Hello.
I live in NY.

I am wondering if it is better to leave a central a/c unit running during
the day when no one is home, either at the normal temperature or at a few
degrees warmer -or- use a set-back thermometer when leaving for work in the
morning and have the A/C unit start up again an hour or 2 before coming home
later in the afternoon?

I use a set-back thermometer and it takes my unit 3 hours of continuous
operation to bring the temperature down 8 degrees.

Some people have told me that it makes more sense, energy wise and cost
wise, to not set the thermostat warmer in the morning and have the unit
struggle for hours on end in the afternoon trying to get back to the
original comfortable temperature. Instead they say to leave the A/C on as
it would probably cost the same if not less to periodically cool an already
cool house instead of cooling a house that is not cool at all.
Con Edison says to turn off the A/C when no one is home but I think they
refer to window units (as they also say to turn the AC back on again via
auto-timer a half hour before returning - a half hour would do nothing for
me)

Thoughts?

Thanks,
Walter

Sorry - set-back THERMOSTAT...

Walter

"Walter Cohen" wrote in message
...
Hello.
I live in NY.

I am wondering if it is better to leave a central a/c unit running during
the day when no one is home, either at the normal temperature or at a few
degrees warmer -or- use a set-back thermometer when leaving for work in
the morning and have the A/C unit start up again an hour or 2 before
coming home later in the afternoon?

I use a set-back thermometer and it takes my unit 3 hours of continuous
operation to bring the temperature down 8 degrees.

Some people have told me that it makes more sense, energy wise and cost
wise, to not set the thermostat warmer in the morning and have the unit
struggle for hours on end in the afternoon trying to get back to the
original comfortable temperature. Instead they say to leave the A/C on as
it would probably cost the same if not less to periodically cool an
already cool house instead of cooling a house that is not cool at all.
Con Edison says to turn off the A/C when no one is home but I think they
refer to window units (as they also say to turn the AC back on again via
auto-timer a half hour before returning - a half hour would do nothing for
me)

Thoughts?

Thanks,
Walter

Starting and stopping are not as efficient as running continuously. AC
units are designed to run continuously. Also the house will gain less heat
during the day as it is warmer so you will be removing less total heat.
When the house is unoccupied it will have no moisture source vs. when you
are home.



"Walter Cohen" wrote in message
...
Hello.
I live in NY.

I am wondering if it is better to leave a central a/c unit running during
the day when no one is home, either at the normal temperature or at a few
degrees warmer -or- use a set-back thermometer when leaving for work in
the morning and have the A/C unit start up again an hour or 2 before
coming home later in the afternoon?

I use a set-back thermometer and it takes my unit 3 hours of continuous
operation to bring the temperature down 8 degrees.

Some people have told me that it makes more sense, energy wise and cost
wise, to not set the thermostat warmer in the morning and have the unit
struggle for hours on end in the afternoon trying to get back to the
original comfortable temperature. Instead they say to leave the A/C on as
it would probably cost the same if not less to periodically cool an
already cool house instead of cooling a house that is not cool at all.
Con Edison says to turn off the A/C when no one is home but I think they
refer to window units (as they also say to turn the AC back on again via
auto-timer a half hour before returning - a half hour would do nothing for
me)

Thoughts?

Thanks,
Walter

In your case it is a toss-up. All of my units are so oversize, it is
like when you check into a motel and in an hour it is livable. So in my
case a set-back works well. I have one thermostat that learns, if it
runs too much to make its goal, it will start earlier the next day
regardless of the set time for run up to normal. That one is a
Honeywell I believe.

I have one of those "learning" thermostats that will turn on the air (or
heat) ahead of time. So if I program it to be 70 degrees at 5pm, it will
turn on ahead of time so that at 5 pm it is 70 degrees. However its been so
humid here lately that I have been running it constantly.

S

"Jeff" wrote:

Starting and stopping are not as efficient as running continuously.

While its true that starting a compressor takes more energy than running it,
that surge of extra use only lasts for a second or two. Most residential
compressors are sized to meet the max load and it most definitely costs more to
run them continuously than to run as required.

High efficiency ACs have two small compressors. They run only one unless higher
cooling is required. This is the only time running continuously or near
continuously makes sense, but most people don't have these types of AC.

AC units are designed to run continuously.

Again, we're not talking about whether the compressor can handle it, we're
talking what is the most efficient. See above.

Also the house will gain less heat during the day as it is warmer so you will be removing less total heat.

Excuse me? That statement makes no sense. The issue is cooling loss from inside
the house. As the interior temps rise, there's less cooling loss. By not running
the AC during the day, you aren't losing cool air through windows, doors and
insulation gaps which gets replaced with warm air, which has to be cooled again.

When the house is unoccupied it will have no moisture source vs. when you
are home.

Which has nothing to do with AC efficiency.

