you will notice it if your heat gain is 60KBTU and your cooling is only 56.5 KBTU. comfort will be moderately poor, inside temp can not be maintained, and temp in house will climb.

full BTUs is desired for nominal size you are considering.

how do you know? only from a professionally performed man j load calc at your design temps-both inside and outside.

IMO

Tigerdunes, that is a 6% difference. How accurate are Manual J calculations when done properly?

Thanks.. for a Lennox XC14 paired with CBX27UH-060 You get 56,500 btus cooling
and XC13 Paired with CBX27UH-060 You get 60,000 btus cooling..

Now I know the XC14 Is a better condenser but why the difference is btus? and which should I choose if I need all the cooling I can get in the big house in South Texas?

An actual manual J calculation is a lot better, especially for central air systems.

Many of the quick window unit calculators are anything but accurate.

Brickeye, my question was aimed at finding out the typical or acceptable error in manual J calculations. Someone must know what the magnitude of error is. ASHRE must know how close good/skilled manual J calculations are to reality. I am too lazy to try to look it up. Maybe more accurately, I doubt it would be easy for me to find that information so I thought I would throw it out and hope that someone here would know.

What prompted my question, several weeks ago, was the 6% difference in the equipment. Trying to think quantitatively, I wanted to know if 6% difference in the equipment was significant compared to the estimation method. If manual J results are good to 3%, then 6% is pretty important. OTOH, if manual J is good to only 20%, a 6% change in equipment is looking not so important.

Thew error in a manual J depends on how well the actual conditions of the structure are surveyed.

Without a blower door test all anyone is doing is guessing at the infiltration loses.
At least with a blower door you have an actual measurement.

Trying to determine the heat loss through walls (especially with older insulation of unknown quality) is also a bit of a guess.

You can take IR pictures and start to figure it out.

Add to all this that the system is normally designed using 95% or 97% worst case weather limits and the last bit of accuracy does not make much difference.

The fact that A/C systems may only be available in 1/2 tin (6,000 BTU/hr) increments makes extrema accuracy not worth the effort.

It is even possible to use existing weather data and how the owner thinks the existing system performed to help adjust sizing.

If the house was comfortable in heating (or cooling) you can use this against the actual weather to assess if the system is sized correctly and have just rolled in infiltration and structural losses.
With a little more work you can start to get an estimate of the houses total losses.

Like most things, greater accuracy costs money, sometimes a lot of money.

The common use of TXVs on newer systems gives some relief on slight mismatches.

Since residential A/C does not normally use humidity as a control input, an oversize system creates cold and clammy (poor humidity removal) and an undersized one simply cannot meet the temperature set point (the humidity load makes the unit undersized in some cases).

Paying for greater accuracy in a manual J when the size increments are large is not money well spent.

Thanks, that is illuminating. I suppose that the best calculations are made in new construction where everything is supposed to be known. OTOH, with all the unknowns in an existing structure, knowing what equipment is in place and some well-placed questions to the homeowner may well work better than the calculations.

"with all the unknowns in an existing structure, knowing what equipment is in place and some well-placed questions to the homeowner may well work better than the calculations."

The existing systems are the experiment, and can be used with owner comments and weather data to evaluate requirements.

That BTU calculator way over sizes needed Btu.

I cool +750-sf with a half-ton room A/C, using a Wind Machine 20" fan to circulate the air though 3 rooms & a hallway. Does fantastic even at a Heat Index of +120-F.

Here is a link that might be useful: Optimizing Room Air Conditioner's EER Performance