Most Efficient Way to Program Thermostat?

[AlwaysLearningToSave]

Has anyone studied whether it is more cost efficient to set the thermostat at a constant temperature day and night versus having the thermostat set to be cooler at night and warmer during the day?  Does it make a difference whether you are heating or cooling the house? 

As background:  I live in a relatively area where summers are hot with fairly high humidity.  We open windows to cool down at night when possible, but sometimes the night temp does not go down significantly and other times we do not want to open the windows to the 60 to 80% humidity.  Basically there is a period in the spring and fall when we take advantage of natural cooling but the house is basically sealed in the heat of the summer.  It seems intuitive to me to cool the house down overnight when the A/C won't have to fight the sun.  Then during the day we set the thermostat higher, allowing the house to warm up as much as tolerable.  In winter, the climate is often quite cold and we follow a similar pattern.  We generally set the thermostat higher in the daytime but overnight allow the temperature to drop so the furnace does not work as hard to keep it warm. 

This is all based on my intuitive hunch which is based on practically nothing and could be straight-up less cost effective than a same-temp-at-all-times approach.  Are we inadvertently making the AC and furnace run less efficiently?

The search function is not working for me right now.  If this topic has already been thoroughly examined, please feel free to point me in the right direction.

[Axecleaver]

It's more efficient to heat/cool only when you're there. Use the programmable thermostat to have your home "ready" when you arrive to maximize efficiency.

It's important to understand that the rate of heat loss (or gain) is relative to the delta in temperature. So, if it's 100 degrees outside, and you have it set to 70 degrees inside, you will gain more heat more quickly than if you have your thermostat set to 80 degrees inside. Even though you have a lot of thermal mass to overcome, it's still better to run the AC for a half hour to cool things down then it is to keep it set to 70 and have it run for ten minutes out of every thirty.

Documentation:
http://class5energy.com/blog/myth-busters-8-ideas-about-energy-efficiency-debunked/
https://campusops.uoregon.edu/utilities-services/energy-conservation-myths
http://ask.metafilter.com/270600/Cost-efficiency-Programmable-thermostat-vs-constant-temperature
http://www.slate.com/articles/health_and_science/the_green_lantern/2008/12/thermostats_tissues_light_bulbs_and_power_strips.html

[Sid Hoffman]

Your A/C will run more the higher the temperature difference is.  Here's an example:

Nighttime
------------
Inside: 75, Outside 80, Difference 5 degrees (not much work)

Daytime
------------
Inside: 75, Outside 90, Difference 15 degrees (plus direct solar gain/greenhouse effect = much more work)

For me, I also have a time of use plan where I pay more for usage from 1pm to 8pm on weekdays, but less at all other times.  In general, I set my thermostat to 78 overnight, 80 at non-peak times, and 83 from 1pm-8pm Monday-Friday.  That has helped me to reduce the total power used as well as optimize the cost by aligning the temperature changes with the electric rates.

It sounds like this is basically what you are already doing, so I would say you can keep doing what you're doing.

[AlwaysLearningToSave]

For me, I also have a time of use plan where I pay more for usage from 1pm to 8pm on weekdays, but less at all other times.  In general, I set my thermostat to 78 overnight, 80 at non-peak times, and 83 from 1pm-8pm Monday-Friday.  That has helped me to reduce the total power used as well as optimize the cost by aligning the temperature changes with the electric rates.

Interesting. I'll have to look into this to see if my electrical utility does this, too. Ive honestly not looked at a bill close enough to see if there are any differences in rates.

[dragoncar]

Turn it off

[teen persuasion]

For heating, it depends on what type of heating system you use.  Heat pumps are more efficient at maintaining a steady temp.  For more traditional heating systems, it can use less energy to raise temps above a baseline only when the house is occupied.

I have no idea about cooling - don't need AC much here, especially in my old farmhouse.

[bogart]

It's more efficient to heat/cool only when you're there. Use the programmable thermostat to have your home "ready" when you arrive to maximize efficiency comfort and/or to increase your willingness to (otherwise) make minimal use of heating/cooling.

:)

[forummm]

We have super cheap rates at night (5 cents/kWh), and it's easiest for the A/C cool when the outside air temp is low, so I run it on 72 all night, which helps keep it cool all day long. Then during the day I use it only when we're here, and never during our on-peak hours (2-7).