For the real scoop from the DOE:

http://www.eere.energy.gov/consumer/.../mytopic=12720

Walter Cohen wrote:
Hello.
I live in NY.

I am wondering if it is better to leave a central a/c unit running
during the day when no one is home, either at the normal temperature
or at a few degrees warmer -or- use a set-back thermometer when
leaving for work in the morning and have the A/C unit start up again
an hour or 2 before coming home later in the afternoon?

I use a set-back thermometer and it takes my unit 3 hours of
continuous operation to bring the temperature down 8 degrees.

Some people have told me that it makes more sense, energy wise and
cost wise, to not set the thermostat warmer in the morning and have
the unit struggle for hours on end in the afternoon trying to get
back to the original comfortable temperature. Instead they say to
leave the A/C on as it would probably cost the same if not less to
periodically cool an already cool house instead of cooling a house
that is not cool at all. Con Edison says to turn off the A/C when no one
is home but I think
they refer to window units (as they also say to turn the AC back on
again via auto-timer a half hour before returning - a half hour would
do nothing for me)

Thoughts?

Thanks,
Walter

I fear your installation is one that is not easy to say. I would guess
that there will not be a great deal of difference in the results either way,
some days you may win and some days you may loose. If it were me, I would
(and do) use a setback thermostat. Even setting it back a few degrees will
help Turning it off or allowing too much of a setback could cause it to run
for a long time during the hottest part of the day when it is less efficient
wiping out all or some of the savings of setting it back.

--
Joseph Meehan

Dia duit

On Mon, 17 Jul 2006 20:04:18 -0400, "Walter Cohen"
wrote:

Hello.
I live in NY.

I am wondering if it is better to leave a central a/c unit running during
the day when no one is home, either at the normal temperature or at a few
degrees warmer -or- use a set-back thermometer when leaving for work in the
morning and have the A/C unit start up again an hour or 2 before coming home
later in the afternoon?

I use a set-back thermometer and it takes my unit 3 hours of continuous
operation to bring the temperature down 8 degrees.

Some people have told me that it makes more sense, energy wise and cost
wise,

CAn they explain that?

Your home loses cold (admits warmth), right? If it didn't you would
only have to cool it off once and you wouldn't have to run the AC
again until you opened the door to leave. So when would the warmth
on the outside most affect the cold on the inside? When it's the most
cold on the inside, right? If it were the same temperature on the
outside and inside, you wouldn't lose any cold, right, and the bigger
the difference in temperature, the more you lose.

to not set the thermostat warmer in the morning and have the unit
struggle for hours on end

The AC does not struggle. You're anthropomorphizing the AC. It just
does what it normally does, at the rate it normally does it.

For you it would be harder to somehow cool off your house if it were
hot in the house when you were doing it, because you get sweaty, but
it doesn't. If anything, it's easier for the evaporator to vaporize
the freon when there is plenty of heat in the house (I don't really
think it is easier. I think it is the same.)

OK cars sometimes struggle and they are not animals. But it's not
like a car that doesn't run well struggling to get up a hill**. The
AC isn't trying to go from 85 to 70 in one minute, and spinning it's
compressor harder than it ever did before, saying "Yes, I can. Yes, I
can." like a car might try to climb 2000 feet in one minute. The AC
just does what it does at the rate it does it.

**By which I mean backfiring, engine missing, faltering. A) I don't
think that happens anymore since there is computer controlled
ignition, and B) when it did happen it was because the car was being
told to do more than it could do. All that would be necessary is to
downshift to a lower gear, or if there were no lower gear, there
should have been, and the car wouldn't struggle, but it wouldn't get
to the top of the hill in the one minute allowed. ACs don't have 1st,
2nd, 3rd gear because there is afaik nothing parallel to that in the
AC world, and because they don't need them. They just cool as much as
they are designed to cool. It's like one workman who is told to
shovel the sand out of a child's sandbox, and another who is told to
shovel the sand out of an Olympic size swimming pool (open at one
end.) If the second guy goes at the same pace as the first guy, he
won't struggle anymore than the first guy does. That's what your AC
does. When the second guy feels frustrated, it's only because he can
think and he knows he won't be finished for a long long time. The AC
doesn't know from that.

in the afternoon trying to get back to the
original comfortable temperature. Instead they say to leave the A/C on as
it would probably cost the same if not less to periodically cool an already
cool house instead of cooling a house that is not cool at all.

Let's extend that theory. Say you went away for a year. Doesn't
their theory say that it would cost the same if not less to
periodically cool an already cool house instead of cooling a house
that is not cool at all.

I don't think these people can explain why they are right. I think
they are just speaking intuitively and their intuition is wrong.

Con Edison says to turn off the A/C when no one is home but I think they
refer to window units (as they also say to turn the AC back on again via
auto-timer a half hour before returning - a half hour would do nothing for
me)

They are not just talking about window units.

