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## Cool down speed

Just curious. Is there a general rule of thumb of how fast a properly working and sized central AC unit should cool a house down x degrees?

For example, if the outside temp is 89 degrees and the inside of the house (because the windows have been open) is the same temp, how long should it take to cool it down, say 10 degrees? 1 hour, 2 hours, 3 hours, etc?

I'm sure there are a ton of variables in the equation, but I am just trying to get a general idea.

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Depends on how humid it is inside the house....if it's very humid it will take longer, if it's extremely dry it won't take long..

3. If its 89 and humid, 12 to 14 hours.
If its 89, and not very humid, 5 to 8 hours.

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## "Cool house" definition

When you talk of the temperature in the house that generally means the temperature of the air around the thermostat.

So, when you first run the air conditioning and get the air cooled ten degrees the thermostat will stop calling for cooling and at first glance you have met your need to "cool the house ten degrees".

BUT, at that moment the contents of the house (furniture, walls, etc) are probably still more than five degrees warmer than the air and they will quickly heat the air so that the thermostat is calling for more cooling quite soon after it first reaches the setpoint.

You will have truly cooled the house ten degrees when all the contents (air, furniture, walls, etc.) are at the same temperature.

Also, remember that Manual J calculates the cooling needed to maintain a defined temperature in the house given a defined outdoor temperature and a calculated heat gain from air infiltration and radiant heat. In theory then, it would be impossible for your AC system to ever reach a lower temperature if the outside temperatures and other conditions were as entered into the Manual J calculation.

Right now I am experimenting with my system (a 3 ton Trane XL19i) to see just how cold I can get the house when the outside temperature is above 100 degrees and I only allow the thermostat to call for first stage cooling (1.5 tons). The answer looks to be somewhere in the mid 70's, and cooling from 80 degrees to 76 degrees takes less than two hours in the morning as the outside temperature climbs from 70 to 90 degrees.

My house is not well insulated and the thick adobe walls retain a lot of heat from the day and release it during the night. The AC is still cycling on at 3 in the morning trying to keep indoors at 80 degrees when it is in the 60's outside! That is why I am painfully aware that cooling the entire house is a very different proposition than just cooling the air in the house.

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Thanks Jeff, that makes sense.

So I pose another question to you then. In the summer time when in cooling mode, is it better to set the stat and leave it at a specified temp (say 75) and not use a setback stat? It would just seem logical that once you get the house cooled and all its contents, it would seem more energy efficient to just keep it at the same level (or am I leaving something out of the equation).

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IMO yes it is best to just set it and forget it, I would also run your fan on low to circulate the air in the house will help also. I live in Wisconsin and have kept my door's and windows closed and have my fan running on low and I have been able to keep the house warm at night when the OD temps drop and cooler during the day when the OD temps rise during the day and still have not needed to run my AC even when it has been alittle humid here with all the rain we have had. Just the last couple of days we have been in the high 80's and still able to retain ID temps of 72*, at night the home retains ID temps of 75 in the evening. If the house has decent insualtion and is somewhat sealed with good doors, and windows and door's and windows closed you should be able to do pretty much the same as me. Oh my indoor RH stays right around 50% with all the rain and on less humid days it keeps right at 45%.

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## I wish I knew!

Perry, Still trying to figure out the economics/comfort issues around that question!

These things I know:

The A/C system does not reach its peak efficiency until it has been running for ten minutes or so. Short runtimes are bad from both an efficiency point of view and also for wear/stress on the system as it starts up.

The colder the interior of the house then the more heat will enter from the outside because the temperature differential is greater.

Based on the above, the optimum arrangement would seem to be to have the smallest possible system, sized so that it has to run continuously just to keep the interior at the desired temperature. That appears to be the best you can do from the A/C side - for economy insulate against outside heat or unconditioned air ever getting in.

Setback for short periods of time are probably not going to result in any savings since the A/C will end up running extra time just to recover (although that might be a good thing for efficiency and the life of the system). Setting back for a week while you are out of town obviously makes sense, just for half an hour while you run to the store is crazy. Don't know what time period makes the best balance between comfort and savings, would depend a lot on how well your home is insulated.

