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HVAC for 2 stories: 2 systems or single zoned unit?

PensacolaPI | Posted in General Questions on

Confused on which way to go, new construction and HVAC….

2 separate HVAC systems vs a single zoned system?

Generally speaking mind you, our new house will be about 3,000 sq ft and 2 stories. I’ve done a bit of reading on a few sites and kind of stuck on which way to approach HVAC.

Option 1: 2 separate systems, 1 for each floor. I like the redundancy factor. The obvious downside, ,the cost of 2 units and maintaining them on a yearly basis. Basically 2X the costs regarding maintenance and the same goes for replacing the systems.

Option 2: 1 PROPERLY installed and zoned system. I’ve dug and dug and then dug more, I’ve got a local company who I am very confident in. They sell and install Carrier equipment but will install any equipment of my choosing. I would lean towards American Standard only due to prior experience with the brand.

When the day is done, it’s all about the install and not so much the brand of equipment.

I’ve been in countless 2 story houses where in just about every case, the upstairs has ALWAYS been hotter when the house had a single unit installed. Either the install was awful or there are other issues. I’ve been in several 2 story houses with 2 units and across the board, a nice even temperature between both floors.

Local company indicates they can install as many as 8 zones on one system. Another expense would be 8 thermostats and I am partial to the Nest Thermostat so there goes another big chunk of change for thermostats.

On the single zoned unit, assuming the installers know what they are doing mind you what additional maintenance issues are there with a zone unit vs a unit that is not zoned? On the flip side if we did a single unit that is zoned and we were not happy and wanted to add a second unit, would that duct work be able to work correctly when a second system was added say for the upstairs?

I’m pretty confident in the 2 systems, not so much so with a single zoned system. We are in Zone 1 in Pensacola and in the summertime, extremely HIGH humidity is the norm here.

Thank you!

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  1. GBA Editor
    Martin Holladay | | #1

    Zoning a forced-air system requires compromises (because it's hard to know what to do with the extra air flow when only one zone is calling for conditioning). So two systems generally result in better comfort. For more information on this issue, see Keeping Cool in a Two-Story House.

    If you can design your system to use ductless minisplits or ducted minisplits, you might end up with an affordable and efficient approach.

  2. PensacolaPI | | #2

    Thanks Martin, I had actually read that prior to posting. Our roof will be a SIMPLE hip roof with a generous "overhang". We have been contemplating a mini-split and that's still up in the air and we're not dead set on either the traditional ducted system vs the mini-splits. Locally, the vast majority of companies do not sell nor support anything except the traditional systems. There is however one company that will do the mini-splits. Having had the ducted systems for as long as I can recall, there's no doubt that we'd be comfortable in a ducted house be it a single or multi story. Having not lived in a house with the mini-splits, that is an unknown. The point of concern is that if we used a mini-split and we were unhappy with the system performance AFTER spending close to $400K on the project and the house is built, now what? HVAC is in, house is done. I don't want to even think about replacing the system and trying to install ductwork. I am personally of the opinion that 2 systems is the way to go. This WILL be our forever house, we are SO DONE with moving!

  3. Expert Member
    Dana Dorsett | | #3

    The key to getting this right is getting aggressive 3rd party generated room by room load calculations. The 1 system vs. 2 is a secondary issue. Leaving it up to an HVAC company to come up with the load numbers is fraught with error. A certified professional engineer or RESNET rater using aggressive assumptions in a Manual-J tool is far more likely to hit near reality.

    Discussions with the HVAC company only begin once you have the load numbers, and oversizing by no more than 1.5x (1.2x is better) for the calculated load is key. Keeping the oversizing factors that low in a 3000' two story might be really tough to do with two separate ducted split systems, much easier with mini-split/multi-split solutions. In new construction with some attention paid to load numbers during the design process you're probably looking at 3 tons or less of cooling (yes, even in Pensacola).

    With a mini-split solution one has the option of running it in "DRY" mode, with a higher latent/sensible cooling ratio when it's tropical-torrid outside, or in a much higher SEER normal cooling mode when it's not.

