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Ask Mr. Electricity about
saving on cooling costs
This page is a Q&A only.
You'll probably find my tips
for saving on cooling more
useful.
Your ceiling fan advice
is all wrong.summer direction should be up not
down.cold air drops and is at the floor. you want
to bring the cool air upward.
--
Steve,
July 2008
Absolutely not. No matter which way you
spin the fan, all the air in the room is going
to get mixed. The point is that by blowing the
air directly down, it washes over your body and
cools you. If you blow the air up then you don't
get the draft, and the draft is the whole point
of using a fan for cooling.
This is ridiculously easy to test: Just sit
under the fan with the air blowing up, then
switch the fan so the air blows down, and feel
the dramatic difference. Clearly you've never
done such a comparison. You might want to do so
before insisting that I (and everyone else) is
wrong about something that has universal,
unanimous agreement.
What do you think of
the $25 homemade
air conditioner
that a Canadian engineering student devised by
running water through copper tubing coiled around a
standard floor fan?
-- Nick
Gressle, July 2006
I think Geoff Milburn is a quite clever
young lad and has a fine career ahead of him in
engineering. But even though his idea works it
won't save money on cooling costs. You're much
better off with a window unit AC.
In Geoff's open system, the cost of
electricity to make ice in your freezer for the
water bucket will approach what you would have
spent by just running a window unit air
conditioner. In his closed system, the cost of
water can easily exceed what you would have
spent to run a window unit AC. Finally, the
homemade AC not only doesn't save money, it
doesn't cool as well as a window unit AC would
in the first place.
Here's a comparison of Geoff's homemade AC to
a traditional small window unit AC.
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Geoff Milburn's
Homemade AC
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Window Unit AC
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Cost of system
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$421
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$902
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Cost of electricity, per hour
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$0.00323
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$0.064
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Cost of water, per hour
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$0.115
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n/a
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Total cost per hour
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11¢/hr.
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6¢/hr.
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Heat removed per hour
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2,000 BTU's
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5,150 BTU's
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Ease of installation
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cumbersome
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simple
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1 Geoff says it's less,
but he's not counting the cost of the
fan or the hose.
2 Sears.com
3 32 watts on low speed.
This from direct measurement of my own
floor fan. Cost assumes
10¢/kWh.
4 600 watts. 0.6 kW x
10¢/kWh = $0.06
5 Water rate of $3.34 / 1000
gl. in Austin, TX in Aug. 2006. 2
L/min. = 31.7 gallons.
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I used the lowest possible rates for water in
Austin. With Austin rates if you use more than
2,000 gallons of water per month, then you're
charged a higher rate and the cost of the
homemade AC is even higher: 25¢/hr. -- or
more than four times what it would cost to run
an easy, simple window unit AC, which would cool
more than twice as well anyway.
Of course you need to use your own local
rates for water and electricity to get a more
accurate comparison for your area, but you get
the idea.
We build 1500-1600 s.f.
homes and our customers are always asking what the
best unit is that will save them money on the
electric bills. What do you
think?-- Tomas
Martinez, Austin, TX, Dec. 2004
The way you design the home is
much more important than which AC unit
you stick in it after it's built. Since you're
building homes from scratch you have a great
opportunity to design homes for energy
efficiency. This is called green
building and it's become especially popular
over the last decade. The City of Austin has a
special green
building program with resources for
homebuilders and homeowners.
No matter what kind of home you build, the
first step in selecting the right AC unit is to
get one that's sized appropriately for the house
it's intended to cool. Units that are too big or
too small won't cool efficiently. Your AC vendor
can help you with this. There's more about the
process on the Department
of Energy's website. The second step is to
look at the SEER rating, which is a measure of
how efficient an AC system is compared to other
systems of the same size. For maximum efficiency
go with the unit with the highest SEER
rating.
Does leaving the AC on
auto save electricity? Or its the same as turning
it on and off
myself?--
Anonymous, Dec. 2004
It's the same as turning it on and off
yourself -- assuming that you turn it on and off
exactly when the auto switch would have done so.
