Incorrectly sized AC unit?

We recently had central AC installed (oh god, it's heaven after 23 years of Southern Ontario summers...), and I'm wondering if the AC unit is correctly sized for the house. The unit the company installed is a 3 ton Amana 14 SEER model, which is actually slightly oversized by the system they use. They recommend 1 ton per 1000 square feet, and the house is about 2500 square feet, not including the basement (and garage of course). The house is a bit weird. It's what they call a "great house" style nowadays, I think. Basically, almost the whole thing is one big room, and just the bedrooms and laundry room are actually distinct rooms with doors. I'm told this screws with air flow to the upstairs quite a bit because of the weird way they have to install ducts. I'm not sure if it would affect the choice of AC unit size.
Anyhow, it obviously works, and the lack of humidity is great. I keep it set at 24C (75F) for the most part, which is a comfortable temperature, and it is able to maintain that even when it gets above 35C (95F) outside. However, if I set it below that, it struggles to maintain the temperature. For instance, today I tried setting it at 22C (72F) in the morning, and it was able to maintain that until late afternoon, when it went up to 24 even with the AC running full time. Outside temperature was 27.5C (82F). Another time, it was not running in the morning (tripped the breaker in an unrelated incident), and by the time we noticed, the temperature had gone up to around 27C. We reset the breaker and set it to 24, and even with it running full time, the temperature did not go down until the evening. On that day it was very hot outside, around 36C (97F). I called and asked the company that installed it about this stuff, and they claim it's normal and not to worry.
Now, 24 is okay, and I am happy to leave it at that for the most part, but I'm getting the feeling the unit they installed is undersized for this house. I have friends who set their AC to 20C (68F) or 21C (70F), and it doesn't have any trouble maintaining that temperature on hot days. I know it's not good to have an oversized unit installed, but it's also not good to have an undersized unit running all the time to maintain the temperature.
I'd love to hear your thoughts on the subject. I'm not sure whether to just live with it or try to have the company install a larger unit (or even if it's actually required).
Chris
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snipped-for-privacy@hotmail.com wrote:

On hot and/or very humid days if it is sized correctly, it should run a lot, even most of the time. It needs long run-time cycles to remove moisture. to achieve the "Human Comfort Zone." http://www.udarrell.com/air-conditioning-latent-heat.html
An oversized unit will get it very cold but humid and clammy! I would keep what you have as it should keep you comfortable. - udarrell
--
Air Conditioning\'s Affordable Path to the "Human Comfort Zone Goal"
http://www.udarrell.com/air-conditioning-total-heat-enthalpy-latent-heat.html
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Thermostat - set it and forget it Running all the time....as long as it is running, it is removing humidity (remember that) In different parts of North America...systems are sized totally different than other parts.
I didn't know the glacier receded that far to give Ontario temps higher than 75 (F) But you don't see temps that high that often... So my $.02 says it is fine

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daytona wrote:

Well, for most of July and August the temperature is usually around 30C, higher (38C) or lower (20C) some days, so it's not exactly cool.
Chris
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snipped-for-privacy@hotmail.com wrote:

On hot and/or very humid days if it is sized correctly, it should run a lot, even most of the time.
I have a 6,000-Btuh half ton window unit that in SW WI cools over 900 sq. ft., that is 1800 sq. ft. per ton. We have many days with quite high humidity making it feel like +95-F to over 100-F. Yet it cools the first floor three rooms and a hallway in an older home to perfect comfort levels. Well, this first link won't work, it's too long & on two lines.
<A HREF="http://www.udarrell.com/airconditioner_current_temperature_btuh_charting.html">
It needs long run-time cycles to remove moisture. to achieve the "Human Comfort Zone." http://www.udarrell.com/air-conditioning-latent-heat.html
An oversized unit will get it very cold but humid and clammy! I would keep what you have as it should keep you comfortable. - udarrell
--
Air Conditioning\'s Affordable Path to the "Human Comfort Zone Goal"
http://www.udarrell.com/air-conditioning-total-heat-enthalpy-latent-heat.html
  Click to see the full signature.
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When air conditioners are correctly sized and the ductwork is properly designed, the system will *MAINTAIN* 75F at the design temps. Nothing more... when the ODT excedes the design temps, the IDT will rise accordingly even though the system is running continuously.
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On 28 Jul 2006 17:53:14 -0700, snipped-for-privacy@hotmail.com wrote:

