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Size of A/C Unit Tom in PA 07-01-2006
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Posted by udarrell on July 2, 2006, 9:53 am
Tom in PA wrote:

>The house is 2076 square feet, new construction, two-story with most of
>the square footage on the ground floor. R-19 insulation in exterior
>walls and ground floor joints, R-30 in ceiling. One big air return,
>about 12 registers. 2x4 (I'm pretty sure, a slight chance it's 2x6)
>frame construction. Furnace in crawl space under house. Have some
>large windows in great room and a glass slider in dining room but these
>all face north, otherwise normal sized windows. Located in Flagstaff,
>AZ, elevation 6900'. Average high temperature in July (the hottest
>month): 82 degrees. All-time record high: 97. Humidity: tends to
>be quite low.... this is the Southwest. This is a town where most
>people don't have A/C, and the unit won't be heavily used. You can
>usually get by with your windows open and ceiling fans. It just gets a
>little toasty now and then, which is why we want the air.
>
>Once contractor says a 3 ton unit is the way to go, another says 5 ton.
> Any opinions regarding who is right? Thanks! Tom in PA
>
>
Here in SW WI we have 2-ton systems on 2000-square footage homes' and
they have proper airflow and cool them perfectly!
Occasionally, we have hot 95-F muggy weather with a heavy latent load,
the 2-ton still gets by, and is optimally sized for 95% of its runtime.

The Summer Design for Flagstaff, AZ (airport), 82-F dry bulb, 55-F wet
bulb or around 13% Relative Humidity.
With that low a humidity, I am betting a 2-Ton with plenty of airflow
would handle any hot weather you have.

Someone needs to do an accurate load calc and sizing job! Never more
than 2.5-Ton.
- udarrell

--
Air Conditioning's Affordable Path to the "Human Comfort Zone Goal"
http://www.udarrell.com/air-conditioning-total-heat-enthalpy-latent-heat.html
http://www.udarrell.com/ac-trouble-shooting-superheat-subcooling.html

Posted by Noon-Air on July 2, 2006, 10:00 am

> Tom in PA wrote:
>
>>The house is 2076 square feet, new construction, two-story with most of
>>the square footage on the ground floor. R-19 insulation in exterior
>>walls and ground floor joints, R-30 in ceiling. One big air return,
>>about 12 registers. 2x4 (I'm pretty sure, a slight chance it's 2x6)
>>frame construction. Furnace in crawl space under house. Have some
>>large windows in great room and a glass slider in dining room but these
>>all face north, otherwise normal sized windows. Located in Flagstaff,
>>AZ, elevation 6900'. Average high temperature in July (the hottest
>>month): 82 degrees. All-time record high: 97. Humidity: tends to
>>be quite low.... this is the Southwest. This is a town where most
>>people don't have A/C, and the unit won't be heavily used. You can
>>usually get by with your windows open and ceiling fans. It just gets a
>>little toasty now and then, which is why we want the air.
>>
>>Once contractor says a 3 ton unit is the way to go, another says 5 ton.
>> Any opinions regarding who is right? Thanks! Tom in PA
>>
> Here in SW WI we have 2-ton systems on 2000-square footage homes' and they
> have proper airflow and cool them perfectly!
> Occasionally, we have hot 95-F muggy weather with a heavy latent load, the
> 2-ton still gets by, and is optimally sized for 95% of its runtime.
>
> The Summer Design for Flagstaff, AZ (airport), 82-F dry bulb, 55-F wet
> bulb or around 13% Relative Humidity.
> With that low a humidity, I am betting a 2-Ton with plenty of airflow
> would handle any hot weather you have.
>
> Someone needs to do an accurate load calc and sizing job! Never more than
> 2.5-Ton.
> - udarrell

Ummm.... Darryll, how do you figure "Never more than 2.5-Ton" on a 2000sqft
home?? Care to come down here and run a couple of loads for me??




