Saving Money With Pre-Cooling

[ano]

I've read some studies on-line that have concluded businesses can save some money with pre-cooling.  I have not seen much for residential and was wondering if anyone has tested it.
 
It works like this if you are on a electric rate plan with higher peak rates during the summer.  For example, my on-peak rates are around $0.24/KWH on Monday to Friday, from noon to 7pm. Other times my rates are around $0.08/KWH.  In addition to higher rates during noon to 7pm I also have to pay a peak demand amount for the highest demand I use in any one hour period from during the noon to 7pm at a rate of about $11 per KW.  So if my highest one hour peak is 7 KW then I pay an additional $77 for that month, even if that peak just lasted for a single hour in the month.  During off-peak rates, they don't look at peak usage.
 
All this means I pay a lot for usage from noon to 7pm.  Of course I live in Phoenix, so guess when its the hottest?  Yup. 
 
So I was reading, that if you "pre-cool" your house, you can save money.  The idea is to keep your house cold for the 8 hours before the rates go up, and this will reduce your peak usage so I thought I would try it.  At 4am I set the temps so they go down to around 72 degrees.  Then at noon, when the rates go up, I raise the temp to 81, which is my normal summer temp.  Hopefully the thermal mass of the house will keep the temps low.
 
We shall see.  Anyone else try this?  Its going to take some time to see the results.

 

[gatchel]

I would think to counter the temperature rise during the day you would have to go lower than 72 in the evening. It really depends on how much heat loss/gain your place has. Is it well insulated?
 
Some business make ice at night and then use it for cooling during the day because the theory has been proven. It sure cant hurt to try it in a residential setting. Maybe add a couple of 55 gallon drums of water to the unused space, if you have any... B)
 
Also, do you have any solar? That would also counter the load during the day.

 

[ano]

I don't have solar. What I have noticed is this. We have two AC units, and in the side of the house we aren't sleeping in, we would raise the nighttime temps to maybe 83.  Overnight, this time of year it cools down pretty good, where its maybe 70 in the morning, but I noticed that inside temps were still 81 - 82, so the thermal mass of the house is holding in the heat.  I have tried in the past to bring in outside air at night to bring down the temps, but it only has been mildly successful because I believe the thermal mass if the house is holding in lots of heat.
 
Today temps should get up to 99.  The test will be at what time does the AC first kick on? 
 
Another benefit I see.  I've noticed in people heat builds up as well.  If my body is not cooled every now and then, heat bothers me much more.  If I spend the mornings a bit chilly, my guess is that the heat will be less bothersome to me in the afternoon, so where before my temp was set at 81, I might be able to raise it to 83 and save more. Just a thought. Of course this will take the summer to determine how much, if any, my costs go down.
 

Maybe add a couple of 55 gallon drums of water to the unused space, if you have any... B)

I have thought about that. But even without that just imagine how much STUFF is in a house, eveything from tile floors to granite countertops.  There is lots of mass already.

 

[apostolakisl]

Consider that
1) Super chilling your house before 12 will make it uncomfortable at that time
2) The unit will use more electricity per btu of heat it expels from the house when it gets down to those lower temps (AC units are less efficient at moving heat as the return air temp becomes lower, all else being equal)
3) The house will pick up those btus from the outside faster because the inside/outside delta will be higher
4) It probably won't make it to 7pm without becoming uncomfortably hot even with an aggressive pre-chill.
 
Overall, I think it would be tricky to get this to work.  If you wanted to spend a bunch of money, you could put in a large insulated water tank and chill the water during off peak hours and then run that water through a heat exchanger to cool the house during the 12 to 7pm hours.  But the cost of doing that really doesn't pay off on a small scale (like a house).
 
If you have a 2 stage unit, it might be possible to program it to stay in stage 1 during those hours, forcing it to run longer and keeping the peak electricity down.  It will probably fall behind in cooling on a hot day, but it probably won't get unbearable.

 

[BraveSirRobbin]

Since you live in a low humidity climate you might want to look at a 'true' pre-cooler for your condenser units.
 
I built one for my old house and it did reduce the compressor current.  They do make commercial 'add-on' ones now.

 

[Automate]

I pre-cooled my house last summer.  Georgia Power has a optional program similar to yours where you pay about 4 times the rate on weekdays between 2:00PM and 7:00PM from June through September.  I programmed my home automation system to use a smart algorithm to determine when to run the AC.   I only ran the AC between 2 and 7 three times last summer.  Once when we were having a party at the house, once when the HA system had a hiccup and once when the outside temperature got above 104 deg.  For Phoenix 104 probably sounds mild but add the Georgia humidity and its pretty bad.
 
I had the HA system calculate a composite temperature using the following formula
40% current main floor temperature + 25% current outside temperature + 25 % forecast high for the day + 5% current attic temperature + 5% current basement temperature
When this composite temperature got above 80 deg the system would lower the cooling setpoint to 74 deg.  Worked great.

