Thanks. I did mention the home is in the Southeast US.

One system with a zoning control should be fairly simple. depending on design, perhaps one zone for living area and another zone for sleeping/bedrooms. As far as basement maybe another zone. i note a SE US location. Finished basements below grade have very small cooling and heating loads. I would think about high end variable heat pump condenser.

TD

Maybe be more specific on location - at least to help with cost. Miami is not Mississippi. Technically Boone, NC is SE and it rarely hits 90.

Is your basement a walkout?

3500 is totally doable for 1 system. Would help for the air handler to be centrally located of course.

My last house had a walkout basement facing East. Levels were about 2000 sqft. 10 ft ceiling. Cary, NC. The basement, once finished, did not really need much conditioning. I put a return in there to force some circulation. This was enough for summer time dehumidification. Then I tied in a few vents. I would open the vents in the winter.

What becomes an issue is a coolness during shoulder seasons. We didn't care but there are times that heating the basement alone would have been nice.

Either way, even a walkout in a typical SE climate doesn't add much to your heating and cooling loads.

Now - on to your specific questions

#1 - most cost effective on installation and ongoing maintenance is a single system. Actual utility costs don't matter much at all for the various options.

#2 Miami or Mississippi? Ballpark $15k but could be $10k to $25k.

#3 No people do not condition unfinished spaces. There are code issues on doing so even if relatively irrelevant in the SE.

Beware zoning. Few do it right.

We have ~2800 on one level with ~600ish on a second level and have a full basement. We are in the southeast as well (KY). We have one full system (heat and cool) for our main level that has two returns. We have a separate smaller system for our upstairs that is an A/C system only per the recommendation of our HVAC guy (since heat rises). We have a couple of vents that shoot off our main level system into the basement. Honestly, basements don't need much heating and cooling naturally anyway.

Before you start costing out your HVAC system, select your passive systems: the windows and insulation package. The more you spend on these, the less you will spend on running the other.

Don't forget dehumidifiers in your system. In most of the South, your unfinished basement will benefit from a dehumidifier system. It will prevent the growth of mold and mildew and improve the health of your home.

If you design the "perfect" HVAC for a home and there is no local expertise to size, install and service such as system it's largely a wasted effort. I suggest you speak with some local HVAC contractors to get a feel for what works best in your area, what will work best for your specific home plan and your specific needs, and a to get a budget estimate for such as system.

Thanks to all for the feedback. The home will be in the Atlanta area and will have a walkout basement. I of course will be reaching out to a professional company for their design and expertise, but thought it'd be good to have some rough idea in mind of what I would actually need.

Here's a good read that will provide sufficient information to make the average homeowner's head hurt:

https://www.buildingscience.com/documents/insights/bsi-022-the-perfect-hvac

JD, first I should state that I am not a pro.

My husband and I have lived in several states in the south, always in humid climates that presented challenges to keeping indoor humidity within a healthy range.

While I have a great deal of respect for the folks at Building Science Corp, it's important to consider when their reports were written and to see what they've learned in the years since before following their advice. For example, Footnote 4 of the linked article states, "It is important to note that an ERV can never, under any circumstances, dry the indoor air. It only reduces the humidity load due to ventilation, it can never eliminate it." I think they'd agree that an ERV cannot reduce the humidity load in much of the southeast for most of the year and particularly during the shoulder seasons. We had an ERV installed years ago thinking it was the answer to our problems. Due to there being several months each year when there's no demand for cooling we were increasing indoor humidity at times when we most needed to decrease it.

The better solution for us, and perhaps for you in Atlanta, has been a whole house ventilating dehumidifier. I'm partial to the Santa Fe brand but no doubt there are others just as good. These days I think they can handle houses up to 5K sq. ft.

We are getting ready to have a house built (yet again) and are looking at two stage variable speed systems plus a whole house ventilating dehu. Our architect is in the early stages of the house design, but we expect we'll end up with about 4K sq.ft. on one floor. Whether that will be served by one system or two, we don't yet know. The only thing I'm confident of is that we'll only use a company that can provide a Manual J and not just size based on square feet, which is all too common here. It's typical for builders to install over sized systems that may quickly cool the house but will never adequately control humidity since they short cycle.

FWIW, in a previous house with a walk-out basement, we handled it similarly to David Cary's approach. It was semi-finished and we put in a return and two vents from the main floor system which was likely too big to start. Once in a blue moon, we ran a portable dehu during a week of heavy rain just as a precaution and that helped the floor above, too.

