A/C efficiency question

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TxAg
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A/C efficiency question

Post by TxAg »

Is it more efficient to leave the unit set at 75 all day and night or 80 during the day and 74 at night?
etarini
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Re: A/C efficiency question

Post by etarini »

The efficiency of your AC is a function of its design and the specific temperature parameters, but I presume you are actually asking which regimen consumes the least energy, and therefore has the lowest electric bill.

If that's the question, then let's divide the 24-hour day into day and night. It's probable that running it at 75 during the hottest part of the day will consume more energy than running it at 80 during the hottest part of the day; running it at 74 at night will consume slightly more energy than running it at 75 at night. Overall, I would expect the 75-degree round-the-clock regimen to consume the most energy, particularly if you have any kind of time-of-day surcharges for the hottest parts of the day, which corresponds to the utility's peak usage.

Just to add fuel to the fire, we live in the greater Boston area (and yes, it's gets both hot and muggy in the summer up here) and we keep the AC at 78 degrees all the time. In winter, we keep it at 72 all the time. It's a very energy-efficient house, so we can enjoy the comfort. We'd consider 74 or 75 way too cold in summer, but then we exercise outside even when it's 85-90, so we're somewhat adjusted to the heat and humidity - but still enjoy 78 degrees for sleeping.

Eric
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TxAg
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Re: A/C efficiency question

Post by TxAg »

^^^ I understand your logic but what about 85, 90, or higher inside the house? We get quite a few 100 degree days here. I wonder how much extra energy is consumed cooling the house back to 75 from 80 (or higher). I don't have a clue, but at some point there is a trade off. Hoping someone here has studied this
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magellan
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Re: A/C efficiency question

Post by magellan »

TxAg wrote:I wonder how much extra energy is consumed cooling the house back to 75 from 80 (or higher). I don't have a clue, but at some point there is a trade off. Hoping someone here has studied this
For most heating and cooling systems, you'll generally save energy with any temperature setback of any length compared to keeping the temperature constant. Some exceptions to this are heat pumps with auxiliary electric heat backup, geothermal systems, and possibly some steam heating systems. The savings is because with conventional heating and cooling systems, the energy required to bring the temperature back to normal after a setback is always less than the energy required to maintain the temperature. Also, as etarini mentioned, running the a/c more at night when it's relatively cooler vs during the day should increase savings further.

With most systems, the savings from large setbacks get compounded even more. This is because the efficiency of a heating or cooling system when running all-out at the end of a setback cycle is much greater than the efficiency of the system when short cycling to maintain a steady temperature. This effect is even more pronounced if you have an older system because most of the energy efficiency improvements of the past couple of decades have been focused on improving cycling efficiency.

This also means that if you use very large setbacks that cause your a/c to run all-out most of the time (vs maintaining temperature and cycling a lot), a decades old low SEER system may cost the same to run as a brand new super efficient high SEER system.
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Re: A/C efficiency question

Post by Mudpuppy »

TxAg wrote:^^^ I understand your logic but what about 85, 90, or higher inside the house? We get quite a few 100 degree days here. I wonder how much extra energy is consumed cooling the house back to 75 from 80 (or higher). I don't have a clue, but at some point there is a trade off. Hoping someone here has studied this
I have real-world data for my house. Normally, I leave the AC at 84F during the day, 80F in the evening, and 75F overnight (I only need it cool while I'm sleeping). For a while, I had an elderly relative staying here to recover from an illness, so I left the AC at 78F all day to keep the relative comfortable while I was at work and I still set it to 75F overnight.

My usage went from an average of 37kWh/day before the relative stayed here to an average of 47kWh/day when the relative was here. So that tells me that maintaining a steady temperature during the heat of the day eats up a lot more power on average than having to cool down by steps during certain transition points.
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Re: A/C efficiency question

Post by Valuethinker »

TxAg wrote:Is it more efficient to leave the unit set at 75 all day and night or 80 during the day and 74 at night?
As per other replies almost certainly the latter.

AC units (exceptions as Magellan notes) are efficient on long cycles, so there is a (probably small) gain of giving it a nice long run at the change in temperature and then fewer on/off cycles because it won't struggle so hard to keep an equilibrium temperature of 75.

