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Timing for air conditioning power consumption


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#1 Linwood

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Posted 07 May 2019 - 11:19 AM

I know there are some pretty scientific types here, and wondering if anyone has any math to go with this question, or has done any experiments along this line.

 

I have to air condition almost year round.  I now have Home Assistant installed where I know inside and outside temperatures, set point on the AC, and can control it all in real time.

 

So... what.

 

Here's my theory.  Make that hypothesis.  First some assumptions (well, facts without quantification): 

  • Cooling is more efficient when the outside air is cooler.  Exactly how much I do not know (I know the theoretical formula, but not how it translates for a real A/C).
  • (Side note: For some people electricity may be less expensive at night as well; it is not here). 
  • Heating and cooling is more about the solid mass of the interior than just cooling the air, e.g. there is a reservoir of heat (or lack thereof) which has, for want of a better term, inertia.

So here's my thinking...

 

Once the air outside temp is near its minimum, crank the set point way down and cool the house substantially below the normal temperature while the cost (due to thermal efficiency) is lowest.  Most days there's a 15F difference in high and low, +/-, outside air temp.  That means the theoretical COP goes from 53 to 28 (give or take), which is pretty big.  What is the actual change?   Not a clue.

 

During the heat of the day go into the period a few degrees cooler, and try to run the A/C less, letting the temperature climb maybe a couple degrees above ideal.

 

As you go into evening bring it back a bit more, with moderate efficiency (it's a bit cooler outside) so people are comfortable for dinner, relaxing, etc.

 

Then repeat over night, crashing the temperature again over night.

 

I have done this with various parameters for some time, and find it is not really noticeable.  Cold temperatures while sleeping (like 70-71) I find pleasant, and do not notice much as I wake.  With it then coasting it is mid-70's by mid-morning.  The problem is mid-afternoon when I let it get to 78 or so, but it is tolerable.

 

I am thinking of adding fan control to this, so I can equalize the temperature more in peripheral parts of the house without cooling.  The 78 is not really the issue, but 78 at the thermostat means 80-81 in some rooms on the sunny side.

 

But... here's the problem... I have no idea if it works, where "works" is defined as being more efficient overall.  After all, I am excessively cooling (in a sense) during part of the day.  Does the reduced cooling mid-day pay it back?

 

I really have no idea.  I have tried a variety of ways to test it, but day to day variations caused by sun vs shade (even if the same temperature), other unrelated power usage (I am not monitoring the A/C specifically for power draw, only run time), cooking, etc. have pretty large impacts.  I'm also told that power usage is not constant, e.g. an hour of runtime at night may different amount of power than during the day (motor loading varies by temperature spread, or so I believe).  So just looking at cumulative runtime on two days with and without the algorithm applied is apparently not a valid comparison either.

 

Most related postings online speak toward presence sensors and such; not to timing and differential costs of cooling.

 

Anyone done this?   Seen math or studies? 

 

Linwood



#2 rockinarmadillo

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Posted 07 May 2019 - 12:30 PM

I have also done some thinking about how I can tie my automation system into my HVAC.     I built a mathematical model to forecast the impact of automating an ERV for ventilation.    I dug through a lot of technical papers online.  In this process, I learned a few things:

 

- Summer: cooling the house down at night and warming during the day is a good thing for two reasons.   First the efficiency of the AC unit is better at night.   Second there is a smaller temp difference from inside to outside  during the day-- which results in less heat transfer (or energy loss).      This second reason is more important if your house is poorly insulated.

-  Humidity is also a consideration.   The heat capacity of air is a function of both temp and humidity.      This becomes a big consideration for ventilation-- if you plan to ventilate with humid outside air.   

-  For winter heat load, the calculation is reversed if you have a heat pump.   Store energy by warming the house during the day and let it cool at night.   However, any sudden changes in set point could trigger the electric heat strips-- which are only 40% as efficient as the heat pump.   You must make slow gradual changes when increasing the set point.   

- My house is very well insulated.   2x6 walls with foam insulation plus 3/4 inch of external foam board.   It will stay constant temp for a long time without AC running.   

 

Based on this I did the following:

 

- Summer time, I set the temp back 3 degrees at night.   Then warm up before we wake up.   I run a ceiling fan on low.     This is for maximum comfort, but also a little efficiency boost.

- I decided it was not worth the hassle to connect the AC to my automation system.   I increase the set point 10 degrees when I go on vacation, and then remotely set the thermostat back before I return home.  Otherwise, I let the setback thermostat run the show with summer and winter programs.

- I have automated the exhaust fan in my master shower.   It comes on with motion-- but only if the average humidity in the house is greater than 40%.   During the winter it does not come on using this program.    This, combined with a variable speed heat pump/AC keeps my humidity 40-60% all year long.   And humidity control is very important for both comfort and efficiency.

- Winter time, I set back 3 degrees at night, but increase it one degree at a time in the morning to avoid kicking on the heat strips.   

- I found a setting in my thermostat that allows me to go straight to heat strips when the outdoor temp falls below a certain point (heat pump is then shut off).   I set mine at 25 degrees F mostly because the system seemed to be overloading when the temp dropped below that.   However, I know that at some low temp around that point, the heat strips become more energy efficient than the heat pump.

