When to best set back A/C temperatures in a house?

[reader2580]

I currently run my A/C at 74 degrees when at home and at 78 degrees when at work. I set to 78 at 6 am and set it to 74 at 5 pm. The thermostat starts running the A/C prior to 5 pm in hopes it will be at 74 by 5 pm. I'm not convinced this is the best schedule since 4 pm to 5 pm is considered a peak time for electricity use, but I do not have TOU rates at this time. Would it be better from a grid perspective, and heat loading, to set the temperature to 74 at 3 pm or 4 pm instead?

An added complication is I have solar so most of the time I'm producing much more electricity than I use even a 4 pm or 5 pm. On the other hand, I'm probably better off sending that excess power to my neighbors during the peak period and get paid for it by the utility.

 

[karoc]

78 down to 74, unless your home is huge or has upstairs it wouldn’t take very long pull temp down to 74. All base if your AC is size correctly.

 

[reader2580]

78 down to 74, unless your home is huge or has upstairs it wouldn’t take very long pull temp down to 74. All base if your AC is size correctly.

It isn’t the length of time so much as what time of day should I be using the electricity to cool my home down? My A/C seems to be properly sized, but it could take 45 minutes to drop the temperature four degree. My A/C never has issues keeping up at the highest temperatures. It is important not to oversize in Minnesota as there are cooler days when you still want humidity control.

 

[danski0224]

Pick a temperature and leave it alone.

 

[bobg03]

Pick a temperature and leave it alone.

Agreed, I leave the AC on 72 from the day I turn it on and that was about 4 weeks ago, if it's cooler like it was last night and this morning it doesn't run.
Usually in October I flip it off and house stays comfortable til I put the heat on at 65 late December/Early January and it runs if needed.
By late February I'm off again til I need AC.

My now deceased wife use to turn the heat and AC up and down based on her perceptions of saving and the bills were wild at times. Nowadays In the extreme heat or cold I am about $160, when I'm in the neutral seasons it's about $55 monthly. When she controlled it the heat of summer bills and cold of winter bills were $225+.
My house is very comfortable at 72 degrees when it's 110 degrees and 90% humidity.

 

[reader2580]

Pick a temperature and leave it alone.

Doesn’t that just waste electricity when nobody is home?

 

[Just_Steve]

I have a split ranch and at night the upstairs is 68 for bedtime but 78 during the day when nobody is in the bedrooms, downstairs is 75 at bedtime but 72 during the day. I have solar so even with a variable speed pool pump that runs 24 hrs a day I am ahead on generation Vs use in the cooling season. I think the power company hates people like me.

 

[danski0224]

Doesn’t that just waste electricity when nobody is home?

Things that warm up now have to be cooled down.

And the AC recovery period would be during the warmer part of the day.

I don't have any empirical data, but I really don't see how temperature setbacks save any energy, especially with forced air cooling. Maybe forced air heating, depending upon the amount of setback.

 

[AA/FC]

78 down to 74, unless your home is huge or has upstairs it wouldn’t take very long pull temp down to 74. All base if your AC is size correctly.

In my old house with zero wall insulation, my A/C unit (which IS sized correctly for my SQ Ft) can BARELY maintain any given temp on hot days even when the unit runs constantly. If I let my house get 4 degrees warmer than I desire, I will never get those 4 degrees back, until outside temp falls low enough, for long enough, to allow my A/C to recover. Thankfully I have a small house.... and because my A/C runs most of the time on hot/humid summer days, my house is usually nice and dry.

 

[AA/FC]

Doesn’t that just waste electricity when nobody is home?

Keeping your home at a steady temp while you're at work probably doesn't take much more power than setting the temp 4 degrees higher for 8-10 hours. Compare the power it takes to maintain the temp all day, VS the amount of power it takes to lower the temp back down before you get home. In a nice, tight, modern house that is properly insulated, it probably wouldn't take much to lower the temp, but in an old leaky house with poor insulation, it can take a lot of power to get the temp back down. Every situation is different. Is your house well insulated, or old and leaky like mine?

