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-   -   To turn off the upstairs AC or not (http://www.diychatroom.com/f17/turn-off-upstairs-ac-not-78343/)

Big Dawg 08-09-2010 09:51 PM

To turn off the upstairs AC or not
 
I have a large home with a two story foyer and Great Room. It has a master on the main level, office/mother-in-law suit in the downstairs daylight basement and 4 bedrooms upstairs. The kids are all grown up and gone so the upstairs is hardly ever used.
Besides the usual registers in the The Great Room, this room also has two large return vents high up on the wall that is connected to the downstairs Furnace.
I've turned the upstairs AC off since no one stays or goes up there much anymore. My HVAC guy said that I should not turn off the upstairs AC. That by not having the upstairs AC running, it is making the downstairs work harder to remove humidity. I thought that since cool air falls, why should I worry about humidity high up in the upstairs area, and besides wouldn't the savings of NOT powering the upstairs compressor and furnace more than offset any additional work the downstairs unit is doing?
Is it costing me more to NOT have the upstairs unit on and working? I realize that the cool air from upstairs would also work its way downstairs, but isn't it at an unnecessary cost?
BM

Proby 08-09-2010 09:54 PM

Some people might tell you to leave the upstairs AC set high, like 78 or 80.

I think in the end the answer depends on your particular situation. I would compare KW usage of similar temperature months.

beenthere 08-09-2010 10:36 PM

Check the humidity in the upstairs since you have the A/C off. if its staying under 60%, no problem. if its going over 60%, then set the upstairs stat to a high temp as suggested above.

Other then that. Your first floor running more and longer, won't harm it. They are made to have long run times.

Big Dawg 08-10-2010 09:30 AM

Thanks for the info.
I kinda thought that running the AC upstairs wasn't really needed. The upstairs doesn't get above 58% humidity.
My only concern was that since the great room spans the main floor and the top floor and it has a double return near the ceiling, that when the downstairs AC kicks on, it would be pulling the hot air accumulating at the top of this room down into the downstairs AC to be re-cooled, thus working the downstairs AC harder than had the upstairs AC been running and cooling the upstairs air. Thus my confusion of what is the most economical and energy efficient thing to do.

beenthere 08-10-2010 09:46 AM

While it will make the first floor A/C run longer. The mix of the air from the high returns and the lower returns will keep the return air temp at the air handler well within safe operating temps for the A/C. And provide a better/lower RH for the first floor.

In some areas. people don't set their thermostat lower then 80 in the summer. And that is on houses with 2 floors. So their return air temp is often above 80 the whole time the A/C is running.

A/C's are rated at a 95 outdoor temp and an indoor temp of 80 at 51% RH. They have a safe continuous operating condition with an indoor temp of 80 and 68% RH.

Which also makes it safe to operate them with an indoor temp of 83 with 59% RH, 85 with 54% RH, or at 90 with a RH of 42%. All of these posted indoor conditions, have the same amount of heat per pound of air. So the A/C is not working any harder at any of the indoor temps I listed in red.

Big Dawg 08-10-2010 10:43 AM

Thanks again...that was very helpful.
Off goes the upstairs AC!

Hmm...just had another thought.
Since the upstairs unit is probably only use once a year for a week, when the grand-kids come to visit. Is it advisable or even necessary to run the upstairs AC (maybe set at 83 or 84 degrees) during the summer even when they are not here (then lower the temp when they come) just to keep the seals lubricated and other parts working properly?
In other words, since I think we have established that it isn't necessary to run the upstairs AC to help the downstairs AC, is it still advisable to run to protect and prolong the life of the upstairs AC itself?

artbuc 08-10-2010 02:44 PM

Quote:

Originally Posted by Big Dawg (Post 483459)
Thanks again...that was very helpful.
Off goes the upstairs AC!

Hmm...just had another thought.
Since the upstairs unit is probably only use once a year for a week, when the grand-kids come to visit. Is it advisable or even necessary to run the upstairs AC (maybe set at 83 or 84 degrees) during the summer even when they are not here (then lower the temp when they come) just to keep the seals lubricated and other parts working properly?
In other words, since I think we have established that it isn't necessary to run the upstairs AC to help the downstairs AC, is it still advisable to run to protect and prolong the life of the upstairs AC itself?