And why would window units be different from central AC?

Thoughts?

Do you live in a house or apartment. How much insulation do you have
in your ceiling and elsewhere? These have maybe nothing to do with
your question, but they are 3 of my thoughts.


Thanks,
Walter

mm wrote:


Your home loses cold (admits warmth), right? If it didn't you would
only have to cool it off once and you wouldn't have to run the AC
again until you opened the door to leave. So when would the warmth
on the outside most affect the cold on the inside? When it's the most
cold on the inside, right? If it were the same temperature on the
outside and inside, you wouldn't lose any cold, right, and the bigger
the difference in temperature, the more you lose.

I believe you and Griswold got it right; so many don't.

The only thing I would add, which complicates things, is a
consideration of what time you turn the AC back on when you come home.
If you turn it on during the blazing hottest part of the day, the AC
has lots of work to do when it is least efficient--working against the
outdoor heat.

wrote:

The only thing I would add, which complicates things, is a
consideration of what time you turn the AC back on when you come home.
If you turn it on during the blazing hottest part of the day, the AC
has lots of work to do when it is least efficient--working against the
outdoor heat.

Well, sort of. A/C compressors are nothing more than heat transfer devices. As
someone else posted, they aren't working harder of easier based on what the
outdoor temps are. They consume the same amount of electricity.

A little refresher course: I'm making up the numbers, but the principles hold.

An AC compressor takes a freon like gas and compresses it. Basic thermodynamics
states that when you compress a gas it heats up. Lets say its 95 outside. By
compressing the gas it goes up 15 degress to 110. If it were 70 outside, it
would go up to 85. The 15 degrees from compressing doesn't change - that's basic
thermo again.

We run the compressed gas through an outdoor radiator and blow a fan across the
radiator. That cools the compressed gas back down to outdoor temp - say 95.
Bring the gas inside and through a heat exchanger, only this time we let the gas
expand.

Basic thermo says the gas gets colder, lets say by the same amount, so the heat
exchanger temp is now 75. By blowing the 90 degree house air through a 75 degree
exchanger, the house gets cooler and the cycle repeats.

Will the house cool faster if the heat exchanger is 50 degrees (ie outdoor
temp/compressed gas is 65)? Sure. Is the compressor running less? Probably. Is
the AC working easier? No. Its working the same.

Will you run the compressor more by leaving it on all day compared to turning
the thermo up while you are gone? Absolutely, because the house isn't perfectly
insulated.

Will the house take a while to cool down? Sure - there's a certain amount of
thermal mass involved besides the air that has to be cooled. But that same mass
also acts as a brake to keep the house from heating up.

On Mon, 17 Jul 2006 22:09:17 -0400, mm
wrote:


The AC does not struggle. You're anthropomorphizing the AC. It just
does what it normally does, at the rate it normally does it.

I forgot one thing I meant to mention. I used to feel that it must be
hard to get an electronic oscillator to start oscillating, and that
eventually it would get tired doing so. Yet I've had tube and
transistor radios and tv that run for hours (or in one case, weeks, on
end, and they never seemed to get tired.

I think I thought it was hard to start them, because I wouldn't want
to oscilate. And that when they started it was sort of like a fluke
(even though, if the device wasn't broken, it always happened. Later
I learned how the circuit works and it didnt' seem amazing anymore.

"Clark W. Griswold, Jr." wrote in message
...
"Jeff" wrote:

Starting and stopping are not as efficient as running continuously.

While its true that starting a compressor takes more energy than running
it,
that surge of extra use only lasts for a second or two. Most residential
compressors are sized to meet the max load and it most definitely costs
more to
run them continuously than to run as required.

High efficiency ACs have two small compressors. They run only one unless
higher
cooling is required. This is the only time running continuously or near
continuously makes sense, but most people don't have these types of AC.


Do What? I've never seen a resi unit like that. I think Trane had one
once..

HeatMan wrote:
"Clark W. Griswold, Jr." wrote in message
...

"Jeff" wrote:


Starting and stopping are not as efficient as running continuously.

While its true that starting a compressor takes more energy than running

it,

that surge of extra use only lasts for a second or two. Most residential
compressors are sized to meet the max load and it most definitely costs

more to

run them continuously than to run as required.

High efficiency ACs have two small compressors. They run only one unless

higher

cooling is required. This is the only time running continuously or near
continuously makes sense, but most people don't have these types of AC.



Do What? I've never seen a resi unit like that. I think Trane had one
once..




I THINK that he really means TWO speed compressors. Heat pumps have
this feature too.

The only two compressor things I am aware of are ultra high end
refrigerator/freezers. Viking and SubZero have a separate compressor
for Refrigeration and for Freezing

"Clark W. Griswold, Jr." wrote in message
...
wrote:

The only thing I would add, which complicates things, is a
consideration of what time you turn the AC back on when you come home.
If you turn it on during the blazing hottest part of the day, the AC
has lots of work to do when it is least efficient--working against the
outdoor heat.

