Keeping the 3rd floor windows open, running the AC...

HardDrive · 08-07-2015, 11:56 AM

Our house has 3 floors. The bottom 2 floors are on a central air system together. The 3rd floor is on a totally separate system. The only connection is a single staircase that comes off the second floor hall (there is no door, its always open). The second floor hall does not have any vents or air returns, they are all in the bedrooms.

SO, how bad do you think it is to run the AC on the bottom 2 floors, while leaving the windows open on the 3rd floor and AC off?

The bottom system doesn't seem to run more often when I do this, and it seems like the mixing of air between the systems is pretty negligible. In my late afternoon caffeine addled daydreams, I can almost convince myself that this makes the lower floor system work less because warm air from those floors rises to the third floor, and get scavenged out the windows.

Expert opinions, bombast and malarky all welcome.

Scott Douglas · Scott Douglas's Garage

Cold air drops, hot air rises, plain old air takes the path of least resistance.

Where is the inlet duct of the bottom floor A/C unit located in all this?

74-911 · 08-07-2015, 01:26 PM

We have exactly the same situation but in a 2 story. Upstairs has windows open and AC turned off. Only exception is there is a door at the upstairs entry.

Hotter than heck outside here in the greater Houston area and temp shown on upstairs thermostat is 93 at the moment but downstairs are ACs having no problem keeping downstairs at 78.

island911 · 08-07-2015, 01:52 PM

Sounds smart. Roofs generally have high heat loads (the sun). Which means you have pretty much ambient air temps on the floor of the 3rd. ..which is likely the better situation.

Heel n Toe · 06:50 PM

It might be a good idea to close those windows and run the upstairs AC to keep it around 78-80 because of the humidity... it may come downstairs since there is no door.

Also, even if there was a door, a lot of heat up there will emanate downward and cause the bottom 2 floors' AC unit to work harder. It depends on how much insulation there is between the floors. If you don't believe it's warmer near the ceiling of the 2nd floor, stand on a chair or a stepladder so your head is closer to the ceiling. Your head will be sweating within a couple of minutes... or you can hold a thermometer near that ceiling.

There's also the question of a surprise rainstorm with the windows open when you're asleep or away... that could get very expensive depending on how much water blew in.

EarlyPorsche · 08-07-2015, 07:42 PM

Run third floor AC harder. Run lower zone less. Vent attic not the conditioned space.

island911 · 08-07-2015, 11:51 PM

Quote:

Originally Posted by EarlyPorsche

Run third floor AC harder. Run lower zone less. Vent attic not the conditioned space.

Yes, this.

Being 3 stories I envisioned a flat roof, with light insulation.

You could get a better idea of temp gradients with one of these:

They are kind of fun, although your wife may look at you strangely. ...and the cat will have frustrated fun.

jyl · jyl's Garage

Seems to me that if warm air is rising from lower floors to exit third floor window, then air has to enter the lower floors from somewhere - outside?

Chocaholic · 08-08-2015, 03:12 AM

I think humidity is the bigger concern in your scenario. If you try that, you may want to run a dehumidifier full time upstairs, but a losing battle with open windows in Houston humidity.

onewhippedpuppy · 08-08-2015, 03:21 AM

Quote:

Originally Posted by EarlyPorsche

Run third floor AC harder. Run lower zone less. Vent attic not the conditioned space.

This. Cool air from the third floor will drop and help with the lower floors. You are also letting in a bunch of humidity which is making your A/C work harder. If you want to improve upstairs efficiency, make sure your attic is well insulated and vented to allow for good circulation.

mattdavis11 · 08-08-2015, 04:41 AM

Dehumidifying is just a by product of cooling. Your air conditioner doesn't turn on when the humidity level rises, it only cares about air temperature.

However, some newer systems claim they can work independently of each other.

I know my thermostat doesn't care what the humidity level is. I can imagine that air temperature will rise faster with higher levels of humidity, but I can't imagine it makes that much difference. It does feel warmer to us, but not to a device. I have been known to be totally wrong though.

The best advice is to cool the 3rd story.

Do you have dual climate zones (on the lower system) and electronically controlled dampeners to shift air to where needed when needed?

There are some pretty slick systems out there these days. Dual compressor/single condenser is but just one.