by Christine Keltner
As engineers, we must make assumptions and estimations as part of the design process. Unfortunately, we don’t always get to explain or discuss our assumptions with the people who are most affected by them. ASHRAE has laid out a few guidelines to help us keep similar assumptions throughout the industry. The Fundamentals Handbook published by ASHRAE every four years gives recommended outdoor design conditions. These conditions include worst-case design dry bulb temperatures and their associated wet bulb, as well as worst wet bulb and the associated dry bulb. We utilize these conditions to run our calculations for sizing mechanical equipment.
ASHRAE also publishes a standard numbered 55 – the standard for “Thermal Environmental Conditions for Human Occupancy.” This document describes many attributes that define human comfort, including five factors of thermal comfort: metabolic rate, clothing insulation, air temperature, radiant temperature, air speed and humidity. There are recommended norms for these five factors that most engineers utilize. This standard outlines the chart below, indicating the limits of human comfort with the normalized metabolic rate and clothing insulation levels (68-82 degrees F and up to 80% relative humidity). This means it is the area in which most people in an office setting will be comfortable. “Most people” is defined as 80% of occupants, which means that 20% of the occupants will complain or be “uncomfortable.”
What do these assumptions and guidelines mean to the building owner and occupant? In Louisville, Kentucky the summer outdoor conditions that are most commonly used are 94 degrees at 45% relative humidity with indoor temperatures of 75 degrees at 50% relative humidity. This means that the mechanical equipment conditioning your space on the hottest day of the summer will only condition your space to 75 degrees. In the past few years we have noticed at Kerr-Greulich that this indoor temperature may not be adequate in the opinion of some occupants. Expectations are getting closer to 72 degrees, even 70 degrees. We have seen similar expectations in the winter as well – desire for 72-75 degrees in lieu of the usual design temperature of 70 degrees.
We as engineers should be discussing temperature and humidity expectations with the building owner/operator during the design phase to ensure we are providing adequately sized equipment for their needs. We also need to discuss temperature and humidity fluctuations. The 2012 International Energy Conservation Code requires a 5 degree dead band between heating and cooling setpoints; this can lead to a 5-8 degree swing in room temperature during the “shoulder” season when HVAC systems are changing back and forth between heating and cooling. For these reasons, an engineer may see a space temperature at 77 degrees and have no cause for concern – a 2 degree fluctuation from setpoint is within design limitations. But an occupant who expects the space temperature to be 72 will think there is a problem. Educating the building owner and occupant on system type, operation and temperature expectations should be part of the normal design/construction process.