 FronkmanAn Apple a day keeps the doctor awayPremium
join:2003-06-23
Saint Louis, MO | AC dispute My wife and I have a dispute that I am hoping someone can answer. We have a 2700 sq ft house with two 2.5 ton 13 SEER Carrier units, one for the first floor and one for the second floor. We generally leave the thermostat at 78.
Is there any advantage to turning off the downstairs AC at night? Will the second floor unit wind up just cooling both floors or should it run for about the same number of hours regardless of whether the downstairs one is on or not? The thought is that that the cold air from the second floor will just go down the stairs and push the hot downstairs air back up.
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 alkizmo
join:2007-06-25
Pierrefonds, QC
kudos:1 | Well, there will certainly be a big loss of cool air going downstairs, but overall you'll still be cooling downstairs LESS than if both units were turned on, and set to cool at the same temperature. In the end, upstairs will still be colder than downstairs.
Then there's the factor of efficiency which usually a single bigger unit would be more efficient than two smaller (but equal when combined) units. |
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| reply to Fronkman
hot air rises,
cold air sinks.
what do you think ? |
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garys_2kPremium
join:2004-05-07
Farmington, MI Reviews:
 ·callwithus
·Callcentric
| reply to Fronkman
If you keep the bedroom and bathroom doors closed at night then most of the cooling will stay upstairs. The hall is often cooled only by air shared from the exterior rooms. If the thermostat is in the hall then it's likely your bedrooms will be really cold due to it never being satisfied. In that case I'd move the 'stat into a bedroom.
If the doors are left open then yes, the cool air will flow to the lower floor and the hotter air will rise. It's up to you to determine how comfortable that may be. |
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FronkmanAn Apple a day keeps the doctor awayPremium
join:2003-06-23
Saint Louis, MO | reply to fluffybunny
said by fluffybunny :hot air rises,
cold air sinks.
what do you think ?
I am not stupid. I am asking a question about RELATIVE efficiency.
Does it use more electricity to operate a second 2.5 ton AC unit on the lower level in order to decrease the temperature differential or does it use more electricity to operate a single unit knowing that some of the output will be lost to the other floor?
For example, take an evening that last 9 hours (10pm-7am). If both units run for about 4 of the 9 hours it will use 18.4 kWh of electricity. If I turn off the downstairs unit the upstairs unit would have to run for 8 of the 9 hours for the power consumption to be the SAME.
My question is how likely is it that the run time of the second floor unit will at least double when the first floor unit is turned off?
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Everyone should own a Mac! Go Bucks! |
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 HallPremium,MVM
join:2000-04-28
Dayton, OH
kudos:2 | reply to Fronkman
To what extent can you isolate the upstairs, if at all ? The systems should operate independently, but technically, with the downstairs unit off, it will get warmer downstairs and as noted, heat rises. Then your upstairs unit will probably have to work a bit harder to offset that.
In reality, it's cooler at night so it's possible it won't get (too) "hot" downstairs. Do you have ceiling fans (downstairs) ? |
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 leiboldPremium,MVM
join:2002-07-09
Sunnyvale, CA
kudos:6
 ·SONIC.NET
| reply to Fronkman
There will be definitely some mixing of the cold air from the upper floor with the hot air from the lower floor. How much depends on your layout of the house and location of vents.
In homes with enclosed staircase and potentially even a door at upper and/or lower end of the staircase (typical for older two story homes in Europe) there will be very little mixing of hot and cold air.
In homes with open staircase (or where entry hall or living room have a double height ceiling) mixing of the air for both floors is almost unavoidable. However even in such homes as long as all air vents are inside the bedrooms with their doors closed, the mixing of hot and cold air may be limited.
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ViperCertified Home Inspector
join:2012-03-22
Toronto | reply to Fronkman
said by Fronkman:My wife and I have a dispute that I am hoping someone can answer. We have a 2700 sq ft house with two 2.5 ton 13 SEER Carrier units, one for the first floor and one for the second floor. We generally leave the thermostat at 78.
Is there any advantage to turning off the downstairs AC at night? Will the second floor unit wind up just cooling both floors or should it run for about the same number of hours regardless of whether the downstairs one is on or not? The thought is that that the cold air from the second floor will just go down the stairs and push the hot downstairs air back up.
Thats interesting because, in my opinion. If you have set both thermostats at 78 then the stat for downstairs AC should be satisfied and should not turn on the entire night without you turning it off manually.
I dont understand why the downstairs AC would turn on at all? If you were setting it to 72 degrees then I could see it happening but at 78, I dont know!
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Certified Home Inspector
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garys_2kPremium
join:2004-05-07
Farmington, MI | reply to Fronkman
Yes, it's all about satisfying the thermostats. If the downstairs' unit is satisfied, it won't run. If the upstairs' is not satisfied because it's in warm air convecting upstairs, then it will run non-stop. |
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 JRW2R.I.P. Mom, Brian, Ziggy, Max and Zen.Premium
join:2004-12-20
La La Land
kudos:5
 ·Optimum Online
| reply to Fronkman
said by Fronkman:said by fluffybunny :hot air rises,
cold air sinks.
what do you think ?
I am not stupid. I am asking a question about RELATIVE efficiency. The simple answer is that you will save little if anything by turning off the downstairs unit.
Here is why...
By running the downstairs unit, you are maintaining the relative temperature in that area, and unless you have the upstairs isolated from the down stairs, any cooler air WILL head downstairs, making the upstairs unit run more and harder to cool the upstairs..
The part all of you are missing is the effect of thermal mass downstairs.
