My house is on three floors, it has central air conditioning, the thermostat is on the middle level, and there's an exhaust fan on its own thermostat in the attic. When the temperature is comfortable on the middle floor, it's too warm on the top level. It's a bit cooler on the ground floor.
What's the most effective, assuming any of them is effective, of the following approaches?
Partly close the incoming air vents on the middle floor and possibly the ground floor. The theory: a relatively larger portion of cooled air will go to the top floor.
Hang a ceiling fan in the stairway between the second and third floors to push the air upwards.
Hang a ceiling fan in the stairway between the second and third floors to push the air downwards.
That is the easiest, cheapest, and can be very effective. Do that first.
You already have air moving in the stairways. Take a candle and hold it near the floor. Watch the flame as you slowly move it towards the top. It is easier to boost the hot air moving up that to boost cold air coming down. After you do step one, experiment with a simple box fan to see what benefits you get.
The most effective way is to have three systems or zones. It also may be rather expensive. I plan on going that route when I replace my system.
The problem with #1 is it may cause problems and even damage your system with too high resistance.
I would say #2 & #3 is not likely to work, but if the room arrangements are just right it may. Keep in mind that if it does push cooler air up it also means that the air pressure will be a little higher up there keeping at lest some of the cooler air from the A/C from getting up there through the existing ducts. It might make more sense to push the warmer air down to the lowest floor.
My suggestion for something that would work, would be to have a professional HVAC do the manuals (maybe just the "D" in this case.
This common situation can be improved by running the evaporator fan continuously. The fan speed for best balance between cost and effectiveness can be determined experimentally. Reducing the heat gain in the upper floors also helps.
blower motor may be adjustable or go to a bigger or faster motor or larger blower. if the blower is belt driiven often a easy pulley and belt change is all thats needed. although in the winter too much airflow can cause drafts.
First, I would check all the ducts and returns to understand what's there and make sure it's all connected. Very common to find tape that's come off and a duct that's become seperated.
Then figure out how the ducts are run and which downstairs registers are on the same ducts as upstairs. Then you can try closing off a couple downstairs registers where it's likely to do the most good in providing additional flow for upstairs.
Also, check out the size of the upstairs returns and make sure they are open. Try taking off the grills for a couple of days. I did that and it made a noticeable difference. Inspect the returns. These typically use the bays between walls for passage and some times they aren't completely open, due to poor installation and the installer having not made complete cutouts all the way along the path, etc.
I agree with the advice about possibly adding another return, or larger grill opening, if possible. Consider adding a radiant barrier in the attic, which can help.
There also are booster fans, of 2 types. One gets inserted into the duct serving upstairs in the basement and is wired into the furnace blower. The other sits over the register, has a sail switch that turns it on. I installed the first type on my system and it made a noticeable difference, though it's not a total solution.
Unfortunately, I agree with Joseph that the best solution is a multiple zone system. Many of these single zone systems would work better if the installers did the job correctly during construction. But too often there are not enough returns, supply ducts for upstairs and it's very hard to fix later.
Thanks to everyone for all the suggestions. Just to follow up:
There is a return on the third floor, at the top of the stairs.
Air is coming into the rooms quite strongly, so that isn't the problem. I lit a candle and held it over one of the vents, and the flame got brighter and roared. I held it near the return, and the flame leaned noticeably toward the return, and its smoke got sucked right on.
Strange thing: while bringing the candle closer to a vent made the flame brighter and more violent, bringing it closer to the return caused it to get weaker and weaker. Holding it steady next to the grill, I saw it barely stay alive, and finally die. Why would that be, I wonder. I would think that fast moving air would be fast moving air, whether headed into or out of a duct.
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