    Installing 8 micro-zones with 8 NEST thermostats is pretty ridiculous (and probably not necessary), but the room-by-room load numbers may tip the balance to more than just two zones, especially if there are doored-off rooms with large amounts of west facing window taking solar gains late in the day.

  4. PensacolaPI | | #4

    Thanks much Dana! West facing windows will be kept to a minimum.

  5. JC72 | | #5

    My two cents regarding a zone system.

    I have a zoned system (3 zones) and it has been nothing but a disappointment. Sizing and duct design are two large compromises with this system.

    Sizing: The system must be sized so that it can cool/heat all of your zones at the same time. Typically the system is at most heating/cooling only 1 zone so it will not be running a peak efficiency and is more likely to short cycle.

    Duct design: There are all sorts of balancing issues to contend with. In order to bleed off excess air typically each zone needs to be "open" by approx. 20 percent when it's not being used and/or the by-pass damper must be of sufficient length so that it doesn't dump super-cooled air back into the system which shortens* compressor life.

    The electronic dampers themselves will eventually fail and replacing them is a PITA because they're usually wrapped up inside flex duct. Replacing it will typically require cutting into the flex duct itself and then meticulously piecing it back together.

    *Super cooled air will not allow all of the liquid refrigerant to convert into a gaseous state. As a result any liquid refrigerant returning to the compressor will cause unnecessary wear/tear on the compressor itself. Our compressor lasted about 12 years.

    Good luck.

  6. PensacolaPI | | #6

    Thanks John, VERY solid info. The dampers were something I wondered about and obviously balancing the air flow.

  7. Jon_R | | #7

    > typically each zone needs to be "open" by approx. 20 percent when it's not being used

    IMO, this should be adjusted on a zone by zone basis. You might want some zone to go to zero (say an unused bedroom) and others would be fine at 80% open even when off.

  8. Expert Member
    Dana Dorsett | | #8

    "You might want some zone to go to zero (say an unused bedroom) and others would be fine at 80% open even when off."

    ...or, get rid of the ducts & zone dampers and make the problem go away. More complexity integrated into zone damper control is likely to incur a higher failure rate.

    BTW: Pensacola is in US climate zone 2, not zone 1. (It's even the COOL edge of zone 2.) In FL only Monroe, Miami-Dade, and Broward counties are in zone 1. But like most Gulf Coast locations, the latent loads are pretty hefty zone-1-ish. The peak cooling loads are also similar to zone 1, despite fewer cooling degree-days.

  9. PensacolaPI | | #9

    Thanks, and that's the reason I put in zone 1. I lived in Broward County for 20 plus years and this is pretty much the same climate here.

  10. JC72 | | #10

    @ Jon R

    Agree, but the issue is how to handle the excess volume of air. My understanding is that leaving dampers partially open on zones which are not being "called" is a must because the volume of air is oversized for each zone individually so excess air must be allowed to travel somewhere.

    For example when our system was commissioned (1999) the dampers would 100 percent closed when the zone wasn't calling. Our blower pushed so much air that it would force the excess out the 2nd floor return in a ceiling above that floors thermostat. Of course it didn't help that air from the by-pass damper was routed from the plenum back into the return via only 6 feet of flex duct. HVAC guys told us "normal operation" and the builder wouldn't warranty a repair/retrofit.

    Our system is currently set up where zone 3 (top level) is permanently open and the system still pushes enough air to trip the by-bass damper when one of the other zones are called. This is my own setup. No HVAC company will touch the flex duct in our house because they said it's made of an older material that's too easy to tear and difficult to repair. *sigh* Where's the lightning when you need it.

    Note: We have a simple passive barometric by-pass damper which consists of a weighted damper arm than require X-force of air to push open the damper.

  11. Jon_R | | #11

    @ John Clark

    If zone 3 is completely open and zones 1&2 are 50-80% open and there still isn't enough air flow, then larger ducts and registers would have been a good idea.

    I'd put a bypass last on the list of options to increase air flow.

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