When the dew point is 75, the upstairs in Murabella is six degrees warmer: zoning, myths first
On a July evening the downstairs thermostat in a Murabella two-story says 75 and the kids' rooms upstairs say 81. The house isn't broken. It has one thermostat for two climates.
A July evening upstairs in Murabella
The afternoon sun has been on the west wall of the second floor for five hours. Heat has been rising through the stairwell all day. The thermostat that runs the whole house is downstairs in the hall, in the coolest spot in the building, and it is satisfied. Upstairs, the bedrooms are 80 or 81, the dew point outside is 75, and the air in those rooms is as damp as it is warm, because the system that could dry them shut off when the hallway reached 75.
That is the climate case for zoning in St. Augustine. It is not just about temperature; a zone that the thermostat can’t see does not get dehumidified either.
Four beliefs about zoning we hear in St. Augustine
- Closing the downstairs vents will push the air upstairs. It pushes the pressure up and the ducts apart. It does not meaningfully move cooling to another floor.
- A bigger unit would fix it. A bigger unit satisfies the hall thermostat faster and shuts off sooner. The upstairs gets less, not more.
- Zoning means buying a second system. Zoning is dampers in the existing ducts and a thermostat per zone. A second system is the other option, not the same one.
- Zoning wrecks humidity by running the system on small zones. On a single-stage system that can be true. On two-stage or variable equipment, which modulates down to match a small zone, it is the opposite: longer, lower runs dry the air better.
What zoning actually does
Motorized dampers go into the supply trunks that feed each area. Each zone gets its own thermostat. A control panel at the air handler reads all of them and opens the dampers for whichever zones are calling, while the equipment runs at the capacity the open zones need. Upstairs calls at 6 p.m.; its damper opens, the downstairs damper closes, and the system cools the second floor without overcooling the first. At midnight the pattern reverses. The equipment needs to be able to modulate for this to work well, which is why zoning usually goes in alongside staged or variable equipment rather than bolted onto a 2005 single-stage unit.
The St. Augustine houses that should have it
- Two-story houses in World Golf Village, Murabella and Palencia, the most common zoning call we get.
- Bungalows in Nelmar Terrace or Fullerwood Park with a rear addition that the retrofit ducts barely reach.
- Houses with a glassed-in Florida room or lanai enclosure on Anastasia Island, where one room has its own weather.
- Homes with a guest wing or in-law suite used seasonally.
- Two-story historic houses in Lincolnville where the upstairs is bedrooms and the downstairs is shut up by day.
And the ones that should not: a single-story block ranch in St. Augustine Shores with one hot room (usually a duct problem), a house already on ductless heads (already zoned by design), and a house with ductwork too fragmented to accept dampers.
What it needs from the rest of the system
Trunk lines with room for dampers, a thermostat location in each zone that is not in direct sun, a return path from every zone, and equipment that can run at part capacity. When a house has all of that, zoning is a single-day job that ends the hot-upstairs complaint for good. When it is missing two of them, we say so and quote the alternative instead.
Zoning questions from St. Augustine's two-story subdivisions
Why does closing the upstairs vents in my World Golf Village house make the air handler whistle?
Because the blower is still pushing the same volume of air through fewer openings. Pressure rises, the ducts strain at their joints, and the whistle is air forcing its way through a gap. Closed registers are a leak generator, not a zoning system.
Can zoning be added to a 1950s Davis Shores house with ceiling ducts?
Usually not well. Zoning needs trunk lines where dampers can go, and furred-in ceiling ducts rarely offer them. For that house, a second small system or a ductless head for the problem room is the practical path.
Does the glassed-in porch on our Anastasia Island house need its own zone or its own system?
If a supply trunk already serves it, a zone with its own damper and thermostat handles it. If it was enclosed after the ducts were run and gets a trickle from one register, it needs its own ductless system. A room with that much glass has a load the rest of the house does not.
Will zoning let us keep the guest wing warm for snowbird visitors without heating the whole house?
Yes. That is one of the quieter uses of zoning in a town where 27 percent of residents are over 65 and winter guests are routine: the guest wing holds 72 while the rest of the house sits at 66, and each zone’s schedule can be set independently.
Upstairs never catches up?
We'll measure the zones and tell you whether dampers or a second system is the fix.