Why is 75ºF the standard indoor design cooling temperature and why should I not operate my home below 75ºF?

When people have a new home built, they want it to be pleasing to the eyes and comfortable to live in. In both cases, the home buyer already has a perception of what is pleasing to the eyes and what comfort is. One of those comforts, maybe the most important comfort, is the heating, ventilation and air conditioning (HVAC) system. The average home buyer, not familiar with Florida’s hot/humid climate, will want an AC system that will cool the home down fast and to 68 degrees Fahrenheit, if so desired.

For two very good reasons, the AC system should not or cannot be sized to cool the home down fast or below 75ºF. The first reason is found in the Florida Building Code-Energy Conservation, Chapter 3, Section R302.1. The section reads “The interior design temperatures used for heating and cooling load calculations shall be a maximum of 72°F (22°C) for heating and minimum of 75°F (24°C) for cooling”.

The second reason is more of a durability issue than an energy or code issue. Our design outdoor dew point is 74ºF. This means that any surface lower than 74 ºF will sweat or condense. Consider the can of soda pulled from the refrigerator, the surface of the can begins to condense. The surface temperature of the can is below the ambient dewpoint, the can is a moisture magnet.

If the outdoor dewpoint is 74ºF and the inside temperature of the home is 70ºF, there is surely condensation taking place someplace in the building materials. Water vapor condensing into water liquid inside building materials causes rot.

It should be noted here, occupants operate their homes at temperatures below 75ºF because the indoor humidity is too high and they perspire. No doubt a bad HVAC system design; bigger is not better.

Conclusion:

A home built to the minimums of the Florida Building Code is the worse home a builder can build without going to jail. If the HVAC design practitioner designs a cooling system to condition a home to lower than 75ºF, the practitioner is not meeting the intent of the building code. The designer will also be liable for any damages due to the home going into failure because of indoor condensation and rotting building materials. It can be very expensive.

The goal of the HVAC system design practitioner is to design an HVAC system that is capable of, at 75ºF thermostat setting, evaporating the perspiration off a person’s body faster than their metabolism can produce it. If this task is met, comfort can easily be achieved at indoor temperatures above the average outdoor dewpoint. Don’t design the home to become a moisture magnet.