A cooling load is not one problem.
It is two:
- Remove heat that raises indoor temperature.
- Remove moisture that raises indoor humidity.
Manual J separates those into sensible and latent load.
If you only look at total cooling BTU, you can miss an equipment mismatch that shows up later as poor comfort.
What is sensible cooling load?
Sensible load is the heat that changes air temperature.
Examples include:
- Heat conducted through walls and ceilings
- Solar gain through windows
- Heat from lights and appliances
- Warm outdoor air entering the building
When the AC removes sensible heat, the thermostat temperature falls.
What is latent cooling load?
Latent load is the energy associated with removing water vapor from the air.
Common sources include:
- Humid outdoor air entering through infiltration
- Mechanical ventilation
- Occupants
- Cooking and bathing
- Other indoor moisture sources
When the AC removes latent heat, water condenses on the evaporator coil and drains away.
The thermostat may not show that process directly, but the homeowner feels the difference.
Why total cooling capacity can be misleading
Suppose a house has:
- 24,000 BTU/hr total cooling load
- 18,000 BTU/hr sensible load
- 6,000 BTU/hr latent load
Now suppose an equipment combination can deliver 25,000 BTU/hr total at the project condition.
That sounds like a pass.
But what if only 17,000 BTU/hr of that is sensible?
The unit may have enough total capacity but still fail to satisfy the sensible requirement.
The reverse can happen too: equipment can handle temperature well while not removing enough moisture.
That is why Manual S must compare more than nominal tonnage.
What drives sensible load?
Major sensible contributors include:
Solar gain
West- and east-facing glass can be major cooling-load drivers.
Envelope conduction
Roofs, ceilings, walls, windows, and floors move heat into the building.
Duct gains
Supply and return ducts in hot unconditioned spaces can add substantial sensible load.
Internal gains
Lighting, appliances, electronics, and occupants all add heat.
What drives latent load?
Infiltration
Humid outdoor air leaking into the house creates a moisture-removal requirement.
Ventilation
A fresh-air system deliberately introduces outdoor air. In a humid climate, that can add significant latent load.
Occupants
People release moisture through respiration and activity.
Building use
Cooking, showers, plants, and unusual occupancy patterns can affect real moisture generation, although Manual J generally uses standardized assumptions rather than chasing every lifestyle variable.
Why humid climates are different
In a humid climate, a house can have a moderate sensible load but substantial latent load.
That is one reason aggressive oversizing is risky.
An oversized single-stage AC may satisfy the thermostat quickly and shut off before the coil runs long enough to remove much moisture.
The result can be:
- 72°F indoor temperature
- 60%+ relative humidity
- A cold but clammy house
Bigger is not automatically more comfortable.
Why dry climates are different
In hot, dry climates, sensible load can dominate.
Large solar gains, hot roofs, and high outdoor dry-bulb temperatures can create a very high sensible requirement with relatively low latent demand.
Equipment selection should reflect that split.
You still need total capacity, but the sensible capacity becomes especially important.
What is SHR?
SHR means sensible heat ratio.
At a simplified level:
SHR = sensible capacity / total capacity
A building has an effective required sensible/total relationship.
Equipment also has an SHR at a specific operating condition.
Manual S compares the selected equipment's performance with the building requirement.
Do not assume the catalog SHR at one rating condition applies at every project condition.
Airflow changes the sensible-latent split
Airflow across the evaporator coil affects performance.
Generally, lower airflow can increase dehumidification, while higher airflow can shift performance toward sensible capacity.
But equipment must stay within manufacturer-approved airflow ranges.
This is not permission to solve a bad equipment match by arbitrarily changing CFM.
The correct sequence is to select equipment that fits the load and then set airflow appropriately.
Common input mistakes that distort latent load
Overstating infiltration
Marking a reasonably tight home as extremely leaky can inflate latent load.
Forgetting mechanical ventilation
The opposite problem occurs when deliberate outdoor air is omitted.
Using the wrong outdoor humidity condition
Moisture load depends on the actual climate assumptions.
Double-counting outdoor air
Do not treat the same airflow as both infiltration and ventilation if the methodology does not call for that.
Why room-by-room latent load can look strange
Moisture does not always stay neatly within one room the way a west-facing window's solar load does.
Depending on software and methodology, latent load allocation may be distributed using standardized assumptions.
The key system-level question is whether the equipment can satisfy the total latent requirement while also meeting sensible load.
How to review a suspicious cooling result
If total cooling looks reasonable but comfort expectations do not, check:
- Sensible load
- Latent load
- Infiltration
- Ventilation
- Window orientation and SHGC
- Duct location
- Equipment SHR
- Airflow
Do not stop at "the tons match."
Bottom line
Manual J splits cooling into sensible and latent loads because homes need both temperature control and moisture control.
A correct total BTU number is not enough to prove that a specific AC or heat pump is a good match.
Calculate both parts of the load, then use Manual S to verify that the selected equipment can actually deliver the right kind of cooling at the project's design conditions.