Saving Money With Pre-Cooling

[ano]

Sounds like you proved that changing the temp a few degrees WILL save money (if done at the right times and you are on a timed rate plan). 
 
Interesting results and it does show that what you are thinking of doing works even with simple methods.  Might be even more effective if started earlier to get more heat out of all the stuff in the house, especially considering your 3x difference in power cost.

I think it becomes a question of how uncomfortable you are willing to be to save money? Mornings at 70 are pretty cool but not bad. As for savings, the jury is still out. If its 100 or lower, then I think no question, it will save some. But when hotter, its still a question. Today it was 110, so the AC could stay off noon to 4 pm, but after that, some AC is needed and it can run up to 50% of the time (currently).  If I'm forced to use much AC from 5 - 7pm, that peak could eliminate any savings, and AC's don't work great when its 110 out.
 
So all-in-all, I need to just try it a few months and cross my fingers, and see if the cost goes down, goes up or stays the same. 

 

[Automate]

@ano
 
On days it goes to 110 is it generally hotter from 12:00-2:00 or 5:00-7:00?  If its 5:00-7:00 then you may be better letting the AC run until 2:00 instead of stopping it at noon.

 

[az1324]

 

@ano
 
On days it goes to 110 is it generally hotter from 12:00-2:00 or 5:00-7:00?  If its 5:00-7:00 then you may be better letting the AC run until 2:00 instead of stopping it at noon.

 
This should answer your question:  https://www.google.com/search?q=phoenix%20hourly%20temperature
 
But it doesn't make sense to run up your demand between 4-7 if you can distribute it over the hours before.  The question is whether it is a losing battle as the rate of rise may be too much under that temperature differential to change the required runtimes significantly during the peak temperature periods.

 

[Automate]

So all-in-all, I need to just try it a few months and cross my fingers, and see if the cost goes down, goes up or stays the same. 

 
I would continue doing your pre-cooling before noon and then from noon to 7 make sure your thermostats will only use stage 1 and that only one AC unit is on at any given time.

 

[ano]

The peak temps do seem to occur around 5pm, so its a challenge.  AC is much less efficient when its 110 out than when its 80 out.  I don't believe its so much losing the cooling through the walls and windows, although that is a factor, as it is the AC having a harder time dissipating its heat. When its hotter outside, the air from the AC is warmer coming into the house, and the ducts in the attic the air travels through is also warmer.
 
So running the AC from 12 to 2pm might make sense even though the rate is higher, because the AC will work better than at 5-7pm. But will that "cool" stay long enough to make a difference? I don't know.
 
Around here, even a properly sized AC will run constantly on stage 2 when its 118 out just to maintain inside temps at 80.  That is almost a 40 degree temp differential so only the best ACs can do that. When you think about it, now it seems logical why June - August electric bills are so expensive. You need the AC the most, but its also running when its least efficient. 
 
So savings comes down to a few factors:
1) Run your AC when its most efficient.
2) Run your AC when utility rates are the lowest.
3) Minimize running the AC when its least efficient.
4) Minimize running the AC when utility rates are the highest. 
5) Use thermal mass to better achieve 1-4.
6) Use insulation to better achieve 1-4.
 
So up to about 100 degrees, pre-cooling and these steps work great and I require no AC during high peak times.
 
On 110 days I can run the AC up to 50% from 4pm - 7pm. Not ideal, but it generally works.
 
For 120, I will need more. So I need to make my AC more efficient. I plan is to run a garage type of evap. cooler next to the outside condenser. If I can drop the air temp 10 - 20 degrees, that would be a plus. My evap. probably isn't as big as I need for this job, but it could be a proof-of-concept.
 
I thought about spraying the condenser with water or mist, but I'm worried about mineral buildup which could hurt in the long-run.
For example check out this device: http://www.coolnsave.com/
Looks like a good idea, BUT I don't want to turn my condenser into a big cake of minerals.  With the evap, the minerals stay on the pad.
Dry AZ makes this ideal for this. Probably wouldn't work as good other places.

