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Incorrectly sized AC unit?

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Incorrectly sized AC unit? chrisbirkett 07-28-2006
`--> Update Chris Birkett08-03-2006
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Posted by Noon-Air on July 30, 2006, 1:56 pm

>> Everyone looked at me like I was stupid when I said I actually do the
>> load calc.
>
> 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.

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.



AppliancePartsPros.com, Inc.
Posted by Travis Jordan on July 29, 2006, 1:00 pm
chrisbirkett@hotmail.com wrote:
> 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.

http://www.fsec.ucf.edu/bldg/pubs/ACsize/index.htm



Posted by Travis Jordan on July 29, 2006, 1:11 pm
chrisbirkett@hotmail.com wrote:
> 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.

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.



Posted by Chris Birkett on July 29, 2006, 3:32 pm
Travis Jordan wrote:
> chrisbirkett@hotmail.com wrote:
> > 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.
>
> 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.

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


Posted by Noon-Air on July 30, 2006, 2:00 pm

> Travis Jordan wrote:
>> chrisbirkett@hotmail.com wrote:
>> > 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.
>>
>> 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.
>
> 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

You can expect the system to *MAINTAIN* 75 degrees @50%RH 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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