If you don't get around to turning off the AC
for 30 minutes after it would have shut off
automatically, then you'll pay for another 30
minutes' worth of electricity. Unless you're
diligent about turning the AC off as soon as it
gets cool enough, the auto switch will save you
money because it turns off the AC right away.
Which uses less
electricity:
- Keeping the AC on
constantly.
- Having the AC on
during the day, then turning it off and opening
the windows instead in the evening when it's
cool, then turning the AC back on when we go to
bed.
The reason I ask is
that my roommates think it takes more electricity
to turn the AC off and then turn it back on
later.-- Julia
Van Voorhies, Sept. 2004
There is no such thing as a
household device needing "extra" electricity
when it's turned on. So the answer is as simple
as you would expect: The less you use the AC,
the less electricity you use. When the AC is
running you're paying for it. When it's off
you're not. So whenever you turn it off, you
save money. It's that simple.
Maybe your roommates are thinking that in the
evening when you open the windows it's still a
little warmer outside than inside, so you're
making the inside air warmer, so the AC will
have to run longer to remove the extra heat when
you do turn it back on. If so, what they're
failing to consider is that your house is having
heat added to it even if you didn't open
the windows. That's because people and light
bulbs generate heat, and because the heat from
the warm air outside will slowly migrate into
your house even with the windows closed. This
means that when you leave the AC on it's
constantly working to remove that heat.
And it definitely takes more energy to remove
that heat constantly rather than removing it
just once when you turn the AC back on later.
See the July 2002 question for more on this.
If I put a fan in the
window, should I have it blowing the cooler air in,
or the hot air
out? --
Bill, Middletown, CA, Aug. 2004
It doesn't really matter. One way vs.
the other may be slightly better for your
particular circumstances, but there's no easy
way to figure out which way that is other than
to try both ways and see which works better for
you. Even so, there probably won't be
that much difference between the better
way and the other way. What will make a lot more
difference is making sure you have at least one
other window or door open to provide
ventilation, and making sure the fan is sealed
in the window (no gaps on the left or right side
of it).
With a central air
conditioning system, will I save energy by closing
the register in an unused room? Our electric
company says that I won't. Would it harm the
central unit in any way to do
this? --
K. Barrera, Aug. 2004
This is a great question, and such a
simple question should have a simple answer, but
I'm afraid it doesn't.
The goal in closing the register in an unused
room is to make more cold air come out of the
registers that are still open. But that may or
may not happen, depending on lots of factors in
your system's design, none of which will be easy
for you to discover. Even worse, if you close
too many registers there's a chance you could
damage the system by building up too much
pressure in the ducts.
So here's my summarized advice:
I have a two-story
home. The landing at the top of the stairs does not
have a door to seal off the second floor. On the
second floor we have a guest room which we use
occasionally (and therefore does not need to be
cooled constantly) and an office room which we use
mainly in the morning and night. All of our other
activities are downstairs. There are two separate
A/C units, one on each floor. Given the usage of
the two floors, what would be the optimal setting
and most efficient usage for each
thermostat?
For our daily
activities downstairs I have the thermostat set at
78 and the upstairs one at 80. (When it gets
warm upstairs, I use the fan in the office room
too.) I am concerned about the load on the
downstairs thermostat given that I can't "seal off"
the upstairs area. We live in the coastal area of
south Texas, where it can get quite hot and
humid! Help!
-- Jaya S.
Goswami, Kingsville, Texas, June
2004
I'm having a hard time understanding
exactly what you're asking. If you're asking
whether it's okay to run just the downstairs AC
and not the upstairs AC when there's nobody
upstairs, then yes, that's fine. If you're not
using the upstairs then there's little reason to
cool it. Remember, hot air rises. I wouldn't
worry about the downstairs unit being overworked
unless it's unable to keep the downstairs cool
when the upstairs AC is off.
