One of the HVAC industry's most familiar measures of cooling demand may be missing an important part of the load.

Humidity.

Combining humidity with temperature can significantly alter estimates of cooling demand compared with conventional cooling degree days. 

Cooling Degree Days Are Built Around Temperature

Cooling degree days are widely used to estimate how much cooling a building or region is likely to need.

The problem is that two cities with the same outdoor temperature can create very different HVAC loads.

One might be hot and dry.

The other hot and humid.

A conventional temperature-only measure can treat those conditions as much more similar than the HVAC system does.

Humidity Is Real Load

Air conditioners do not only reduce temperature.

They remove moisture.

That latent load can represent a substantial portion of cooling demand, particularly in humid climates.

Ignoring it can affect estimates around:

Energy consumption.

Peak demand.

Regional cooling needs.

Building performance.

System operation.

Why Contractors Should Care

Most technicians are not calculating nationwide cooling-degree-day models.

But the underlying principle is directly relevant to system design.

A piece of equipment can satisfy the sensible temperature load and still deliver poor comfort if it cannot control humidity effectively.

That is especially important with:

Oversized equipment.

Variable-capacity systems.

High-performance homes.

Humid climates.

Shoulder-season operation.

Better Metrics Could Change Forecasting

Utilities, energy analysts and manufacturers use weather metrics to predict HVAC demand.

If those metrics systematically understate humidity effects in certain regions, forecasts of electricity consumption and cooling requirements can also be distorted.

Contractor Takeaway

The industry has spent decades using outdoor dry-bulb temperature as one of the clearest shorthand measures for cooling demand.

The new research argues that may not be enough.

For HVAC, 90°F in Phoenix and 90°F in Houston were never the same problem — and our cooling-demand metrics may finally need to reflect that.