I don't understand tonnage measurements for central air conditioners. I have a 1500 sq foot house, 3 bedrooms and 2 baths, Oklahoma summers average hot day is 93* according to Trane's website. How much AC do I need? 2 ton, 3 ton, 5 ton?

I vote for as big as you can get

Quoted: I vote for as big as you can get

Wrong answer. THe AC unit needs to run for a sufficient period of time to dehumidify the structure. It's not just about dumping lots of cold air. There are calculations that must be done to size a unit properly.

Most of the websites discourage overkill with the AC due to "short cycling" the compressor which results in the AC cooling the house too fast and not enough moisture being removed from the air causing mold and other problems, so oversize won't do...

Any HVAC people in the forum tonight?

I vote for as big as you can get

Agreed.  The HVAC guys love to sell you undersized units to make sure you'll soon be calling them back to pay them more money.  I've had too many relatives and friends that were ripped-off by HVAC people.  I've also managed rental property for decades, and I've seen the crap they pull too many times.  For example, I recommended to my great-nephew to buy a 4-ton unit for his house.  The idiot HVAC guy said he only needed a 1.5-ton unit.  I know from experience that a 1.5-ton unit isn't big enough around here for a 800 sq foot apartment that has several large south-facing windows.  Well, my great-nephew ended-up replacing the unit two years later.  The other professional whined about having to put in a large enough unit.  Well, he bought the 5-ton two years ago and is very happy with it.

Another example, the owner of my company just built a 5,000 sq foot house.  The idiot HVAC contractor put a smaller unit than I've used before in an apartment.  The contractor had a HVAC professional engineer(PE) approve the plans, so my boss decided to go ahead and use the tiny unit even though he knew it was wrong from experience.  He already has trouble keeping his house cool, and it's just May.  Since the unit is already running 24/7, it probably isn't going to last through the summer.  I know my boss's patience isn't.

Add multiple return vents.  The idiot HVAC guys now are too lazy to put in more than one.  Every contractor I've worked with whined about having to add more.  The units work better with lower return grille velocities, and I've found that good air exchange is important to comfort even though the HVAC guys disagree.

Also, use some common sense when sizing the unit.  The "professionals" never seem to do that.  If you have a house with little shade, then buy a bigger unit.  If you have a lot of windows, especially those that face south, buy a bigger unit.  If you keep the house warmer in the summer and colder in the winter to save money, then buy a slightly smaller unit.

Make sure the condenser is properly sized to the coil.  In 1998, I found that a local contractor had ripped us off 97 different times (yes, 97!).  They used a York 1 ton condenser with a 1.5 ton coil.  We found-out about it the hard way when tenants started complaining about their AC.  It was late July before we got some of the condensers replaced, and those apartments were over 80 degrees with the A/C running full blast.  We sued the contractor and lost since their PE testified that their calculations showed you only need 3/4 of a ton of AC for our apartments.    The PE claimed that even though the 1 ton units were not big enough in practice, that 3/4 ton was all they should have installed.  We had the quote for 1.5 ton units, but facts apparently don't matter to HVAC people.

Also, make sure you look the ductwork over carefully for leaks.  I haven't seen a contractor in years that does a competent job with the ductwork.  I found more than three dozen leaks in my boss's new house, and our maintenance guy spent more than a month fixing leaks in an apartment complex my boss bought a few years ago.z

I had a 3 ton unit for my 1440 SF, 4 bedroom, 2 story condo in San Diego Co. Worked fine and wasn't excessive in energy use. be sure to get a good programmable thermostat.

Quoted: Most of the websites discourage overkill with the AC due to "short cycling" the compressor which results in the AC cooling the house too fast and not enough moisture being removed from the air causing mold and other problems, so oversize won't do... Any HVAC people in the forum tonight?

+1


IM "Ops" for some advise.

My buddy ownes an AC biz. He uses a formula that includes alot of different factors. You can't go by sq. footage alone.
Get someone to come out and "run a load"( that's his term) on your house. If they don't come out and take measurements and go back and plug the numbers into a computer you might have problems in the future.