Well, sort of. A/C compressors are nothing more than heat transfer
devices. As
someone else posted, they aren't working harder of easier based on what
the
outdoor temps are. They consume the same amount of electricity.

AC compressors are nothing but a vapor pump. That's it. Kinda like the
water pump in your car.

A little refresher course: I'm making up the numbers, but the principles
hold.


Making up more than numbers.

An AC compressor takes a freon like gas and compresses it. Basic
thermodynamics
states that when you compress a gas it heats up. Lets say its 95 outside.
By
compressing the gas it goes up 15 degress to 110. If it were 70 outside,
it
would go up to 85. The 15 degrees from compressing doesn't change - that's
basic
thermo again.


Grab the discharge line of a properly charged AC compressor.

We run the compressed gas through an outdoor radiator and blow a fan
across the
radiator. That cools the compressed gas back down to outdoor temp - say
95.
Bring the gas inside and through a heat exchanger, only this time we let
the gas
expand.

Close.

Basic thermo says the gas gets colder, lets say by the same amount, so the
heat
exchanger temp is now 75. By blowing the 90 degree house air through a 75
degree
exchanger, the house gets cooler and the cycle repeats.

Again, close.

"Robert Gammon" wrote in message
y.net...
HeatMan wrote:
"Clark W. Griswold, Jr." wrote in message
...

"Jeff" wrote:


Starting and stopping are not as efficient as running continuously.

While its true that starting a compressor takes more energy than
running

it,

that surge of extra use only lasts for a second or two. Most
residential
compressors are sized to meet the max load and it most definitely costs

more to

run them continuously than to run as required.

High efficiency ACs have two small compressors. They run only one
unless

higher

cooling is required. This is the only time running continuously or near
continuously makes sense, but most people don't have these types of AC.



Do What? I've never seen a resi unit like that. I think Trane had one
once..




I THINK that he really means TWO speed compressors. Heat pumps have
this feature too.


That statement is something I can agree with. And yes, HP's can be 2 stage.

The only two compressor things I am aware of are ultra high end
refrigerator/freezers. Viking and SubZero have a separate compressor
for Refrigeration and for Freezing

Yep, but that is for 2 separate refrigeration circuits.

HeatMan wrote:
"Clark W. Griswold, Jr." wrote in message
...
wrote:

The only thing I would add, which complicates things, is a
consideration of what time you turn the AC back on when you come home.
If you turn it on during the blazing hottest part of the day, the AC
has lots of work to do when it is least efficient--working against the
outdoor heat.

Well, sort of. A/C compressors are nothing more than heat transfer
devices. As
someone else posted, they aren't working harder of easier based on what
the
outdoor temps are. They consume the same amount of electricity.

AC compressors are nothing but a vapor pump. That's it. Kinda like the
water pump in your car.

A little refresher course: I'm making up the numbers, but the principles
hold.


Making up more than numbers.

An AC compressor takes a freon like gas and compresses it. Basic
thermodynamics
states that when you compress a gas it heats up. Lets say its 95 outside.
By
compressing the gas it goes up 15 degress to 110. If it were 70 outside,
it
would go up to 85. The 15 degrees from compressing doesn't change - that's
basic
thermo again.


Grab the discharge line of a properly charged AC compressor.

We run the compressed gas through an outdoor radiator and blow a fan
across the
radiator. That cools the compressed gas back down to outdoor temp - say
95.
Bring the gas inside and through a heat exchanger, only this time we let
the gas
expand.

Close.

Basic thermo says the gas gets colder, lets say by the same amount, so the
heat
exchanger temp is now 75. By blowing the 90 degree house air through a 75
degree
exchanger, the house gets cooler and the cycle repeats.

Again, close.



And what exactly was your point heatman? While his numbers are off,
Clark's description of the process and how AC works is spot on.

When you leave the AC on, it has to work harder to keep the house cool
(versus keep the house warm). This uses more electricity.

However, if the recovery time is very long, it might be more important
to keep cool, compared to the energy saving. Since you describe it
"struggling", I'd leave it on. I'd also call a HVAC company to clean
and service the unit. Might be running very inefficently.

--

Christopher A. Young
You can't shout down a troll.
You have to starve them.
..

"Walter Cohen" wrote in message
...
Hello.
I live in NY.

I am wondering if it is better to leave a central a/c unit running
during
the day when no one is home, either at the normal temperature or at a
few
degrees warmer -or- use a set-back thermometer when leaving for work
in the
morning and have the A/C unit start up again an hour or 2 before
coming home
later in the afternoon?

I use a set-back thermometer and it takes my unit 3 hours of
continuous
operation to bring the temperature down 8 degrees.

