I am sure you'll get a quantitative answer from someone more knowledgeable.

Before I had my new system installed last year, I used to keep the thermostat set at 79 degrees, like you.

Had I increased the set point to 81 or 82 degrees it would have COST me the expense of a divorce.

Instead I got a new heat pump and oil furnace meeting current efficiency standards and now have the set point ay 75 degrees, use less electricity, and have a much happier spouse!

I posted the numbers her re my savings a month or two ago.

If you are raising the set point with the idea to lower it (recover) at some later point, you will pay, both in comfort and $. Set it, and leave it at whatever temp you decide upon.

V

Saltidog has a point when it comes to divorce costs. Not knowing the area you live in I can only say, the less the unit runs, the more money you will save. At what point does it make sense to just turn the unit off and open windows and use fans? Your comfort thresh hold would determine that. I think it is important to note that the higher the temperature, the more moisture the air will hold. To allow the temp. to rise too much would defeat the de-humidification efforts of the unit and comfort could begin to suffer.

I disagree with veesubotee. There is absolutely a savings with setbacks.

The answer to your question is hard to figure without more inputs. If you have poor insulation qualities, there is a greater savings. If you have great insulation, then heat gain from the windows becomes a greater part of your cooling load and this doesn't change with interior temperature.

Heat gain is made up of window solar gain, infiltration, interior gains and gain thru walls/attic. By lowering the delta T with a higher interior temp, you will reduce the gain thru walls/attic but not the other variables.

In a model I was doing, the wall/attic gain was about 30% of the gain. You might cut this by 20% with a 2 degree change. So then you are cutting heat gain 6% - see, not much. That actually overestimates it since that is assuming an outside temp average of 91 - which is likely higher than reality.

David, the information you provide is absolutely correct since the heat transfer rate would be slowed down. However, at the temperatures the OP is suggesting and of course depending on where he lives, I don't think the humidity would have a chance to be lowered to baseline comfort levels. To each his/her own but, those would not be very comfortable unless one sat perfectly still. Here in Maryland where it gets hot (will be close to 100 today)and very humid, going under a tree initially feels very cool however, to move around under that tree becomes very uncomfortable and the A/C at 76 or 78 degrees with much lower humidity feels great.

For me personally, you could hang meat in a room if it were as cool as I like it.

There is a savings with set backs unless one does what I think VEESUBOTEE was getting at. To allow the temperature and humidity to rise too much, the walls, furniture, etc. in the space also become warmer and also absorb humidity. To do a radical set back and then come back later for a big recovery and run the unit to cool the space down to a lower temperature would use more energy. That has already been proven, find your comfort zone and leave the thermostat alone. The initial work the unit will do is remove humidity, sending that water with the heat removed right down the drain. The unit will not cool to its optimum until that is done. I would guesstimate a 3 degree differential would be ok at the temperatures the OP is suggesting, nothing more, and only if they find that to be comfortable.

Last night, the outdoor temperature here was 71 degrees with an 80% humidity and it felt very uncomfortable. The house was at 74 degrees with a 50% humidity level and felt much better although for me, that might be too warm but, I'm not the only one here!!!.

Your points about comfort and humidity are valid, but this hand-waving statement simply is not true. There is no physical way to use more energy by setting a steeper set-back.

"To do a radical set back and then come back later for a big recovery and run the unit to cool the space down to a lower temperature would use more energy"

In a hot environment you use energy to maintain a temperature gradient against radiation, convection and conduction. In a humid environment you use energy to maintain a moisture gradient against air leaks and diffusion. If you have both, you maintain both gradients with even more energy.

Setting back the thermostat will cut your energy use because you are not maintaining either gradient. The only thing you need to do is set the timer so that it is comfortable when you get home. That can be trickier to do in an optimal fashion than with heating.

Sources of humidity and heat in a building ARE NOT just those that are caused by radiation, convection and conduction. The same with humidity, there are a whole truckload of sources from within a structure as well as without. In the case of a building, you are not just conditioning a box, there are many other caveats to be considered.

