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My home is one level. Have a new unit (2 years old). I find that are great room with gets sun all day. Stay about 5 degrees hotter than the rest of the has. During the hotter day. Blinds closed and use a floor model fan to move the air around in this room. Seem to get the temp down to the rest of the home. Cozy at 73.
Keep in mind that all A/C systems sizes are specifically selected to NOT handle the worst case, hottest hour of the hottest day outside.
When the A/C unit(s) for your house were selected by the builder, the size (capacity) selected was for it to be able to cool your house for those 15 or so days of the year where there are 1-2 hours in the afternoon where it is hotter, the system is expected to NOT "fully" handle the load and the inside temperature will creep hotter than the thermostat setting. In the later afternoon when the sun starts going down, the A/C will continue to run and pull the inside temperature down back to where you want it.
This is standard design practice not only for houses but all buildings (like office buildings, schools, churches, etc.), and for two reasons:
1) Pure economics, The incremental cost to get the next larger capacity unit is pretty large and would only be useful to you those 15-30 or so hours per year. Only critical facilities like hospital ORs, labs and such are designed more strict, and even they usually are expected to be too small, but for maybe 5 hours a year.
2) The energy saved by not cooling that extra during those few hours is considerable, and these few hours of the entire year are the ones that hurt the electricity companies the worst. Therefore selecting the units to be slightly less than 100% capacity sized is a requirement by the energy portions of the legally mandated Building Codes.
This also apples to heating equipment selection too, but because people freezing is a bit worse than people sweating a lot so units are selected for the 99% worst case heating for all uses.
FYI: for commercial buildings in Raleigh, the energy code Design Temperature for Cooling is 91 dF, and heating is 21 dF.
When the A/C unit(s) for your house were selected by the builder, the size (capacity) selected was for it to be able to cool your house for those 15 or so days of the year where there are 1-2 hours in the afternoon where it is hotter, the system is expected to NOT "fully" handle the load and the inside temperature will creep hotter than the thermostat setting.
Nice post.
I understand that in many cases, residential AC systems are over-sized because the company did not do an accurate calculation of needed capacity. Bigger is not always better. One thing an AC system does is to remove moisture. A system that is too large can run short cycles and not do a good job of moisture removal.
VeniceBound I am sure your system is fine. Remember heat rises so that is why your upstairs is baking. 5-7 minutes on and off sounds ok as well. Mine sometimes does that on very hot days. I also have increased attic insulation in my 3 year old home and it still happens. My technique is to keep AC off during the day. I find it silly to cool a house that no one is in. When I come home I cool the first floor to 1 degree cooler then I want the upstairs. An hour before I want to go to bed I turn the upstairs unit to 78/79. It takes about 1 to 2 hours to cool from 86 degrees.
What do you expect to happen...it to remain at 75 and only kick on 1 time an hour? I am sorry that is unrealistic if the temp outside is 95+. To compare the unit at my place of work stays on most of the day. Your attic temp is probably 120 to 135 degrees. I actually did a 5 day attic temperature test using a dual probe type monitor. On August 10th 2010 at 3pm the attic air temp reached its peak of 126.4 degrees.
See below:
RED – Probe under cellulose insulation.
BLUE - Probe mounted at peak inside attic.
Avg outdoor high : 92.7 | Avg outdoor low: 77
Last edited by StanFischlerHockE; 06-10-2011 at 06:26 AM..
I find having all the ceiling fans running {low-medium speed} really makes a difference as well, keeps the air moving and keeps it cooler, which allows me to keep the thermostat at 77/78 for the AC. Even on the 90+ days...
You could consider having an energy audit done on your attic. Its my experience that the extent of the typical inspection is to verify the insulation is of the proper depth to meet the code.
A good audit will evaluate leaks from your conditioned living space. You might also be a candidate for a radiant barrier - I've seen results that show a 10-15 degree attic temp drop in high sun situations. Remember that your upstairs AC air handler and duct work probably reside on the "hot" side of your attic insulation so a radiant barrier in the attic at the roof benefits your upstairs AC system as well.
I understand that in many cases, residential AC systems are over-sized because the company did not do an accurate calculation of needed capacity. Bigger is not always better. One thing an AC system does is to remove moisture. A system that is too large can run short cycles and not do a good job of moisture removal.
This is probably right.
I believe that house designers have to do calculations (to verify compliance with the energy and mechanical codes), but I know those calculation processes are not super-precise). Also, as you said, the equipment available comes in steps. You can't get a 4.82974 ton unit, it's either a 4-1/2 or bum to a 5, and they will bump it.
The down sides to getting a unit larger than necessary is that the DX type A/C units (direct expansion, IE: with a condensing unit outside), runs or it doesn't run. So if it's over-sized, it will cycle more often, and that increases wear, reducing lifetime. (And decreases de-humidification a bit)
I'm not sure of your setup, but we have had 3 level (2 stories plus finished basement) and 2 level (single story plus finished basement) homes with just one thermostat. We found that especially during extreme temperatures the home were overall more comfortable when we set the fan to "on" instead of "auto." This keeps air circulating and more uniform. A lot of people think the "on" setting means you are running the compressor or furnace constantly, but you are just running the fan constantly and making temperatures uniform. The heat or cool still only comes on based on the temperature setting.
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