Some people have told me that it makes more sense, energy wise and
cost
wise, to not set the thermostat warmer in the morning and have the
unit
struggle for hours on end in the afternoon trying to get back to the
original comfortable temperature. Instead they say to leave the A/C
on as
it would probably cost the same if not less to periodically cool an
already
cool house instead of cooling a house that is not cool at all.
Con Edison says to turn off the A/C when no one is home but I think
they
refer to window units (as they also say to turn the AC back on again
via
auto-timer a half hour before returning - a half hour would do nothing
for
me)

Thoughts?

Thanks,
Walter

Stormin Mormon wrote:
When you leave the AC on, it has to work harder to keep the house cool
(versus keep the house warm). This uses more electricity.

However, if the recovery time is very long, it might be more important
to keep cool, compared to the energy saving. Since you describe it
"struggling", I'd leave it on. I'd also call a HVAC company to clean
and service the unit. Might be running very inefficently.


I had a unit that ran almost constantly even with a setback thermo. It
got so bad that if more than two people were in the house (i.e.
grandkids came over), we had to go escape to the mall to get cool. AC
bill more than doubled. Time for a new unit.

Now in spite of at least 50% increase in electricity rates, house stays
at a reasonable temp and AC bill is staying close to the peak before the
old unit got REALLY bad.

Stormin Mormon wrote:
Dear Jeff,
Your answer is confusing. "less efficient to stop and start".
Well, most AC I've seen stop and start, unless you crank em down to
freezer temps.
"house will gain less heat during the day when it's warmer". Well,
when it's warmer out, the house will gain more heat, compared to night
when it's cooler.
"no moisture source". Such as through the cellar floor, or around
doors and windows?
Also you didn't say if you suggested leave on, or setback.


My newsreader chops off stuff.

I agree that starting/stopping a central AC is NOT a problem, unless the
interval between a stop and a restart is too short, i.e. hard start
conditions exist.

On balance, a house with adequate insulation and adequate air leaks
plugged will need little more than a bit of cooling and a bit of
dehumidification, depending on where the house is located. The rub is
'adequate' insulation and 'adequate' plugging of air leaks. Adequate in
one climate is inadequate in another. The concerns of someone in
Calgary Canada are quite different from those who live in Mobile
Alabama. With adequate insulation and sealing even on the Gulf Coast,
peak summertime AC expense for a 2000 sq ft home CAN be less than $100 a
month.

Another issue is that with 'adequate' sealing of air leaks, odors can
accumulate in the home, so a controlled air exchange with the outside
air then becomes necessary. AprilAire's Energy Recovery Ventilator is an
example of such a product. www.ourcoolhouse.com shows one of these in
operation with minute by minute readings of temperatures and power
consumption.

Clark W. Griswold, Jr. wrote:
wrote:

The only thing I would add, which complicates things, is a
consideration of what time you turn the AC back on when you come home.
If you turn it on during the blazing hottest part of the day, the AC
has lots of work to do when it is least efficient--working against the
outdoor heat.

Well, sort of. A/C compressors are nothing more than heat transfer devices. As
someone else posted, they aren't working harder of easier based on what the
outdoor temps are. They consume the same amount of electricity.

Sorry, not true. The compressor is a pump, pumping a fluid against some
head pressure. The simple fact, readily measured, is that this
pressure depends on outside temp. As Joe Meehan alluded to. Distilled:
high-side gauge pressure is proportional to ambient temp. Higher
pressure demands more power output of pump, thus higher power draw.

With a window unit and a "Kill-A-Watt" you can see this directly.
Excuse my ignoring the bogus physics/thermo. Rather, consider that an
a/c is pumping heat from one reservoir to another, and the cost of
doing that depends on the "vertical" (read: temperature) difference.

A little refresher course: I'm making up the numbers, but the principles hold.

An AC compressor takes a freon like gas and compresses it. Basic thermodynamics
states that when you compress a gas it heats up. Lets say its 95 outside. By
compressing the gas it goes up 15 degress to 110. If it were 70 outside, it
would go up to 85. The 15 degrees from compressing doesn't change - that's basic
thermo again.

We run the compressed gas through an outdoor radiator and blow a fan across the
radiator. That cools the compressed gas back down to outdoor temp - say 95.
Bring the gas inside and through a heat exchanger, only this time we let the gas
expand.

Basic thermo says the gas gets colder, lets say by the same amount, so the heat
exchanger temp is now 75. By blowing the 90 degree house air through a 75 degree
exchanger, the house gets cooler and the cycle repeats.

Will the house cool faster if the heat exchanger is 50 degrees (ie outdoor
temp/compressed gas is 65)? Sure. Is the compressor running less? Probably. Is
the AC working easier? No. Its working the same.

Will you run the compressor more by leaving it on all day compared to turning
the thermo up while you are gone? Absolutely, because the house isn't perfectly
insulated.