One has to assume that while the downstairs unit is off, the temperature is rising, so when they turn it back on, it has to then cool the entire downstairs again. It is cheaper and more efficient to maintain the downstairs temperature than to have to cool it back down everyday.
Unless you are talking about a significant period, like twelve hours, that the unit is off....
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| reply to leibold
said by leibold:There will be definitely some mixing of the cold air from the upper floor with the hot air from the lower floor. How much depends on your layout of the house and location of vents.
In homes with enclosed staircase and potentially even a door at upper and/or lower end of the staircase (typical for older two story homes in Europe) there will be very little mixing of hot and cold air.
In homes with open staircase (or where entry hall or living room have a double height ceiling) mixing of the air for both floors is almost unavoidable. However even in such homes as long as all air vents are inside the bedrooms with their doors closed, the mixing of hot and cold air may be limited.
Unless....
That home is like mine with both ac system RETURN grilles located next to each other in the ceiling of the 2nd floor.... Making just one big box with the hot / cool air stratification issues. Even with ducted AC to each room it gets mixed with all of the return air in the central open area.
I will try to use an example.
During the summer we try to stay down stairs to minimize AC cooling. The problem is that the return is on the ceiling of the 2nd floor where it is the hottest. If the return was down low then we could passively stratify the 1st and 2nd floors (even with out doors). With the lower unit seeing the hottest temps it ends up not really "saving" much. I hope this example helps or at least makes sense. |
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alkizmo
join:2007-06-25
Pierrefonds, QC
kudos:1 | I still think that focusing the A/C into a smaller zone to reach a certain comfort temperature while the rest of the house is warmer will save energy compared to keeping the whole house at said comfort temperature. No matter what the level of compartimentation. |
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 XBL2009------
join:2001-01-03
Chicago, IL
1 edit | reply to Fronkman
Anytime you are cooling less air mass then the AC will run less and use less electricity. However in your scenario it depends on where the thermostat's are, I presume there are two, one upstairs and one downstairs. Also as others have indicated the return ducts plus whether the upstairs can be isolated from the downstairs will have some factor. Close the downstairs return vents or cover them at night.
I would definitely turn the downstairs one off at night though and see if the bill goes down.
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Reviews:
 ·AT&T U-Verse
| reply to Fronkman
Technically, if both units operate properly you will use less electricity by turning one zone off. That is regardless of which direction the cold air flows on Tuesdays.
The heat losses (or gain in this case) to the zone being off will be lower if the temperature difference between in and out is lower. Simple physics.
Yes the other unit will run longer, but overall your energy use will be lower too. Think 1 unit running 1.5 hrs vs 2 units running 1hr each for a thought process. |
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 Lurch77Premium
join:2001-11-22
Oconto, WI
kudos:4 | reply to fluffybunny
said by fluffybunny :hot air rises,
cold air sinks.
what do you think ?
That doesn't mean anything. I've had plenty of service calls where lower levels are not cooling when the upper is, or upper levels are not heating when the lower levels are. |
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 KrKHeavy Artillery For The Little GuyPremium
join:2000-01-17
Tulsa, OK | reply to Fronkman
Easiest way to tell is to go out and read your meter one night, then read it again in the morning. Now the next night, assuming similar weather, try it with the lower zone off, and then again read the meter in the morning.
As long as the weather is approximately the same, you'll have a easy to carry out test of both scenarios.
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 MashikiBalking The Enemy's Plans
join:2002-02-04
Woodstock, ON
kudos:1 | reply to Fronkman
You could probably get better efficiency if you used duct pumps between the downstairs and upstairs. Though whether or not you want to use that as an option to balance out the cooling between both levels is up to you. |
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 djrobx
join:2000-05-31
Valencia, CA
kudos:1
·VOIPo
 ·Verizon Wireless..
·RoadRunner Cable
·AT&T U-Verse
| reply to JRW2
The part all of you are missing is the effect of thermal mass downstairs.
One has to assume that while the downstairs unit is off, the temperature is rising, so when they turn it back on, it has to then cool the entire downstairs again. It is cheaper and more efficient to maintain the downstairs temperature than to have to cool it back down everyday.
The most succinct explanation I've found from »physics.info/conduction/ :
Newton's law of cooling Q/t ∝ ΔT. Heat leaks faster from a cool house than a warm house. Thus, it's more cost effective to turn your air conditioner off when you're away, than to leave it on hoping to keep your house cool.
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 AVDRespice, Adspice, ProspicePremium
join:2003-02-06
Onion, NJ
kudos:1 | reply to Fronkman
I turn off my downstairs at night. The upstairs cycles more frequently, so it feels colder than it actually is, the downstairs stays relatively cool with no additional cooling. Either split unit cools the whole house, its just a question of precise temperature control.
You should just run a test, if you leave the downstairs off, how much hotter than the upstairs is it in the morning?
In my house the differential is only 2-3 degrees depending on the outside temperature, so even if I dial up the downstairs, it is the same as shutting it off.
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--Standard disclaimers apply.--
The preceding posting is null and void in Arizona and any other jurisdiction where prohibited by law. |
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 cdruGo ColtsPremium,MVM
join:2003-05-14
Fort Wayne, IN
kudos:7 | reply to cowboyro
said by cowboyro:Yes the other unit will run longer, but overall your energy use will be lower too. Think 1 unit running 1.5 hrs vs 2 units running 1hr each for a thought process. You are taking an extremely complex physics problem and oversimplifying it in order to make that statement. I'm not saying that it's incorrect 100% of the time, but that there is much more then meets the eye and does not apply at all times. |
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