Questioning Assumptions

Posted on September 3, 2008. Filed under: Uncategorized | Tags: , |

There are times when we stumble into our lessons.

A few years ago, when I was first getting my feet wet as a campus sustainability professional, I was unexpectedly called upon in the middle of a design meeting for a new building and asked to lead a conversation about energy efficiency. As my mind began to race, I found myself thinking that there was about 200 years of combined design and construction experience assembled at the table, and that I was without a doubt the least experienced of the group. My eyes must have been as wide as saucers.

I began by charting the rapid increase in energy costs to our university. I explained that every dollar spent on utilities is a dollar that can’t be spent on teaching, research, students, or any other aspect of the school’s educational mission. I encouraged the design team to make every effort to embed energy efficiency into the building’s design. It was a pep rally speech, and it was going nowhere.

I was grasping, so I asked the mechanical engineers to briefly explain their design to me. I have no background in mechanical engineering, and I struggled to follow their answer. I then asked a question that has since served me well: “Tell me about the assumptions that guided your design.”

I soon learned that the HVAC systems of buildings are designed to particular summer- and winter-time thresholds set by ASHRAE (the American Society of Heating, Refrigerating, and Air-Conditioning Engineers) using fifteen years of historical hourly weather data. The summer design temperature is the point that contains 99% of all of the hourly summer readings, meaning the threshold will only be exceeded 1% of the time, on average. Similarly, the winter design temperature is set at the 99% condition, with only 1 hour in every 100 falling below this bound during a typical winter.

For the project at hand, the upper bound was 97 degrees, which seemed reasonable enough. However, the design team reported that they were using a winter design temperature of 20 degrees. Suddenly, there was no doubt who in the room was from Houston and who was not.

I’ve lived in Houston for a combined 26 years. I remarked that I could probably count on one hand the number of times that the temperature has dropped below 20 degrees during those 26 years, and that’s probably true. The coldest month in Houston is January, where the average high is 63 degrees and the average low is 45. Since the beginning of record-keeping for such things, the temperature has never been colder than 10 degrees in January in Houston.[1] This year, the coldest recorded temperature in January was 33.8 degrees, and that happened twice. All told, we’ve only spent 44 hours this year to date below 40 degrees. In 2007, the mercury dropped below 40 degrees for a grand total of about 120 hours, with a minimum recorded temperature of 32. We haven’t had a single hour below freezing since early March of 2002.[2] That winter design condition of 20 degrees just did not seem right to the Houstonians in the room.

With a few minutes of pouring through design guides, we found that the appropriate winter design temperature for Houston as established by ASHRAE is either 29 degrees or 27 degrees, depending on the site location in the city. Even those values seemed low to us, but nevertheless far more reasonable than 20 degrees. Somewhere along the way, someone had padded an established standard with a significant “factor of comfort”.

So why does this even matter? The capacity of the HVAC equipment is designed in part to meet these conditions. We were over-designing for the winter, and doing so would cost us energy and money. We might not have ever uncovered this had we not questioned the design team’s assumptions.

With that issue resolved, we examined several other assumptions, and over the course of the meeting developed a series of proposed changes – most of which were adopted – that saved up-front equipment costs while slashing the building’s anticipated energy consumption by 15-20%. We weren’t sacrificing the functionality of the building – we simply were correcting a previously undetected over-design. Who knows how many we didn’t catch?!

A challenge for campus sustainability professionals is that we participate in so many different kinds of conversations with such a broad range of people that we can’t be experts in everything. There are going to be moments – perhaps many – when we are the least experienced people in the room. Bringing the conversation to a fundamental level enables us to partially overcome the experience gap, to start to play to our strengths, and to make constructive comments. This is such a young profession that we’re all climbing the learning curve together. I’d be interested to know if others have similar tips to share. We need all the lessons we can get!

[2] Temperature data drawn from the Bayland Park weather station, located at the corner of Hillcroft and Bissonnet in southwest Houston. Data available online at


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