Why a Bigger A/C When Current one Cools Adequately???

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O.K. Experts--- So, far 2 contractors have come out to my Tidewater VA 2280 square foot home currently cooled by a 30 year old Kenmore 3 ton unit. I am replacing it before it dies as the wrong time, and will probably replace the ducting also. Both gave me the Rule of Thumb (which is also used by Virginia Power), 1 ton/600 sq. ft., therefore 4 ton unit should be installed. One did a Manual J and came up with the same size. He doesn't understand why and is coming back out with his boss and his laptop on Monday. Now, the Sears runs much (but not all) of the time at an ambient air temp of 90+ but does cool. the house to the 75 degrees it is set for and keeps the inside humidity at a good level. I have been told in the past (and don't know the basis for that either) that a unit "works better" when running much of the time All y'all have have constantly said---Don't get a unit that is too big, it is worse than too small and among other things will not properly dehumidify the house. So, is a 1970's 3 ton unit, bigger than a 2004 3 ton unit (both specify 36K BTU). Why the difference. I could see maybe going with a 3 1/2 ton unit max---but I feel I'm on shaky ground even doing that. Help please! I would be like to be able to hold an intelligent discussion with these gents on Monday
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Roy wrote:O.K. Experts---

They've made some efficiency developments in the past coupla' decades...Check SEER ratings. Tom Work at your leisure!
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I don't know much about HVAC, except that we just had our 3 ton unit replaced with another 3 ton unit. Neither one had any problem cooling our 2800sf house to 74 on a 95 degree day. Unless it is regularly 110 degrees there, I think 4 tons for 2200sf is a bit high.
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royroy wrote:

3 tons is 36,000 BTU, regardless of the make, model, or SEER rating of the unit. "Rules of thumb" don't work for sizing air conditioners. You have to do a heat gain calculation.
Ask the contractor that did the Manual J to walk you through his input to the program. Check the design temperatures. Make sure s/he didn't "pad" the results with any kind of safety factor.
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Rules of thumb are BS. If 3 ton is good , why even consider 4. I have a 2 ton 1825 sq ft super insulated , yesterday was 90 with 100 heat index, I still had to run 2 new dehumidifiers to add load and remove moisture. I had no load calc done or I would have gotten a 1 ton. Figure your new unit will work even better than the old. Get someone that knows what they are doing instead of guessing. That rule of thumb you mentioned is only a guess.
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So, the rule of thumb is, ignore rules of thumb?
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On 23 Jul 2004 23:30:24 GMT, snipped-for-privacy@aol.com (HA HA Budys Here) wrote:

...eggs_zack_er_eee !
BTZ
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Except for the rule of thumb that "when installing, take extra care not to smash your thumb."

--
Mike

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On Sun, 25 Jul 2004 04:41:12 GMT, The Michael

..impossible in Plum Pie :-D
BTZ
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m Ransley wrote:

If its too big, it won't run log enough to remove moisture. Humidity is what you want to get rid of, not necessarly making it cooler.
My house is usually set at 80, and the ac unit runs enough at that temp to dry it out nicely. Saves electricity that way also.
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royroy wrote:

This one that did the Manual J is the ONLY one you should consider at this time. Rule of thumb is worthless. A Manual J is what is needed. (You will need a Manual D for the ductwork evaluation)
Yes, it could be he erred or it could be you are on the edge with what you have. Hard to tell from here.
Don't let someone size your equipment by rule of thumb and don't use anyone who wants to.
--
Joseph E. Meehan

26 + 6 = 1 It's Irish Math
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Good.
Then both dont havea damn clue...the ONLY Rule of Thumb is that most people have two. There are NO RULES OF THUMB in HVAC...NONE...

No..it removes more humidity the longer it runs...

Righto..
36K is 36K... The higher SEER ratings do not matter. IF you are replacing a heat pump, Manual J is the wrong one. Manual T is the one that needs to be ran.