Our current home has three systems for the first floor (>4K SF) plus a separate one for the basement and more upstairs. I've wondered if zoning could have worked just as well, but we bought the house after it was 99% complete and had no say. One nice thing is that we keep the system serving the master suite set cooler than the rest of the house, summer and winter, which we prefer for sleeping comfort.

Our son and his wife live in NC and had a zoned system for a two story house which was okay, while their current house, approx. 3600 SF, has separate systems for each floor which they prefer. I think the cost for replacing the two systems a couple of years ago was $16K, which did not include replacing ducts. We replaced a small heat pump system in a suite over a garage around the same time and the cost was about $5K. No doubt costs have risen.

Good luck!

Here's 2 vids to give you ideas for thought and to discuss when choosing a hvac contractor.

Tight house needs less dehumidification. Here in NC, certainly no need for supplemental dehumidification. Large house needs less also - since people/pets/plants give off humidity and that is dissipated in a large space. So 3 people showering and living in 1500 sqft is a lot more of an issue than 5,000 sqft. Just build it tight if the house is large and you likely won't need another mechanical device to potentially break down.

That is my experience at least. Atlanta is a bit warmer but not sure if it is more humid.

One my current house which finished 18 months ago, we spent 15k for 3800 sqft. Two Trane XR15s (two ton each) and a ducted Mitsu 1 ton. My experience is that replacement units are a larger profit item so they are a lot more expensive relative to what they do.

Hi, J.D.

Atlanta is in climate zone 3A; Cary, NC is in climate zone 4A. The air conditioning design criteria for each location are different.

You noted that your home will have a basement. Some level of dehumidification is generally indicated for basements.

Here in coastal Virginia I find dehumidifiers to be especially useful for maintaining indoor relative humidity levels--both for comfort and to preclude mold growth. I have a 3,500 SF 2-story home with two HVAC systems, each with an integrated dehumidifier. They are great in the hot, humid summers, but also in the "shoulder" seasons when no heating or cooling is called for, but indoor relative humidity can be high.

One of the best ways to select a good HVAC contractor is to share with them your preferences for comfort level, zoning/control if applicable, relative humidity range and indoor air quality. Then ask them what they recommend to achieve your objectives. Listen. The difference from contractor to contractor will be, well, telling.

Isn't the relative humidity problem mostly independent of climate zones and more dependent on distance to coast? At least in the SE? And we are about 20 miles from the 3/4 line - talk about arbitrary borders. And we are about 100 miles closer to the ocean than Atlanta.

I find humidity to be a tougher issue at the coast of NC. But even there, if you have enough solar gain, you tip the balance away from needing a dehumidifier - except on cloudy days. But, of course, there will be times a dehumidifier is nice to have. We have a house with huge SE windows, so you need a/c most of the time and that handles dehumidification. So house design has a lot to do with it.

Just like with humidifiers, there is rarely any absolute answers. Shady lot, small windows, coastal house - favors dehumidification.

I suspect the vast majority of new houses with walkout basements in Atlanta do not have whole house dehumidification and most have low humidity levels inside. The same is true in Raleigh. Our last house with walkout ran 46-48% most of the summer. We were staged which helps.

@David Cary

With the notable exception of the marine climate zone along the Pacific coast, climate zones tend to vary more as a function of latitude than longitude.

There are many factors to consider when designing an HVAC system in any climate zone. That's why a one-size-fits-all prescription is inappropriate.

You are right in my wheel house. I currently live outside of Atlanta, too. Humidity.

Times have changed and technology for basements and crawlspaces in our area has changed, too. It used to be they would put some grates in the foundation every few feet or so to vent the crawl space with an open dirt floor. That is a highway to mold and mildew.

In our area, good building practices will include a sealed crawl or unfinished basement. This is commonly referred to as encapsulation. Don't use the spray foam for this area. The heavy plastic sheeting is the best encapsulation system. Foam in this area is a mess with all the mechanicals that will be in the space and you can't see water infiltration if there is any.

Even with the encapsulated basement or crawl, you will need a dehumidifier for the spaces. I like the Aprilaire. You will need one for the basement and one for the main floor. I also like the GPS, Global Plasma Solutions system if you have family members who have allergies. Everyone in Atlanta has allergies, so you may want to include one of these to remove mold and contaminants. Don't forget to spec returns in every room. Proper airflow into the system is critical.

In the ATL, heat pumps are the way to go. You can do a gas backup system, gas is cheap in Atlanta, But, the good heat pumps have gotten so efficient that you won't need backup very often and the electric strip backup probably will suffice. Have them cost it out both ways and see if you want to spring for gas backup.

Spend the bulk of your money on good windows and a great insulation package. It's passive and will continue to pay your dividends for many years to come.