Restarting...again

Ok, so it's been close to a year since my last post, when I recommitted myself to blogging more often and focusing on writing. Hmmm.....oh well. I have been writing, just not blogging. So I guess now is the time when I tell myself the same thing all over again. Here is an article that I just wrote for our local ASHRAE chapter newsleter. Nothing too exciting, but at least it's something....See you in a year!

Back to the Classroom: So…Does Heat Rise or what?

So there I was, lounging on my living room sofa on a lazy Saturday afternoon, being endlessly and inexplicably amused by a YouTube video of a 1970’s Soviet vocalizer, while listening to my daughter bemoan the fact that on such a perfect Southern California day she was stuck inside working on a 20-page research paper for her Geology class. Like a good parent I offered little sympathy, but rather reminded her of how fortunate she was to be living in the age of the internet, where she could do all the research from the comfort of her bedroom while enjoying the latest Charlie the Unicorn videos at the same time. After all, back when I was her age I had to actually go to a library, track down books using the Dewey Decimal System, check them out, and then craft my papers on a painfully unforgiving typewriter. Charlie the Unicorn hadn’t even been invented yet. I also reminded her of the great opportunity that was before her, to learn more about the interesting and exciting world of…geology. “It’s not like I’m planning to become a geologist,” she explained. “I mean – how often do you actually use what you learned in school?” And she had a point.

I can’t remember the last time I used a Laplace Transform to solve a function, or the last time I had to balance a stoichiometric chemical equation (I’m sure you guys do it ALL the time….sure…). I think that while we were in school, we learned the fundamental concepts behind the science that we now practice, but that once we actually go to put those concepts into practice, we discard them in favor of easy-to-remember rules of thumb and standardized processes. We’ve come to rely so heavily on the computer programs that we use to improve our efficiencies, that we’ve forgotten the basic principles that those programs are based on. So I thought that it would be good time to go Back to the Classroom – to take a look at some the fundamental concepts of how the science of heat transfer and thermodynamics actually work when we attempt to harness them to design efficient, safe, and comfortable indoor environments.

A few days ago, I had the opportunity to speak on radiant cooling at the annual Radiant Panel Association convention in Reno, Nevada. The day before my presentation, I sat in on a discussion on radiant floor heating and the viability of radiant floor cooling. The well-intentioned main speaker stated that radiant floor heating works fine, but that radiant floor cooling just doesn’t work. When asked why, his response was simple: “Because heat rises. It’s physics.”

Everyone in the room seemed to agree. And so I started thinking – Is this simple physics after all? Was his argument sound?

POP QUIZ: Raise your hand if you think that heat rises.

It’s natural to think that heat rises. After all, we recognize that floor registers work better in heating applications. We see concepts like the chimney effect employed in local projects like the New San Diego Children’s Museum to draw hot air out and cool air in by allowing the sun to heat a tower of glass. We understand how stratification works within a high-ceiling area or in a multi-story atrium as the heat rises to the upper regions. So we know that heat rises – right?

Wrong. A simple review of your trusty, but dusty physics or heat transfer text shows that heat does not rise. Hot air rises. Hot air rises because of the difference in density between the hot air and the surrounding air. Following Archimede's principle, the buoyant force pushing the hot air up is equal to the weight of the displaced air. Because the hotter air is less dense than the cooler air that it is displacing, it will naturally rise. This is referred to as natural convection. This principle is applicable when we are relying on convection for heat transfer. In the case of radiant floor cooling, however, we are looking at another form of heat transfer: radiation. Radiation heat transfer doesn’t rely on air. It relies on electromagnetic waves. You remember the Stefan-Boltzmann Constant, don’t you? Ok, neither do I. But the important thing here is this – if we are saying that heat does not specifically rise, then what direction does it go?

Give yourself an extra credit point if the Second Law of Thermodynamics just popped in your head. The second formulation, or the Clausius Statement of the Second Law, states that heat will always naturally flow from hot to cold. So heat transfer naturally occurs whenever there is a temperature difference. Therefore, in the case of radiant cooling, it doesn’t matter if the cooled surface is the floor or the ceiling or the walls or objects within the space. What matters is the relationship between that cooled surface and the space around it. Regardless of where the cooled surface is, up or down, the heat from your body will naturally flow towards it, leaving you with a cooling sensation.

Therefore, radiant floor cooling should not be dismissed based on the misconception that heat rises. I’m sure there will be those who will adamantly declare that cooled ceilings work better. I would agree, not because of the spatial relationship between the cooled surface and the occupant, but because a cooled ceiling can be driven to lower temperatures, dew point permitting, and because of the convective component that was discussed previously. Both are viable alternatives for energy-efficiency and have their own sets of pros and cons. The point of this rambling was not to sell anyone on the idea of radiant floor cooling, but to challenge some preconceived notions that many of us may have regarding fundamental heat transfer because of the extended time we’ve spent away from the classroom.

Okay, that’s all for now. Class dismissed. I’ve got some YouTube videos to catch up on and a geology paper to read…