 

[JimS]

You could use a ground source heat pump.  That allows you to dump heat into the cooler ground rather than warmer air.
 
Couple ideas that are a bit farther from the mainstream...
How about a solar powered ice maker?
http://hackaday.com/2008/08/29/solar-powered-ice-maker/
These aren't new.  There was even a company that made residential size natural gas powered AC systems not too many years ago.  Seems like an ideal technology to convert to solar power.  Fairly big collector though.  And the meth heads would try to steal your amonia. 
 
Then there is ice storage.  Make it at night when rates are low and let it melt through the day.
http://www.treehugger.com/clean-technology/cool-idea-using-ice-to-chill-buildings-in-nyc.html

 

[homeautomationhound]

Pre-Cooling with Air-Conditioner Condensate
Energy efficiency is a balancing act between simplicity versus complexity, cost versus savings. In the HVAC industry many of the more energy efficient solutions can seem prohibitively complex, not to mention the wide array of choices can be downright daunting. Do you choose a variable-speed indirect evaporative cooler, or maybe an evaporative pre-cooler retrofit—will a building owner save money through energy savings and rebates? What about the added costs of training facilities managers to be able to actually service these newer systems? Is there an option out there that will save energy and money without the need for someone with an engineering degree to install and maintain?
 

 

[ano]

Is there an option out there that will save energy and money without the need for someone with an engineering degree to install and maintain?

I have nothing against some of the solutions mentioned, and I spoke a bit about using pool water to store cooling, but I need to repeat for those that missed it: My method of pre-cooling uses absolutely NOTHING that isn't in the majority of homes already.  Yes, I do some elaborate switching of AC units and temps using 4 OmniStat2 thermostats connected to an Omni Pro II, but outside that, nothing.  And even without the home automation system, you can achieve almost all of what I do with just a programmable thermostat. The pre-cooling saves the most money. The complex thermostat control is really just to reduce costs a bit when I do have to use AC above 105.  
 
So got the first bill after pre-cooling, and I really only used it for two weeks of the month, meaning, no savings in peak demand. Bill was 10% less than last year, even though rates went up.  But that doesn't tell the full story, we are SO much more comfortable.  Average temp inside last year was 84, this year its like 77. 
 
Next month will be very telling. Last year the bill was $350. Hoping under $300 this year.

 

[tmbrown97]

I can tell you that right now I'm living 100% off pre-cooling so I've been studying it closely!  I don't have a working AC so I can't speak for running 12-2 vs. 5-7 but the precooling in a tightly sealed house makes all the difference.
 
Since we're without AC for the time being, we use the whole house fan to cool the house.  I watch the outside temp very closely, then when it reaches a point where it's cooler outside than in, I kick on the fan - sometimes just upstairs if downstairs is still cooler - this kicks the hot air out of the attic and starts bringing the inside temperature closer to outside and cools down the whole house building materials.
 
I also have some data being graphed showing me the temps at any given time and was able to determine that at 6:15AM right now is basically when we hit our absolute coldest time - so if we turn off the fan and seal up the house at that time, we'll get maximum cooling - and the house will stay cooler that much longer.  The difference is noticeable by several degrees at the end of the day.
 
While this is nothing scientific, it's something I've been having to do for weeks now and my real world results show that if I am 5 degrees cooler at 8:00AM, I'll be about 3-4 degrees cooler at 5:00PM - and the difference between 75 and 79 is pretty substantial.  Luckily in our case it wasn't so much when to run the AC, as it turned out to be when to wake up and close up the house and kill the fan.

 

[ano]

So today it hit 117 here. This morning I set the pre-cooling to 68 as opposed to the 70 we have used in the past. It was a bit chilly for us but not that bad. So at noon the AC turned off and my guess was it was about 108 then. At 5 pm the warmer parts of my house with Western exposure was about 83. The coolest parts were still 81. Between 5 and 6 my AC ran once for 20 minutes, but 83 or 84 isn't bad for an hour or two.
 
