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Re: [Phys-l] cooling hints



I believe the answer to this is... it depends.

Assumptions... The outside temperature is higher than the desired inside temperature at least during the day, and maybe also at night. This situation persists during the time period for which you are trying to discern the most cost-effective mode of operation.

[1] It depends on how long will you leave it off.

When the outside temperature is higher than the desired inside temperature, turning it off and leaving it off forever, or for a month, or probably for a week, will cost less than leaving it set to a comfortable temperature 24/7 during the time period. For the rest of this discussion we will assume we are talking about daily setbacks or daily shutdowns. That is, the setback period or shutdown period is less than 24 hours.

[2] It depends on the humidity.

The answer can be different in high humidity situations compared to low humidity situations. It also depends on how much humidity enters from outdoors (which includes both air exchange from opening and closing doors as well as infiltration even when the doors and windows are shut). It depends on inside humidity sources such as cooking, showers, human respiration, etc. It depends on whether the house is "opened up" during the setback period.

The problem is that if the humidity has a chance to increase, when the A/C comes back on it takes a lot of energy to remove the heat of vaporization of the water vapor that needs to be removed before the A/C efficiently regulates the internal temperature of the building.

An example is some friends of mine that figured they would save money on A/C by shutting the A/C off at night, opening windows, and running a whole house fan. At night they were bringing in 70F to 78F air at over 90% humidity. The next morning they closed the windows. The outside air temperature quickly rose to a temperature between 85F and 100F (depending on the day, of course), so once the inside temperature exceeded the set temperature of 75F (which might be immediately, because the temperature never got below 75F during the night) the A/C came on. At this point the A/C ran continuously for several hours as it mostly just removed the water vapor that had come in from opening the windows at night.

I recommended that they just run the A/C all night, or if they still want to turn it off, at least don't open the windows. They initially turned the A/C off at night and left the windows closed. This did indeed cost less than opening the windows. But, in their minds, the closed house at night without the A/C running felt stuffy. They preferred either to open the windows at night, or run the A/C at night. They tried running the A/C all night and found that it cost the same or slightly less to keep it on all night as it did to turn the A/C off and open the windows at night. So they now run the A/C all the time unless they are going to be away from home for a few days.

[3] It depends on whether you have peak metering.

The example given in [2] was for a household that has the same electricity rate 24 hours a day. At my church we have a peak/off-peak metering system. If we turn the A/C off at night (or to a higher temperature at night), it runs less during the off-peak time when the electricity is cheaper, then it has to catch up just before people arrive in the morning, and this catch-up occurs at the peak rate. It cost more money to do a "setback" at night than just letting it stay the same temperature all night long.

[4] It depends on whether you have equipment that suffers from humidity and temperature changes.

At the church we have to spend more money tuning the piano and tuning the organ if we allow the humidity and temperature to fluctuate. We have also periodically spent big bucks to work on the organ pipe valves, and the the organ company said we were doing that too often because we weren't regulating the humidity well enough. It appears to us that any savings (if indeed there were any savings) from setting the A/C off or at higher temperature at night were offset by the cost of maintaining the piano and organ.

At the Bluffton University science building, we had to pay $15,000 to replace the NaI and KBr optics in our FT-IR spectrophotometer because we tried turning the A/C off at night, and the higher humidity ruined the optics. You can buy a lot of electricity for $15,000. If we had thought of it, we could have installed a continuous dry-nitrogen purge for the instrument so it would be safe (safer?) in a high humidity environment, but that is not free, and someone has to pay attention to it.

We are now running the science building A/C 24/7, and it is on a peak/off-peak meter.

[5] At my house I run the A/C 24/7 even when I am away for a few days because I have an expensive 7-foot grand piano that can take a beating from humidity changes. However, I don't feel too bad about this because my experiments with set backs or shutdowns haven't shown much savings compared to running the A/C continuously. Unless I can save a reasonable hunk of change from doing set-backs or shut-downs, I don't see any reason to bother with it. When I run the A/C continuously from late May through September, my electricity cost only goes up from $60 to $80 per month (depending on the weather), and I am comfortable that whole time, and I don't worry about it.

This summer has been a very hot summer in Ohio, although not as bad as Texas and some other states. My average electricity usage in the months when I do not air condition the house is 1680 kW-hrs/month, and I pay about 10-cents per kW-hr, for a typical bill of $168/month. My usage this past June and July averaged 2390 kW-hr/month which is an increase of 710 kW-hr/month at an extra cost of about $71 per month. My house is about is about 2500 sq ft. I am not complaining.

Some of the reasons I believe I do so well are... some large shade trees, good insulation in the house, an A/C that is not sized too large, and operating the A/C in 24/7 mode.


Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Chair, Division of Natural and Applied Sciences
Bluffton University
1 University Drive
Bluffton, OH 45817

419.358.3270 (office)
edmiston@bluffton.edu
-----Original Message----- From: Aburr@aol.com
Sent: Wednesday, August 24, 2011 1:08 AM
To: phys-l@carnot.physics.buffalo.edu
Subject: [Phys-l] cooling hints

In a recent newspaper article about energy efficiency, an energy company
spokesperson said
"If you actually turn your air conditioner off for periods of time, the
amount of energy it will use as it tries to "catch up" and make the house cool
again is more than if it comes on periodically throughout the day."
Am I missing something practical here or is this statement as wet as the
condensation from the air conditioner?
Alex. F. Burr
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