Advisability of zoning in this situation
Hi GBA – I just discovered this forum and would appreciate if you could offer an opinion or point me in the right direction where I can learn more. My question has to do with the advisability of zoning to increase energy efficiency in my situation. I have a two-floor home, 1850 square feet, open floor plan, in North Carolina (Zone 4A). Upstairs is a guest bedroom, which isn’t used that often, and an upstairs office, used during the day a few times a week. However, most of my time is spent downstairs. I am replacing my aging HVAC system with a 3-ton two-stage 17 SEER heat pump and variable speed air handler. I was thinking of zoning (two zones), as I would prefer to heat or cool upstairs primarily on an as-needed basis rather than having energy continuously directed to an area when I am not using it. However, two different contractors said that zoning would not be more energy efficient in this situation – that I should zoning primarily because of comfort – and the biggest bang for the buck in energy efficiency will come from the new HVAC system, with nothing much added by zoning. A third contractor agreed the biggest change would come with the new high efficiency HVAC system, but that there would be an increment in additional energy efficiency (and the additional costs might pay themselves back in ~10 years). I am surprised with these assessments as I was thinking there would be greater energy efficiency, and wondering if this sounds right? Can you please point me in the direction of resources where I can learn more about this? Thanks in advance
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Zoning isn't going to buy you much energy savings with a variable speed high efficiency heat pump. Getting the sizing (and duct sizing) of the replacement system correct and the air flows balanced is the most important aspect of this. Zoning dampers create higher static pressures, cutting into efficiency when serving just one zone, and can make the system more un-balanced creating higher air-handler driven outdoor air infiltration. Assuming the ducts & air hander are all inside the insulation and pressure boundary of the home (not in an attic, above the insulation) designing for a duct velocity <<400 fpm under all operating conditions (both zones calling, or just one zone calling) keeps the static pressures low, and the air flow through the coil high, with high heat transfer efficiency. That's easier to do with reasonable sized ducts if it's not zoned.
If the ducts are in an unconditioned attic it's even more important to keep the system balanced, but the duct velocity needs to be a bit higher (and duct sizes smaller) to control direct duct losses. Smaller ducts & higher duct velocities increase static pressures, making the problem even worse when one of the zones is dampered off. High end air handlers and compressors can modulate, but the modulation range isn't infinite.
Three tons is a LOT for an 1850 square foot house that has glass in the windows and insulation in the walls. Most 2 story homes that size would have a 1% cooling load of LESS than TWO tons, and depending on your 99% outside design temps but 2 or 2.5 tons might be about right for the design heating load in a reasonably tight 2 story insulated house that size.
If you can put off the decision it's possible to measure the cooling load directly this summer by measuring the duty-cycle on the existing equipment on afternoons that cross the 1% outside design temp. See:
https://www.greenbuildingadvisor.com/article/how-to-tell-if-your-air-conditioner-is-oversized
Note: it's best to measure the duty cycle on the compressor itself, (or at least at the condenser), and NOT rely on a smart thermostat's call for cooling to make that call. It's quite possible that during long calls for cooling the compressor will cycle on/off, depending on the equipment and how it is controlled internally.
Three tons for 1850' is about a ton per 600 square feet. Typical cooling loads for houses that size are in the ton per 1200-1400' range, though it is highly variable. Allison Bailes compiled this graphic of square feet per ton ratios against house size for a few dozen carefully done Manual-J calculations on houses (mostly in the southeastern US):
https://i1.wp.com/www.energyvanguard.com/wp-content/uploads/2016/08/square-feet-per-ton-air-conditioner-sizing.png?resize=1024%2C618&ssl=1
Only the very WORST house in the 1500-2000 square foot range came in at under 700 square feet per ton, and half of the houses in that size ran more than 1000' per ton, with a couple close to 1500' per ton. How crummy is your house? Do you have lots of unshaded west facing window?
It's also possible to measure the heating load directly from wintertime energy use, using variations on this method:
https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new
If the current system is a heat pump or resistance heater (say, central AC with auxiliary heat strips) some adjustments need to be made for the other uses of electricity when making those calculations. But no matter what it would be easy to establish an absolute upper bound. In zone 4A it's important to use energy use for just the two coldest months when doing this sort of calculation, typically December & January, since milder days with high direct solar gains in November & February can skew the numbers. (In zones 5 or higher using the three coldest months is fine.)
If the heating and cooling delivered to 1/2 of your space could actually be eliminated, then you would use about 1/2 of the heating/cooling energy. But without insulation and air sealing between floors and with a reasonable lower limit on the upstairs Winter temperature, it's going to be much less.
I'll let others comment about energy efficiency. I will suggest that if you do end up choosing to zone, you might be better off with a fully variable speed system (Trane XV18, XV20, Carrier Greenspeed, etc.) than a 2-stage system.