Cooling rate for lil' brick bungalow

Little brick bungalow in the midwest US, 800 sq. feet. New Heil HVAC system installed Mar. '06. The central AC is 2-ton, r-22, 10-seer.
The hi yesterday was around 100 F. At 5:05 it was 87 in the house, and I set the Tstat to 79. At 6:45 it had cooled to only 81 F.
So, it cooled only 6 F. in 100 min. with outside temp of 100.
The system is under warranty. This is I think the first time it's been hot enough to really stress-test it. There's no way I'd expect really fast cooling with a 2-ton unit, but 17 min. per degree of cooling sounds kinda suspect.
I'm thinking maybe low charge, maybe even tiny leak.
Your knowledgable opinions are enlisted.
Thx, Puddin'
"Mit der Dummheit kaempfen Goetter selbst vergebens!" -Friedrich Schiller
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Puddin' Man wrote:

Hi, Are the coils and filter clean?
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Condenser coil is more-or-less spotless.
Filter is reasonably clean.
Humidity was at or a little under 50%.
P
"Mit der Dummheit kaempfen Goetter selbst vergebens!" -Friedrich Schiller
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I'd check the temperature drop. But I suspect that your system is working fine.
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Puddin' Man <Pudding Dot Man At Gmail Dot Com> wrote:

Small, with (say) R11 insulation and G = (800+900)ft^2/R11 = 155 Btu/h-F room air conductance to outdoors? Is the brick exposed to room air?

... 2 tons is 24K Btu/h, like this, viewed in a fixed font:
24K Btu/h --- |-----|<--|------- T --- | | 1/155 | 100 F ---www--| | --- C --- | _

So that wasn't very important.
Here's an equivalent circuit, with a Thevenin equivalent temperature of 100-24K/155 = -55 F:
1/155 ---www--------- T | | | | | -55 F | C --- --- - --- | | - _
If T = 81 = -55+(87-(-55))exp(-1.67h/RC), RC = -1.67/ln(136/142) = 39 hours, so C = RCxG = 39x155 = 6K Btu/F, a bit high for such a small house, unless you have some exposed brick or water-filled 55 gallon accent drums.
Nick
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I asked my HVAC contractor (who did NOT install the system in this house) the same basic question last week. Here's his email response. Extrapolate from what you see.
"A properly sized ac system will run almost non-stop when the outdoor temperature gets above 88. That's assuming the system was sized correctly to begin with. The design parameters for Upstate NY are a 15-20 differential at 88-90 outdoor ambient, which means that at 88 outdoor the system is only designed to maintain a 15-20 difference with the indoor temperature or approximately 73. It's not uncommon for systems to be oversized, so we get used to systems cycling when they really shouldn't. But the disadvantage to that is that when the outdoor temp is mid-80's and the humidity is very high the system won't run long enough to dehumidify the home. Actually a smaller system (or one closer to the design conditions described above) is preferred. If it runs a lot when the temp is near 90 that's fine - it's doing its job. Remember half of the battle of air conditioning is dehumidification which involves air moving over the ac coil. If it doesn't run long enough to circulate the air sufficiently you end up with a cool clammy/humid room."
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JoeSpareBedroom wrote:

I am a retired HVAC/R Contractor & Tech, let's see if we can figure something out.
We normally design for a 75-F 50 to 55% Relative Humidity at the Summer Design Conditions for Upstate Rochester, NY is 88 Dry Bulb 71 Wet Bulb. Where I live it is 88 DB 73 WB, a little more humid.
If my little Half-Ton 9.7-EER Room AC will bring nearly 900sf down to 76-F, 50 to 55% RH with outdoor conditions at +100-F with 104-F Heat Index. This is in an old 1937 home with a lot of windows. (With a Tougher Summer Design.)
Therefore, why does it take four times more BTUH to get far less results? I have witnesses, it performs. - udarrell
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If you expect MOST people to read what you just wrote, rewrite it for a magazine audience. Otherwise, you're just showing off.
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That may well be normal. There was probably a lot of humidity that had to be removed before it could get down to the business of lowering the temperature. Only good test is to see if it can hold a cool temperature through the day.
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Brick bungalow with how much insulation? Seems like it should be enough cooling, but 100 degrees and little or no insulation will make it struggle.
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R-30 or so in the attic. Nothing in the exterior walls. Built in 1954.

There's tons of the pink roll insulation over rock wool in the attic. It's not perfect insulation, but it's far from uninsulated.
Humidity was maybe 48%.
P
"Mit der Dummheit kaempfen Goetter selbst vergebens!" -Friedrich Schiller
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I'm surprised it did that well! But 2 tons for 800 sf is pretty big, unless you have no insulation.
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I have a ranch built of concrete block faced with granite. The walls are about R1. My experience is similar to yours, so now I simply never raise the thermostat setting in the summer. It does cost a ton of money to keep it that way, but my husband has some health issues.
Cindy Hamilton
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Puddin' Man wrote:

Well, according to my little Half Ton 6,000-BTUH 9.7-EER Rated window jammer that is cooling close to 900sf in an old 1937 home. At above 100-F & a 104-Heat Index, it will take the temp down to 76-F & 50 to 55% Relative Humidity, - that is comfort Plus!
You need a manual J Heat-Gain Calc done on your home, plus the ductwork checked & insulated if need be. Then, do everything possible to REDUCE the heat-gain, including the radiant solar heat, & the air infiltration rate.
No, I would NOT be at all satisfied with that performance with a 2-Ton 10-SEER System; "it appears to be," either the system or an out of control home heat-gain condition that needs attention!
A 1.5-Ton condenser with a 2-Ton evaporator and a TXV refrigerant metering device along with a Copeland Scroll compressor - should cool your home down to a perfect comfort level. http://www.udarrell.com/proper_cfm_btuh_duct_sizing_air_conditioning_systems.html
http://www.udarrell.com/air-conditioning-total-heat-enthalpy-latent-heat.html
Provide us the indoor temp drop & percent of Relative Humidity, also the outdoor condenser air discharge split! That will provide us some clues, but not a bullseye diagnosis. It is up to you to do the work here, - in order to gain some needed clues! - udarrell
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What it boils down to is that unless you have a grossly oversized system, do not expect to only turn it on when you get home and get rapid cool-down like you do when you get in your car. An air conditioning system should run almost continuously when outside temperatures are very high. Some installers will not even consider checking performance unless the system has been on for 24-48 hours without cycling off. It seems to me that your cool-down rate under those conditions is certainly adequate and probably more.
Don Young
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Don Young wrote:

Don, while you are probably correct concerning what installer's will not consider, - I do not go along with their reasoning. I always want to run all the checks to make sure that AC system is performing at, or very near, its rated BTUH. All those performance checks including the compressor discharge line temp, should always be completed upon completion of the installation.
A compressor could overheat to the point of breaking down the oil & also causing damage to the insulation of the compressor's motor windings. I have known of installations where the systems were allowed to operate for months & even a number of years with damaging & costly defective installs.
Do all the proper testing & then leave a record with the unit of its proper operating data, then if a problem occurs there is proper performance reference data for the tech. I also would leave easy ways for the owner to check basic operating performance of the AC system. Check my Web pages for some of them. - udarrell
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For where you live that A/C is plenty big enough except on realy hot days like the last couple of weeks. It's not designed to cool your house completey under extreme circumstances like this. Just do what most people would do and ust live with it since it's temporary in nature, or get a small window unit for your bedroom so you can get it cool enough to sleep at night.
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