Not sure what you mean by the hand waving comments. Here we go with the condescending comments. I'm sure we can disagree on opinions without that crap.

Practical application will always prove or disprove a theory.

In theory it is scientifically impossible for a bumble bee to fly, here's why.........
In actuality, a bumble bee fly's, here's why..........

When I was single and worked nights I let the temp to rise to over 80 degrees at 3pm. and when I got off work it started working on bringing it to 77 which was midnight. I saved a fortune because it was only me and when I wanted my setback it was already cooled off outside.

Now that I am married and have people in and out throughout the day there is no way a setback would work without the big "D".

Setbacks can save a nice chunk of change in the right environment under the right circumstances. They can also cost more in the long run especially in winter.

A large number of people out there are laboring under a myth that setting back the thermostat too much costs more that it saves. It is time to kill that myth because it only applies to heat pump heating, with resistance back up, that is equipped with obsolete controls.

maryland_irisman:

Sources of humidity and heat in a building ARE NOT just those that are caused by radiation, convection and conduction. The same with humidity, there are a whole truckload of sources from within a structure as well as without. In the case of a building, you are not just conditioning a box, there are many other caveats to be considered.

Yes, there are internal sources of heat and humidity. How does that affect your setback theory? What warnings or cautions "to be considered" are you referring to?

Not sure what you mean by the hand waving comments. Here we go with the condescending comments. I'm sure we can disagree on opinions without that crap.

It was not meant to be condescending, only descriptive. When there is no supporting data or theory, it is "hand-waving."

Practical application will always prove or disprove a theory.
In theory it is scientifically impossible for a bumble bee to fly, here's why......... 
In actuality, a bumble bee fly's, here's why..........

Do you have any information indicates that the flying ability of Bombus species is not well understood? I think that you do not understand science. It may be that fixed-brain engineers that understand only fixed-wing aircraft do not understand how bees can fly, but I assure you that others do.

If practical application can prove that setbacks use more energy than not, show me the data.

Countryboymo
...They can also cost more in the long run especially in winter.

This is only true if an expensive, secondary source of heat is called upon. If smart controls are used instead of stone-hatchet thermostats, there is not a problem with huge setbacks.

general expectations----using setpoints at 78 degrees on cooling and 70 degrees on heating..... operating cost goes up by 3% to 8% for each degree lower setting in summer and for each degree higher setting in winter (depends on the type of heating system.)

Disregarding infiltration for the moment, the inside of a typical home has several sources of heat generation and moisture constantly being added into the space. Too much humidity in a home also has an effect on the building and contents in it. A typical home these days has a frost-free refrigerator and maybe a freezer. We all know how frost-free's work so it's obvious a substantial amount of heat and moisture are introduced. Evaporation from toilets, traps and houseplants adds to that. Hot water heaters, entertainment devices even if just in the standby mode, basements, crawl spaces and so on add heat and moisture. Add to that the amounts of moisture and heat added by each human, which increases according to activity. If you go from room to room, most people can identify additional sources of heat and moisture if there are any (washing machines, dryers, dishwashers, etc.).

Humidity levels play a big part regarding human comfort. Higher humidity in winter makes a space feel warmer, lower humidity in summer makes a higher temperature feel cooler. So with cooling, we want to dehumidify as well as cool. Higher temperatures in summer tend to have a high humidity content, of course depending on the climate the home is in.

The OP indicates they want to set the thermostat up to 81 degrees. He didn't indicate he was going to lower the temperature at another point. If he intends to drop the thermostat to, bring the temp. and humidity down, it's going to cost as I attempted to agree with VEESUBEE. The OP indicates he would like to know what the savings would be if he set it up to 81 and I assume left it there. That would save a bit of money but I doubt it would be comfortable and, the trade off might be a larger expense elsewhere. At that temperature, there would be a substantial amount of humidity that would remain and also be generated inside the home. When the unit starts up, most of the work by the unit will be removing moisture. The evaporator coil will be soaked with condensation. Most of the cooling will be cooling that water and most of the cooling effect will be washed away as the cold moisture drains out. Moisture will be added back into the structure from wall, furniture, carpeting, etc. Yes, there will be some cooling of the air but not as much as if the air were a bit cooler. The thermostat may be satisfied long before a substantial amount of humidity is removed or it's quite conceivable the unit will run for a long time or not shut off at all, depending on the rate of heat removal against heat infiltration and peripheral generation.

