but the smallest of current offerings are still 10x+ overkill for a PassiveHouse. If you're the type who boils every meal and takes 30 minute showers, you'll need SOME mechanical dehumidification no matter what.

Even in hot humid climates a combination of-

*ERV

*controlling solar gain

*limiting interior-activity latent & sensible loading

*hygric buffering within the pressure envelope

*judicious nighttime passive (open window) ventilation

..should be sufficient for all but the worst of days/weeks.

Additional ventilation penetrations of the thermal & pressure envelope to boost air-flow are likely to create significant net loss in overall efficiency (and cost). Enhancing the ERV with dessication on the inlet air at the ERV (after the underground ducting) would be the lowest-power strategy (whether a chilled hydronic coil, dessicant wheel, or some other method), but try to avoid punching any extra holes in the envelope to deal with a 2% problem at the expense of the 98%.

Dana1 & Jetgraphics: Thanks - for this and all your other posts I have learned from!

Jetgraphics - I agree, heating load will be virtually non-existant, but the solar heat is needed for DHW so I'm also using it for any makeup heat needed.

Dana1 - We do have more than our share of gadgets and internal heat loads. One halo lan party throws a lot of heat! The plan is for thick cellulose (double stud or TJI stud) walls, so we will be good with the hygric buffer. Judicious nighttime passive cooling is what I was hoping to automate. The idea would be to switch to ventilation mode (via computer) when temperature and humidity were lower outside. This could be the early hours of the morning when no one is going to wake up to open their windows. The thought was that thermal mass would keep the house temperature from rising more during the day than what could be cooled at night. A straightup passivhaus erv wouldn't move enough air in that early morning window to make a dent in the thermal mass.

Also , anyone know where I can find an ultra-tiny absorption-chiller? The tiniest I found was 10 tons!

If you can heat a house in the winter in Saskatchewan on waste heat then how do you not get cooked in the summer? After all, the refrigerator and other appliances still runs in the summer.

I would think you'd need passive solar gain in the winter, which you lose in the summer, due to the higher arc the sun takes at that time of year.

http://www.rotartica.com/pub/marcos_1024.htm

http://www.rotartica.com/pub/ingl/pdf_s/caracteristicas.pdf

They're about 1.3 tons (still way overkill for a PassiveHouse), and designed to run off solar thermal input.  (But they have a natural-gas fired version on the way for those too cheap to spring for the solar panels to run it, or need to run it all night long to stay ahead of the heat from the rave party. :-) )

But truly, you should still be able to do all-but-dehumidification with night-ventilation as long as you control the solar gain, even if it means oversizing the ERV a bit and giving it a higher night-time duty cycle.  Automating it further, with separate (and annually lossy) systems seems silly.

OTOH, if you're committed to 24/7 automated cooling with greater capacity, hydronic radiant-cooling-ceiling panels (or walls, but not floors) using earth-coupling (but no compressors or active chilling) would likely be cheap & effective, and need only very tiny & subterranean penetrations of the thermal envelope to work.  You'd need to actually design it, but using something like swimming-pool solar collectors as earth-coupled heat exchangers (buried below the water table for maximum effect). A big ball o' PEX might work too, but it would likely need a much bigger pump to overcome all that head.  It would only remove sensible heat, not humidity, but even 60-65F water can work, with sufficient panel area.  And it would have zero-chance of pumping humidity into the space the way a temperature-only night ventilation control scheme could.

But control your lighting & plug-loads, eh?  (Gonna go all halo lan party 24/365, pump the heat out with an oversized chiller are ya? ;-) )   It's the 24/7 plug loads that can really add up to a cooling load. Controlling your ventilation system with a 200watt PC instead of a sub-1/4 watt humidistat or timer is sort of a "solution-problem", creating the heat than needs to be pumped out under it's own control.  Being judicious in purchases, avoiding high-standby plug loads is something to think about (be it a superinsulated house or a borrowed FEMA trailer.)  Ten watts here, a hundred there, add up (which is why the average 'merican home represents about a kilowatt of continuous grid load and climbing.) An extra 500watts of background power in a superinsulated house is called a "heating system".

