a/c overwhelmed by heat

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.
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The obvious question is: How cold is the air coming out of the vents? Also, is your attic properly vented (or have an exhaulst fan)?
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The heat load on the AC is the sum of the conduction loss through walls, windows, etc. and that incurred in cooling (and dehumidifying- no small challenge in SE VA) infiltration from leakage.
Can we assume the system was running non-stop?
Not knowing what energy-conservation measures are in place, it's a SWAG as to what measures would benefit you most. SWAG taken.
Suggestions: 1) Replace filter- too cheap to quibble unless it's new. See that all air-movers are moving air. Probs- see 5. 2) See that all windows, including storms are closed. Tightly. 3) Look for the obvious air-leaks, like all door-seals, especially at top and bottom of enclosed volume of house. Chimney-effect thing. 4) Close shades, especially facing S & SW. No shades? Good investment. 5) If A/C has not been checked/serviced within a year, schedule it now. Better techs will be busy, and you don't want a hack/salesman. Ask what checks or basic servicing you should do regularly.
Vaulted ceilings will make it difficult to retrofit proper ventilation, but you might check that you've sufficient soffit vents if there's a ridge vent. Which I'm assuming. (There really should be a "channel" for air-flow from soffit vents, between insulation and roof-deck, out ridge vent. To reduce heating of house and frying of shingles.)
Ceiling fan in "great room" mainly mixes hot air above with cooler below. Sub-optimum. You'd get better perceived cooling with small circulators moving only air down at human-level. "Vornado" comes to mind- works for me.
OTHO, 84 vice 102 is good. :')
HTH, J
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On 28 Jul 2005 06:38:35 -0700, snipped-for-privacy@sme-online.com wrote:

You mean the passive heat load. The total heat load is quite a bit more than that. You forget about active load, unless there are no humans, pets, lighting fixtures or heat generating appliances living or in use inside the building.
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badgolferman wrote:

Yes to both. BTW fans do not cool your house they cool the people in the house. You might get free advice from the electric company. They come to the house in NJ.
Lou
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badgolferman wrote:

It could be both!

SE VA Norfolk summer design: 91-DB, 76-WB 48% Relative Humidity; near a lot of water. Don't use the ceiling fans in a vaulted room; hot air rises, don't bring it down or mix it with the cooler lower level air! Leave the hot air stratified a few feet above the occupant space level.
Where are the Return Air inlets and Supply Air outlets located; high or low, ceiling or walls? I don't like vaulted ceilings for cooling or heating!
Can you close off or cover the skylights?
Also, the A/C system may not be working properly! Provide us with the make and model number of the condenser!
Check the air discharge temp rise off the condenser! http://www.udarrell.com/ac-trouble-shooting-chart.html - udarrell
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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:

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wrote:

I think that is an urban legend. Distribution voltage can drop a lot more than that at each pole pig, due to load alone. Sometimes the pigs can be taxed to more than twice their rated KVA, during on-peak hours. They are built to take a lot of punishment.
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~^Johnny^~ wrote:

Its not an urban legend. If you check out PJM's website (thats the grid for the mid Atlantic states), part of their emergency procedures, during periods of very high load, is to drop the voltage on the system by 5%. Its the last step before they initiate rolling blackouts
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OK, I'm convinced, but that's crazy. The maximum theoretical power savings would be 9.75%, in a purely resistive (heating) load, with no motors or computers, etc.
Most computer switching supplies, ac motors, etc, will simply draw more current as the mains potential drops. All cutting the voltage wilt do is dim the lights, and they'd be lucky to save 3 or 4% at the most in the process, with rolling blackouts (rotating outages) imminent anyway.
During our last power shortage here on the west coast, PG&E actually had informed the media of which Rotating Outage blocks were scheduled for blackout. With, like, 30 or so RO blocks in most areas, the threat of blackouts wasn't much of a deterrent, because the customers in the rest of the blocks knew they would not lose service during that particular scheduled outage.
If they would black out the blocks at random, with little or no notice, it would be a much greater deterrent to excessive usage, as everybody would cut back - not just those in the next few scheduled RO blocks.
And I don't buy the arg "they have to give customers ample notice before disconnect..."; it is, after all, an emergency.
my two cents
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wrote:

A-ha! That's why it is common to see window boxes installed in homes along with properly sized central air, in places like Los Angeles. An extra 8000 BTU/hr can really help out in a kitchen during a heat wave. A 5000 BTU/hr unit can help in the bedroom of a house full of children, when the parents want to keep the door sealed for privacy. And there are other situations...
But I disagree about oversizing, to a point. 20 or 25% oversize isn't going to hurt that much, with a well designed package and a variable or multi speed blower. The better systems can be made to unload quite nicely on milder days, without excessive short cycling. There will be a very slight loss of efficiency, if any. But a too-small system can't perform any magic.
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badgolferman wrote:

Must be tat global warming I keep hearing about.
Too many cattle belching up methane.
Jeff
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badgolferman wrote:

You have been given some good advice and information. I will only add a few thoughts.
A/C should be sized based on the home's construction, including insulation. There for if you A/C is not keeping you cool under "normal" conditions, it may not be functioning as it should or it may be undersized. Of course in large parts of the world right now, temperature is not normal and it is much hotter than normal.
If you are unsure of the insulation, have it checked and consider adding more. More is always better.
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#1 complaint in VA and NC, SC the past week has been this one.
Normally, no one took the time to explain something called design temp to the homeowner, and prob 50% of the calls were based upon that.
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Assuming the A/C is working properly and that you were just temporarily overwelmed, you could on those really hot days, close off a few rooms by closing the doors and vents (just not the room with the thermostat) and cool less of the house if that is paractcal.
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This is Turtle.
Well you need to get some one to explain to you what design temperature is and why the contractor did not put a unit sized to take care of any temperature over 95F and when it get 102F outdoors your going to get hot. Maybe it will make you feel better while your hot in your home.
I would have your unit serviced to see if it is anything wrong there.
TURTLE
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