Houses leak heat (out or in) so thermal equilibrium with the outside will always be what the house is trying to achieve, and your AC fighting that. So let it get hotter in the day, and cool it off in the evening.
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Re: A/C efficiency question

Post by Call_Me_Op »

Your question is not really an efficiency question. Your question is which approach uses less energy. Efficiency is related to the ratio of dissipated energy to delivered energy.
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etarini
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Re: A/C efficiency question

Post by etarini »

Mudpuppy wrote:I have real-world data for my house. Normally, I leave the AC at 84F during the day, 80F in the evening, and 75F overnight (I only need it cool while I'm sleeping). For a while, I had an elderly relative staying here to recover from an illness, so I left the AC at 78F all day to keep the relative comfortable while I was at work and I still set it to 75F overnight.

My usage went from an average of 37kWh/day before the relative stayed here to an average of 47kWh/day when the relative was here. So that tells me that maintaining a steady temperature during the heat of the day eats up a lot more power on average than having to cool down by steps during certain transition points.
Actually, doesn't that just tell you that running your A/C at 78 during the day instead of 84 uses significantly more electricity, irrespective of what you do at other times?

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Svensk Anga
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Re: A/C efficiency question

Post by Svensk Anga »

I endorse the idea that hotter setting during the day uses less energy. In the first year that we had programmable thermostats, our electric bill went down by $330/year. For A/C season, I programmed 85 while we were at work and 78 evenings, nights and weekends. Previously we left it at 78 all the time. We are in Florida and need very little heating, so most of that savings was over the summer. That was a pretty nice return on two $35 gizmos. I would suspect that less cycling also means less wear and tear on the AC units.
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Re: A/C efficiency question

Post by Stonebr »

TxAg wrote:Is it more efficient to leave the unit set at 75 all day and night or 80 during the day and 74 at night?
You could get a "Kill-A-Watt" meter and measure for yourself. They are not expensive, and some public libraries can loan them just like a book.
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just frank
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Re: A/C efficiency question

Post by just frank »

AC setbacks reliably save energy and improve dehumidification.
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Re: A/C efficiency question

Post by Valuethinker »

Svensk Anga wrote:I endorse the idea that hotter setting during the day uses less energy. In the first year that we had programmable thermostats, our electric bill went down by $330/year. For A/C season, I programmed 85 while we were at work and 78 evenings, nights and weekends. Previously we left it at 78 all the time. We are in Florida and need very little heating, so most of that savings was over the summer. That was a pretty nice return on two $35 gizmos. I would suspect that less cycling also means less wear and tear on the AC units.
Less cycling would mean less wear and tear though I am not sure how significant a factor that would be in service life.

What you did basically was save yourself consumption at the hottest part of the day. Since the biggest load is when the gap between air temp and desired temp is the greatest, you cut 8 hours per day out of that.
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magellan
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Re: A/C efficiency question

Post by magellan »

The other nice thing about using deep setbacks with a/c is that the system will dehumidify a lot better when running in long cycles compared to shorter maintenance cycles.

During a typical cycle, it takes the evaporator coils several minutes to reach their lowest operating temperature. During this time, cooling btus get delivered into the living areas, but with little or no dehumidification because the coil temperature is close to the dew point rather than solidly below it. The same effect occurs at the end of the cycle when the blower fan runs for a few extra minutes after the compressor stops to push that last bit of cold out of the evaporator and into the living space.
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Re: A/C efficiency question

Post by walkabout »

Some people that set their AC way back during the day and then set it back to a more cool setting after work find that the AC runs for a really, really long time trying to cool the house.