Posted by DIDO on July 2, 2006, 12:00 pm

> Hi!
>
> I'll start by saying that I know the correct answer is "consult a
> professional to do a load calculation", but I've consulted two
> professionals who are coming up with much different answers so I'm
> feeling stuck.
>
> The house is 2076 square feet, new construction, two-story with most of
> the square footage on the ground floor. R-19 insulation in exterior
> walls and ground floor joints, R-30 in ceiling. One big air return,
> about 12 registers. 2x4 (I'm pretty sure, a slight chance it's 2x6)
> frame construction. Furnace in crawl space under house. Have some
> large windows in great room and a glass slider in dining room but these
> all face north, otherwise normal sized windows. Located in Flagstaff,
> AZ, elevation 6900'. Average high temperature in July (the hottest
> month): 82 degrees. All-time record high: 97. Humidity: tends to
> be quite low.... this is the Southwest. This is a town where most
> people don't have A/C, and the unit won't be heavily used. You can
> usually get by with your windows open and ceiling fans. It just gets a
> little toasty now and then, which is why we want the air.
> Once contractor says a 3 ton unit is the way to go, another says 5 ton.

That is so esey get one between (4 ton) sound good to me
dido

> Any opinions regarding who is right?
>
> Thanks!
>
> Tom in PA
>



Posted by Murdentech on July 4, 2006, 4:30 pm

> Hi!
>
> I'll start by saying that I know the correct answer is "consult a
> professional to do a load calculation", but I've consulted two
> professionals who are coming up with much different answers so I'm
> feeling stuck.
>
> The house is 2076 square feet, new construction, two-story with most of
> the square footage on the ground floor. R-19 insulation in exterior
> walls and ground floor joints, R-30 in ceiling. One big air return,
> about 12 registers. 2x4 (I'm pretty sure, a slight chance it's 2x6)
> frame construction. Furnace in crawl space under house. Have some
> large windows in great room and a glass slider in dining room but these
> all face north, otherwise normal sized windows. Located in Flagstaff,
> AZ, elevation 6900'. Average high temperature in July (the hottest
> month): 82 degrees. All-time record high: 97. Humidity: tends to
> be quite low.... this is the Southwest. This is a town where most
> people don't have A/C, and the unit won't be heavily used. You can
> usually get by with your windows open and ceiling fans. It just gets a
> little toasty now and then, which is why we want the air.
>
> Once contractor says a 3 ton unit is the way to go, another says 5 ton.
> Any opinions regarding who is right?
>
> Thanks!
>
> Tom in PA
>

Somewhere between 3 and 5 tons... LOL. Actually, you are probably closer to
5 with that square footage... figure one CFM per square foot at 450 CFM per
ton. Puts you at 4.6 tons. However, there is not a 4.5 ton unit
manufactured. Also, you are in a low humidity region, so that will lower
your calcs. Yet, there are other factors to consider. If you want to be
exact, then have a Manual J calculation performed on the structure and you
will be sure to have a properly sized system.

If what you say is true about a/c not being needed very often, then you will
probably end up being satisfied with a 3.5 or 4 ton unit with a 5 ton blower
inside. IMHO you can never have too much airflow thru the system,
particularly since you have a relatively low latent load.




Posted by Fat Eddy on July 4, 2006, 11:18 pm

Tom in PA wrote:
> Hi!
>
> I'll start by saying that I know the correct answer is "consult a
> professional to do a load calculation", but I've consulted two
> professionals who are coming up with much different answers so I'm
> feeling stuck.
>
> The house is 2076 square feet, new construction, two-story with most of
> the square footage on the ground floor. R-19 insulation in exterior
> walls and ground floor joints, R-30 in ceiling. One big air return,
> about 12 registers. 2x4 (I'm pretty sure, a slight chance it's 2x6)
> frame construction. Furnace in crawl space under house. Have some
> large windows in great room and a glass slider in dining room but these
> all face north, otherwise normal sized windows. Located in Flagstaff,
> AZ, elevation 6900'. Average high temperature in July (the hottest
> month): 82 degrees. All-time record high: 97. Humidity: tends to
> be quite low.... this is the Southwest. This is a town where most
> people don't have A/C, and the unit won't be heavily used. You can
> usually get by with your windows open and ceiling fans. It just gets a
> little toasty now and then, which is why we want the air.
>
> Once contractor says a 3 ton unit is the way to go, another says 5 ton.
> Any opinions regarding who is right?
>
> Thanks!
>
> Tom in PA



FatEddy www.hvactalkforum.com


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