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3 ton A/C unit too big?

TheoRichter's picture

I just had a York package unit delivered and installed today. While the tech was unboxing the unit he said he had some good news, they were giving me a 3 ton unit instead of a 2 1/2 ton as specified in the proposal. Now Im always up for a bargain but I have heard that it is counter-productive to oversize a unit beyond a certain point. My living area is just under 800 sq. ft. with another 250 sq. ft. that I would like to enclose and air condition in the future.My old unit was 26 years old and still working, I replaced it in hopes of saving money on my large electric bill. Is a 3 ton unit going to be effecient and save me money or did I just replace one expensive to run unit with another?

(post #109351, reply #1 of 15)

Only if you want to run a butcher shop on the side.!

(post #109351, reply #2 of 15)

Only way to know for sure is to get the complete specs for the 2 units from York or your dealer (don't think York has full specs on the web, IHS catalog service has them, but it's a fee) and compare the COP for typical temperatures in your area. Forget looking at Seer and that stuff.

As an example, in the 1-5T range, the Rheem 4T is more efficient that any of the others. Other makers will vary.

(post #109351, reply #3 of 15)

Where are you located?  I don't see 3 tons until houses are around 2,500 -3,000 sq feet, in NW Ohio.  I bet you'd need more in, say, anywhewre warmer.!

The problem with oversizing is the loss of effective dehumidification.


======================================== "Man's capacity for justice makes democracy possible, but man's inclination to injustice makes democracy necessary." Reinhold Niebuhr: 'The Children of Light and the Children of Darkness' ========================================

(post #109351, reply #4 of 15)

Probably shoulda mentioned I'm in SW Florida.4 seasons here, warm,hot,hotter, hottest. My house is south facing to boot. Older house (leaky) little or no insulation. (Will fix insulation and caulk)

(post #109351, reply #5 of 15)


3 tons is a lot of cooling for 1000 sf (2-1/2 is too, for that matter). Modern equiment is much more efficient that something 26 years old. You will not be able to find COP info for an AC unit since it doesn't apply. The SEER rating will tell you something to compare similar units. For instance, a 12 SEER unit is 20% more efficient than a 10 SEER unit, for the same conditions.

Does the 2.5 T unit run constantly on a very hot day? The difference between 2.5 and 3 is minor and if 2.5 is well suited for your house, the 3 isn't going to be too big. If the 2.5 is already too big, the 3 ton will be worse. Two problems with oversized AC's: the run time during "off peak, high humidity" loadsis too short to dehumidify and the space will be "clammy" and uncomfortable; and the constant cycling of the unit will be inefficient and wear out the equipment quicker.

What is really important is what is the cooling load on your house? Without that info, nobody can tell you what size AC you need.

Another question for the company doing the work: "Why can't you fill an order properly?" If they think there is no difference or cannot tell the difference, they have problems.

(post #109351, reply #6 of 15)

"not be able to find COP info for an AC unit since it doesn't apply"

HUH? -- you mean my Thermodynamics prof (and textbooks, and Trane manual, etc.) are spouting BS and last year's Rheem spec sheets were made up for AC??  The rest of your post is right on.

IMHO, SEER and HSFC are mostly meaningless numbers for marketing people and gov. bureaucrats to tout, had a heck of a time last year finding out the specific test parameters to get to SEER et. al.  As a specific example, if you buy by SEER for cooling but need winter HP operation also, a 4T Rheem 10 SEER actually is MORE efficient than a 12 SEER in the 30 F to 52F regime, so in some areas the 10 will show better than 12 over the entire year if you run all the basic numbers yourself.

Edited 3/30/2002 7:49:21 PM ET by JUNKHOUND

(post #109351, reply #7 of 15)

Three tons is three tons, no matter what the efficiency of the unit or how old it is. I can say it sounds big enough for sure though.

(post #109351, reply #9 of 15)



What I mean is that COP applies to HPs not AC only units. At least not that I've seen, but I'm always willing to be educated by someone who has more than I. COP also applies to chillers, but that's not what we're talking about.

Edited 3/30/2002 11:48:03 PM ET by Tim

(post #109351, reply #10 of 15)

I'm no expert in AC but if it was my house I would get a second opinion on the cooling load calculations and AC sizing. Check with your power company. The ones in this area will do an energy audit for free. The last audit I saw was quite detailed, they included all the worksheets, and gave an unbiased opinion for comparison.

If necessary get another contractor. Some contractors may do the analysis for a nominal cost. Handling the second analysis on the QT may be best. I would think a reputable contractor would not be overly miffed. If he gets rude I would toss him out. I think I would rather deal with an angry AC contractor than have to live in a damp, clammy, house and pay extra money to the power company every  month for the privilege of having an oversized AC unit.

(post #109351, reply #11 of 15)

Onlyest time I ever saw COP applied to HP and AC was when cousin clem was hiding some of that marijeweenie stuff in the ductwork down there at wal-mart.

You know, they almost throwed him in jail. I don't really understand why. Sales in the junk food aisles more than tripled. And nobody was fighting over shoes in the women's shoe department anymore.

Whattaya mean, I can't be three people at once ???!!?

(post #109351, reply #12 of 15)


Wasn't trying to criticize, only trying to make the point that over the last 30 years, engineering terminology has been so badly corrupted and dumbed down by marketers and bureaucrats that it's hard to get real technical data on retail items.


(post #109351, reply #13 of 15)

I don't have 30 years of experience in HVAC, but I know what is available in all the engineering data that I use regularly. I do not specify Rheem products and seldom see submitals for Rheem. Never see COP listed. If you want it, you have to calculate it. I do not know the details of how SEER is determined. So are you an HVAC installer, designer, contrator, or what?

(post #109351, reply #14 of 15)


I'm an aerospace engineer. See one of Ron Teti's thread on who I used to be. You are absolutely correct in that you have to calculate COP in most cases, which was my point.

Not in the HVAC business but have installed HVAC on volunteer basis in churches/private schools, relatives homes, etc. Have been part of teams that built Environmental Control Systems (HVAC is called ECS in the aerospace arena, everybody has a different terminology) for a few large military systems.

(post #109351, reply #8 of 15)

What about the duct work is it big enough? is the air handler did you get a Matching indoor coil ? How bout a heat loss heat gain run on the house ? that will tell all

(post #109351, reply #15 of 15)

Hi 123,

I got some news for you if it's not too late.  I have 1200 sq foot home.  It was always uncomfortable.  I had the thermostat set on 70 to make me feel like I had air condition.  I had a 3 ton unit with seer of 8.  It short cycled and never got rid of the humidity.  It is a older home I have 6 inches of insulation in the ceiling, and 4 inches in the floor.  No insulation in walls.  I just had to replace it because no parts are available.  The unit had a stainless steel heat exchanger.  The galv. flue box kept rusting out.  Put a new 2 1/2 ton unit in and am comfortable at 75.  The installer corrected many items that should of been done on the first installation.  They used some type of masking liquid and closed off someleaking return items.  They insulated the return. No more short cycling.  New unit is 12seer.  Hope this helps.  Larry

Edited 4/14/2002 8:51:07 PM ET by Larry