That doesn't necessarily have a lot to do with the question about efficiency, but it is something to think about if you set your AC way back and then wonder why it takes so long for your house to get to a comfortable temperature.
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Re: A/C efficiency question

Post by pshonore »

Stonebr wrote:
TxAg wrote:Is it more efficient to leave the unit set at 75 all day and night or 80 during the day and 74 at night?
You could get a "Kill-A-Watt" meter and measure for yourself. They are not expensive, and some public libraries can loan them just like a book.
I think we're talking Central AC. Kill-A-Watt requires you to "plug-in" somewhere. I've never seen a Central AC system that was not wired directly to the Service Panel. They're also usually 220V.
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Re: A/C efficiency question

Post by Mudpuppy »

wageoghe wrote:Some people that set their AC way back during the day and then set it back to a more cool setting after work find that the AC runs for a really, really long time trying to cool the house.

That doesn't necessarily have a lot to do with the question about efficiency, but it is something to think about if you set your AC way back and then wonder why it takes so long for your house to get to a comfortable temperature.
One can just determine how long it takes for their AC to achieve a comfortable temperature and program their thermostat appropriately. If you'll be home at 7:00pm and it takes half an hour, program the thermostat to go to 78F at 6:30pm. If it takes an hour, program it to drop down at 6:00pm. Then you'll only be uncomfortable if you come home early for some reason, and you can use a room fan while waiting in such a situation.
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Re: A/C efficiency question

Post by DSInvestor »

pshonore wrote:
Stonebr wrote:
TxAg wrote:Is it more efficient to leave the unit set at 75 all day and night or 80 during the day and 74 at night?
You could get a "Kill-A-Watt" meter and measure for yourself. They are not expensive, and some public libraries can loan them just like a book.
I think we're talking Central AC. Kill-A-Watt requires you to "plug-in" somewhere. I've never seen a Central AC system that was not wired directly to the Service Panel. They're also usually 220V.
I suppose one could read the electric meter every day and see if there's a difference in usage with either method. If you're billed different rates for different times, watch out. If your AC has work hard in the early evening to get back to temp, that may be exactly at a time when the rates are highest. Some utilities that have time of use billing set higher prices for early morning and evenings when demand is highest.

In areas that that time of use billing, the peak rate could be 2-3X the off-peak rate.
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Re: A/C efficiency question

Post by sharpjm »

wageoghe wrote:Some people that set their AC way back during the day and then set it back to a more cool setting after work find that the AC runs for a really, really long time trying to cool the house.

That doesn't necessarily have a lot to do with the question about efficiency, but it is something to think about if you set your AC way back and then wonder why it takes so long for your house to get to a comfortable temperature.
Part of this problem is related to heat transfer. The peak heat of the day may occur mid-afternoon, but the peak heat transferred to the inside of one's house may be several hours later due to a "thermal lag" as the heat slowly transfers through the walls/insulation of a house. This is a pretty common analysis performed by HVAC engineers. But its just one other situation that convolutes the OPs question. If one turns the AC down when they get home, the AC is trying to cool down the house when peak heat is entering the house, which makes the process take longer.
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Re: A/C efficiency question

Post by Valuethinker »

DSInvestor wrote:
pshonore wrote:
Stonebr wrote:
TxAg wrote:Is it more efficient to leave the unit set at 75 all day and night or 80 during the day and 74 at night?
You could get a "Kill-A-Watt" meter and measure for yourself. They are not expensive, and some public libraries can loan them just like a book.
I think we're talking Central AC. Kill-A-Watt requires you to "plug-in" somewhere. I've never seen a Central AC system that was not wired directly to the Service Panel. They're also usually 220V.
I suppose one could read the electric meter every day and see if there's a difference in usage with either method. If you're billed different rates for different times, watch out. If your AC has work hard in the early evening to get back to temp, that may be exactly at a time when the rates are highest. Some utilities that have time of use billing set higher prices for early morning and evenings when demand is highest.

In areas that that time of use billing, the peak rate could be 2-3X the off-peak rate.
The time of day factor could be huge on the final bill. It is not so much about kwhr used as when you use them.

The thermostat should then be used to cool 'full on' until say 4pm (when the rate jumps) and then 'off' until 7pm.

My mother is on an interruptible power tariff with the utility that lets them turn the air con off, I think for 30 mins in a 2 hour period. Hot water heating is also done that way (electrically heated hot water)-- at the control of the utility.
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Re: A/C efficiency question

Post by drawpoker »

As far as setting the temp at night (I am assuming the OP is referring to the overnight hours when they are sleeping?) does anyone have the problem I encounter in the summertime.

