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  • Dumb HVAC Question...

    This will probably be a dumb question for the HVAC educated folks.

    Why does any A/C unit use more energy if the thermostat is lower and left alone?

    Example: If the outside temp is consistently 90 degrees, and the conditioned air loss from a structure is a constant exchange, then why would a unit use more energy to keep an interior at 70 as opposed to 75?

    Does the exterior temp on the unit cause this?

    Thanks.

    J.C.

  • #2
    Re: Dumb HVAC Question...

    There is always a migration from warmer to cooler. The insulation in your house is TRYING to keep the heat out, just like in the winter it TRIES to keep your warm air in. But it is not perfect. Your house will try to equalize the inside temp with the outside. The colder you want to keep it inside, the harder your unit has to work.

    Close up all the windows real tight. Turn everything off. Leave the house. After a certain number of hours, the inside temp will equal the outside temp

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    • #3
      Re: Dumb HVAC Question...

      Every BTU that sneaks in or is generated indoors (lights, cooking, body heat, etc) has to be moved outside. And the greater the differential between indoors and outdoors the more work required to move those additional BTUs and maintain that temperature delta.
      ---------------
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      • #4
        Re: Dumb HVAC Question...

        Thanks guys. I'm slowly absorbing it.

        J.C.

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        • #5
          Re: Dumb HVAC Question...

          Originally posted by Bob D. View Post
          Every BTU that sneaks in or is generated indoors (lights, cooking, body heat, etc) has to be moved outside. And the greater the differential between indoors and outdoors the more work required to move those additional BTUs and maintain that temperature delta.
          I think this in some sense answers your question.
          energy saving ac drive, frequency converter

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          • #6
            Re: Dumb HVAC Question...

            It's refered to as heat load. If I only have to get a space to 75 when it's 90 (not to factor the latent load (humidity)) the unit will work considerably less as opposed to 70. If the humidity is factored in,then the AC works a lot harder (initially). If you let a house get much higher than 80, and then try and cool it down, you'll kill the efficiency cause the majority of the work is to remove the humidity. 75ish constant is better in that regard but dont be afraid to go to 78 ish if your gone a good part of the day to help save money.

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            • #7
              Re: Dumb HVAC Question...

              How about heat transfer is proportional to temperature difference?

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              • #8
                Re: Dumb HVAC Question...

                you are maintaining a lower temp inside, therefore you need to remove more heat. the greater the difference between outside and inside equals higher operating costs!

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                • #9
                  Re: Dumb HVAC Question...

                  It's all about delta-T.

                  Let me try a plumbing analogy . . .

                  Say you have a leaking water heater in the basement. Supply pressure at 40 PSI. Leaking a gallon an hour. There's a sump pump down there, and it cycles every hour to pump that gallon outside.

                  Now the PRV fails and supply pressure goes to 90 PSI. Water heater has the same leaks/holes, but now leaks 2.5 gals/hour. Sump pump runs three times an hour to keep up.

                  The reason the water heater leaks more at higher pressure is due to the higher pressure differential, inside to outside: delta-P.

                  1) The leaking water heater is equivalent to the insulation value / leak rate of your building. Unlike a water heater, a building never has a zero leakage rate -- temperature always migrates.

                  2) Supply pressure is equivalent to the outside temperature.

                  3) The pressure outside the water heater (roughly 14.7 lbs/in² at sea level) is analogous to the ambient temperature inside your building.

                  4) The sump pump is your A/C system.

                  Summary: the greater the difference between the inside & outside temperature, the higher the heat leakage rate, and the greater the amount of work the A/C system has to perform to maintain that difference: it has to move more heat from the inside to the outside.

                  Hope that helped

                  [I'm no engineer, but I've sure worked with roomfuls of them, and it does kind of rub off on you.]
                  Regards,
                  Al S.

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                  • #10
                    Re: Dumb HVAC Question...

                    The only dumb question is the one you're too embarrassed to ask, and my Pawpaw told me when I was 7 that if I didn't learn something every day that I'd better check to make sure that I was still alive.

                    That more energy thing is easy and complicated at the same time. You are part of the way there with air leakage. In our heat load for our house we call that "Infiltration". Depending on the quality of house construction and the average wind speed for your location that can be somewhere .5 and 1.75/ hr. in the summer. Say it's 1.0/hr, that would mean that every hour all the air in your house (8,000 CF for a 1,000sf house with an 8' ceiling throughout) would "leak" out and be replaced by outside air. But that's only part of the heat gain. this makes it sound like living in a airtight structure would be good but that would be a serious mistake. For health reasons minimum air exchange from outside to inside in the US is .35/hour (studies are indicating that that number should be .5/hr). The greater the difference between indoor and outdoor temp the more BTU's must be removed (or added in the winter).

                    Next we have conductive loads through the wall based upon the construction properties of building units of the structure. These are rated in the average r-value /sf surface area. In the case of your 90deg/ 75deg example a net R-15 wall panel assembly could maintain that 15 degree difference for 1 hour, where as if you attempted to maintain a 20 degree difference with the same R-15 wall assembly the BTU's would travel by conduction through the wall 33% faster. It wouldn't matter if the outdoor temperature increased to 95 or the indoor temperature was lowered to 70 BTU loss would increase.

                    Since your A/C unit is designed to remove a set quantity of BTU's/hr @ 95 deg Outdoor temperature, and your system is sized to maintain or keep up with the BTU gain @ 75 degrees indoor temperature vs. your local design outdoor temperature, the lower you keep the indoor temperature the more btu's you will gain and the more btu's you will have to pay to remove. Eventually you will reach the point of equilibrium where with the unit operating constantly it can only maintain a temperature difference to a point matching the BTU/h capacity of the equipment.

                    I hope that this helps.

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