AC Window Unit Size, 10k or 12k?

Old 06-08-10, 11:39 AM
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Question AC Window Unit Size, 10k or 12k?

I live in Tucson, AZ where the temp hovers around 105F+ (40.5C). It's super dry outside until monsoon season rolls around.

Anyway, my central AC doesn't seem to want to stop running. My electricity bill comes in at around $200/mo. during the summer months (which seems to be a 8 mo. season in Souther AZ.) The kicker is, my house is only 974sqf of living area. My wife is a stay at home mom and likes the air temp no higher than 74F.

As she spends most her time in the Living/Dining/Kitchen area (all connected thus ONE Room in my house), I'm really thinking about getting a Window AC Unit.

The total sq. footage I need cooled is exactly 355sqf. Just basing off sqf, an 8000-BTU unit would suffice.

Thing is:

1- There's a kitchen attached (but incredibly small, seriously)

2- Vaulted Ceilings (12ft high at it's peak)

3- The way my house is orientated, ALL exterior walls get sunlight. (Corner of my square house faces almost directly to the rising sun in summer.)

4- It gets EXTREMELY HOT in the summer time outside, BUT... my house is new-ish (built 1998), and has good insulation, and double-pane windows.

I'm THINKING about a 10,000-BTU unit, but will I better be served with a 12,000-BTU?

Though it is dry here, I don't want to lose efficiency with an over-sized unit that's not de-humidifying. On the other-hand, I don't want to run into the same situation now where the AC unit NEVER stops running.

The unit will be placed in the Dining Room (which seperates the living from kitchen area).

PLEASE! Help me decide!
Old 06-08-10, 12:12 PM
Tolyn Ironhand's Avatar
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Not helping you decide, but have you had anybody do a checkup on your AC system? Most service places can come out and get your system running in top shape for about $150 or less I would guess. Call around and get some prices for an AC tune up.
Old 06-08-10, 05:45 PM
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How big is your central a/c? I don't think a window unit is going to be more efficient than your central a/c. Especially since it will be very leaky around the unit. You will also wind up with really hot bedrooms.
Old 06-09-10, 10:45 AM
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Even in AZ IMO it shouldn't cost that much to cool a 974-sf home

Even in AZ, IMO it shouldn't cost that much to cool a 974-sf home! Also, try 76-F with floor fans moving air.

The first procedure is always to do a manual J room by room heat-gain calc with the option shown so you can do everything possible to reduce all sources of heat-gain & heat-loss, greatly reducing both heat & cooling BTUH equipment sizing.

A reduction in equipment sizing will usually greatly improve the duct system performance.

The duct system airflow must be checked & be correct, before performing any charging procedures!

This ordered procedure must be followed to achieve an optimally charged efficient operating air conditioner.

Do not leave out any of the steps and always do these procedures in the order sequence illustrated.

First, check that there are NO air leaks in the Supply and Return Air duct system.

Next, Check to see if Indoor Squirrel Cage Blower wheel blades are free of lint or other build-up & Filter.
Before checking the refrigerant charge always check the actual air flow with an anemometer & apply the math shown below!

Check for a dirty lint clogged Evaporator Coil fins then check the Condenser Coil fins, check both coils on the air entering sides as well as between the fins, if needed, get it cleaned.

At 50 relative humidity indoors & normal airflow the temp drop should be 18 to 20-F. On a 12-SEER condenser, the outdoor condenser discharge air temp also should be 18 to 20-F.

On smaller tonnage equipment & in most climates that are not overly humid, I like 425 to 450-CFM per/ton of cooling on a wet coil.

Contact me...
Old 06-10-10, 09:28 AM
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Thanks for the tips.

I have no idea what the size of my central AZ unit is. It looks really small, but I'm not an hvac person.

I did find I had some insulation leaking around the door's weatherstripping, so I've replaced that. I shouldn't say my central AC "never" stops running. It's just as soon as the inside air hits 74F, my ac kicks off, but my house immediatly jumps to 75F, and it kicks on again. This is a problem because even at 10pm at night, it's near 100F outside!

All my windows are double-paned, I have heavy curatins, UV window film, and make sure they are closed properly. Hopefully, replacing my door weatherstripping will help.

I'm not familiar with Manaul J calcs (but I've read about it while investigating about purchasing an AC window unit).

I have no attic, so the duct work is impossible to get to. I can't even find the indoor blower. The furnace is even in the ceiling's crawl space.

I'm just suprised that a 10k BTU unit only comsumes 925watts of power under load.

My computer (I'm a gamer), which has an overclocked quad-core CPU, overclocked dual video cards, 4-fans, 2x HDs, and energy savings disabled, consumes around 550watts of power when under load.

I'm really looking at Energy Star certified AC Frigidaire's 10k BTU (FRA106CT1).

The only really cool room is the master bedroom and the hallway (where the thermostat is). So when the hallway is around 75F, the living room (which is the hottest) is about 2F degrees warmer. So to compensate, we turn the thermostat down.

Our plan is, getting a window ac unit to cool down the living/dining/kitchen, and turning the central AC up to 80F.

Question: How much wattage does an AC unit the entire system uses under load for say... a 1-ton unit?

It's just.. winter time, and highest Electric bill is $50. Then come June, and it's topping $200.

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