Saving Money With Pre-Cooling

[tmbrown97]

Wow - seriously busy thread today!!  Some thoughts:

1. I was in Phoenix last week - and I feel for you.  I'm in Sacramento so we're only a few degrees behind but I'll take them.  I only wish I'd taken swim shorts as I was in the doubletree with an awesome looking pool.
2. Your idea of "cold" actually sounds miserably hot to me!!  Anything over 74 and I'm cranky; 76 (as it is right now in my office) and I'm sweating and irritable.  If my house is higher than 76°, I'm not here AND something must be broken - I don't even torture my cat with temperatures above 78° while on vacation.
3. I don't have the energy to search for it right now, but someone on this board, a year or two ago, actually detailed out his custom controllers for his homegrown HVAC controllers - and he did it specifically for time of use billing and precooling.  It's worth searching for - he detailed his results and they really backed his plan.
4. I live in CA - and while we are a few degrees cooler, I hate you for your LOW electric rates.  Yeah - I don't have time of use rates - just penalty rates for usage; my peak will be about 2300kWH and I'll pay nearly $900 for it.  That's June/July/August here.  And I work from home, and wife is a nurse working nights - people are home and upstairs all day long so there's no setting back of thermostats; so keeping even a modern 4K sq ft. house at 74° is miserably expensive.
5. Precooling does work.  But as you've noticed, HVAC may not be the best way to do it.  Last year we installed a large whole house fan.  I use the now discontinued RCS communicating zone controller and I have Elve watching the temperature readings from a house 100' away (via weather underground) - when the outside temperature is lower than the upstairs temperature, all the thermostats start flashing a red light... that's my signal to kill the A/C and turn on the whole house fan.  We'll get the house miserably cold to low-to-mid 60's, then we've noticed that we can sometimes go to 2-3PM before needing AC... it's paid for itself 20 times over just in the first year.  Yeah - it's miserable being so cold, and I have about the worst hayfever known to man - but it can lower the electric bill considerably.  As you've seen, thermal mass is a bitch -  I can cool the house 'till I'm cold then turn it off - just to have the thermal mass of the house bring the room temp right back to 77°.  I end up switching the whole house fan to low and leaving it on a few more hours.
6. Double check the manual for your omnistat - I ran the original omnistat in my last house and I had full control of the "swing" setting - I really can't imagine they took away that feature.

 

[JimS]

Thermal mass is really your friend.  Without it your house wouldn't stay cool when it gets hot outside.  But it makes an effective strategy more complicated.  Maybe run the outside air in shorter cycles so the house doesn't get so cold but for a longer total time - something like 20 minutes on/20 minutes off perhaps.
 
I think Work2Play has it right about using outside air for cooling, especially in climates with low humidity like yours.  Why use the AC at night when the outside air is dry and cool?
 
I live in the midwest - we have high humidity often that makes using outside air a lot less effective but we use a whole house fan quite a bit in the spring and fall to cool the house at night and keep the power bills lower (even though we don't have time of use rates and our rate for electricity is pretty low).  I want to automate it based on outside temp and humidity but do it manually still.

 

[apostolakisl]

Wow - seriously busy thread today!!  Some thoughts:
 

1. I live in CA - and while we are a few degrees cooler, I hate you for your LOW electric rates.  Yeah - I don't have time of use rates - just penalty rates for usage; my peak will be about 2300kWH and I'll pay nearly $900 for it.  That's June/July/August here.  And I work from home, and wife is a nurse working nights - people are home and upstairs all day long so there's no setting back of thermostats; so keeping even a modern 4K sq ft. house at 74° is miserably expensive.

 
You know there is a great solution to this problem. . . . move :mellow:   Where I live we complain about .12/kwh.  We also have no income tax.  Property taxes are high, but I bet you have those too.  Highest bill I ever had was just over $400 and that was actually a bizarre cold spell where it got down to 16 at night for quite a while.  I have all electric and at those temps the heat pump has to switch to regular heat strips which is not efficient.  This last winter we barely topped $200.  And that is for more than 6000sf.  August bills usually get up close to $400, but that is with highs of 105 and lows of 82. . . so no rest for the AC.  The Phoenix guys at least get the cool nights.  I am sure that has a lot to do with the longer warmups during the day, especially with a brick or stone house with a lot of external thermal mass.

 

[bpwwer]

Wow - seriously busy thread today!!  Some thoughts:

1. Precooling does work.  But as you've noticed, HVAC may not be the best way to do it.  Last year we installed a large whole house fan.  I use the now discontinued RCS communicating zone controller and I have Elve watching the temperature readings from a house 100' away (via weather underground) - when the outside temperature is lower than the upstairs temperature, all the thermostats start flashing a red light... that's my signal to kill the A/C and turn on the whole house fan.  We'll get the house miserably cold to low-to-mid 60's, then we've noticed that we can sometimes go to 2-3PM before needing AC... it's paid for itself 20 times over just in the first year.  Yeah - it's miserable being so cold, and I have about the worst hayfever known to man - but it can lower the electric bill considerably.  As you've seen, thermal mass is a bitch -  I can cool the house 'till I'm cold then turn it off - just to have the thermal mass of the house bring the room temp right back to 77°.  I end up switching the whole house fan to low and leaving it on a few more hours.