If you're asking about what temperature you
should set each thermostat at, only you can
answer that because only you know your own
comfort level. Just set each thermostat for your
comfort level, remembering that the lower you
set either thermostat the more energy you'll
use. That's really all there is to it. If you
want the downstairs to be cooler then it's the
downstairs thermometer you'll adjust, not the
upstairs thermometer. Same story for cooling the
upstairs. There's no magic here -- just set each
thermostat for your comfort level for that part
of the house, remembering that the lower you set
either thermostat, the more energy you'll
use.
What about installing a
thermostatically controlled fan to vent the warm
air out of the attic?
-- Guy Meades, CA, June
2004
There is no question that venting hot
air out of an attic with an electric-powered fan
will keep the attic cooler than it is now. But
that may not be the best question. The better
questions are:
(1) How much cooler will the attic be with a
powered fan vs. inexpensive passive vents?
(those things that are spun by the wind)
(2) Could you get similar savings in cooling
by simply adding extra insulation?
(3) Is the savings you get in cooling your
home from venting the attic air more than the
extra money you'll spend to purchase, install,
and power the fan?
Unfortunately the answers to these questions
depend on a lot of variables that are difficult
to process. I suggest you check with your local
utility company to see if they can perform an
analysis of your particular situation.
If your goal is to save energy for
environmental reasons and cost is not an issue,
you can install an inexpensive solor-powered
vent fan. That way even if you don't save enough
energy to pay for the solar vent fan right away,
you're still saving energy.
We have a two story
home but only one A/C unit. The downstairs usually
stays a comfortable temp in the 70's with no A/C,
but the upstairs can get up to 88, so, we just had
central A/C installed. Should we shut off the
registers to the downstairs, since the downstairs
is always cooler and doesn't need the A/C anyway?
My husband fears that not cooling the downstairs
will force the heat upstairs causing the A/C unit
to work even harder to cool the upstairs. We also
have ceiling fans in our house. How should we be
using the A/C and ceiling fans together to maximize
efficiency? Your comments and advice are
greatly appreciated. I learned a lot from your
website. --
Tina Griffith, San Marcos, CA, June
2004
Why did you install AC downstairs if
the downstairs was already cool enough without
AC? Maybe you already had a central heating
system so you just installed an HVAC unit to
operate through the existing ducts?
Yes, closing the registers in the downstairs
area might help, but maybe by not as much as you
think. And if you close too many registers you
could damage your HVAC system. Check with the
company that installed it to see what they
advise.
Assuming that they say it's okay to close the
downstairs registers, I can't see how this would
overwork the AC system in cooling the second
story because the system will still removing the
same amount of heat from the house -- the only
difference is how the heat is distributed. Since
almost all the heat was upstairs to begin with I
can't see how cooling the upstairs only would be
problematic for your system. As for how to use
ceiling fans, just turn them on and make sure
they're spinning in the correct direction,
usually counterclockwise.Good luck!
Does the fan switch on
a central AC use a lot of energy?
-- McBroom
Family, Nov. 2002
No it doesn't. It's the cooling that
uses the bulk of the energy. You can measure
exactly how much using the methods described
on this website.
For those who have been puzzled by the fan
switch, here's an explanation: If the AC is off,
then turning the fan on will simply recirculate
the inside air without cooling it. (It draws in
through the intake and blows out through the
ceiling vents as it normally does.) There's not
much advantage to this, because it doesn't make
the house any cooler, but it can help keep the
air "fresh" since it's being drawn through the
filter and it's being moved around a lot. When
the AC is on, the air's already being
circulated, so in that case there's no
difference whether the fan switch is on or
off.
I was reading about
turning the air conditioning off during the day to
save on costs. I have argued this with my fiancee
and others for so long. They believe it should run
all day to avoid the cost of trying to get the air
back to a 75 degree comfort level. My question is
this, you state turning it completely off. Is this
efficient to do in the middle of a 95 degree
summer, or should I just turn it up to around 85
degrees?