Will the house take a while to cool down? Sure - there's a certain amount of
thermal mass involved besides the air that has to be cooled. But that same mass
also acts as a brake to keep the house from heating up.

"HeatMan" wrote:

I think Trane had one once..

They still do: The XL-19i or something simliar.

http://www.signonsandiego.com/uniont...s09dulley.html

I saved about 25 percent of my summer electric bill when I used set
back during the day. That's a huge savings when you are talking
about $400 electric bills.

It started when I bought a digital thermostat that allowed me to
easily control my AC.

We like 74 for sleeping, 78 up until bedtime and 86 setback when we
are not home.

Huge savings. Don't listen to those that say otherwise. They are
lying thru their teeth.

The only other advice I can offer is that it takes about 45 minutes
to make the house livable, so use your digital thermostat to start
cooling 45 minutes before you get home from work.






On Mon, 17 Jul 2006 20:04:18 -0400, "Walter Cohen"
wrote:

Hello.
I live in NY.

I am wondering if it is better to leave a central a/c unit running during
the day when no one is home, either at the normal temperature or at a few
degrees warmer -or- use a set-back thermometer when leaving for work in the
morning and have the A/C unit start up again an hour or 2 before coming home
later in the afternoon?

I use a set-back thermometer and it takes my unit 3 hours of continuous
operation to bring the temperature down 8 degrees.

Some people have told me that it makes more sense, energy wise and cost
wise, to not set the thermostat warmer in the morning and have the unit
struggle for hours on end in the afternoon trying to get back to the
original comfortable temperature. Instead they say to leave the A/C on as
it would probably cost the same if not less to periodically cool an already
cool house instead of cooling a house that is not cool at all.
Con Edison says to turn off the A/C when no one is home but I think they
refer to window units (as they also say to turn the AC back on again via
auto-timer a half hour before returning - a half hour would do nothing for
me)

Thoughts?

Thanks,
Walter

On 18 Jul 2006 06:52:30 -0700, wrote:


Clark W. Griswold, Jr. wrote:
wrote:

The only thing I would add, which complicates things, is a
consideration of what time you turn the AC back on when you come home.
If you turn it on during the blazing hottest part of the day, the AC
has lots of work to do when it is least efficient--working against the
outdoor heat.

Well, sort of. A/C compressors are nothing more than heat transfer devices. As
someone else posted, they aren't working harder of easier based on what the
outdoor temps are. They consume the same amount of electricity.

Sorry, not true. The compressor is a pump, pumping a fluid against some
head pressure. The simple fact, readily measured, is that this
pressure depends on outside temp. As Joe Meehan alluded to. Distilled:
high-side gauge pressure is proportional to ambient temp. Higher
pressure demands more power output of pump, thus higher power draw.

With a window unit and a "Kill-A-Watt" you can see this directly.
Excuse my ignoring the bogus physics/thermo. Rather, consider that an
a/c is pumping heat from one reservoir to another, and the cost of
doing that depends on the "vertical" (read: temperature) difference.
snip

True, but not sure how much effect this would have on the equation
generating monthly electric bill. Probably not over $5.00 on the
worst month of the year.

Here is some info from the manufacturer of a small AC unit showing
increased wattage due to temp differences:

80 outside air 420 watts
95 outside air 510 watts
110 outside air 540 watts

That shows a significant increase in wattage used when the ambient
temp is higher. Certaintly not intuitive for the general
population, but good hvac guys know this and use an amp meter to
troubleshoot performance.

With myself, if it is going to be a very hot day, I'll leave my A/C on all
day. If a mild day, then I'll leave it off.

On mild days, I can come home and turn on the A/C and it will cool down the
house in not too much time. On really hot days, the A/C takes forever to
cool down the house.

Or depending on if I will be away all day and not return until 10:00 PM,
then I will leave it off. Then when I get home, I can open the doors/windows
and cool the house down quickly.

JimL wrote:
I saved about 25 percent of my summer electric bill when I used set
back during the day. That's a huge savings when you are talking
about $400 electric bills.

It started when I bought a digital thermostat that allowed me to
easily control my AC.

We like 74 for sleeping, 78 up until bedtime and 86 setback when we
are not home.


Much the same here. My thermostat is set for 83 on the second floor
with a comfort setting of 80. About a half hour before I get home from
work the thermostat switches to the comfort setting. Seeing as how heat
rises that means the ground floor is quite cool yet the up stairs is
quite comfy at bedtime.

The scroll compressor I had installed years ago has paid for itself many
times over. Runs steady but doesn't have the high draw of standard
compressors.

On Tue, 18 Jul 2006 17:27:02 -0400, Jim wrote:

JimL wrote:
I saved about 25 percent of my summer electric bill when I used set
back during the day. That's a huge savings when you are talking
about $400 electric bills.

It started when I bought a digital thermostat that allowed me to
easily control my AC.