YMMV

Quoted: I vote for as big as you can get Agreed.  The HVAC guys love to sell you undersized units to make sure you'll soon be calling them back to pay them more money.  I've had too many relatives and friends that were ripped-off by HVAC people.  I've also managed rental property for decades, and I've seen the crap they pull too many times.  For example, I recommended to my great-nephew to buy a 4-ton unit for his house.  The idiot HVAC guy said he only needed a 1.5-ton unit.  I know from experience that a 1.5-ton unit isn't big enough around here for a 800 sq foot apartment that has several large south-facing windows.  Well, my great-nephew ended-up replacing the unit two years later.  The other professional whined about having to put in a large enough unit.  Well, he bought the 5-ton two years ago and is very happy with it. Another example, the owner of my company just built a 5,000 sq foot house.  The idiot HVAC contractor put a smaller unit than I've used before in an apartment.  The contractor had a HVAC professional engineer(PE) approve the plans, so my boss decided to go ahead and use the tiny unit even though he knew it was wrong from experience.  He already has trouble keeping his house cool, and it's just May.  Since the unit is already running 24/7, it probably isn't going to last through the summer.  I know my boss's patience isn't. Add multiple return vents.  The idiot HVAC guys now are too lazy to put in more than one.  Every contractor I've worked with whined about having to add more.  The units work better with lower return grille velocities, and I've found that good air exchange is important to comfort even though the HVAC guys disagree. Also, use some common sense when sizing the unit.  The "professionals" never seem to do that.  If you have a house with little shade, then buy a bigger unit.  If you have a lot of windows, especially those that face south, buy a bigger unit.  If you keep the house warmer in the summer and colder in the winter to save money, then buy a slightly smaller unit. Make sure the condenser is properly sized to the coil.  In 1998, I found that a local contractor had ripped us off 97 different times (yes, 97!).  They used a York 1 ton condenser with a 1.5 ton coil.  We found-out about it the hard way when tenants started complaining about their AC.  It was late July before we got some of the condensers replaced, and those apartments were over 80 degrees with the A/C running full blast.  We sued the contractor and lost since their PE testified that their calculations showed you only need 3/4 of a ton of AC for our apartments.    The PE claimed that even though the 1 ton units were not big enough in practice, that 3/4 ton was all they should have installed.  We had the quote for 1.5 ton units, but facts apparently don't matter to HVAC people. Also, make sure you look the ductwork over carefully for leaks.  I haven't seen a contractor in years that does a competent job with the ductwork.  I found more than three dozen leaks in my boss's new house, and our maintenance guy spent more than a month fixing leaks in an apartment complex my boss bought a few years ago.z

Get a new contractor.

If too small you'll have problems and if too large you'll have different problems.

A good rule of thumb is 1 ton for every 500 sq. ft.You need a 3 ton in your place.

Sorry to be so long!

  I guess I got on just in time. As for Tonnage. One Ton of cooling = 12,000 BTUH. That's British Thermal Units per Hour. Usually for a typical house with a eight foot ceiling thru out. You could go by the old rule of thumb of One Ton of cooling per every 500 sq. ft. If all or half of your house has valted ceilings then that rule is thrown out. You can also get the model number of your existing unit and you should be good to go. Or you can have a load calculation done on your house. Most all manufactures have the tonnage of a system in the model number. Such as 024 = 2 Ton, 030 = 2 1/2 Ton, 036 = 3 Ton, 042 = 3 1/2 Ton, 048 = 4 Ton, and 060 = 5 Ton. A five ton unit is as big as a residential unit gets. If your house is bigger than 2,500 sq. ft. , you'd need multipile units.

 I lived in and worked as a A/C contractor in Oklahoma about 15 years ago. The humidity is a bitch. Also a A/C is a De-humidifier. So you don't want to over size. Forget about Tim the Tool man Taylor.

 So for your 1,500 sq. ft. home. Go with a Trane 3 Ton A/C. Now that we have your tonnage size. The next step is to decide on the SEER . That stands for Seasonal Energy Efficeincey Ratio. On the market today there is 10 SEER =lowest, 12 SEER = Builders model. These two 10 and 12 SEER will no longer be manufactured after January,2006. So my advice is to go with at least the XL14i. This should come with the indoor Variable Speed indoor blower. It's nice and quiet and helps ring out humidity out of your house. Next is the Trane XL/XR19i. This is the top of the line unit. It's a two stage cooling system. It comes with a Variable speed indoor and outdoor blower/condensing motors. It also come with two compressors. A 1 1/2 ton for low speed and 3 ton for high speed.

 As I advise everyone, call the register of contractors and the BBB, before calling any HVAC  company out to give you a proposal. Only use licensed, bonded and insured contractors. Get at least three bids, after above screening.

 Also If you go with the 19i / 18 SEER unit. You more than likely will have to increase your duct return size and line set size. If you don't, then that 18 SEER will only give you about 15 SEER.

 If you have any more questions lay it on me.