Hey Dawg, consider yourself VERY lucky that you have adequate returns located high in your great room/2 story foyer. I have the same house design as you with one puny return up high tied into the 2nd floor A/C. I have a massive pocket of hot air even with both units running. I can't see any practical way to resolve the problem so we just live with it with the help of box fans.

Big Dawg 08-10-2010 05:04 PM

artbuc
Sorry about that...when we designed and built our home, I worked with the HVAC people to make sure that with all the open space from the downstairs to the upstairs, the large 2 story palladium window in the foyer and the large arched windows over both sets of french doors on either side of the fire place in the Great Room would not create hot and cold spots in the house.
I guess I am lucky because we have never suffered from too hot upstairs in the summer nor too cool downstairs in the winter. Of course double pane windows and way over code insulation helped.
I curious though, our HVAC company put the large return vents high up on the wall in the great room as well as down low to both be controlled by the main HVAC system, not the upstairs. Are you sure that the returns you see on the wall high up in your great room is connected to the upstairs unit not the main level unit? Also, if for some reason your upper returns are controlled by the upstairs system, I would think you could gain access to the small returns entry point on the wall and increase the size back to the furnace. Your HVAC people may have to re-balance everything, but I've been told you can never have too many returns.

artbuc 08-10-2010 05:36 PM

Yep, it is tied into upstairs unit, no doubt about that. It is physically possible to get it connected to the downstairs unit and/or increase it (ie, use two wall cavites instead of one), but it isn't worth the effort. Five or ten years ago I would have done it, but not now.

artbuc 08-10-2010 08:38 PM

Quote:

Originally Posted by beenthere (Post 483239)
Check the humidity in the upstairs since you have the A/C off. if its staying under 60%, no problem. if its going over 60%, then set the upstairs stat to a high temp as suggested above.

Other then that. Your first floor running more and longer, won't harm it. They are made to have long run times.

Beenthere, what is a "long" run time? My understanding is that a properly designed system will run almost 100% of the time at design conditions, so I assume running 4 or 5 hours straight would not be a problem?

beenthere 08-10-2010 08:41 PM

Long run times are times of 3 hours or more.

I think mine is running long anytime it runs for more then an hour when its 75 outside. But mine has special tweaks done to it.

artbuc 08-10-2010 08:48 PM

Quote:

Originally Posted by beenthere (Post 483750)
Long run times are times of 3 hours or more.

I think mine is running long anytime it runs for more then an hour when its 75 outside. But mine has special tweaks done to it.

So running 3,4 or even 5 hours straight would not overstress equiopment?

beenthere 08-10-2010 08:58 PM

Quote:

Originally Posted by artbuc (Post 483753)
So running 3,4 or even 5 hours straight would not overstress equiopment?


Heck no.

The other summer when it was 98 here. Mine would run for 6 hours straight from 12 to 6PM. Shut off for 15 minutes or so, and then run for 4 more hours.

If it was harmfull to it. I wouldn't leave mine run like that. I can increase air flow and have it shut off in 3 hours on 98 degree days.

I have my blower slowed way down. A bit slower then I would for a customer.

gregzoll 08-10-2010 09:13 PM

There are humidistat/thermometer unites such as Radio Shack sells, so that you can see what the temps are in various parts of the house. If you really want to foot the costs, you could go with a thermostat that you can control from your computer, plus see what the air temp, etc is in the home.

Big Dawg 08-10-2010 09:39 PM

The other summer when it was 98 here. Mine would run for 6 hours straight from 12 to 6PM. Shut off for 15 minutes or so, and then run for 4 more hours.

Yikes! No unit in any of my previous homes ever ran continuously for that many hours at a time. Living in the South, we have plenty of high 90 degree days during the summer. Could the amount or lack of insulation have anything to do with how long your units are needing to run to properly cool the home or are you keeping your thermostat really low?
Most of the time my units will come on for maybe a 1/2 hour and then shut off for and hour or more before starting again. The home stays quite comfortable...no really large temp swings. Man, I can imagine the AC bills you guys must be dealing with.

By the way, as I asked above...
Since the upstairs unit is probably only use once a year for a week, when the grand-kids come to visit. Is it advisable or even necessary to run the upstairs AC (maybe set at 83 or 84 degrees) during the summer even when they are not here (then lower the temp when they come) just to keep the seals lubricated and other parts working properly?
In other words, since I think we have established that it isn't necessary to run the upstairs AC to help the downstairs AC, is it still advisable to run to protect and prolong the life of the upstairs AC itself?


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