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Did he SHOW you the manual J that he did or did he just SAY he did one?
You CANNOT go by rule-of-thumb. I have a 2300 sq ft house and my unit is a 2 1/2 ton. It works perfectly (has for 22 years).
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Dr. Hardcrab wrote:

Would a 2300 sq ft house with 11ft ceilings require more tonnage than a 2300 sq ft house with 8ft ceilings?
I would guess the heat leakage (transfer??) of the house might be higher, thus requiring a bit more tonnage.
Seems to me there'd be a lot more air to 'condition' in a house with higher ceilings too.
a couple of guesses by, mikey.
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Hi mikey, hope you are having a nice day
On 25-Jul-04 At About 23:33:28, mikey wrote to All Subject: Re: Why a Bigger A/C When Current one Cools Adequately???
m> Would a 2300 sq ft house with 11ft ceilings require more tonnage than m> a 2300 sq ft house with 8ft ceilings?
Not necessarily. there are quite a few factors involved in sizing an A/C system. it could be that they both take the same size. it all depends on insulation, house orientation etc...
-=> HvacTech2 <=-
.. URA Redneck if your house warming involves removing the tires.
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Thank yoiu for the many replies to my initial message. Here's what I got out of it. New systems are more efficient, i.e. higher SEER Knew that. Really what lead to the initial question. I can't understand why a more efficient system needs to be bigger than a less efficient system. I'm looking at about 13-14 SEER. My current unit is what---perhaps 5?
Rules of Thumb. I know they don't hold water. I would suspect that some numbers are being fudged to drive the results to that desired.
Manual J: He took the measurements he wanted with him. It was his working them out in the office that caused him to say he didn't believe what he got, because it was too high and he was in a comfortable house on a hot day. Hence he is coming back with his boss and laptop.
For sake of discussion (for which I thank you profusely), cqn anyone think of some factor(s) that would necessitate a bigger system in current house. I know you can't see it. It is a brick ranch with 2280 sq. ft. of A/C space. Pretty well insulated. 267 sq ft of Anderson double pane windows and sliding glass doors plus 137 sq feet of 1/4" plate glass in family room (this room is a bitch to cool, due the glass) Looks like a separate zone, thermostat, vent dampers will be the best solution for this one. I'm not looking for everyone to "do the math". What I'm trying to get to is a decision I'm comfortable with that it will be a cold day in hell before I let anyone put in a bigger system than what is there now. So I grasping at what kind of straws could drive it the other way To me, a bigger system means less running, hence uneven cooling and more humidity. So, does not a higher SEER same size system also mean less running and more humidity? Maybe I need to go down in size with the efficiency I will gain. Thanks again for your efforts to get this clear in my mind
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royroy wrote: /snip/

I think we said this before, but the energy efficiency has nothing to do with BTU output (and therefore, nothing to do with run times). The efficiency comes from consuming less electrical power to produce the same heat removal in BTU.
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royroy wrote:

I can't think of one, but I can't rule it out.
--
Joseph E. Meehan

26 + 6 = 1 It's Irish Math
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I haven't previously engaged in this discussion but I offer my opinion.
SEER has absolutely nothing to do with the amount of cooling you'll get per ton of a/c. It has only to do with how much it will cost you to run it.
If you're conmpletely satisfied with the performance of your current system, logic would dictate that you do not need a larger one.
It's possible that they are recommending a larger unit because of the hard to cool family room. Diverting air flow from the rest of the house to increase it in this space could possibly increase temperature in othe areas.
Bottom line, AFAIC, is if you're skeptical of the recommendations you're getting, have other contractors calculate and recommend. If they all tell you essentially the same thing, then there must be a reason.
You can get opinions from posters here in a.h.r., but they are only unfounded opinions unless they have personally been to your house and calculated your heat/humidity load. Stick with the pros who can actually be in your house and rely on their judgement. Several estimates should give you enough information to make a decisions.
--
Wayne in Phoenix

If there's a nit to pick, some nitwit will pick it.
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You would be better off winter and summer and get rid of that 137 sq ft of 1/4" R -1 plate glass and get Low E Argon that has a R 3.4 Or a Tri Pane of R 5 If that room gets any sun the temp loading will take 10 -20 f off that room easily with the benifits of Low E Argon. Cellular shades can be added for an additional 3.2 R . Its allot. With Tri pane and cellular shades you have an equivalant of R 8.2 or apx 2.5" of fiberglass insulation. That will reduce your load and you will be sure not to need extra cooling there. But if 3 tons works now , It will work fine with a new unit.
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