 

[Wrench97]

Late November early December is the time to set the A/C tmps back.

 

[reader2580]

Nice tight house that is reasonably well insulated. Both my heating and cooling bills are pretty low.

The real question is should I change the time I switch the temperature from 78 to 74 from 5 pm to something earlier like 3 pm? I suspect I should not be dropping four degrees during high heat and peak electricity usage.

 

[fitter30]

Call your electric company ask if they offer a energy audit a blower door test tells how tight the house is.

 

[BillK]

We have it turn down from 75 to 72 at about 10PM. We both like it cool to sleep and by 11 it is fine. Turns back up at 6 AM 75 is plenty cool during the day. We really didnt see any noticeable savings in the bill when we got the programmable thremostat. Just didnt have to mess with turning it down and up manually.

Most HVAC guys will tell you to leave it alone. I have two thermostats at the shop. One for the heat and one for the AC. I dont think I have touched either one in the last 5 years. If it gets cold the heat comes on. If it gets hot the AC starts

 

[jblnut]

We built a SIP home a few years ago and I put an Emporia Vue on each panel on the property and I messed with setback temps and such. I noticed very very little actual energy savings setting the temp warmer when we weren’t home vs just leaving it one constant temp.

I actually do the opposite with my heat and set it higher during the day when we’re not home as I can fill the outdoor boiler during the day and the house acts like a big heat sink and it doesn’t hammer on the stove as hard when the sun goes down.

 

[chinboys]

How well your home is insulated, how many air exchanges per hour, and solar radiation heat gain will determine what set back temperature you will want.
And this is relative to the high and low temperature of each day too.
High levels of heat and humidity are driven into areas of lower levels relative to your insulation value and humid air leaking in.
Your AC system will first reduce the humidity level before lowering the temperature if properly designed (manual J, blower fan speed, and high flow ducts, correct amount of refrigerant, correct to age of cooling, and etc).
Hence, I would prioritize on latent cooling... removing the humidity then lowering the temperature... Sensible cooling as a factor to deciding what the set back temperature is.
In my experience, I try to set my set back temperature to be 10 to 15 degrees lower than the peak day temperature. And I have the system begin its dehumidificatiin and cooling 2 hours before I get home, around 7PM. On the days home from work, I run the fan continuously and set my temperature 12 to 20 degrees cooler than the high temperature of the day. I control these temperatures via my smartphone to a wifi thermostat.

 

[bobg03]

Doesn’t that just waste electricity when nobody is home?

The hottest part of the day (normally) is from 1400 hours on to sunset.
My deceased wife use to turn it up during the day, and then come home at 1500 to 1700 hours and turn it down. I'm in the same house alone and the electric bills were always over $220 in the summer.

My bills since she passed almost two years ago, have never gone over a $170 a month for heating or cooling (I only have an HVAC unit & mini-split in garage, no furnace). My house is almost 10 years old and it was built for us on our property. That $170 includes the mini-split in the garage, she hated to run it. I had to have a good reason as the utility bills were paid by her. I run it when I want now and pay less..YMMV

 

[Jeepster04]

I wouldn't mess with it for such a short period of time.

Sometimes I'll change mine but I have hills on the east and west side of me. I wont turn the A/C back down till the sun is over the hill behind my house. It makes a huge difference.

On the other hand, my parents house gets pure sun all day. Changing the A/C to anything would be useless since it heats up so quick in their house. Might keep the A/C from running for an extra 30minutes, then it will run all day no matter what. That unit may be slightly undersized.

 

[niget2002]

The only way we save money adjusting the temperature is if I wait until after dark to lower the temperature. We like to sleep with it cooler, so I lower it at dark and let it warm up around 7am. I work from home and don't mind warmer temps.

