1 Ton = 400 CFM??

I have adjusted away from the 100 CFM per 3000 BTU of cooling standard on many occasions. Most of my work was done in the southern tip of New Jersey. Cape May County is a peninsula at the southern most part of New Jersey that is surrounded by large bodies of water. The Atlantic Ocean to the east and south, and the Delaware Bay to the west. So the prevailing wind will often blow humid air from the evaporating waters around this area causing an unusually high amounts of relative humidity. The latent heat that must be removed from that air, in order to allow the sensible temperature to drop, often requires a slightly larger piece of equipment for the same exact home that may be located in the central and northern part of the state. This often sets the number at 80 CFM per 3000 BTU of cooling. or 100 CFM per 3750 BTU of cooling.

As @pecmsg so eloquently stated: "it's a starting point".

Do you know why 12,000 is called one ton of cooling?

Back when refrigeration was in the early stages it was determined that it took a certain amount of heat to melt one ton of ice (2000 lbs.) over a 24 hour period. That number was approximately 288,000 BTU. In order to get the BTU per hour, you would divide 288,000 by 24 Hours in order to come up with 12,000 BTU per hour. That number became the standard way to measure refrigeration equipment in 1903 or 1904. Only a few short months after Willis Carrier invented Air Conditioning.

Several factors must be balanced: sensible versus latent heat (humidity) removal, and transferring enough heat through the evaporator coil to make the cooling process efficient. Modern high efficiency equipment needs higher airflow per ton of cooling to achieve that higher efficiency.

28 deg TD with 1 ton and 400cfm will only be with 0 humidity removal and no moisture in the air.

In most (but not all) areas the SHR is 75-80% sensible and 20-25% latent. So 75% of the 28 degrees is around 20-21 degree TD is usually what you see.

From what I've seen they use a much larger evaporator but stick with 400 cfm per ton. Some of the Goodman equipment I was looking at recently used lower airflow in their SEER data, something like 360 cfm per ton. I was surprised.

Just for fun, I ran some numbers. If your indoor temperature is 74F, your air handler is running at 400 CFM and your AC is delivering 12,000 BTU/hr:

• At a dewpoint of 46F or below (37% RH) the air comes out at 46F
• At a dewpoint of 50F (42% RH) the air comes out at 48F
• At a dewpoint of 60F (63% RH) the air comes out at 55F
• At a dewpoint of 70F (90% RH) the air comes out at 63F

The SHR ratio for those four scenarios is 100%, 92%, 65%, 38%.

AC is kind of magic this way, the more humid it is, the more dehumidification it does.

or the higher the Dew Point the higher the latent load.

I'm not sure I understand, Is there any other way you can describe this phenomenon?

I'm sure someone can come up with another way given the time.

With a matched evap and condenser the higher the RH in the return air the more humidity will be removed. The more humidity removed requires more btus to condense the water so there are less btus available to lower the sensible temperature.

Same job with running a lower evap temp then above will have the same issue.

most package units and common split systems the evap coil is selected to be 75-80% sensible and 20-25 latent. thats what the SHR is.

When I used to do labs and computer rooms etc that required a lower than normal cooling temp or a lower than normal RH we gave the MFG the required CFM and room condition we needed and had them select a coil (usually thicker with more rows) that would do the job and we would often use a split system so we could play with the air flow (belt drive) if needed.

Some MFGs (AAon is one) will build a package unit that will do the job if you give them the specs.

There has to be more ways to say this?????

You guys are not trying hard enough!!!