 

[ano]

I had the HA system calculate a composite temperature using the following formula
40% current main floor temperature + 25% current outside temperature + 25 % forecast high for the day + 5% current attic temperature + 5% current basement temperature
When this composite temperature got above 80 deg the system would lower the cooling setpoint to 74 deg.  Worked great.

That is interesting. I do need a way to decide when its an advantage and when not.  I don't expect to make it with a 120 degree day and not use AC, but anything I can defer is an advantage.  Savings in cost (not electrical usage) can be 20% or more.
 

Since you live in a low humidity climate you might want to look at a 'true' pre-cooler for your condenser units.
 
I built one for my old house and it did reduce the compressor current.  They do make commercial 'add-on' ones now.

I think you might be referring to systems outside that attempt to spay a mist of water in front of the outside unit in an attempt to make a bit of an evap. cooler. The theory being if you cool the outside unit, its more efficient. It sounds like a good idea, and I see units on the Internet for about $80, but my fear is minerals would form on the condenser fins, and over a period of time, it would actually be less efficient.  A thought would be to use cooler pads so the minerals would form on that instead.
 

Consider that
1) Super chilling your house before 12 will make it uncomfortable at that time
2) The unit will use more electricity per btu of heat it expels from the house when it gets down to those lower temps (AC units are less efficient at moving heat as the return air temp becomes lower, all else being equal)
3) The house will pick up those btus from the outside faster because the inside/outside delta will be higher
4) It probably won't make it to 7pm without becoming uncomfortably hot even with an aggressive pre-chill.
 
Overall, I think it would be tricky to get this to work.  If you wanted to spend a bunch of money, you could put in a large insulated water tank and chill the water during off peak hours and then run that water through a heat exchanger to cool the house during the 12 to 7pm hours.  But the cost of doing that really doesn't pay off on a small scale (like a house).
 
If you have a 2 stage unit, it might be possible to program it to stay in stage 1 during those hours, forcing it to run longer and keeping the peak electricity down.  It will probably fall behind in cooling on a hot day, but it probably won't get unbearable.

 
> 1) Super chilling your house before 12 will make it uncomfortable at that time
72 is a bit chilly, but not that bad. Its about the temps they keep most businesses in AZ.
 
> 2) The unit will use more electricity per btu of heat it expels from the house when it gets down to those lower temps (AC units are less efficient at moving heat as the return air temp becomes lower, all else being equal)
Actually I don't think so.  I start it at 4am when its the coldest outside, and that sure is easier for the AC then running when its 120 outside.
 
> 3) The house will pick up those btus from the outside faster because the inside/outside delta will be higher
That is true, and as I said, I don't expect to use NO AC, just less.  My homes insulation isn't bad.
 
> 4) It probably won't make it to 7pm without becoming uncomfortably hot even with an aggressive pre-chill.
Not when its 120 certainly. Today its going to be 99. 
 
> Overall, I think it would be tricky to get this to work.  If you wanted to spend a bunch of money, you could put in a large insulated water tank and chill the water during off peak hours and then run that water through a heat exchanger to cool the house during the 12 to 7pm hours.  But the cost of doing that really doesn't pay off on a small scale (like a house).
 
As I said, check the Internet. Others say it works to some degree.
 
> If you have a 2 stage unit, it might be possible to program it to stay in stage 1 during those hours, forcing it to run longer and keeping the peak electricity down.  It will probably fall behind in cooling on a hot day, but it probably won't get unbearable.
 
I have two brand new 17 SEER 2-stage units. So far (its April) they only have run at stage 1, but in the morning when rates are low, it does run in stage 2. They are controlled by OmniStat2's so I can pretty much pick the stage.

 

[apostolakisl]

Regarding point 2.  
 
"All else being equal".  In other words if you try to cool your house from 70 to 69, it uses more electricity than going from 80 to 79, with all else being equal. . . namely outside temp.
 
But you are correct to say that an AC unit will run more efficient when it is cooler outside, all else being equal.  
 
Now if you are asking what is more efficient, cooling from 70 to 69 when it is 80 outside, or cooling from 80 to 79 when it is 100 outside, well that is not so simple to know.
 
If you are pre-chilling to 72, well that is not much of a pre-chill.  You'll just have to do some experimenting to see how fast the temp climbs in your house when you shut your unit off.  I bet if it is 100 outside, it won't take too long for your house to get up in the 80's.
 
Also realize that pre-cooling is more effective with large thermal mass.  So if your house is full of stuff this will work better.  Like if you had a big tank of water in your living room.  But also realize that you actually have to have the house pre-chilled long enough for the stuff inside to get cold or the thermal mass of the objects will cause the air temp to quickly rise when you shut the AC off.  The air itself has very little thermal mass and quickly moves to the temp of the stuff it is touching.  Getting your foundation cooled off would be an example of something with a lot of thermal mass.  But you probably don't have an insulated foundation.
 
I think your best bet to save money is to set your compressor to not use stage 2 during the 12 to 7 pm hours.  You will keep your actual usage down during those expensive hours and keep your peak demand down.  Chilling your house to 72 several hours ahead of noon will have some impact, but I bet it is very small.
 
