Optimal setting for 2 A/C unit in two story house

[jasguild]

Hi I have a two storey house with two separate A/C units: one for each floor. I have a normal work schedule during the week 8 to 5. When I get home I typically stay down stairs until about 11:30 pm before I go upstairs to bed. My prefered temperature here in tampa is 76 degrees

I am thinking about programing my ac, to do the followirng

1) increase the temp for upstairs and downstairs to 80 during the day when I am not at home

2) at 5 pm lower the temp downstairs to 76 for my enjoyment downstairs while leaving upstairs at 80

3) at 11:30pm increase downstairs to 80 and lower upstairs to 76 while I sleep

4) at 8am the following morning increase upstairs to 80 and maintain down stairs at 80while I go to work

Repeat this every day of the week.

Is this recommended? the builder of the house suggested that I should always keep both units within one temperature degree of each other. However this makes more sense to me.

Your comments would be appreciated.

thanks

jasguild

[sosessyithurts]

you say you can program the units, that's nice. this way you can set them and forget them right...

anyway

cold air sinks, hot air rises

depending on the interior of your home, your upstairs unit is going to work harder to acheive the same thing as the downstairs unit

hot air rises.

the upstairs is taking a more direct hit from the sun

that's why I say the upstairs unit is going to work harder

I did a little research for a friend on this same subject about the effieciency of a fluctuating temperature scheme...

it is actually cheaper to fluctuate... some argue that you pay to cool a house back down... in my research, I discovered it does not cost more to cool a house back down

So I like your scheme, I think your maximizing your resources.

I've got this down to a science, I have 2 windows units

for the one in my bedroom, i decided it is better to leave the ceiling fan off. reason: cold air sinks... so.. i let the ac run on low.. it fills the lower portion of the room with cold air... as the room fills up with cool air the target temperature is reached quicker! than if I had to cool the upper portion of the room as well.

by turning the ceiling fan on, I have to cool the whole cube! by leaving the ceiling fan off.. i only have to cool the lower portion of the room.

another fine example of this. a friend installed a sun room, a sun room with glass ceilings can gather alot of heat up in the top...

I told him.. why not install a vent at the gable end of the sunroom (where it ties onto the house)... put an air vent hose on the vent.. and suck the hot air out of the upper portion of the room.

he did it. it worked.

hot air was gathering in the top.. it had nowhere to go but compact itself back down into the living space. which made his stand alone wall ac work harder.. in fact.. it wasn't doing the job UNTIILL he removed the cumulative hot air from the upper portion of the sunroom.

now his ac cuts off and on. it cools the room just fine.

why cool the upper portion of a room when you do not use that part of the room.

my theory is... don't stir the air around. EXCEPT in the winter.

in the winter all you warm air is on the ceiling... so it needs to be directed back down to where you can feel it.

it's just the opposite, you wants to have ventilation that allows sinking cold air to escape... it's just like water, cold air will settle to the lowest spot so cut a hole in the floor at the lowest spot..and put a vent there to allow the cold air to escape. (i think im just kidding about that...)...

we'll talk about that when winter gets here

[Eric in North TX]

That's pretty much what I do, only more so. I have a setback thermostat upstairs that is set to 90 between 9 am and 9 pm, and 73 between 9pm and 9am. I find that this works well, as the long cool down cycle really dehumidifies the upstairs, it really doesn't do it at once, since the thermostat is a "smart" one and it calculates how well it did at achieving its goal and adjusts the next days attempt to start earlier or later. The downstairs one I completely shut off at night and manually start in the morning. We are home all day, so it isn't exactly your scenario, but I see no real reason to keep them all that close in temperature, a little cold or hot probably transfers between the two, but it isn't all that noticeable, and I can't see how it could adversely effect the equipment.

[John Gilmer]

If humidity control is important to you than you might want to only have one unit operating at a time (unless it's a VERY hot day).

Use the scheme you outline but make the set back temperature of the "other unit" high enough that it usually doesn't come on at all when in "set back" mode. This will may be non set back unit operate almost continuously which helps control humidity.

You might pick a set back temperature of up to 85. This isn't super uncomfortable if you have to go into the hot zone to get something. You will have to experiment to see how long it will take to bring that zone down to 76F while the downstairs unit is also on. The goal is that you shut down the downstairs A/C just before the upstairs is cooled to 76F.

[jasguild]

Thanks for all your response. One thing I probably did not elaborate on is what exactly l am trying to accomplish.

My goal is to lower my electric bill without adding any excessive wear and tear or strain on my equipment.

Humidity is not really an important factor to me.

With that goal in mind, do your opniions change??

jasguild

[Eric in North TX]

Not at all, that is my original goal, but the reduced humidity is icing on the cake. Keep in mind that a humid space at 75 degrees may "feel" warmer than a dry space at 80.