An A/C system should be sized for the design point in your area. If it
was sized for the max temp (100F+), the system would be oversized,
causing a humidity problem in your home during evenings and cooler days.

Normally, a system has a swing of 3 F. Meaning, if you set the
thermostat at 75 F, it could go up as high as 78 F during the peak heat
gain hours (normally 3:00 to 6:00 pm).

If your A/C is only having a problem during this unusually hot weather,
the system is probably working fine. The hotter it gets outside, the
lower the cooling capacity/efficiency goes for the system. High SEER
systems especially see a sharp drop in efficiency (compared to a 10 SEER).

What you need to do is reduce the heat gain going into your home. Most
designers assume that you have shades on your windows that you can close
during hot days. In well insulated homes, light through
windows/skylites can account for 40% to 50% of the heat gain in the
home. Blocking out that light, especially east and west facing windows
will help a lot.

Also during really hot weather, utilities will reduce voltage about 5%.
This will result in a further capacity drop in your A/C system.



badgolferman wrote:
Yesterday in SE VA the heat was so oppressive my home AC was
overwhelmed. At 4 PM when I came home the temperature outside was 102.
My AC is set to 78 and the inside of the house was 84. By the time I
went to bed at 11 PM it had come down to 81 inside the house. This
morning it is at 78 again.

Is this a function of the poor insulation of my house or is it likely
my system needs to be serviced? My house is 1650 sq ft with vaulted
ceilings and one ceiling fan in the great room and ceiling fans in the
bedrooms. I also have two skylights in the great room that I hate
which contribute to heat in there too. I believe the system is 2 or
2.5 tons; the original barely adequate system installed by the builder.