 One more thing, get a complete price/no hidden cost. Replacing a complete system should consist of the following. New unit, complete removal of old unit, tax, labor, new pad for outside unit if needed, new electrical disconnect, new wiring from disconnect to new unit, new fuses, new breaker, new electrical pigtail for indoor unit, re-piping and flushing out of condensate line, re-piping of gas line, re-piping of  new flue pipe, and new thermostat. Also pay the extra $350.00 or $450.00 for Tranes 10 year parts and labor warranty. This way no cost to you for ten years after buying new unit. WoW

I just purchased a 2.0 Ton for my 1500 sq foot house.   It was a Trane XR11?  mated to an XE90 furnace.   In OR we only get a few really hot days, mostly 90s in the summer and not terribly humid.   They figured 8 registers, did a bunch of measurements and had it installed in a day.   Never had central AC prior to that.   Hit me for $2600, it was within $150 of the cheapest contractor but an established company.   I have no idea how well my setup will work though as it hasnt broke 70 since it was installed last week.   Hopefully it'll be the right setup, if not I hope they care about their reputation.

Also,get the highest SEER rating you can afford.It will definately pay you back and then some on your electrical bill.

Quoted: I vote for as big as you can get Agreed.  The HVAC guys love to sell you undersized units to make sure you'll soon be calling them back to pay them more money.  I've had too many relatives and friends that were ripped-off by HVAC people.  I've also managed rental property for decades, and I've seen the crap they pull too many times.  For example, I recommended to my great-nephew to buy a 4-ton unit for his house.  The idiot HVAC guy said he only needed a 1.5-ton unit.  I know from experience that a 1.5-ton unit isn't big enough around here for a 800 sq foot apartment that has several large south-facing windows.  Well, my great-nephew ended-up replacing the unit two years later.  The other professional whined about having to put in a large enough unit.  Well, he bought the 5-ton two years ago and is very happy with it. Another example, the owner of my company just built a 5,000 sq foot house.  The idiot HVAC contractor put a smaller unit than I've used before in an apartment.  The contractor had a HVAC professional engineer(PE) approve the plans, so my boss decided to go ahead and use the tiny unit even though he knew it was wrong from experience.  He already has trouble keeping his house cool, and it's just May.  Since the unit is already running 24/7, it probably isn't going to last through the summer.  I know my boss's patience isn't. Add multiple return vents.  The idiot HVAC guys now are too lazy to put in more than one.  Every contractor I've worked with whined about having to add more.  The units work better with lower return grille velocities, and I've found that good air exchange is important to comfort even though the HVAC guys disagree. Also, use some common sense when sizing the unit.  The "professionals" never seem to do that.  If you have a house with little shade, then buy a bigger unit.  If you have a lot of windows, especially those that face south, buy a bigger unit.  If you keep the house warmer in the summer and colder in the winter to save money, then buy a slightly smaller unit. Make sure the condenser is properly sized to the coil.  In 1998, I found that a local contractor had ripped us off 97 different times (yes, 97!).  They used a York 1 ton condenser with a 1.5 ton coil.  We found-out about it the hard way when tenants started complaining about their AC.  It was late July before we got some of the condensers replaced, and those apartments were over 80 degrees with the A/C running full blast.  We sued the contractor and lost since their PE testified that their calculations showed you only need 3/4 of a ton of AC for our apartments.    The PE claimed that even though the 1 ton units were not big enough in practice, that 3/4 ton was all they should have installed.  We had the quote for 1.5 ton units, but facts apparently don't matter to HVAC people. Also, make sure you look the ductwork over carefully for leaks.  I haven't seen a contractor in years that does a competent job with the ductwork.  I found more than three dozen leaks in my boss's new house, and our maintenance guy spent more than a month fixing leaks in an apartment complex my boss bought a few years ago.z

Ouch!  

I agree , 3 tons ought to do it. Carbine gave good advice but if you want a trane a/c your going to pay extra for it. Shop around for contractors.

Without knowing about the construction or type of the house or any more about the house Carbine is on the money.

How important is local climate to selection of tonnage?   I'm just curious why they came up with 2 ton for my 1500 sq ft home.  

3 ton will be enough. make sure you have a return in every room as well. The system will cool  better  that way. For an added bonus have a ( the name escapes me at the momment) system that cycles the hot air from the compressor  through the water heater. that way you can heat your water without actually having the water heater run. that will help with your electrical bill.


J

Quoted: How important is local climate to selection of tonnage?   I'm just curious why they came up with 2 ton for my 1500 sq ft home.

Have you ever been in Oklahoma in July?

ETA: I was installing 2 ton units for 1000 sq.ft. apartments at my last property. North and east 3rd. floor units worked great. South and west units still had a hard time holding their own even with matched coil and condenser units. The thing that made me shake my head was that I was ripping out 1.5 ton units in them.

We have 2400 sq. ft and a 4-ton unit...works just fine even on the hottest days...