I'm building a 4300 SF (heated/cooled) ICF house with sprayed foam insulation in attic. Location is gulf-coast Alabama. I intend to install a GCHP system. My floorplan has a large room open to two stories (about 22' ceiling height), the living room. The kitchen (on the first floor) is open to the living room, and there's a loft directly above the kitchen that is also open to the living room. The floor space is about 550 SF living room, 350 SF kitchen, 350 SF loft. I like windows; there's about 400 SF of glass (window/door) facing NW in the living room.

I've had several contractors make load calculations as part of their bidding process, but they're treating the floorplan as two separate floors with separate rooms (no consideration for the open to two stories, nor kitchen, LR, loft conjunction.)

I'm offered designs with one, zoned 6-ton unit; two 3-ton units; and a 2-ton/4-ton combination, all proposals using 2-speed WaterFurnace products. (At least everybody seems to agree on the total load size.) I'm concerned with dehumidification on low-cooling-required days, so I'm looking for a hot-gas reheat. If I have two units, I'll include a de-superheater on one, as I'm told I can't have the reheat and de-superheater on the same unit. 

My concern is zoning and ducting. Both dual-unit proposals are upstairs/downstairs, no zoning. So, though I have potential for fine control over the amount of heating and cooling, I'm not sure how it should be allocated to the living space. Should I just treat the kitchen/living room as a single room and the loft as another? Or, should I treat it all as one big room?

Is there someone who can help me understand how to deal with the large, open room from an HVAC sizing and zoning perspective?

If I understand this correctly (1st and 2nd floor connected by a big 2 story room), figure on most of the cooling load from the common volume ending up having to be dealt with by the upper floor equipment (heat rises).

Six tons seems on the high side for 4300 SF ICF / Sprayfoam (I have 3k SF in north Florida, 3 stories, 4 zones, single 3 ton 2 speed Waterfurnace which I locked into low stage only for the entire summer (~ 2.5 tons actual cooling)

That said, your heavy glass may make the difference - I'll defer to the locals who loaded your house. Do consider specifying glass with a low SHGC - makes a big difference on western exposures, then make sure accurate SHGC factor and whatever shading is considered in load calc.

I'd lean away from a single six ton solution - on low stage it won't go below approx 4 tons, and that's a lot of potentially noisy air moving around when just a few bedrooms need chilling at night, and dehumidification will suffer. I'm debugging a 3 story 3 zone beachfront house with that setup for my builder and it ain't pretty.

Assuming 6 tons is correct I'd take a long look at individual zones' loads to make the call as to how to split it up. I don't think WF makes a 2 speed 2 ton - smallest 2 speed is 2.5 tons.

With two 2 speed units and ICF construction I doubt you'll need separate provision for dehumidification. I provided for it, and despite running my one unit as a single speed all summer humidity stays 40-55%, so I'll likely never need it. A significant benefit of ICF construction is that its tightness limits infiltration, reducing latent load (better dehumidification) and its thermal mass transfers sensible load from hot afternoons into damp nights, giving the HVAC reason to run more often after dark - this really helps with nighttime dehumidification.

I was unaware WaterFurnace has a hot gas reheat dehu option - are you considering a separate system or a non-OEM modification of the WF units for dehu?

Consider an extra zone or two if you do go with two 2 speed units. My dream house darn sure was gonna have a thermostat in the master bedroom, it is nice to have the kids' rooms arrayed along the east side be a zone to deal with morning sun on that side, and the main kitchen / family room has a stat as well, allowing a precise response to a crowd, heavy cooking, or afternoon sun. Finally, the basement has its own to deal with dampness there as well as future finishing off as a teen space or whatever.

A smaller unit on low stage moving just 600-900 CFM just into the zone in need should be almost inaudible and run times should be long enough to ensure good dehu.

Finally, favor the contractor who provides for a preheat tank for superheat recovery - its the right way to go.

Thank you, engineer. I was beginning to think nobody was going to offer a response.

I appreciate your help on the heat load question. You got the question right. I ran the manual J calculations and got a figure that's pretty close to what the pro's got. So, I'm ok comfortable with that. However, I had no good ideas about the two-story situation. The contractor with the 4/2 solution put the 4-ton unit on the bottom floor (which caused me some concern for the reason you identified.) So, thanks agaiin for giving me a warm fuzzy that I'm not out in left field somewhere.

