Manual 'J'

Have read several posts that state that the manual j oversizes systems. Please tell me how they are oversized ?
In my world, it seems to undersize them.

I bought a ten year old home wih 1950 sq. ft. The system was a 2.5 ton HP. At 85 degrees or higher, would not bring the temp below 75, at 90'f' would not go below 78. It was installed per manual 'j', per the paperwork from the original homeowner.
In my experience, manual 'J' sizes for the 'new' concept.
once windows & doors have been opened and closed a few times, the seals are not the same. More infiltration. The
unit gets older & does not produce what it did when new.The combination causes the system to be short a little after about five years.
Have been on many calls where the system was put in by competitor, and worked well for a few years, but just doesn't seem to be keeping up now. Most of these, to me, was sized 1/2 ton short, at least.
Manual 'j' works well here for people who set their thermostats on 78 'f' in the summer. But if you are the next owner of this system & you like it to be 73 in the summer, forget it.
I do manual'j' calcs, but I allow for the other 'possibilities'

Manual J is a "tool" of our industry and should be used as such. I also believe it is vital that we utilize just such tools as these to provide the best possible product for our customer. Common sense is also a tool to be used. For example, if a load calculation determines that the cooling load is 32,000 btu, then common sense would dictate that a 3-ton system be installed as opposed to a 2.5 ton. Now, having said that, ideally, you would match components up to dial in to the 32,000 load whenever possible. If the design conditions are correct for the geographical area in which the home is located, correct data has been input, and no extenuating circumstances have been overlooked, then the Manual J calculation will provide the correct size for the information entered. Manual J is just one component of properly sizing a system. Correct duct design and installation is also critical to the operation of the system. I have seen many correctly sized systems not perform due to a poor duct system. I always go into the equipment specs to determine exactly what my capacities are. For example, a 3 ton system may only produce 34,000 btu at design conditions, so if my load is in fact 36,000 btu, I cannot make the assumption that a '3-ton' will be correct. I have to match up the OD with ID to satisfy my sensible and latent loads.

1.Manual J,allows you to design for 78 or 72°F ,or whatever indoor temp.,you want within reason.Yes if it's designed for 78 and a new owner wants 72,it probably won't do it.

2.If you design for 72 and run it at 78,it won't dehumidify properly,because it's too large for 78°.

3.If you design for 72° and fail to follow Man S,for proper size selection,it will likely be too small.

4.If you do Man. J&S correctly,but undersize the duct system
it won't cool properly,due to lack of airflow,which reduces sensible capacity.

5.After a few years,you could have duct leakage,or dirty coil and blower,causing lack of airflow.

If you do a Manual J correctly and that system comes up short on capacity,Then there is other problems. The manual J oversizes consistently and so does all of the software programs on the net, ( do you think they would sell you a program that is accurate then let you do the math and oversize the units and give there program a bad rap)

Tests performed, by Proctor Engineering, on many real homes have shown actual heatloss/gain to be only 66-70 % of accurate manual j calcs. The biggest variable that does not usually enter manual J calcs is the actual infiltration rates. Very few do a blower door test. Also leaky ducts can make a big difference. The fact is, manual J done properly does over size slightly, but it is the only reliable tool we have.

The ACCA "manuals" do NOT oversize or undersize systems, the individuals entering the information into the worksheets or programs do that.

A full understanding of building construction and how these buildings go together is important when entering data.

Remember, you pour in the info, the only thing coming out is the answer to the info you fed it. The load calcs are not to blame, the data entry people are, when the results are "wrong".

I can also see when using software programs equipment can be sized incorrectly. The program I use does a calculation with the Sensible Heat Ratio to spit out a tonnage size like a 3-ton. Problem is this size is incorrect if you follow the methods in manual S.

Sorry dave but I have to disagree--Building America homes have HVAC systems that are 10% under a correct manual J, and they work fine because Manual J does oversize. In fact Hank Ratowski, who was on the commitee for ACCA that designed manual J states that because of leaky ducts there had to be oversizing.