I guess those new windows I put in a couple years ago have made the living room (where the wall thermostat is) just too darn well-insulated. Meaning, if I set my central air for 74 (or even 72) at night, the system seems to be able to hold that temp for hours, and the blower does not come on (often enough) to give me fresh air circulating back in my BR. :(

I would constantly wake, feeling the room was "stuffy", and go out to the LR to turn down the thermostat and make the blower come on. :annoyed

Finally, I just gave up, installed a ceiling fan over the bed, set up a table fan nearby. Turn them both on "high" and now I seem to sleep "good".

Guess I am saving some $$$ this way :confused :confused
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Re: A/C efficiency question

Post by Mudpuppy »

drawpoker wrote:I guess those new windows I put in a couple years ago have made the living room (where the wall thermostat is) just too darn well-insulated. Meaning, if I set my central air for 74 (or even 72) at night, the system seems to be able to hold that temp for hours, and the blower does not come on (often enough) to give me fresh air circulating back in my BR. :(
Newer thermostats can be programmed to run the fan for x minutes every y hours to provide circulation for those who have this issue. You might also have an HVAC company come see if your duct work needs to be "rebalanced", assuming the duct work has baffles to adjust airflow.
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Re: A/C efficiency question

Post by fredjohnson »

I just keep the thermostat at 80 all the time. But, I live in MN and don't use A/C much. I bet our whole house A/C only runs 4 weeks a year at most.
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Re: A/C efficiency question

Post by deanbrew »

I don't have much to add regarding the original question. There is little doubt that "setting back" the thermostat saves money, unless time-of-day electricity rates come into play. The question becomes at what cost to comfort in terms of cooling back down and perhaps removing humidity.

I am amazed at all of the responses that mention keeping the thermostat set to 78 or 80 degrees. Wow. That is way too warm for me, though probably not for my wife. I have our thermostat set at about 74-75. I would be sweating and very uncomfortable if it were set at 78 or 80, and I don't think it would remove enough humidity at that level. I have a split system in my office, and the thermostat can only be set on even numbers. I keep it on 74, as 72 is a bit too cool, and 76 is way too warm for me. FWIW, I set the thermostat at 69 in the winter, and would bump it up a degree or so if I weren't trying to save a bit of money.
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Re: A/C efficiency question

Post by btenny »

Out in Arizona many new AC units use two stage compressors and a continuously variable speed fan. These are typically 16 SEER (or more) units. Yes they cost a bundle but they save a ton on electricity. Some people report savings of 50% or more on summer bills. This is what all the new homes and AC contractors are recommending. These units run at low fan speed and low compressor settings for hours and keep the house at a constant temperature with no hot spots. People love them. See here
http://blog.conditionedair.com/2010/07/ ... t-comfort/ and here
http://theairgenie.com/2-stage-heating-mean/

But I suspect they may work differently if you use them on a set back temperature plan. Plus many of us out west have deep discount for time of day electricity so a big surge to cool off the house at 5 PM would be crazy expensive. So the current approach is to set the temperature to your upper comfort level and let the AC run low speed all day and hold the house cool.

.
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Re: A/C efficiency question

Post by Spirit Rider »

Mudpuppy wrote:
wageoghe wrote:Some people that set their AC way back during the day and then set it back to a more cool setting after work find that the AC runs for a really, really long time trying to cool the house.

That doesn't necessarily have a lot to do with the question about efficiency, but it is something to think about if you set your AC way back and then wonder why it takes so long for your house to get to a comfortable temperature.
One can just determine how long it takes for their AC to achieve a comfortable temperature and program their thermostat appropriately. If you'll be home at 7:00pm and it takes half an hour, program the thermostat to go to 78F at 6:30pm. If it takes an hour, program it to drop down at 6:00pm. Then you'll only be uncomfortable if you come home early for some reason, and you can use a room fan while waiting in such a situation.
Or get a new thermostat. Almost all the current programmable thermostats except for the low end models have a predictive capability. You set the temperature for 75 at 6pm. It adaptively adjusts the time it comes on so the temperature will be 75 by 6pm. This is done by modeling previous behavior so it is not perfect. If the day to day outside changes are modest, it works pretty well. If you want it to work even when there is a substantial increase in outside temperature, there are models which work with an outside thermometer.
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Re: A/C efficiency question