 

[Jim greengo]

I set mine on 1 temp,and leave it there.

 

[BrandonV]

In my old house with zero wall insulation, my A/C unit (which IS sized correctly for my SQ Ft) can BARELY maintain any given temp on hot days even when the unit runs constantly. If I let my house get 4 degrees warmer than I desire, I will never get those 4 degrees back, until outside temp falls low enough, for long enough, to allow my A/C to recover. Thankfully I have a small house.... and because my A/C runs most of the time on hot/humid summer days, my house is usually nice and dry.

Yup. Too many factors to give a "time" to the OP.

Similarly if I move my thermostat from 78F at any time during the summer it's not going to recover until well after the sun goes down.

 

[theoldwizard1]

Personally, I would set the "at home" temp for 70 and start at 3 PM.

My local PoCo made it "very expensive" to install any system that feeds excess power to the grid by charging a large amount for permitting, review, and second meter installation.

Plus, they now "buy back" at the "commercial" rate, not the "residential" rate. Considering how little annual sunshine we get in SE MI, it is not worth the cost. Probably better to go "hybrid". (Solar cells charge batteries; inverter provides power to the house; PoCo kicks in when batteries are low. PoCo can also recharge batteries during "off peak" hours.)

 

[AA/FC]

I set mine on 1 temp,and leave it there.

Yep, that's what I do....

And it's what I would do even if I didn't have a leaky old house. lol.

 

[reader2580]

I wish I had a way to monitor how much power just my central A/C uses so I could experiment with different temperature settings and see the impact they make on power usage. I know there are power monitors I could buy, but they tend to be expensive for a 240 volt hardwired appliance.

 

[jlv03]

Most of the smart thermostats will tell you total runtime. It would be easy to figure out % of total by taking runtime x ratings on the unit and subtract from your monthly bill.

When I installed my first Nest thermostat (when they first launched and you had to pre-order it), there were big numbers claimed on energy savings. I didn't see anything close, maybe I did save a bit. But what I gained was a bunch more comfort. Setting back during the day when no one was home meant I could feel a bit better about getting the exact temp I wanted when I was home.

 

[BrandonV]

Most of the smart thermostats will tell you total runtime. It would be easy to figure out % of total by taking runtime x ratings on the unit and subtract from your monthly bill.

When I installed my first Nest thermostat (when they first launched and you had to pre-order it), there were big numbers claimed on energy savings. I didn't see anything close, maybe I did save a bit. But what I gained was a bunch more comfort. Setting back during the day when no one was home meant I could feel a bit better about getting the exact temp I wanted when I was home.

The problem is I doubt they show the runtime for a multi-stage system however. That could be a big difference in terms of power.

 

[u3b3rg33k]

Nice tight house that is reasonably well insulated. Both my heating and cooling bills are pretty low.

The real question is should I change the time I switch the temperature from 78 to 74 from 5 pm to something earlier like 3 pm? I suspect I should not be dropping four degrees during high heat and peak electricity usage.

you probably won't be, even if you try.

We have it turn down from 75 to 72 at about 10PM. We both like it cool to sleep and by 11 it is fine. Turns back up at 6 AM 75 is plenty cool during the day. We really didnt see any noticeable savings in the bill when we got the programmable thremostat. Just didnt have to mess with turning it down and up manually.

Most HVAC guys will tell you to leave it alone. I have two thermostats at the shop. One for the heat and one for the AC. I dont think I have touched either one in the last 5 years. If it gets cold the heat comes on. If it gets hot the AC starts

yeah, your average "properly sized" system is going to be able to pull down temp once the heat load is gone. at night.
I used to pre-cool on forecasted hot days with TOU. often got a good couple hours of on-peak without the AC running.

Personally, I would set the "at home" temp for 70 and start at 3 PM.

My local PoCo made it "very expensive" to install any system that feeds excess power to the grid by charging a large amount for permitting, review, and second meter installation.

