Home AC Question

John T · Jul 22, 2011

Okay, last time we figured out my RV AC problem, so this question concerns my home AC.

The upstairs has a heat pump and I set the t stat to 75 at night (bedrooms upstairs) but when I get up in the AM I turn it off (were not up there all day). But then in this mid 90"s heat storm we been havin it gets up to like 90 to 96 when I go upstairs to turn it back on to 75 for the nights sleep, and it takes like 2 to 3 hrs before it cools back down to 75. So the "first wife" tells me I should leave it on during the day set at maybe 80 or so tellin me its more efficient that way and will save us electricity. Im an electrical engineer but not a thermodynamics or mechanical engineer so I just dont know if Im best to leave it off during the day n let it reach 94 then turn it on at night, or maybe set it to 80 or so during the day so it dont have to work so long n hard each evening.

Whats best and why??????????????????????????

Ol John T

T_Bone · Jul 22, 2011

Listen to your wife, a very smart woman.

You answered your own question as too why! No more than a 10º set-back is normal.

T_Bone

Lamar Fudd · Jul 22, 2011

Leave it on during the day; reason being that upstairs gets hottest because the roof and attic make right above your ceiling act like a broiler. The walls, furniture and floor also get warmed up and taking 4 hours to get this all cooled off is actually normal. It is, however, harder on the equipment to run steadily in the afternoon/evening heat. My opinion is that any electricity saved will be offset by the accelerated equipment wear. Remember, the warmer air holds more humidity, which in our area, has to be removed from the air before the actual room temperature takes a dive.

JerseyJoe · Jul 22, 2011

Well JohnT,now you know why she is the first,hasn"t been too far off in the past! Joe

old art · Jul 22, 2011

think back 60 years when you first got married.
no one had A/C. you slept in your shorts on the floor had great things on your mind and finaly decided to go to sleep. now every one has A/C.. butttttttttttttt you are 75+ and want to sleep.
old art. may your day get better.

IaGary · Jul 22, 2011

.

I have Geothermal and was told to leave it set at the same temp all the time.

It cost more to move the temp 5 degrees everyday than it does to maintain the temp. This is for Heating and cooling.

Don't know how this compares to a heat pump.

Leave it on some day and see how many minutes out of an hour it runs. Total run time in those 12 daytime hours might be less than 3 hours.

Gary

JMOR · Jul 22, 2011

(quoted from post at 21:56:07 07/22/11) Okay, last time we figured out my RV AC problem, so this question concerns my home AC.

The upstairs has a heat pump and I set the t stat to 75 at night (bedrooms upstairs) but when I get up in the AM I turn it off (were not up there all day). But then in this mid 90"s heat storm we been havin it gets up to like 90 to 96 when I go upstairs to turn it back on to 75 for the nights sleep, and it takes like 2 to 3 hrs before it cools back down to 75. So the "first wife" tells me I should leave it on during the day set at maybe 80 or so tellin me its more efficient that way and will save us electricity. Im an electrical engineer but not a thermodynamics or mechanical engineer so I just dont know if Im best to leave it off during the day n let it reach 94 then turn it on at night, or maybe set it to 80 or so during the day so it dont have to work so long n hard each evening.

Whats best and why??????????????????????????

Ol John T

f I went up to go to bed & had to suffer 2 to 3 hours waiting for cool down, it would be a real simple decision for me to make! :idea:

ShadetreeRet · Jul 22, 2011

Tbone is right. Talk to a rep at your local power co. Ours tell us not to vary the temp much as it will offset any savings when you try to adjust the temp.to where you want it, either heating or cooling As someone suggested, try it and compare the run times. If at all possible, it would be good to add insulation to the ceiling and walls of the upstairs. This would likely help more than anything.

M-MAN · Jul 22, 2011

John T;
Back when I in HVAC schooling the one thing the instructor's drilled in to us on heat pump's was to set it and forget it. That's the only way they save you money.