Trying to figure this out in theory is going to be quite the challenge.  Your best course would be to experiment.  When you have several days in a row of similar weather, try doing it different ways and see what works best as far as comfort and cost.  Of course you need to be careful about all of your non-hvac usage as well and keep it consistent from day to day.

 

[ano]

So its been a bit over 2.5 hours without AC.  It hit 99 outside. Kitchen is 76, Family Rm is 77, Office is 76, and Master is 78.  I suspect the Master doesn't have the thermal mass of the rest of the house, which is O.K. since we are not in there during the day. The AC will turn on at 82 (85 in the Master)

 

[video321]

I just started, as in 2 days ago, changing my deltas so there would be a 2 degree swing between cycles (I don't have tiered rates). When the body is cool and the humidity level is low, the house temp doesn't need to be as low - especially when you have ceiling fans running. Well, at least that's how I feel. So I figured I would set my thermostat to have to run for a 2 degree swing thus running longer and more efficiently as well as pulling out more humidity during each cycle and set my temp accordingly.
 
....We'll see how it goes!

 

[ano]

I have HAI OmniStat2s which don't let me change the "swing" but they do have a minimum ON time and a minimum OFF time.  I agree with you, the longer the better, but you get larger temp swings.  I have set my minimum ON time to 10 minutes.  I set minimum OFF time to 12 minutes, So that is a minimum cycle of 2.7 per hour.
 
My wife sometimes gets hot and cold, so I have been experimenting with switching on and off the ceiling fans in response to the temp as well.

 

[apostolakisl]

So its been a bit over 2.5 hours without AC.  It hit 99 outside. Kitchen is 76, Family Rm is 77, Office is 76, and Master is 78.  I suspect the Master doesn't have the thermal mass of the rest of the house, which is O.K. since we are not in there during the day. The AC will turn on at 82 (85 in the Master)

 
You are a lot more tolerant of being hot than I.  Personally I run 74 during the day and sleep at 70.
 
You should look at your settings on your hvac regarding stage 1/2.  Mine has all kinds of parameters for controlling when it will do stage 1 vs 2.  One of them is outside temp above "x".  It might be that your unit goes from off directly to stage 2 on a hot day.
 
How do they calculate your peak?  It might be that you can turn stuff on for short periods and keep it from being counted.

 

[az1324]

Have been doing this for a while. I've found it more useful to limit the amount of time the A/C runs per hour so for example it runs every 30 minutes for 8 minutes or so, no matter whether the temp is above or below the desired setpoint. This limits the peak demand. Combined with some precooling in the morning it works out pretty well, though for me precooling hasn't been as beneficial as hoped as the temp climbs pretty quickly. At the very end of the on-peak cycle the house is a bit warmer than ideal, but the duty cycle can be tweaked without affecting the peak demand too much. Saved at least 15% energy costs over keeping a single setpoint.

 

[ano]

You are a lot more tolerant of being hot than I.  Personally I run 74 during the day and sleep at 70.
 
You should look at your settings on your hvac regarding stage 1/2.  Mine has all kinds of parameters for controlling when it will do stage 1 vs 2.  One of them is outside temp above "x".  It might be that your unit goes from off directly to stage 2 on a hot day.
 
How do they calculate your peak?  It might be that you can turn stuff on for short periods and keep it from being counted.

80 is pretty typical for people living in AZ and accumulated to the heat. We do 74 at night which is really cold for me but wife likes it. Shell put on a blanket or coat when its under 82 otherwise. Humidity here is also very low so that helps.
 
OmniStats have lots of options for stage 1 and 2. Typically they watch how fast or slow the temp changes. I think 5 degrees per hour is the cutoff point. I have three OmniStats going through a zone controller which has more options still. 
 
My units are Trane which operate at 75% and 100%. They make a unit that has two compressors that can run 50% or 100%, but its not well suited for AZ.
 
Today AC didn't kick on until 6 pm so 6 hours of peak with no AC needed wasn't bad and it was 100 today. And I think my bedroom, set to 84 never went on.
 
I could limit the time on, but I really don't see that different than setting it to a higher temp. The only limiting I do is I won't let my two units operate at the same time. When all three of the thermostats on my zoned system are not calling for AC, I turn them OFF for 12 minutes and turn on my bedroom unit for the same 12 minutes. 
 

Have been doing this for a while. I've found it more useful to limit the amount of time the A/C runs per hour so for example it runs every 30 minutes for 8 minutes or so, no matter whether the temp is above or below the desired setpoint.

 
You may way to make it longer than 8 minutes. An AC unit can take 10 minutes before it even reaches peak efficiency. Under 12 minutes and your spending a lot of time in the low efficiency zone.

 

[az1324]

You may way to make it longer than 8 minutes. An AC unit can take 10 minutes before it even reaches peak efficiency. Under 12 minutes and your spending a lot of time in the low efficiency zone.

I should look into that, however if I run it 16 minutes every hour that doubles the time between operations to 44 minutes instead of 22 minutes meaning more temperature variation.