Previously above 82 I had the AC come on for 15 minutes per half an hour, but in speaking with the wife, she wasn't really hot, so I scaled it back to 20min per hour. Even with no AC, I don't think it would get over 85. At 7pm rates drop and AC runs.
 
This weekend could be 120, but weekend rates are LOW.

 

[ano]

O.K. I just received my July electric bill, and finally have some pre-cooling results.  Results are this year July (mostly June actually) compared to the same days last year.  In the spirit of full disclosure I have upgraded my two HVAC systems from last year to this year.  Both new systems are higher efficiency, and one is zoned.  But also, a "typical" summer with my old system was my kitchen in the center of the house set to around 83, with my family room on one side, and office and two bedrooms on the other side hitting about 88-90 during the day.  (The old system couldn't keep the house uniformly cool.) A second system cooled my master which reached about 87 on the hottest days. The only automation on my old system is the two units would alternate to keep my 12 noon - 7 pm peak charge lower.  The Kitchen/family rm/bedrooms/office unit would have priority, with the master unit only running when the other unit was off.
 
My new systems are more efficient, and I have three zones on one side; 1) family room, 2) Kitchen & 1 bedroom, 3) Office & 1 bedroom.  My set temps are now 81 in the kitchen, 82 in the office and family room.  On weekdays I precool the house to 70, then shut it off at noon. Between noon and 7pm I allow the AC to run for 20 minutes per hour if the set temps are exceeded. 
 
So the drum roll ......
Billing Days: This month 32 days, last year 31 days. So 1 more billing day this year.
Average Outdoor Temp: This month 92 degrees, last year 90 degrees. So this year it was 2 degrees hotter.
Total KWH used: This bill 3210 KWH, last year 3127 KWH. So used slightly more total electricity this year.
Percentage on-peak use: This year 7%, last year 22% So much less peak usage.
Peak demand Charge: This year 4.1 kW, last year 9.6 kW
Total Charges: This year $285.55, last year $356.26  So, I saved over $71 while being much more comfortable.
 
Also note that electric rates went UP this year, not surprisingly. 
 
So it works. 

 

[apostolakisl]

Good to hear your efforts saved you some money.  I hope you also consider this a hobby since you probably could have made a lot more money doing whatever it is you do for a living for the same amount of time you've spent on this. :unsure:
 
But anyway.  You might consider trying to use the forecast in your algorithm.
 
If the forecast is for 110.  Keep your pre-cooling going for 15 minutes or so per hour during the first few hours after you hit the noon peak rate time.
 
And here is to rain and cool weather!!  You guys should be getting the same cool (less than 90) weather we are having and some rain too!  It is pouring here and only 73 outside.  That is CRAZY for July.

 

[tmbrown97]

Your set temps are 88-90°?  When our AC failed, there was a weekend where it was 90° at 9:00PM - I had to put the kids in the car and drive for 3.5 hours with the kids sleeping in the car while the whole-house-fan ran getting the house down to 83° before I came home and put them to bed - with fans on them.
 
72° is the usual set temp - because my new system won't go to stage 2 until 2 degrees variance - so it keeps the house @ 73.  We're comfortable but we pay for it.

 

[ano]

Your set temps are 88-90°?  When our AC failed, there was a weekend where it was 90° at 9:00PM - I had to put the kids in the car and drive for 3.5 hours with the kids sleeping in the car while the whole-house-fan ran getting the house down to 83° before I came home and put them to bed - with fans on them.
 
72° is the usual set temp - because my new system won't go to stage 2 until 2 degrees variance - so it keeps the house @ 73.  We're comfortable but we pay for it.