 
Hmm, I know we're in the same general area, but that's not my weather underground station is it?  KCAELDOR9
 
I do the same thing with the whole house fan but let the house get a bit warmer during the day since we're not normally around.  I'll let it go up to 78 and unless it's close to 100, the AC doesn't come on until 5pm or so when it's dropped down to 76 for our return.  Once it's in the high 90's and above, it gets harder to cool the house down at night and the AC will be on in the early afternoon.

 

 

[ano]

We definitely use outside cooling air when possible, but like most things, its complicated.  We have outside air intakes for each unit, that go through a filter, then a booster fan, then a damper, then into the units air intake.  The dampers and fans turn on and open when the outside temp is between 75 and 55.  Since my wife has allergies, we have to use two fairly restrictive filters outside, so that reduces our airflow a bit. 
 
Still, this only helps certain times of the year.  According to my logs, it got to 100 yesterday, and last night it was 75 or below from 12:30am to around 7am, but it was only 70 or below for an hour and the low was about 68.  In a week or two it won't be below 75 at all, and in July and August, the lows will be in the 80's or 90's.  While I have nothing against outside air, and it is cheap cooling, it really has to be below about 60 for several hours to offer significant cooling. 

 

[ano]

3.  I don't have the energy to search for it right now, but someone on this board, a year or two ago, actually detailed out his custom controllers for his homegrown HVAC controllers - and he did it specifically for time of use billing and precooling.  It's worth searching for - he detailed his results and they really backed his plan.

I will check that out. Thanks.

 

[apostolakisl]

We definitely use outside cooling air when possible, but like most things, its complicated.  We have outside air intakes for each unit, that go through a filter, then a booster fan, then a damper, then into the units air intake.  The dampers and fans turn on and open when the outside temp is between 75 and 55.  Since my wife has allergies, we have to use two fairly restrictive filters outside, so that reduces our airflow a bit. 
 
Still, this only helps certain times of the year.  According to my logs, it got to 100 yesterday, and last night it was 75 or below from 12:30am to around 7am, but it was only 70 or below for an hour and the low was about 68.  In a week or two it won't be below 75 at all, and in July and August, the lows will be in the 80's or 90's.  While I have nothing against outside air, and it is cheap cooling, it really has to be below about 60 for several hours to offer significant cooling. 

 
Yeah, really the only way to make much of a dent in your hvac load using outside air is if the outside air is quite cool or you are using a large CFM whole house fan like work2play.  Letting your hvac fan draw in a relatively paltry amount of outdoor 70 degree air for a few hours isn't going to go far in cooling your house.  And if the night time gets down to 50, you probably don't need to draw in outside air to keep the house comfortable with minimal hvac use.  Most folks along the Southern California coast with their 80 degree days and 50 degree nights don't even have AC.  And as far as drawing in massive amounts of outside air, well I sealed my house up tight as a drum and put in all kinds of high end filters because I wanted dust and pollen free air inside the house.  And it works.  No allergy problems and dust is very minimal.  Part of climate control in my book is not just the temp and humidity, but the particulate matter in the air.  Even when it is perfect outside, we don't open the house.   We so rarely open windows that we actually took all of the screens out for a crisper view of the outdoors.  Plus it is just a pain running around opening and closing windows all the time.
 
In the end, you just have to figure your balance between comfort, convenience, and money.

 

[ano]

Success.  Yesterday I precooled to 70 degrees.  My two AC units started running at 4am, and by 6am the temp was down to 70.  From 6am to close to noon, the AC units continued to run almost constantly to extract the stored heat out of the house.  At noon, all AC units shut off.  At 7pm, 7 hours later, it ranged from 81 to 83, which is close to the high-end limit.  Note that the outside temp shows over 110 degrees, but this is due to some sun exposure on the sensor. It really got up to about 100. 
 

 
This shows its reasonable that I can eliminate on-peak AC use up to about when the temps reach 100.  After that I will need to use some peak demand AC, but it should be quite a bit less than otherwise.
 
The remaining big question is, will this save money?  My two AC units run a pretty solid 8 hours in the morning.  That costs the same as about 2.5 hours of peak operation not counting my hourly peak usage charge.  But also, running an AC constantly is its most efficient mode of operation.  Turning it on and off is less efficient. 
 
It will take a year to really see if this saves money. 

 

[Automate]

What stage of the AC were you running between 6am and noon?  I'm thinking you would be better off starting the cooling a little later but force it to stage 2.  You were at 70 degrees for 6 hours.  At 70 you are going to have a lot more heat gain than at say 78.  A couple hours later would help as long as you can get it down to 70 by noon.

