As my post stated, I have ran across a manual j that is showing me the wrong size system for the 1st time. I know what you are thinking the input details have to be wrong but I have triple checked them.

Now let me explain. I have a customer with a 1800 square foot home that was built in the 50's which is rasied frame home with no insulation in attic or the floor. The walls are more then likely not insulated as well. The house can't be much tighter then average.

He has been running a 4 ton system in the home for 10 plus years with no problem keeping it 72 on a 95 degree day. Some more info on the home has a good bit of windows all single wood with metal storms on them.

My load came out to be 6.5 tons of air needed to cool the home due to no insulation but this man home has done fine for years with his existing (4) ton system as we have been doing all his work for the past 5 years.

Other notes the system is even mismatched using a goodman a/c unit with a tempstar coil. The system has done the job. So Made a call to install another (4) ton system rated at 48,000 btus and we have installed it and per customer it is doing good. The duct system would only handle a 4 tons of air anyways along with returning it.

Has anyone one else ever had this happen to them where rule of thumb has worked for them over manual j? I hate even saying that as I belive strongly in manual j but this time it was wrong! Any input to help me understand this would be great...