Passive dehumidifier
http://www.semcoinc.com/products/energyrecovery/pinnacle/howitworks.php

Great thread! thank you everyone.

Passive dehumidifier and tiny absorption chiller - nice! I don't need an absorption chiller but I really want one anyway. They even have one model that doesn't need a cooling tower. That's sweet.

I would like to point out that many of the sample PassivHaus designs for warm/humid climates are not all that different from my proposal. For example the passive-on & UK designs most appear to drag air from the ground level or roof level and eject it at roof level. Some simply have all their ventilation equipment at roof level and bring in the fresh air from there. They allow for some small level of active cooling as well as controlled amounts of make up heat. These warm/humid designs are short on details and short on experience since most PassivHaus's have been built for colder climates without much need for cooling. The designer of the first US PassivHaus admits that her house was uncomfortably hot as originally built. Like Dana1 I am very impressed by the science of PassivHaus, but I think the science is still a work in progress for warm/humid climates. Given the experimental nature, I don't think a 10W pc is excessive usage - it's in the name of science! I plan to instrument the house anyway and have instantaneous data feed available on the internet.

That said, I think Dana1 raises all good points. We are going all out to limit solar gain as pointed out- outside sun shades, overhangs, etc. Absolutely the top priority.

I considered earth tubes, but so far have rejected them because the Girja Sharan study demonstrated COP's similar to heat pumps considering the cost of the fan. The buried swim pool heater is potentially brilliant. If no one else steps up to measure this I might have to be the first. I think it's possible given the efficiency of moving water this could outperform earth tubes and no tube to keep clean.

Most PassiveHaus designs are half the size of the house I am building. The house will be multi-generational and will be home for 3 families. Multiple families will make sound control imperative, thus more reliant on ducts compared to most PassivHaus designs that tend to have open floor plans. The size of the house will make it tough for the blower of a typical passivhaus ventilation system to circulate air far away from the system without a little help (most non-PassivHaus designs in our area have air handlers in the basement and attic). The attic chiller was proposed to give the system a little kick for little cost. I considered radiant ceilings but I thought perhaps this was more expensive and difficult to control condensation than needed, so that made me think maybe I could get away with one strategically placed radiant ceiling to get the most bang for the buck.

To expand on the Halo party use case, I have some concern that not much has been written about PassivHaus parties. Our big old American watt guzzlers handle this case easily - crank up the 5 ton condenser and make it just as cold as you want. I think it's potentially interesting case for PassivHaus designs. Thermal mass only goes so far, and ceiling chillers are notoriously slow (probably one reason they are so comfortable!). It's socially not an option to cancel most parties because of a heat wave. A 1 ton solar absorption chiller, with a big insulated tank of chilled water? That would be insanely great. I want to buy stock in the company making it.... It's rated starting at 176F, too, which should be reachable with normal solar panels even in our non-solar friendly sky.

And just for the sake of argument...
An ERV in our climate is not a slam dunk. ERV's consume about 230W. Solar heat is pretty darn cheap, 230W would collect a lot of solar heat. It would be sometimes be nice to move some heat and humidity from the intake air to the exhaust air during the summer, but if you can stratify the air a bit and take out the warmest / moistest air, you might be replacing it with cooler/drier air much of the time anyway. Personally I'd like to experiment a bit, so I am thinking in terms of a design that would be sensible with and without an ERV, then measure it.

Again, thanks, and keep the ideas coming!

This is a downer - I'm not 100% sure I'm reading this right, but apparently the rotartica chiller draws 1.11 Kw(!). It's derated for 180F water (it wants 194F water), so "all in" the COP is barely better than the best electric units, even considering the heat as free.

You will ned a BIG house for that!  That is the type of kit we use in commercial office buildings.