I suspect that there is an excellent way to do a quick check on the Manual J results and the sizing of the equipment installed by looking at the rate of temperature change both with and without the A/C running. The only variables that would need to be considered would be the temperature differential between indoors and outdoors and the amount of solar radiant heat. Indoors would have to be stable (no cold air/hot furniture situation), but I think that would make for an interesting project for some aspiring thermodynamics student.

Surprised that the professionals here have not offered more rules of thumb. It was a little difficult to answer your original question since you used X as your desired temperature drop and got back specific answers as to the time it would take.
Last edited by jeff520; 06-16-2008 at 09:34 AM. Reason: Typos!

8. Its go time jeff! work work work!

The A/C system does not reach its peak efficiency until it has been running for ten minutes or so. Short runtimes are bad from both an efficiency point of view and also for wear/stress on the system as it starts up.
I will go into the office with a smile on my face because of this though, which will be quickly removed from my face as I face the horror of the office.

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I guess I was more thinking of setback from 9am - 4pm (at work... but also peak heat time of the day) and 11pm - 5am.

In the winter time, the setback works just fine and we can heat the house up quite quickly, but the AC is a different animal. My Honeywell stat has a nice feature which will "learn" how long it takes to reach a desired temp (at least on the heat side I have seen this). On the cooling side, I don't think it has yet figured out how long it will take.

On a positive note, I have started to add heat reflective tint to my west windows. We have 4 windows in the family room, a slider in the kitchen, a back door, and a bathroom window on the first floor that all face west. On the 2nd floor, we have 6 windows that all face west that will all be treated as well. Hopefully this will help at reducing the heating of the house in the afternoon.

After that is all done, time to do the east side of the house.

10. How much money you save using set back varies with how long the set back is. The outdoor temp during the set back period. The amount you set the temp back. Weather your area is a high or low humidity area. And if you try to recover during the high or low solar gain period of the day.

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Okay, so from the sounds of it, in the summer, set it and forget it should be the best. Winter time is a different story (at least in my house).

12. I am a homeowner, rather than HVAC schooling my qualifications are from other sources. But that includes a basic knowledge of physics and math, and working 15 years for an electric utility. I firmly believe that setbacks save energy, although in some cases the amount may be too small to care about. It would be comfort reasons IMO, not economic reasons, which tell you to leave it at one stat setting 24/7. Still good reason, just not dollars and cents.

It is very hard for me to believe the amount of energy spent in pulldown mode, will exceed the amount *not* spent in setback mode. I believe the utility claims of a few percent savings are well researched and truthful -- these claims are usually overseen by the PUC and not just salesmans' jabber. You can look for 3rd-party verification via FSEC publications (Florida Solar Energy Center, a very good resource for all hot-humid states), or possibly Building Science Corporation or other sources.

In my own house I find myself using setbacks of just 1-2 degrees, and the pulldown process in my house is highly variable but may take a couple hours (any oversizing is relevant to pulldown). Last year I had some wider setbacks from day to night, and found to my surprise there were quite long runtimes after midnight. This seemed to teach me there were a lot of daytime BTUs stored in all the building structure.

If you can find any attic duct leakage (assuming not a sealed attic), that would possibly outweigh benefits from using large thermostat setbacks. Let me suggest solar screens on the west facing windows rather than tint -- screens are inexpensive and they work very well for me. In my house there were several east facing windows which led to an uncomfortable room in the morning sun, and solar screens were an effective solution to that comfort problem.

Hope this helps -- Pstu

P.S. I misspoke a bit, forgot that my setback on one system is 3 degrees, 78 day and 75 night. Overstates my case some.

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Well, my setback degrees were more than just 1 - 2 degrees. We are talking 75 degrees while at home and 85 when away (or at night).

I think the other problem is that my system is not working efficiently yet, so the cool down takes days, not hours. I am waiting for the installer to come back out and take care of that.

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