Smart or Wasteful: 65F Nights; 77 F Days

13 years ago

I asked one more question of you experts at the end of my post "How Long to Cool Nine Degrees".

Part of the question is whether it is wasteful to set A/C to run at 77F from 6 a.m. to 6 p.m. then drop to 68F overnight.

Sometimes the system only cools to 70F, but even then the system doesn't kick in until 5 p.m. (House temp has remained below 77 F until then).

I'm asthmatic and sleep better in a cool, dry room. Would it save much to leave the house temp at 77F day and night and install a casement A/C in the MBR? How large a room unit would I need? MBR is 16 X 19 X 9 (2736 cu ft) and faces SW. There is a ceiling fan. I'm 30 miles north of Chicago.

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Comments (3)

david_cary
13 years ago
Wasteful compared to what. Would it be cheaper to run at 77 all the time? Yes. Would it be cheaper to run at 68 all the time? No.

I've never heard the term casement for a window a/c. It is really hard to answer your question because the it depends comes up. If you have leaky ducts and Seer 10 equipment, then yes, a window unit would be cheaper. If you have good ducts and Seer 16 equipment and good insulation, then the window unit would likely cost more.

Then there is the issue of solar gain, size of the house, number of inhabitants, number of other heat loads etc etc.

You are increasing the delta T in other parts of the house when you do what you are doing. But - if the outside temp at night is only 70, then you really don't have a delta T. You are just gradually getting rid of your solar loads from the day (which are the same no matter what) and the other heat loads (lighting, animals, humans) which are also the same.

Now there will be a time when the house is 73 and the outside temp is still 90 (like maybe a really hot night at 8 pm). 17 degrees is a pretty small delta T but it is more than the 13 that you would have had if you left the rest of the house at 77. The heat gain difference between 13 and 17 degrees is not much....

So sleep well - I bet the cost of the window unit couldn't be recovered in 100 years.
weedmeister
13 years ago
A casement window is one that opens side to side, not up and down. There are window AC units made for this, but they usually cost a lot more due to their scarcity.

What your asking is, if I leave the temperature of the house at 77 and cool my bedroom at night with a window unit will I save money? I would say, yes, you'll save money in this situation. But I can't say how much per month, nor how many months per year and therefore how many years it would take to make up the cost difference of a casement unit. From the description, I think an 8000 to 12000 BTU unit should be large enough for a single room.
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chisue
Original Author
13 years ago
Yes, I rather thought the cost of buying the window A/C would be a show stopper.

Maybe I didn't phrase the other question correctly. I'm wondering just how 'wasteful' it is to make the system work at night (dropping 9 degrees) if then the house stays below 77F almost until evening. (System doesn't have to work all day.)

This summer we're having a lot of 90F days with high humidity (80% and higher).

Smart or Wasteful: 65F Nights; 77 F Days

Comments (3)

david_cary

weedmeister

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Smart or Wasteful: 65F Nights; 77 F Days

Comments (3)

david_cary

weedmeister

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chisue
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