Plus, they now "buy back" at the "commercial" rate, not the "residential" rate. Considering how little annual sunshine we get in SE MI, it is not worth the cost. Probably better to go "hybrid". (Solar cells charge batteries; inverter provides power to the house; PoCo kicks in when batteries are low. PoCo can also recharge batteries during "off peak" hours.)

you got WE/WEC? sounds like it.

I wish I had a way to monitor how much power just my central A/C uses so I could experiment with different temperature settings and see the impact they make on power usage. I know there are power monitors I could buy, but they tend to be expensive for a 240 volt hardwired appliance.

check out "Sense". not too spendy, pretty useful.

 

[theoldwizard1]

you got WE/WEC? sounds like it.

DTE

 

[Snapped-off]

I just leave mine set on 74. It's hard to get an accurate number month to month on how little could be saved by fluctuating. It's peanuts.

 

[bonneyman]

When customers asked me what temperature to set the thermostat back (say later digital stats), I'd tell them 5 degrees or so. If you set the time to come on 30 minutes before you arrive (like have it warmer all day and then have it come on when you leave work so it'd be the right temp when you get home) I found that most systems could do that. Your commute could vary - adjust it assuming a 2 degree drop every 10 minutes. That should get you in the ballpark.

 

[Old Man Roger]

When customers asked me what temperature to set the thermostat back (say later digital stats), I'd tell them 5 degrees or so. If you set the time to come on 30 minutes before you arrive (like have it warmer all day and then have it come on when you leave work so it'd be the right temp when you get home) I found that most systems could do that. Your commute could vary - adjust it assuming a 2 degree drop every 10 minutes. That should get you in the ballpark.

But does it save run time/money?

 

[bonneyman]

But does it save run time/money?

I have found that if you "set and forget" the stat, over time the cycles will even out and you could see a savings of 5-8%. But that's the kicker - so many people keep fidgeting with the digital stats. They were constantly changing the temps up or down during the day as they go about their activities, which totally screws up the learning function of the stat in try to customize the cycles to maintain temps with a minimum of unit running. Over time the stat would "learn" that your particular system didn't need to start 30 minutes before you got home to cool it to the temp setting you had. It might only take 22 minutes. So it would alter it's programming to accommodate this data. Over time, those 8 saved minutes of run time per day add up. As I recall Honeywell called this "adaptive recovery".
Alot of customers were sold expensive digital stats and didn't realize the savings they were promised, simply because they didn't change their behavior. I mean, it doesn't take too many unplanned compressor start-ups to erase the minute electrical savings of the day.

 

[gba2331]

I wish I had a way to monitor how much power just my central A/C uses so I could experiment with different temperature settings and see the impact they make on power usage. I know there are power monitors I could buy, but they tend to be expensive for a 240 volt hardwired appliance.

I think the Empora Vue can monitor 240v

240V (double-wide breaker) installation? – Emporia Community

community.emporiaenergy.com

 

[InsanePyro]

74? In the summer? Are you a frog or lizard? Cuz you apparently live in a terrarium

 

[Old Man Roger]

74? In the summer? Are you frog or lizard? Cuz you apparently live in a terrarium

76 inside when it’s 98 out is pretty cool.

 

[bobg03]

74? In the summer? Are you frog or lizard? Cuz you apparently live in a terrarium

I tend to like it on the cool side, but I have discovered that by leaving the house at 72 when I put it on in April keeps me very comfortable til sometime in October when I turn it off. We ain't reached hells front porch just yet but it's coming, I have had plenty of 90 degree days with 85% humidity and it's very comfy indoors. I do not like it too hot, but I must be comfortable to sleep.

 

[75gmck25]

Where you set the A/C temp for comfort depends on how hot it is outside.

When I lived in San Antonio our daytime setting was usually 78 degrees, since the outside temp was usually close to 100 degrees and a 22 degree differential made it comfortable. IIRC, we set it back to 75 degrees at night.