Bruce

I am following this with interest because we will be building near the gulf coast and, due to of views and topography, the long side of the house will face 248°....not optimum for 29° latitude.

When we bought the lot, we knew that there is no way to do solar protection and maintain views with anythng other than active exterior shading (probably Hunter Douglas).  

Due to allergies, we will (almost) never open the house at night for cooling.   We are also on solid limestone with avg soild temps >70, so earthtubes are not a solution.  I wish there was more we could do with passive cooling.

DX GSHP in vertical wells is viable (and expensive), but we hope to get the loads low enough that a good ASHP with an ERV will be the most cost effective (the lower the loads, the longer the payback for GSHP).

I have more faith in low tech shading, insulation and interior load management than I do hi tech absorbers and solar collectors, although we will likey do solar for domestic hot water.  

Bruce

Bruce, it sounds like your strategy is similar to mine, as I'm building in Louisiana. So I find this thread very interesting, too. I am familiar with the Passivhaus concept from my time in Germany, but I always wonder how it could best be adapted to a climate like my current one.

Vernacular architecture strategies from before the civil war follow the siting tenets of passive solar. And in addition to orientation, one would add high ceilings and transoms to direct hot air flow up and out (plus large fly swatters over the table and mosquito netting on the bed). That probably made life livable for the 18th and 19th century, but I can assure you that's not enough for modern comfort!

So we're hoping an air source heat pump with an ERV, coupled with no glazing on the east and west as well as active shading on the south, will be good enough for a steel SIP house near the hot humid gulf coast.

There is a good information about low-energy building techniques in your hot/humid climates in the PHIUS book: http://www.passivehouse.us/passiveHouse/PassiveHouseBook.html

The project under conversation is the Lakeland Zero-Energy by the Florida Solar Energy Center, there's extensive documentation of their techniques at this link: http://fsec.ucf.edu/en/research/buildings/zero_energy/lakeland/index.htm

Going to a 36" overhang was more important than increasing wall insulation, keeping the exterior walls shaded from any direct sun was key. White roof was also top on the list, as was as much tile floor as possible to create a natural heat sink. They also worked hard at shifting the cooling load off peak hours using the PV and thermal mass.

Thanks for the links!  I had not seen them before.

Bruce

My ideas about this have changed based on the input from this thread and from reading papers like this:
http://www.buildingsciencecorp.com/resources/presentations/Rudd-Residential_Dehumidification_and_Ventilation_Systems.pdf

The fatal flaw of my original plan is the lack of latent cooling. Earth chilled water will do little or nothing to dehumidify, and the night time ventilation could easily increase the latent load when its needed most. As pointed out in the Rudd paper, highly efficient designs (like PassivHaus) have more latent load than sensible load.

The most cost effective solution I've found is to use small A/C unit configured for maximum dehumidification.
Because this produces sensible load cooling as a by-product, it reduces the value of other cooling schemes.

The problem is this product doesn't seem to exist. Any suggestions? The expensive 2 stage, variable speed blower with thermidistat got kicked to the curb in the test above. The smallest central A/C available is 1.5 tons which is potentially an issue since maximum dehumidification would demand that the system stay running to keep the coil as cold as possible. There are a few small geothermal units but they are relatively expensive for this application. I could put a mini-split in the main living area and just rely on the central system to mix the air around, but I doubt any minisplit is going to have a really cold coil since it could be uncomfortable next to it and their SEER is suspiciously high. Going this cold would probably kill a heat pump's efficiency rating because of the higher delta T. Building Science's solution of running a dehumidifier and A/C at the same time, while eminently practical with off the shelf parts, just seems very unsatisfactory. Why dump the heat from the dehumidifier back into the room and then cool it with A/C?


I am also exploring liquid desiccant technology. http://www.solarteam.org/page.php?id=641
It's probably not practical yet, but it would reopen the other energy efficient sensible cooling techniques if I could use desiccant technology to handle the latent load. Desiccant wheels don't seem cost effective for this application, but are more proven in commercial applications than liquid based desiccants.