Post by drawpoker »

Mudpuppy wrote:
drawpoker wrote:I guess those new windows I put in a couple years ago have made the living room (where the wall thermostat is) just too darn well-insulated. Meaning, if I set my central air for 74 (or even 72) at night, the system seems to be able to hold that temp for hours, and the blower does not come on (often enough) to give me fresh air circulating back in my BR. :(
Newer thermostats can be programmed to run the fan for x minutes every y hours to provide circulation for those who have this issue. You might also have an HVAC company come see if your duct work needs to be "rebalanced", assuming the duct work has baffles to adjust airflow.
Not sure, but I wonder if I should just leave the system's fan setting in "on" position at night instead of "auto".

Not sure of this, but I think that means that the blower will run continuously (circulate air all the time ) but the compressor will only come on (blow cooled air) when the need arises. Based on the temp setting on the thermostat.
I've heard of older systems being able to do this, but I am embarrassed to say I'm not really sure about mine. :confused

I have pulled out the user's manual and re-read it over and over but it doesn't seem to address this question. (Have a Kenmore brand installed in late 2006 but not a two-phase system, which I sincerely regret not getting. As, after the fact, found out two-phase technology was available at time of purchase. :annoyed
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Re: A/C efficiency question

Post by walkabout »

I don't know that there is 100% consensus about putting the AC fan in "always on" mode, but I have read a number of articles over the years that recommend against doing that. Here is one such article:

http://www.energyvanguard.com/blog-buil ... -Like-This
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Re: A/C efficiency question

Post by Mudpuppy »

drawpoker wrote:
Mudpuppy wrote:
drawpoker wrote:I guess those new windows I put in a couple years ago have made the living room (where the wall thermostat is) just too darn well-insulated. Meaning, if I set my central air for 74 (or even 72) at night, the system seems to be able to hold that temp for hours, and the blower does not come on (often enough) to give me fresh air circulating back in my BR. :(
Newer thermostats can be programmed to run the fan for x minutes every y hours to provide circulation for those who have this issue. You might also have an HVAC company come see if your duct work needs to be "rebalanced", assuming the duct work has baffles to adjust airflow.
Not sure, but I wonder if I should just leave the system's fan setting in "on" position at night instead of "auto".

Not sure of this, but I think that means that the blower will run continuously (circulate air all the time ) but the compressor will only come on (blow cooled air) when the need arises. Based on the temp setting on the thermostat.
I've heard of older systems being able to do this, but I am embarrassed to say I'm not really sure about mine. :confused
I can do that with my HVAC unit, but I don't like to leave it on the "on" position for more than an hour or two at a time. I most frequently do that in the Spring and Fall when the unit doesn't otherwise come on and I want to change the air over in the house. But I don't want to put unnecessary wear and tear on the blower by leaving it on for long stretches. On the rare occasion I've fallen asleep and have forgotten to turn it back to "auto", the unit seems to function properly. I'd just be concerned about the wear and tear cause by doing that every single night. Maybe that's an unwarranted concern.

It really isn't that hard to switch out a thermostat though, as long as you label every wire with the name of the terminal it was originally connected to and follow the directions on the new thermostat precisely. The directions should tell you how to change over the wires from the old terminal names to the new terminal names (the names might even be the same). The hardest part for me on the last switch-over was finding paint to match the wall color, as whomever the prior owner had asked to paint the walls had painted around the thermostat, so removing it revealed a patch of the prior wall color.
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Re: A/C efficiency question

Post by deanbrew »