- I decided to forego the ERV.    Instead, I ventilate with fresh air supplied into the the HVAC supply.   Based on the studies I found, this is nearly as efficient, and results in a positive pressure in the house.   This reduces the amount of humid air that infiltrates through the building envelope. 

 

The most efficient system for you will depend on your climate, the type of HVAC you have, and your electric rate structure (including peak rates).



#3 Linwood

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Posted 07 May 2019 - 12:58 PM

Thanks, @rockinarmadillo, glad to see I am not too off the wall in thinking about this.

 

FWIW I did the bathroom and am pretty happy with it, though I had not considered low humidity cutoff in it.  What I do is monitor relative humidity and when the current exceeds the median by 15% (of the value) I turn the fan on, and turn it off when it drops to 7% of the value.  I may tweak those a bit, but so far it works fairly well, seems to strike a good balance.  I tried doing it relative to the rest of the house with bad results, also tried doing a rate-of-rise, which I think has more potential, but the median worked pretty well. 

 

My problem here is that a lot of what I learned up north does not apply well.  Insulation has less impact than you might think, since it all gets heat soaked during the summer when the low temperature is still 5-10 degrees above interior for weeks on end.  It's kind of like having thicker walls on a crock pot -- the inside still gets cooked, just takes a bit longer to reach temperature.  Radiant heat transfer is so critical as well, and window shades and outside shade is very important (but also dangerous during hurricanes, i.e. trees).  I'm pretty sure humans were not meant to live in S. Florida, only visit in the winter months.   :angry2:



#4 Linwood

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Posted 09 May 2019 - 04:06 PM

So if anyone is interested in this sort of thing here is what it did today (which is very similar to the prior day -- all of two days use).

 

Attached File  Temperature.jpg   89.57K   5 downloads

 

Top bar shows colors for operating mode.  The dark blue is idle, the orange is cooling and the purple is a timeout that is idle-but-waiting.  Light blue on the left I had the fan on.  

 

Buttom is the temperature at the thermostat.

 

As you can see, about 1am it went from around 77-78 to 72.  Well, sort of -- there's an issue there.  It held that during the evening, then I started raising the set point and it warmed to around 78 without any A/C running until about 4:30pm.  So it missed the very hottest part of the day running at all, though with a 6 degree +/- rise, which is substantial.

 

The actual set point is not doing what I expect.  Prior to yesterday it was alternating between the set point and 1 degree above (so say 76 -> 77, cool comes on, when it hits 76 it goes off).  I thought that was too fast, and found what I thought was a setting to cool one degree below (e.g. as it came on at 77 it would go down to 75 before turning off).  But what it seems to have onde is just move up a degree.  All that 73-74 should be 71-73.  So I need to go through the manual again.

 

But... it's doing what I wanted, shifting most of the run time to the off hours.

 

Is it working though?   Is the cost of this cheaper than the cost of just holding one temperature all the time? 

 

Still have no real idea.  

 

Linwood



#5 ano

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Posted 09 May 2019 - 11:58 PM

I live in AZ and have been doing something similar for years now. Mine is really based on my utility rates, which the utility company has changed several times in an effort to screw solar users. I do not have solar, so I have been benefiting with every change, and my electric bill has been dropping, if you can believe that.
 
So we have several rate choices, but they basically have high rates (sometimes really high) and low rates (sometimes really low.)  Our high rates used to be 9am to 9pm, then they were changed to 12pm to 7pm, and now they are 3pm to 8pm.  So solar output goes down in the later afternoon, but the temp goes up, so by changing the high rates to 3pm to 8pm, they try to "get" those solar users. Again I am not one.  For me, being able to use AC until 3pm rather than 12 noon is fantastic.
 
So we have several rate choices, but they all increase in cost, weekdays 3pm to 8pm. One plan just uses a higher rate weekdays 3pm to 8pm and gives you a lower rate at other times. The plan I am on does that also, BUT also adds a peak demand of over $11/KWH peak for the month during that 3pm to 8pm time. So, just using lots of power in ANY hour 3pm to 8pm ANY hour of the month can be VERY costly.

So my Omni Pro II does all the calculating and controlling of the AC and heat. An Omni is not very high-power when it comes to calculations, but it does a good job. I have about 5 or 6 "programs" that are followed based on the temperature at sunrise for that day. It can be cold or very hot here, so I have to monitor temps to decide. On the hottest days, the temp is pre-cooled to 72 or 71 until 3pm, and then AC goes off until 8pm. Actually its more complex than that.

So my house is maybe 20 years old, I'd say average insulation, but when we remodeled it after we bought it, we added lots of granite countertops and stone tiles, and it has worked. On the hottest days, and they can be hot, inside temps won't usually exceed 83 by 8pm. Not terribly warm. I have a backup, where every hour after 5pm it checks temps, and if they exceed 83, then the AC runs for up to 20 minutes per hour. I don't think that has ever happened.