Your thoughts echo mine on the 1 vs 2 unit design. That ties back to the dehumidification, too. We like it warm in the house, so I was afraid a 4-ton low speed would force me to choose between comfortable temp or comfortable humidity in the summer. My kid rooms are on the East side, also, so it's good to hear from someone with a similar arrangement.

I saw that WF offered dehu reheat on the commercial units and asked if it was available on the Envision units. The answer was yes, but a unit can have the de-superheat option OR reheat dehu, not both.

Since you're ICF also, what provision did you provide for fresh-air ventilation? I'm looking for a ERV to provide controllable fresh-air without all the loss of just sucking in unconditioned outside air. It seems this isn't a common configuration in this area; I guess most trade fresher-air for cooling efficiency.

Just bear in mind that a single two stage unit with zone controls can perform like two units depending on room x room or floor x floor loads. It is wonderful for you that 3 contractors agree on the load so you can move on from there. Ask each about design and contact their refrences. Also ask them about other blogs here on WF warranties. No system is perfect, find out how they compensate for that. Ask for the phone number (or E-mail address) of somebody who had things go wrong but was satisfied anway due to the contractor's commitment.
Good luck,
Joe

No provision for fresh air so far - I figure cheap / leaky dryer vent, 3 bath vents, range hood outlet opening, 7 exterior doors (includes porches) and 40-odd windows all leak enough to keep us in the clear. Kids routinely leave doors open anyway...

We still haven't installed some door stripping and hardware - kind of deliberate procrastination to let a new house off gas - When we get it tightened up I'll revisit the issue.

I'm a fan of WF but the point of warranty support is well taken - a recent poster here seems to be getting jerked around badly.

Engineer, no fresh air provision? What code was your house built under?
J

Florida. My builder is fairly progressive and has never had to do one in 10+ years. Neither code (or more accurately, inspectors in ~5 counties) nor customer has required it so far. I originally intended to do it, but he advised was not needed / required. I don't remember it arising in my licensing exam, either.

He's had to deal with it on commercial jobs but not resi.

Tight homes, no ventilation, kind of takes the mystery out of the recent mold scares doesn't it.
Old leaky homes have few mold occurences. It's new homes that suffer from this.
Is your code a Florida one? Is it based at all on the IMC?
J

Not sure about the code base.

My understanding of mold is that it occurs not due to a lack of fresh air but rather as a result of two conditions: 1) RH>~60% and 2) presence of mold food - anything cellulosic. With reference to cellulose as mold food, the more the cellulose has been processed, the tastier it is to mold. Framing lumber is not nearly as yummy to mold as paper, such as that on drywall.

Failure to control all moisture - all sources ranging from duct / pipe sweat, improper drainage, plumbing leaks, infiltration of humid air into cool crawl spaces, wall cavities will lead to mold. Ventilation doesn't fix these problems, and can actually serve to mask them.

Poor ventilation leads to other IAQ problems ranging from oxygen depletion to buildups of contaminants as building materials and finishes off gas (paints, plastic, vinyl, upholstery, carpets. This is a real issue owing to the witches brew of chemicals contained in building materials.

OTOH I'm a bit skeptical about the mold scare - if the stuff is that bad, a walk in the woods should be fatal - think about it - shade and moisture in the woods mean RH >> 60% much of the time, and vegetation is continually decaying (being eaten by mold). There are a few hyper-allergic folks who probably can't take a walk in the woods, and for them mold is a huge issue, but for the rest of us, I don't think so.

True that "mold" scare is overblown. Ignorant private inspectors tend to identify any mold or mildew that is dark in color as "black mold". I agree that mold is rooted in humidity; stale air of course is a root of humidity. Mold and IAQ problems were not "discovered" recently, they were "created" with our tighter building envelopes (in my opinion).
We can argue the point about mold but it sounds as though we're in agreement about about poor ventilation = poor IAQ. Which I believe would mean that we agree ventilation is a must (you just haven't gotten around to it yet), yes?
Curiously the walk in the woods may be ok for some with allergies due to dilution of air outside the wood's envelope. One man's masking is another's dilution (a ventilation strategy).
Not trying to belabor a point, but I think you'll want to know what code over sees installations in areas you wish to contract in. I'm also curious as to wether your one size fit all local inspector actually did his job or is negligent like so many others.
Joe

I just returned from an ACCA vendor and education event and listened to Tom Mooney talk about dehumidifying a home on the Gulf in Corpus Christi.  I doubt if the experience in Corpus is any different in Alabama.  I googled for something on Tom just now and found this on Contracting Business.com that might provide helpful insight to the discussion.