With 2 pump's like you have one will alway's be dominant. When you turn off the upstair's the downstairs unit will work a little harder due to the hot blanket on top so it's using more power. Not as much as both but still more. When you fire up the upstairs unit the compressor will be screaming for mercy & pulling maximum amp's untill the load is reduced to normal. The lower unit is still working overtime as well untill the set temp is reached. You might save a nickel a day on power but it's gonna cost you a lot more in reduced equipment life. Remember, design temp on a heat pump is 90 degrees. Anything over that and you're loosing ground.

We're supposed to have this heat a while longer so why don't you experiment? Read the meter & do it your way 2 day's. Read the meter again and try the first wife's suggestion & see what the difference is on the meter. That ought to give you a better idea.

greygoat · Jul 23, 2011

We"ve had a heat pump for 30 years. We set it at
80, in the summer, and forget it, Works great.
But if I had to do it over I"d just get Central
air...we can"t afford to run the heat pump on
"heat", in the Wisconsin winter...it use"s too much electricity.

guido · Jul 23, 2011

Hello John T,
The wife is right! The higher temperature requires more amperage draw. As other said raise the T stat 10 degrees max. A/C recovers a lot slower then the heating system.
Ceiling fans and a power roof ventilator also are a big help!
Guido.

oldtanker · Jul 23, 2011

When you change the temp in a room or the whole house you have to heat or cool everything in that room/house. Appliances, furniture, flooring, walls, toys, pictures....everything. Untill everything is the same temp they will affect the air around them as they cool/heat.

Rick

Jiles · Jul 23, 2011

If I were in your situation, I would definately consider more insulation :!:
I live in NW Alabama in a 2800 sq ft. home with uncooled and unheated partial basement. It is single level with heat pump.
I have a separate unheated and uncooled double garage that is well insulated with the proper attic ventilation and it has never had an interior temperature the same or higher then outside temperature
In the summer, my home thermostat is set at 78 and in the winter at 68. As suggested, it is never moved except when we are gone on vacation for a few days and then it is raised up or down 4 or 5 degrees.

OldFarmTractor · Jul 23, 2011

I was having a similar problem and decided to do three things. First I needed to replace the roofing so I went with metal. I went with the best energy efficient. This makes my attic much cooler and I will NEVER need to replace it and it adds lightning protection.

I next was going to put in a solar operated attic vent. Instead, I went with a bigger ridge vent and found the attic has a better heat flow from eave to ridge. So no Solar Powered Vent.

Lastly I will be putting in better insulation in the attic. But even now, it is much cooler up there.

I did so some new Low-E windows two years ago. Those also are a big help in all seasons. They open more easily and reduce heat flow in winter and summer.

I refuse to pay the utility companies every month more than I need to.
Energy Effecient Roofing

Kirk Grau · Jul 23, 2011

As others have noted, you are probably not doing yourself any favors on either energy consumption or equipment wear and tear. Think about it thermodynamically, all of that solar radiant energy has to be removed whether or not you do it over the course of the day or try to do it in a short period of time. So to use your method would probably require twice the capacity (maybe more, depending on how fast you wanted it to cool down). Further consider that we generally try not to oversize AC equipment much since that reduces the run time during less than peak periods, this results in poor humidity control. Thermostat will be satisfied, but room feels "clammy". About the only justification I would consider for your situation is if you have some sort of "time of day" adjustment for your electric rates, but even at that it is probably not worth it.

Kirk

Sam#3 · Jul 23, 2011

Another part of the equation is 'creature comfort.' How conforable were you while you're waiting 2 or 3 hours for the temperature to come down?
I was out of town last week and turned the AC off. When I came home about 1830 the house was 95, the same as outside. It took my system over 4 hours to bring it down to 79 and kept running often and long all night so it was a long sweaty night.
My experence with AC is don't change it at all because the heat is more difficult to tolerate.

Home AC Question

John T

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T_Bone

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Lamar Fudd

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JerseyJoe

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old art

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IaGary

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JMOR

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ShadetreeRet

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M-MAN

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greygoat

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guido

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oldtanker

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