Our co-housing community has been discussing the a/c dilemma - and busting some myths in the process. Efficiency and energy/emmissions savings is top priority around here, so our green minds wanted to know the final word - during the hot, humid dog days of Summer, is it best to leave the a/c running when you are not home or turn it off? I found information on this issue - see below - but I’ll let our resident, off-grid, sustainability “expert” and pioneer, Giles, have the last word. Read on. Tao

The idea that it uses more energy to cool a warmer room is a myth  - the unit doesn’t “crank out” more cooling even though it’s easy to perceive it as so -see below, copied from this link:  http://srmi.biz/Tips.Residential_Heating.Thermostat_myths.htm:

“Another myth regards the efficiency of setting your thermostat down when you don’t need heating or cooling, such as at night or when no one is home. This myth states that a furnace works harder than normal to heat your home back to a comfortable temperature after the thermostat has been set back, resulting in little or no savings. This is not true, as has been proven by years of research and field observations. The longer your house stays at a reduced temperature when heating–or at an increased temperature when cooling–the more energy and money you’ll save. This is because your heating or cooling cost depends mostly on the temperature difference between indoors and outdoors. When you adjust your thermostat down in the winter–or up in the summer–you simply reduce this temperature difference. If you set your temperature back 10 to 15 degrees for 8 hours while you’re asleep or at work, your energy savings can be 5% to 15% on your energy bill.”

from Giles Blunden, sustainable architect, Carrboro, NC:

I would like to do a little myth busting myself about HVAC energy use -

The amount of energy used to heat or cool a building is a direct function of the difference between the outside of the envelope (building exterior) and the inside of the building since heat can only escape or enter at that surface  plus any internal loads. Therefore the lower the differential in the outside temp and the inside temp times the time it is lower is the amount of energy saved

Single stage cooling units like ours only work at one speed. Because of this there is no extra work invloved in reducing the temperature quickly. In fact there is more power used each time the motor starts from a stop.

Humidity is another function of summer comfort and leaving the windows open all of the time and closing them when you start cooling does make the cooling system work harder because it is using energy to both remove the water and reduce the temperature.

The best cooling non occupant mode would be to keep the windows closed and let the unit run very early in the am for a couple of hours, when the compressor is most efficient, to keep the humidity level down.

Then turn it up (or down) when occupied.

Giles

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