1 ton per 1000 sq. ft.? Who the hell ever told you that. Everyone knows that proper sizing is done by counting all the door knobs in the home and dividing by 7. Geez, Chris. Dont you ever listen? Bubba
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Hahahaa Doorknobs...
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wrote:

Found out some interesting stuff today as far as sizing. Since title 24 came along our company is real big on using load calc sheets as a lot of the counties around here will not let you pull a permit without a load calc, that then gets transferred to the title 24 sheet(some counties don't give a shit one way or the other).
Found out today that everyone at the shop is doing their own thing. One guy said he uses the 500 sqft per ton rule, another (the friggin service manager no less) said he does the same and then adds a half a ton to be safe. They then make up, out of thin air, the numbers on the load calc sheet to match the tonnage they came up with and then submit it.
Everyone looked at me like I was stupid when I said I actually do the load calc. Best part was, I have not sized a system wrong yet, which was the subject of today's service meeting, with the owner wanting to know why we were getting all these call backs for improperly sized equipment. Boy did they get their asses handed to them.
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I do mostly repair, but the few times I've done a load calc, I find them a pain in the ass, some of the information you require can be difficult if not imposssible to determine, like the R value of the walls, the ceilings,...information that would require opening up the walls or tearing down drywall to find out. How do others in the trade cope with this? Honestly, I'll bet the percentage of guys who do a load calc is below 5%. I find with older homes, you have to guess at a lot of the numbers for the calc. On newer homes, the buiding department has a heat loss/cooling load done by the builder which they will share with you. If the truth be known, few of us do them. Most just don't want to admit it.
--
Respectfully, Bob

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wrote

What about the gals?
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wrote

Bob,
Its a real good idea to know what kinds of construction is most prevelant in your area so you can at least make an educated guess on what kind of insulation and how much(if any) is in the walls. R11 is pretty standard in the homes here that were built after 1970...Before then is kind of a crapshoot. Sometimes you can drill a single small hole(1/4 inch) in an inconspicuous spot on wall and see if there is any insulation there. FWIW, 80% of the time I actually *DO* a room by room, heat load/loss calculation.....especially since Katrina. A lot of homes have had a lot of damage, and a lot of additions have been done as well as enclosing carports, etc. Unless there is a mechanical sheet with the blueprints for new construction with a PE stamp on it, I will do a heat load/loss on those too.
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snipped-for-privacy@hotmail.com wrote:

http://www.fsec.ucf.edu/bldg/pubs/ACsize/index.htm
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snipped-for-privacy@hotmail.com wrote:

A system designer can design the system to maintain any reasonable indoor temperature (called the "indoor design temperature") at any outdoor temperature (the "outdoor design temperature") and any ambient weather conditions (humidity, etc.), taking into consideration all the parameters of the home such as window and door types and sizes, ceiling heights, orientation on the lot, insulation, etc.. This is called doing a heat gain calculation, something that the hack company that installed your air conditioner did not do.
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Travis Jordan wrote:

I did research it beforehand, and I wasn't happy about them using the "tons per square footage" method, but every company I called used pretty much the same method. The guy from the gas company just read the size off the furnace!
Regardless of how they chose the size, I'm really not sure what to reasonably expect from a properly sized AC system. Is what I described normal? I'm not even sure what people typically set their AC at.
Chris
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at the OD design temp for cooling and also to *MAINTAIN* 72 degrees at the OD design temp. Keep in mind that when the design temps are reached, the system should be running constantly.
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snipped-for-privacy@hotmail.com wrote: *snip*
Okay, it seems the original issue is not really an issue. I do have one other question about the AC system, though. I went around the house checking temperatures with the AC running and set at 24C. Pretty much everywhere downstairs is 24-24.5C, which is great. Upstairs is a different story. I measure around 28C in most rooms during the daytime, which goes up to 30C at night with the doors closed. It is still better than outside because of the lower humidity, but that seems awfully high to me. We have to keep the ceiling fans running full time, and still wake up sweaty. As I mentioned previously, because of the design of the house, the duct work is full of comprimises, and airflow to the bedrooms seems quite low compared to downstairs (in the downstairs bathroom we actually have to block off the vent or it becomes frigid). There is definitely cool air coming through the vents, though.
I'm wondering a) if again, this is normal and b) if it's possible to improve the situation with anything short of installing a second unit for the upstairs (four bedrooms, a bathroom, and a small landing area).
Thanks,
Chris
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