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
What's the most effective, assuming any of them is effective, of the
1. 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.
2. Hang a ceiling fan in the stairway between the second and third
floors to push the air upwards.
3. 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
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
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
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.
to reduce heat gain close blinds stuff like that.
do you have air returns from the 3rd floor?
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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