What you may not realize is that people's sense of "comfort" and their body can change with the conditions.  Its takes about a year to adopt, but I think you will find that most people living in hot climates like Phoenix adapt quite easily to the temperatures.  If the temps in the house are below 82 or 83, my wife will wear a jacket.  82 or 83 is probably like 72 or 73 for you.  Remember, of course, that the humidity here is also in the single digits most days. 
 
Even if I loved it at 70 degrees, that would not be possible.  When I "pre-cool" to 70 or 69 in the morning, and when its 95+ outside, my AC struggles to bring the house lower than 71 or 72.  I have that same 2 degree differential for stage1/stage 2 you do, but even at stage 2, my AC struggles to drop temps below 70 or 71, and that is when its 95 outside.
 
Now look at when its 119 outside like it was a few weeks ago. Under those conditions, at BEST, the AC can keep it below around 82 or 83 inside.  The temperature differential is just to great, no matter how good your insulation is.  And Energy Star requirements don't allow installers to oversize units, not that this would really help anyway.
 

Good to hear your efforts saved you some money.  I hope you also consider this a hobby since you probably could have made a lot more money doing whatever it is you do for a living for the same amount of time you've spent on this. :unsure:
 
But anyway.  You might consider trying to use the forecast in your algorithm.
 
If the forecast is for 110.  Keep your pre-cooling going for 15 minutes or so per hour during the first few hours after you hit the noon peak rate time.
 
And here is to rain and cool weather!!  You guys should be getting the same cool (less than 90) weather we are having and some rain too!  It is pouring here and only 73 outside.  That is CRAZY for July.

That is a good idea and it has crossed my mind.  I use CQC so that probably could do such a thing if I had the time to program it.  I could use forecasting to determine pre-cool temperature needed.  When its hot, however, I don't have many options.  When we had a stretch of 118 degree days, I tried to pre-cool a bit cooler, but the AC just couldn't pull it off. 70 or 71 is about the minimum.  If its not going to be too hot, I could use a higher pre-cool temp, but not much higher.

 

[apostolakisl]

What you may not realize is that people's sense of "comfort" and their body can change with the conditions.  Its takes about a year to adopt, but I think you will find that most people living in hot climates like Phoenix adapt quite easily to the temperatures.  If the temps in the house are below 82 or 83, my wife will wear a jacket.  82 or 83 is probably like 72 or 73 for you.  Remember, of course, that the humidity here is also in the single digits most days. 
 
Even if I loved it at 70 degrees, that would not be possible.  When I "pre-cool" to 70 or 69 in the morning, and when its 95+ outside, my AC struggles to bring the house lower than 71 or 72.  I have that same 2 degree differential for stage1/stage 2 you do, but even at stage 2, my AC struggles to drop temps below 70 or 71, and that is when its 95 outside.
 
Now look at when its 119 outside like it was a few weeks ago. Under those conditions, at BEST, the AC can keep it below around 82 or 83 inside.  The temperature differential is just to great, no matter how good your insulation is.  And Energy Star requirements don't allow installers to oversize units, not that this would really help anyway.
 
That is a good idea and it has crossed my mind.  I use CQC so that probably could do such a thing if I had the time to program it.  I could use forecasting to determine pre-cool temperature needed.  When its hot, however, I don't have many options.  When we had a stretch of 118 degree days, I tried to pre-cool a bit cooler, but the AC just couldn't pull it off. 70 or 71 is about the minimum.  If its not going to be too hot, I could use a higher pre-cool temp, but not much higher.

 
I don't know about CQC, I have ISY which I can post variables to.  Probably I would have to parse some weather page and then post it.  The ISY has weatherbug, but the forecast high doesn't seem to be part of the package there.  Perhaps you do it manually every morning.  Like you could have 3 options.  "Precool" for 10 minutes each hour starting at noon, 20 min, or 30 min, based on predicted high.  Something like that.  Like with Insteon, I could program 3 buttons on a keypadlinc to be those 3 options.