During more mild times of the year, no pre-cooling is used or less is used. Like today, I think it got to 90 outside, below normal actually, but just dropping the temp to 75 between 1pm and 3pm was enough to keep it comfortable 3pm to 8pm. Our "typical" temp for weekends and holidays is 78. (Yes, it knows holidays, which high rates don't apply.)

Overnight we drop it to 73 so my wife is comfortable. A few days the heat goes on in the morning when its a bit chilly out, even when the AC goes on later in the day. I'm trying to reduce that. (We have gas heat.) We also have a gas stove and gas hot water, so that's not a problem. We have an electric dryer, so that is only used during low rates.

The great thing about our rates, yes peak rates and the peak charge are high, the off-peak rates are very low. They used to be like $0.04 a KWH.
 
With my temps, its interesting. So on the hot days, June to September say, it does get cold in the morning, and wife wears a blanket (yes 71 is cold for AZ people) but because of this, its much easier to live with the higher temps later in the day. If you keep temps high all day and night, then you really feel hot and tired and get overheated. We never really feel overheated.
 
I'm sure we have talked about this in the past, since I have been doing it for years. The tricky part was that I had to talk my HVAC provider to install a bigger unit then what they would recommend, because they are taught to undersize the units for efficiency. But for me, that wouldn't work. Its ALL or Nothing some of the year. :-)
 
So do we save lots of money? I don't know, but I'd estimate we save maybe $50 - $100 month over just keeping it 78 or 79, and we are more comfortable I think.

Edited by ano, 10 May 2019 - 12:04 AM.


#6 ano

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Posted 10 May 2019 - 12:16 AM

Linwood, on 07 May 2019 - 09:26, said:

Here's my theory. Make that hypothesis. First some assumptions (well, facts without quantification):

  • Cooling is more efficient when the outside air is cooler. Exactly how much I do not know (I know the theoretical formula, but not how it translates for a real A/C).
  • (Side note: For some people electricity may be less expensive at night as well; it is not here).
  • Heating and cooling is more about the solid mass of the interior than just cooling the air, e.g. there is a reservoir of heat (or lack thereof) which has, for want of a better term, inertia.
So I THINK AC is most efficient when the temperature inside is most away from the outside temp. If inside temp is constant, then yes, cooling is most efficient when outside is cooler. I think efficiencies vary some, but I'm not sure a lot. Even when its 120 here, ACs work fine, but the problem is heat gain through windows and walls. When we precool, efficiency drops since our inside temp drops, but outside temp is also relatively low. Also, your AC is very unefficient when it starts as it has to cool the ducts, but gets more efficient as it runs.

Here, the biggest factor is electric rates. We are talking a 6X to 10X difference low to high.

And yes, its amazing how much heat and "cool" the mass of your house can hold. If I had a pool and lots of money to experiment, I'm sure I could store LOTS of thermal mass with 14K gallons of pool water. But that's another post...

Edited by ano, 10 May 2019 - 12:20 AM.


#7 ano

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Posted 10 May 2019 - 12:28 AM

Oh, and I forgot. We installed automated blinds on all our windows, and that is tied in as well. Above certain temps outside, blinds close to keep out the heat. This makes a big difference. Some also have sun sensors to keep funiture and cabinets from fading.



#8 Linwood

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Posted 12 May 2019 - 07:08 PM

So I THINK AC is most efficient when the temperature inside is most away from the outside temp. If inside temp is constant, then yes, cooling is most efficient when outside is cooler. I think efficiencies vary some, but I'm not sure a lot. Even when its 120 here, ACs work fine, but the problem is heat gain through windows and walls. When we precool, efficiency drops since our inside temp drops, but outside temp is also relatively low. Also, your AC is very unefficient when it starts as it has to cool the ducts, but gets more efficient as it runs.

Here, the biggest factor is electric rates. We are talking a 6X to 10X difference low to high.

And yes, its amazing how much heat and "cool" the mass of your house can hold. If I had a pool and lots of money to experiment, I'm sure I could store LOTS of thermal mass with 14K gallons of pool water. But that's another post...

 

 

The second part is true, the first is backwards.  Efficiency is better with a smaller difference in temperatures, at least theoretical efficiency.  Whether the realities of starting, stopping, mechanical loses, humidity, etc. affect I do not know.  Formulae can be found here but notably the theoretical value is (outside / (outside - inside)) in degrees kelvin, and you can see it gets really large when they get similar.



#9 ano

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Posted 13 May 2019 - 08:07 PM

The second part is true, the first is backwards.  Efficiency is better with a smaller difference in temperatures, at least theoretical efficiency.  Whether the realities of starting, stopping, mechanical loses, humidity, etc. affect I do not know.  Formulae can be found here but notably the theoretical value is (outside / (outside - inside)) in degrees kelvin, and you can see it gets really large when they get similar.

There are a few different factors at play.  Sure, if its cool out and hot inside your house, efficiency will be greater. This is maybe true here in spring and fall at night. Electric rates are low and at night you want temps lower, so life is good.

 

Unfortunately that is not the conditions I run under most often. Its hot outside, 110+ often, and I attempt to precool inside, 72 or below. As this difference gets LARGER, the AC has a tougher and tougher time. AC becomes less efficient and will run constantly. So the trick is make it cooler sooner before the difference becomes too great.






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