I doubt very seriously the occupants will be comfortable under that condition unless they enjoy warm humid conditions but yes, they will save a few dollars. They will also experience the effects commonly related to high humidity in the home. I say to the OP, give it a try and see what happens. Comfort levels are obviously different from person to person. The most economical setting for heating or, cooling, would be approximately 10 degrees from the outside temperatures. Infiltration would certainly be minimal on a moderately insulated building. On average, this doesn't work if it is 10 degrees outside in winter or 100 degrees in summer. Any adjustment for comfort is going to cost you.

You really have to wonder how the laws of thermodynamics keep coming up for debate.

I'm not sure where you come up with "the most economical setting ... is 10 degrees from outside temp" - wouldn't you agree that zero degrees from outside temp would be most economical?

"Infiltration would certainly be minimal on a moderately insulated building" - I'll just tell you that a manual J I just did, infiltration was the largest btu item on a new house assuming best qualities (which defaulted to .2 ACH summer). That was the largest source of heat gain.

If OP sets to 81 and he is comfortable there, more power to him. Some places would be absolutely fine with that - ie Arizona. With low humidity, 81 is quite comfortable especially with some air flow.

Maybe we should agree on a few things. Heat gain or loss is lowest when the delta T is lower. Any argument there?

Now - an a/c runs most efficiently with long run times? Right?

Next - when doing a large setback, you will create a very long run time.

Can we agree on these 3 things?

maryland_irisman

That is all great information, but you don't say why a small set-back saves more money than a large set-back. You don't say why you can no be comfortable with a large set-back as long as the the HVAC system is active for long enough for before re-occupation.

david_cary

No argument from me. I love knowing that my compressors are running for three hours straight from 4-7 PM. In the old days, my boiler had to run long enough to bring the house temps from low 50s up to 65 in the afternoon and from the high 50s in the morning. It would nice to have adaptive controls, but I did not and do not.

I'm flattered you use my very own examples to present your argument, thank you.

I think you are taking a few facts and positioning them to discredit my information. I'm flattered you would spend so much time critiquing my posts it's just unfortunate you need to pick fly specs out of pepper to fulfill your agenda.

Long run times are economical as long as they augment off times. The longer the unit runs, it's only common sense the meter is spinning. To cause the unit to run longer to recover a dramatic setback and then to continue on to now condition the space to effect a longer off time is not efficiency. Some setback does help, a large set back does not help.

My response was 2 fold, one to give my opinion as to why I don't think the OP should assume setting the thermostat up is going to give him the desired comfort with a savings of money. He can indeed have an adverse effect.

At the same time, I am agreeing with the statement VEESUBOTEE offered, find your desired temperature and leave the thermostat alone and let the system do it's job.

Huh?
I get frustrated with your viewpoint because it unnecessarily wastes energy. I encourage anyone who needs clarity on this subject to check the large body of studies that address this.

Ok, here is a response to your most recent post that is certainly not picking fly specs [sic] out of pepper as you characterized it:

Your most recent post is totally horse hockey.

Shoot, where I live, the longer the dang thing stays off, the more money I thot I was saving. Mine is set at 76 when it gets to 76, it shuts off. Must be broke. Any you boys near DFW who can come out and make that rascal run longer?

"Huh? ... check the large body of studies that address this."

Or check with windslam who expressed it with exquisite brevity.