We like 74 for sleeping, 78 up until bedtime and 86 setback when we
are not home.


Much the same here. My thermostat is set for 83 on the second floor
with a comfort setting of 80. About a half hour before I get home from
work the thermostat switches to the comfort setting. Seeing as how heat
rises that means the ground floor is quite cool yet the up stairs is
quite comfy at bedtime.

The scroll compressor I had installed years ago has paid for itself many
times over. Runs steady but doesn't have the high draw of standard
compressors.

I am so jealous.

I have an expensive Trane compressor and wish I could trade it for a
goodman scroll.

Sorry, but 45 minutes would be a joke for me. That might bring the temp
down only 2 degrees.
As I mentioned, it took 3 hours to bring the temperature down to 'livable'
conditions.

Walter
"JimL" wrote in message
...

I saved about 25 percent of my summer electric bill when I used set
back during the day. That's a huge savings when you are talking
about $400 electric bills.

It started when I bought a digital thermostat that allowed me to
easily control my AC.

We like 74 for sleeping, 78 up until bedtime and 86 setback when we
are not home.

Huge savings. Don't listen to those that say otherwise. They are
lying thru their teeth.

The only other advice I can offer is that it takes about 45 minutes
to make the house livable, so use your digital thermostat to start
cooling 45 minutes before you get home from work.






On Mon, 17 Jul 2006 20:04:18 -0400, "Walter Cohen"
wrote:

Hello.
I live in NY.

I am wondering if it is better to leave a central a/c unit running during
the day when no one is home, either at the normal temperature or at a few
degrees warmer -or- use a set-back thermometer when leaving for work in
the
morning and have the A/C unit start up again an hour or 2 before coming
home
later in the afternoon?

I use a set-back thermometer and it takes my unit 3 hours of continuous
operation to bring the temperature down 8 degrees.

Some people have told me that it makes more sense, energy wise and cost
wise, to not set the thermostat warmer in the morning and have the unit
struggle for hours on end in the afternoon trying to get back to the
original comfortable temperature. Instead they say to leave the A/C on as
it would probably cost the same if not less to periodically cool an
already
cool house instead of cooling a house that is not cool at all.
Con Edison says to turn off the A/C when no one is home but I think they
refer to window units (as they also say to turn the AC back on again via
auto-timer a half hour before returning - a half hour would do nothing for
me)

Thoughts?

Thanks,
Walter

"Walter Cohen" wrote in message
...

I am wondering if it is better to leave a central a/c unit running during
the day when no one is home, either at the normal temperature or at a few
degrees warmer -or- use a set-back thermometer when leaving for work in
the
morning and have the A/C unit start up again an hour or 2 before coming
home
later in the afternoon?

The question omitted whether better = feeling
more comfortable or better = cheaper. If you can
read your electricity meter, you can try both methods,
record meters for similar sample days, and see how
elec. consumption compares.

--
Don Phillipson
Carlsbad Springs
(Ottawa, Canada)

JimL wrote:
On 18 Jul 2006 06:52:30 -0700, wrote:


Clark W. Griswold, Jr. wrote:
wrote:

The only thing I would add, which complicates things, is a
consideration of what time you turn the AC back on when you come home.
If you turn it on during the blazing hottest part of the day, the AC
has lots of work to do when it is least efficient--working against the
outdoor heat.

Well, sort of. A/C compressors are nothing more than heat transfer devices. As
someone else posted, they aren't working harder of easier based on what the
outdoor temps are. They consume the same amount of electricity.

Sorry, not true. The compressor is a pump, pumping a fluid against some
head pressure. The simple fact, readily measured, is that this
pressure depends on outside temp. As Joe Meehan alluded to. Distilled:
high-side gauge pressure is proportional to ambient temp. Higher
pressure demands more power output of pump, thus higher power draw.

With a window unit and a "Kill-A-Watt" you can see this directly.
Excuse my ignoring the bogus physics/thermo. Rather, consider that an
a/c is pumping heat from one reservoir to another, and the cost of
doing that depends on the "vertical" (read: temperature) difference.
snip

True, but not sure how much effect this would have on the equation
generating monthly electric bill. Probably not over $5.00 on the
worst month of the year.

Here is some info from the manufacturer of a small AC unit showing
increased wattage due to temp differences:

80 outside air 420 watts
95 outside air 510 watts
110 outside air 540 watts

That shows a significant increase in wattage used when the ambient
temp is higher. Certaintly not intuitive for the general
population, but good hvac guys know this and use an amp meter to
troubleshoot performance.

Thanks for everyone sharing their ideas.