Opinions: dehumidification technology and controls

Tom Mooney, a systems analyst with CCAC Air Conditioning, Corpus Christi, TX, has gained a solid reputation as a dependable dehumidification expert.

Mooney sells as many as a dozen Thermastore whole house dehumidifiers each month in the humid climate of Southern Texas, as a successful antidote for dust mites, which he says are the world's number one indoor air quality problem.

Mooney helps customers understand that an air conditioning system alone cannot control humidity in a home year-round, because there are many days each year when it's not in use.

"We have nighttime, mornings, winter days, cloudy days, rainy days; so many days where the air conditioner isn't going to run unless you turn the thermostat down," says Mooney. "But cranking the thermostat down in my part of the country turns the house into an evaporator coil, and you create indoor air quality problems."

The overcooling by the homeowner because they are not comfortable with the high humidity level in the home brings the whole house temperature down below the dew point. This can cause interior wall spaces to sweat, and develop water and mold problems. Running the house that cold can also cause equipment, ductwork, and supply registers to sweat.

Once CCAC's customers have control of the humidity in their homes, they tend to turn their thermostats up to 76 or 78 degrees and feel much more comfortable than they did before at 68 to 70. The energy savings of turning the thermostat up, in most cases, more than pays for the energy used by the dehumidifiers.

Thanks for the info, Dale.

That confirms what I've read elsewhere about cooling and dehumidification in an area with high latent load. Interestingly, none of the contractors I've talked to here thought that I'd have any problem with humidity and cooling, even though I told them that I liked it warm in the house (~80 in summer). They always came back to "you're going to get a two-speed pump, and it'll run plenty to keep the humidity down." When I pushed, they offered a whole-house dehumidifier, but I didn't like that solution because it would provide me with a net heat gain, which I'd then have to eliminate with the air conditioner. Not a good solution.

The heat pump with reheat accomplishes the same function as the whole-house dehumidifier, but provides a simpler, more elegant solution (to my taste), with greater capacity, if it's needed.

I found a good document that compared various solutions, including whole-house dehumidifiers and multi-speed air conditioners. Though it's not an end-all resource, I think it does a good job of outlining the problem and a few approaches with some empirical data on effectiveness. The document is NREL 36643 from the National Renewable Energy Laboratory. It's from 2005, but I think that's recent enough to be relevant. Here's the link: www.nrel.gov/docs/fy05osti/36643.pdf

Based on what I learned today and given the trend that is forming towards energy efficiency, this issue is causing the mold problem to re-emerge and is a sign of trouble on the horizon for builders and HVAC contractors who work in high humidity areas.  Tom get's quite a bit of work from insurance companys dealing with mold issues down his way and he was showing me pictures of homes the insurance company's bulldozed, the mold was so bad. 

Historically, the insurance industry has shifted the risk to the homeowner by withholding coverage for mold damage.  As people scramble for ways to save energy, the homes will get tighter, the humidity control problems will increase and the mold will follow right along in lock step.  I agree the health concerns about mold are likely over dramatized.  However, mold is ugly and hairy and no one is interested in having this kind of guest in the house.  Tom showed me pictures of whole walls that were covered with mold.

Get enough voting homeowners with hairy walls, no insurance coverage to resolve the problems, and politicians in Austin looking for a parade to lead and the papers will have plenty to write about.

I actually incorporated a "whole house" house dehumidifier in my plan for B. Gillett's indoor pool area, but where geo and reheat are available, that's not always the best choice. DeHu strategy favors the largest coil (such as a geo) in most circumstances. In Ben's case, the size of the mechanical room as well as pool heating and enclosure heating requirements made it an economic solution (3 less tons of excavation and avoids putting everything on a relatively low COP water to water). In the average home re heat may be cheaper (not much average about Ben's house).
J