Very interesting discussions and feedbacks. I am the OP and wanted to provide a little more info. I live in Arizona, where humidity is a non factor and is usually under 15%. My house was built in 1995 and so was the 5 ton A/C unit that came with the house. The A/C unit has had about 11 summers of use (first owners never used the house in the summer). I don't know what number insulation the house has, but it's blow in, in the attic vs thick pads. I don't change the thermostat, but rather leave it on all the time at a set temperature. For the past few days, I have had the thermostat set at 81 and so far I can tolerate it. I turned the ceiling fans on in the rooms I most frequent.

Please explain how well setbacks work out in a place that has variable electrical rates and peak charges? Like I said previously there are situations where setbacks aren't so great.

There are some electric utilities that advise to set it and forget it in the summer and will only pay a rebate on a hybrid gas furnace/heat pump system or geothermal. The cost of buying power at a peak time over top of what the utility already has allocated is stupid expensive.

If the system has a decent stat and the house is vacant 10 or so hours a day and the utility rates aren't screwy and the climate isn't horribly humid I would say by all means go for it.

The peak rates are a thing that will eventually haunt more and more of us and we will have to deal with. The huge setbacks tend to make for longer peak usage times which cost the utility even if they have plenty of generation to cover for it because that is power that they cannot sell at a peak rate.

Setbacks are great in moderation if the system is set up correctly and fairly well insulated and the occupants will be out for an extended period and the rate schedule doesn't fluctuate.

Sure - peak rates could change everything. I bet very little of the country is currently under that system but that it will probably move to that. Initially though, it will probably be simple with nighttime being cheaper. This will be for charging cars. Won't affect daytime setbacks.

Then the computations get really complicated. It might even make sense to go colder at night than desired. I know around here, if I can chill the house out at night without driving humidity up, the a/c won't need to run even at 90 outside. So with a tight house, you might actually benefit to drive it to 65 overnight just to minimize peak. Certainly you will want to make sure the a/c is running the last hour of nighttime rates.

But funny how the OP was not even talking about setbacks. I take some pride in guessing he might be from AZ.

We don't have peak rates where I live in NJ. But I assume peak rates would be charged during normal business hours, so doing a cooling temprature set back while you are at work could potentially save more money.

Do you have a link to an electric utility which advises their customers to not set back their cooling temperature? I would like to read their explanation as to why this would be the most efficient use of an AC.

Oooh, I had not thought of the rate variation complication and what a calculation problem it is. Thanks for bringing it up. We might all end up doing what some of the big electricity user do -- freeze ice at night and use it for cooling during the day.

Mike - the true peak is 3-5pm or so. So if you get home at 5:00 with a daytime setback, you have really added to the peak. In most of the summer, for most of us, you would need to run the a/c straight for a couple of hours to recover the setback.

FWIW - my utility specifically states to turn the temp up a couple of degrees during the day if you are not home.

Here in the Maryland Suburbs of Washington DC we pay quite a lot for electricity. The Time of Use Servive provides for three different rates depending on the time of day.

For Weekdays:

PEAK USAGE: 1200-2000
INTERMEDIATE USAGE: 0800-1200 AND 2000-2400
OFF-PEAK USAGE: 0000-0800

The rate for Generation ONLY is .10529 cents/Kwh Peak. .09021 cents per Kwh Intermediate. And .08315 cents per Kwh Off-Peak.

All three periods need to have the rates upgraded to include distribution, taxes, fees, etc.

As you can see, PEAK cost per Kwh is some 26% more that OFF-PEAK cost per Kwh.

All kinds of schemes come to mind... cool the house way down from midnight (0000) until either 0800 or 1200 and avoid running the A/C as long as possible after 1200. Or, keep the temp fixed except for 1200-2000 PEAK hours at which time let temp creep upwards.

Etc, etc, etc

With my new efficient system I have set the thermostat to 75 degrees and do not change it at all. This makes my wife happy and I am paying less than I did with the old system where I cooled way down at night and let the temp creep up in the afternoon.

YMMV

In NJ PSE&G avoids all confusion by charging one simple rate of 18 cents per KWH.

I use temperature set backs as much as possible.