I'm no refrigeration guy, but have had a tinge of thermodynamics in my
background. With that, it makes no sense to me that the outside
temperature wouldn't be an issue. The compressor is doing work,
effectively moving BTUs from inside to outside The gas in the outside
condenser is being force to condense. This will be more difficult if
the outside ambient--and therefore condenser--temperature is higher.
The higher the outside temp, the higher the condenser coil temp and
therefore its pressure (I THINK this to be true; there may be something
about the system that says otherwise). Some people put evaporative
cooler nets (wet) around the outside condenser to lower the temps at
the condenser coil and thus improve efficiency. (That's not an option
in humid climates or places where water is at a premium.)

From just a basic thermodynamic point of view, it SEEMS intuitively
obvious that if the heat sink is at a higher temperature, the spread
between inside and outside temps is greater and therefore the system
has to put in more energy (work harder) to keep the BTUs flowing at the
same rate.

"Don Phillipson" wrote:

The question omitted whether better = feeling
more comfortable or better = cheaper.

Very good!

Although you could probably argue that for any duration greater than a few
hours, either criteria could be met by using a setback thermostat and adjust the
restart time to allow for sufficient cooldown.

On Tue, 18 Jul 2006 19:20:17 -0400, "Walter Cohen"
wrote:

Sorry, but 45 minutes would be a joke for me. That might bring the temp
down only 2 degrees.
As I mentioned, it took 3 hours to bring the temperature down to 'livable'
conditions.

Walter

You would be surprised how much reduction of humidity can be coupled
with that 2 degrees. The house would feel much cooler due to the
reduction in humidity.





"JimL" wrote in message
.. .

I saved about 25 percent of my summer electric bill when I used set
back during the day. That's a huge savings when you are talking
about $400 electric bills.

It started when I bought a digital thermostat that allowed me to
easily control my AC.

We like 74 for sleeping, 78 up until bedtime and 86 setback when we
are not home.

Huge savings. Don't listen to those that say otherwise. They are
lying thru their teeth.

The only other advice I can offer is that it takes about 45 minutes
to make the house livable, so use your digital thermostat to start
cooling 45 minutes before you get home from work.






On Mon, 17 Jul 2006 20:04:18 -0400, "Walter Cohen"
wrote:

Hello.
I live in NY.

I am wondering if it is better to leave a central a/c unit running during
the day when no one is home, either at the normal temperature or at a few
degrees warmer -or- use a set-back thermometer when leaving for work in
the
morning and have the A/C unit start up again an hour or 2 before coming
home
later in the afternoon?

I use a set-back thermometer and it takes my unit 3 hours of continuous
operation to bring the temperature down 8 degrees.

Some people have told me that it makes more sense, energy wise and cost
wise, to not set the thermostat warmer in the morning and have the unit
struggle for hours on end in the afternoon trying to get back to the
original comfortable temperature. Instead they say to leave the A/C on as
it would probably cost the same if not less to periodically cool an
already
cool house instead of cooling a house that is not cool at all.
Con Edison says to turn off the A/C when no one is home but I think they
refer to window units (as they also say to turn the AC back on again via
auto-timer a half hour before returning - a half hour would do nothing for
me)

Thoughts?

Thanks,
Walter

JimL wrote:

You would be surprised how much reduction of humidity can be coupled
with that 2 degrees. The house would feel much cooler due to the
reduction in humidity.

Yes. And wouldn't it be nice if the builders these days would put in a
vapor barrier that was worth more than the ink to print the words, so
that the moisture in the house wouldn't rise so much even when no one
is home.....

That piece of kraft paper on the face of the horribly outdated R13
insulation batt is about as good a vapor barrier as is a kitchen
colander.

It's 110 outside, and 90+% of the new homes hereabouts still have only
the R13 in the walls, improperly installed. Homebuyers don't
understand, so builders don't care, so we are stuck..

Rant off.

Bill wrote:
With myself, if it is going to be a very hot day, I'll leave my A/C on all
day. If a mild day, then I'll leave it off.

On mild days, I can come home and turn on the A/C and it will cool down the
house in not too much time. On really hot days, the A/C takes forever to
cool down the house.

Or depending on if I will be away all day and not return until 10:00 PM,
then I will leave it off. Then when I get home, I can open the doors/windows
and cool the house down quickly.

Coincidentally, I saw this article in teh paper on Thursday. The
information indirrectly from SRP, an Arizona power producer:

"Turning your unit off in the morning when you leave the house and then
waiting until you return at night to turn it back on will save you the
most money. The problem is that not only will the air in your house
have a chance to heat up, but the furniture, carpet, walls, etc. will
aso heat up. With all this heat, it will take your air conditioner
longer to bring the temperature back down to a comfortable level.
"The continuous operation of the units is actually very efficient.
Without all the starts and stops of a typical cycling unit, you're able
to run the system at its peak efficiency. However, setting the
thermostat at 85 will aso save you money (perhaps not as much as
turning the unit off), while not taking as long to recool your house.
Both options work; it's just a matter of how much discomfort you're
willing to put up with."

Sounds good to me!....