I put my thermostat on the "fan on" mode when I first turn on the AC in the spring, or in the summer if it's been off for a day or two, to equalize the temperature throughout the house. If the AC has been off, the upstairs is hot as hades and the downstairs is much cooler, so the AC won't stay on long enough to cool the upstairs. After six or eight hours, I turn the thermostat fan switch back to "auto" and things stay OK, although the upstairs is still hotter than downstairs. As for leaving the fan setting on "on" all the time, it would certainly help even out temps, but I know there are drawbacks, as mentioned above.
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Re: A/C efficiency question

Post by bloom2708 »

Let's say it is 80 degrees outside. I have our thermostat set to "76" during the day (at least 1 person is usually home). The inside house temp starts at 72 from overnight and heats up to 76 at some point during the day. At that point the AC runs as often as needed to keep the temp at 76. Then at say 8:00 pm I bring the temp down to 73.

Does the relationship of 80/76/73 matter? What if it is 90 outside so 90/76/73? My assumption is it is "easier" for the AC when the temp is closer to the outside temp. No matter the outside temp, going from 76 to 73 takes a "long run" (goes to stage 2), but this seems to be good at removing humidity as another post pointed out.

We have a single zone, 2 stage AC, SEER 16 with a smart Ecobee3 thermostat. Our house is a two story so we have wide temperature swings between the top level (3 bedrooms) and basement (1 bedroom). It might be 79 upstairs and 69 downstairs and 75 on the main level. No real good way after the fact to add multiple zones.

I never seem to be able to pick a setting that works for very long. We have a number of "heat during the night/cool during the day" days. Then very cold and very hot days. So our house is all over the place. I have just decided to tinker with the settings and lower level vents based on the type of day.
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Re: A/C efficiency question

Post by drawpoker »

bloom2708 wrote:....never seem to be able to pick a setting that works for very long. We have a number of "heat during the night/cool during the day" days. Then very cold and very hot days. So our house is all over the place. I have just decided to tinker with the settings and lower level vents based on the type of day.
You need the kind of system that was in the house I bought around 20 years ago. The kind where the system had five settings - "off" "heat" "emerg heat" "AC" and "auto".

The auto mode I am referring to is not the auto mode for the fan switch, no, this was for system itself. The idea was to set the thermostat at 70, or 72, whatever your taste, then when placed in "auto" function the system would automatically cycle back and forth between heat & AC as needed.

I always thought this was very strange (why not just get up and do the changeover yourself - why risk more premature wear and tear on the parts - particularly with the coil having to reverse itself more times than a sidewinder snake? )

Anyway, I thought it strange, maybe I'm the only one who did. Now your post makes me curious if these type systems are still readily available for residential installations. (I can see how it might be practical in big, commercial bldgs. where a constant temp is desired and electric cost is not a big issue)
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Re: A/C efficiency question

Post by bloom2708 »

The ecobee3 thermostat I have has 3 modes. Heat only, Cool only or Auto.

The only requirement for Auto is that the desired heat temp and cool temps have to be 5 degrees apart. So you can heat to 70 and cool to 75. If you want to cool to 74, heat has to be 69 or lower. This mode does work pretty good in the spring/fall when we are in a mixed bag of heating and cooling.

Here is my big issue. When we heat for extended periods, I close all (or most) of the vents on the 2nd floor (two story with full basement) and open the 3 vents in the basement. When we cool for extended periods I close the 3 vents in the basement and open all (most) of the vents on the top floor.

This vent opening closing is what a zoned system would help to fix. The ecobee3 has 3 remote sensors and the main thermostat is a sensor. So I have one sensor upstairs, one in the basement and the third in a often used room a good distance from the main thermostat.

In the perfect system I would replace all vents with "smart vents". Each smart vent is tied to one of the temp/humidity sensors. If I am cooling and the basement sensor is already below the desired temp, the sensor signals those vents to close. The upstairs vents are open because the upstairs sensor is several degrees above the desired temp. The main floor is usually ok, but if those vents are at the desired temp, they could be closed to push more air to the top level. I know some "smart vents" exist. I actually sent this idea to ecobee when they have their suggestion times.

Theoretically you wouldn't have to replace the vent, just mount a controlled flap inside the vent that could shut off airflow when closed. Power would be an issue and also linking them to the correct sensor.

Vent open. Vent close. :annoyed
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