-- Kevin Work,
Houston TX, Ju/y 2002
It doesn't matter what you set your
thermostat to (before or after you get home) or
how hot it is outside -- you'll always save by
keeping the AC off when you're not home. Think
about it: Heat constantly penetrates your home,
and that's what your AC removes. If you turn on
the AC when you get home, then your AC has to
remove the accumulated heat only once. If
you leave it on during the day, your AC must
repeatedly remove heat that enters your
home.
You might think that there's no difference,
because your house should absorb the same amount
of heat either way. Not true. With the AC off,
at some point your house will be so hot that it
can't absorb any more heat. But with the AC on,
your house will always be cool enough to absorb
more heat -- and you'll be paying to remove that
heat, over and over again.
By the way, if your fiancee isn't comfortable
unless the temperature is at a meat-locker-like
75 degrees, then you should probably marry
somebody else. Thanks for writing!
How can I determine if
my bedroom circuit is capable of handling a window
unit air conditioner? It has a 20-amp
breaker.
-- Nick
Sibilia, July 2002
First you need to find out how many
amps your air conditioner draws. It's probably
listed on the AC unit somewhere, probably where
the plug goes in.
If the amps aren't listed, then it will at
least tell you the number of watts, and you can
divide the watts by 120 to get the amps. (For
example, if the AC draws 1500 watts, then
1500/120 = 12.5 amps. This assumes that your AC
is a normal 120-volt AC. If it has a normal plug
that goes into a normal outlet, it's a 120-volt
AC.)
Next, add up the amps of everything else on
your bedroom circuit, especially the lights. (If
you have a 100-watt bulb, that's 0.83 amps:
100/120 = 0.83.) (How
to find out what's on the same circuit.)
As long as you don't go over 20, you should
be fine.
Make sure you don't miss anything else that
might be on that same circuit. For example, that
same circuit might be powering TWO different
rooms.
I have ceiling fans
throughout my house. Should I keep them on all the
time in every room even when I am not home? How
does having them on or off impact my electric
bill? --
David Weber, Phoenix, AZ, 8-01
Good question. Fans don't actually
lower the temperature of the room, so it doesn't
help to leave them on when you're not home. We
added a detailed
explanation and a picture about this.
As for how using fans will impact your
electric bill, the How
much your appliances use page shows that a
typical ceiling fan uses between 55-90 watts,
while a 2.5-ton central AC system uses 3500
watts. If you have the fan on when you're home
instead of using central AC, you'll save a ton
of electricity. If you run the fan when you're
NOT home instead of running nothing, then you'll
be wasting 55-90 watts every hour the fan is
on.
Our new home has a
split-level A/C system (one unit for upstairs and
one unit for downstairs), each with a programmable
thermostat. We're downstairs during the day, so
during the day I set the thermostats to keep the
downstairs cool during the day and the upstairs
cool at night. The downstairs AC, which is bigger,
runs 3-4 hrs/day while the upstairs unit runs 7-10
hrs/day! The AC system has been checked for
leaks/fan malfunction etc. and it was given a clean
bill. Also, could I turn off the first floor unit
at nights instead of just setting at a higher
temperature?
-- Aprash,
Houston TX, 7-01
The second floor unit is probably
running more because (a) it's smaller and less
powerful than the downstairs unit, (b) hot air
rises, so the second floor unit has more work to
do, (c) the second floor is closer to the hot
roof, or (d) some combination of the above.
If your downstairs unit is larger than the
upstairs, that could have been a mistake made
during installation. It might make sense to have
them swapped; check with an AC professional to
have them analyze your situation. Another
possibility is to locate the air return for the
downstairs unit closer to the second floor. That
way you shift some of the responsibility for
cooling the second floor to the downstairs
unit.
You could certainly turn off the first floor
unit at night, but it would be more convenient
to just set a temperature so high that it
wouldn't kick in. That will save you the hassle
of having to turn it on/off every day.
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