 

[ano]

What stage of the AC were you running between 6am and noon?  I'm thinking you would be better off starting the cooling a little later but force it to stage 2.  You were at 70 degrees for 6 hours.  At 70 you are going to have a lot more heat gain than at say 78.  A couple hours later would help as long as you can get it down to 70 by noon.

 
I'm going to have to experiment a bit. My feeling is that I may need to run it LONGER not shorter.  The AC is in stage 2 until the temp gets down. (My AC is probably a bit oversized as well.)
 
I think you might be fooled by the same thing that I though initially.  Once you get down to the right temp your done. WRONG.  In order to cool everything in a house it takes hours and hours.  If you don't spend long enough to cool EVERYTHING in the house adequately, this stored heat will quickly warm up the house again, EVEN if its a cooler temp outside.  Its a misconception to believe that inside temps will follow outside temps.  The temperature of your contents has a greater effect on inside temperature, at least if your insulation is doing its job. 

 

[Automate]

Yea, I know what you are saying.  My house has a large brick fireplace all the way from the basement through the roof, old plaster walls and hardwood floors so it has a lot of thermal inertia.  Maybe place a temperature sensor directly on the highest thermal mass in the home and compare its temperature to the room air temperature.

 

[apostolakisl]

I have to wonder what is up with the temp staying at 70 with the unit continuously running for hours on end.  I really don't think that stuff in your house should be unloading as much heat as your AC is able to pump out.  At least not from having gone from 80 to 70.  What is your air grill temp when it hits that steady state?  I would also measure the temps of the all the objects in your house and the walls and stuff.  The IR laser pointer thermometers work great for this.
 
My suspicion is that the stuff in your house is already down to 70 much sooner than noon and that your AC isn't putting out cold enough air to overcome the load of the exterior wall heat transfer from outside to inside.

 

[ano]

There is no question there is lots of stored energy. That is what this tread is about. In the morning the air from my ducts runs about 46 degrees. That is because its so nice and cool out and the nighttime cooling is more efficient than running my AC when its 120 outside..
 
It takes 1 BTU to drop the temp of one pound of water 1 degree.  So I'd be dropping the temp about 12 degrees in the morning (82 to 70). I have a total of 7 tons of refrigeration which is 84000 BTU per hour.  So simple math says under ideal conditions, by my calculations, my AC can cool 56,000 pounds of stuff 12 degrees in 8 hours. (Assuming everything is perfect, AC running constantly, etc.)  So assuming things aren't perfect, AC not running constantly, etc., a good estimate would be  maybe 35,000 lbs. 
 
Now, how much does my house inside insulation weigh? I have no idea, but for the "stuff," house movers typically say 10 - 12 lb. per sf.  My house is 3000 so that alone would be about 33,000 lbs.  Also note this doesn't include the actual house like the granite counters, tile floors, etc.  (I have a fair amount of stone) I wouldn't be surprised by a weight total of 60,000+ lb.
 
So the reality is 8 hours probably isn't near enough time to fully cool the house 12 degrees. I'm going to experiment with longer cooling times.

 

[apostolakisl]

Water has one of the highest specific heats of any substance out there. Most of the stuff in your house is going to have a specific heat that is a small fraction of what water's is.
 
Fiberglass quilt for example has a C of 840J/kgK.  Water is 4,200.  Also consider that insulation weighs almost nothing (12kg/cubic meter vs 1000kg for water) and you realize that it contributes very little to the thermal mass of a house.
 
Take a look at this chart.
 
http://people.bath.ac.uk/absmaw/BEnv1/properties.pdf
 
You'll notice that most everything has a specific heat of around 1000 J/kdK plus/minus a couple hundred.  The main difference between things is the density, so the dense stuff has more thermal mass per volume of space it takes up.
 
The main thermal mass of your house is going to be the slab. . . if it is built on one.  The sheetrock will probably be number 2.
 
But all of this thermal mass stuff I just wrote isn't the point.  Your system totally flat lined the temp at 70.  If thermal mass unloading was the only cause, it should not have flat lined.  Thermal mass will only slow the temp change, it can not flat line it.  As items unload their thermal energy, they get colder, and thus the house temp should have continued to fall.  There are 2 reasons this can happen.  Either you hvac isn't putting out air that is much colder than 70 (which apparently is not the case) or you have heat being added to the system as fast as your hvac can take it out.  This could be appliances or other energy consuming items in the house (or heat from people's body), or you taking on heat from the outside world.

 

[JimS]

Interesting thread.  I have thought a lot about cooling with outside air which has similar issues to consider.  Lou has some good points.  Your graph indicates the air cools 10 degrees fairly quickly - a few hours.  But cooling the "stuff" takes longer.  You could measure the temperature of the countertops and see how quickly they cool - could put a sensor on the bottom out of the way.  Maybe embed a sensor in the floor slab (if that's the construction) under a baseboard or some interior corner - I think I would avoid an outside wall.  Depending on how the insulation is done you may be getting a lot of heat transfer from the earth up into the floor.  I used 2" foam under the slab but that isn't standard, at least around here.  Might also put a sensor in the duct - you could easily see when the system was running and what the outlet temp was.