Tom Székely, P.E., LEED AP

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May 9, 2006

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Digital vs. Analog Heat


As is often the case, as life becomes more and more technologically complex, everything old is new again (those words sound like they should be in a song), and we sometimes forget the “old” which we understood, suffocating ourselves under the now “new” old idea.  A case in point revolves around heating systems and the imperfect understanding, especially of the lay person, of what heat is all about, and how, as I’ve mentioned before, it differs from temperature.  The difficulty is compounded by the fact that we live in a digital age, and haven’t internalized the huge change this is from all that has come before.


As I am wont to do, let me start with the last thing and work my way back to where I started from. We know (or should know) that the words digital and analog mean, on-off and “continuously representable” respectively. Wha?  Well, the former is so obvious [not so fast], I didn’t feel the need to put it in quotes, but “continuously representable”??? 


An analogy is a comparison (which a literary person would call a simile when describing the analogy using particular words) between two things, where for every characteristic or value of an original thing, there is a corresponding characteristic or value in the analogous thing.  Where we’re only interested in a single characteristic or value of something, we can create an analogy to represent it by a similar characteristic or value in the different thing.


In fact, there still exists analogue instrumentation where, you measure, say the resistance of a wire as it gets hotter and hotter, and display it on the scale of an ohmmeter, calibrated and marked to read not in ohms, but in degrees Fahrenheit, Celsius, Kelvin, or whatever.  This works because we know from empirical experimentation that the resistance of a wire varies directly with its temperature.  You could then point a close-up television camera at the meter, project the image on a wall-sized TV so large as to fill the screen with the space between, say, 65°F and 66°F, and watch the needle move smoothly from 65 to 66. The natural world thus appears to be mostly analogue, in that things change as part of a smoothly continuous process, not in discrete jerky steps.


Of, course why should we be content to leave things in their simple natural state when we can muck it up with stuff like progress? In the old days, we built fires to heat us and to provide light.  Want more light or heat?  Make more fire.  There are, of course problems with this approach which call for things like fireplaces, stoves, and oil lamps, to replace things like fire pits and oil dipped flax torches.  There is nothing, however, digital about any of this.


Electricity, however, is weird. It is generated and transmitted by analogue devices, switched digitally, and lately acts as though it’s being controlled analogously when it’s really digitally. 


I know, I know, the headline of the piece is about heat.  I’m getting there.


Let’s start with the simple phrase, “switched digitally,” which is a bit redundant; I mean, digital means on-off.  That is, it’s discontinuous.  As is on-off.


Go to the light switch on your wall, and very, very, very slowly move it from on to off or vice-versa.  Did your lamp go dim before it turned off?  Did it gradually brighten as it turned on?  (Actually this last thing does happen in compact fluorescent and other gas filled lamps, but the electricity is full on and the lamp’s conductance increases only relatively slowly.)


So switches are, because they’re on-off devices, digital.  Dimmers used to be analogue [Autotransformers], continuously varying the voltage to a lamp to dim it, but now they’re digital [Silicon Controlled Rectifiers or SCR’s], doing the same thing in a set of discrete steps so small, they seem to us to be continuous. Inverters are things which make fake AC out of DC in this manner.


The computers we now seem to be unable to do without are digital devices. In World War Two, analogue computers were used to compute artillery trajectories and submarine torpedo firing solutions.


Steam heating is digital. Hydronic [hot water] heating is analogue.  Everything old is new again.


Which brings us back to the difference between temperature and heat.


As I alluded to in Vol1No4 in August of 1991, and again in Vol5No1 at the beginning of last year, it’s that heat is different from temperature that makes steam heating digital rather than analog.


That is, as I’ve said before (and think I’ve written, though I can’t remember where), you can’t light a log with a match.


If wood’s highest ignition temperature is 500°F and the lowest temperature of a match flame is 1100°F, why not?


The mistake we make is that when we say something is hot, we think it means it has a lot of heat, when what it really means is that it’s at a high temperature.  As illustrated by the match and the log, the two are not the same. Low temperature, (or low grade) heat is fine if there’s enough of it, and it’s why solar heating works.  We don’t have to boil water to heat a house, and this is where the old analogue heating of hot water systems seems new.


British Thermal Units (or BTU’s) and Calories are measures of the same thing, heat, in different units.  The former is the amount of heat required to raise the temperature of a pound (about a pint) of water, by 1°F while the latter is the amount required to raise the temperature of a liter (about 2.2 pounds) of water 1°C.  This continues until reaching the boiling point, when 970 BTU’s go into that pound of water to turn it into steam, during which transition, its temperature never changes.  When the steam condenses back to water vapor, it returns that 970 BTU’s per pound, again without a temperature change, which is what makes it “digital.”


On a mild day, I can circulate water in a hydronic heating system, at, say, 170°F, and when the water returns to the boiler at 150°F, it has given up 20 BTU of heat to its surroundings for each pound of water.  When it’s colder outside, I can crank up the water temperature to say 190°F, and if it still comes back at 150°F, it will have given up twice as much heat to its surroundings.  Each pound of steam gives up no heat until it condenses, and then it dumps 970 BTU to its surroundings.  You can’t throttle that.  Hence, it’s “digital.”


Unless . . .  you make the heating system think it’s trying to boil water in Denver. Make it a two-pipe vacuum return system, and you can boil water at any temperature you like, with the latent heat of steam (970 BTU/lb. at standard atmospheric pressure) varying inversely with the vacuum.


Unfortunately, such systems leak as they age, reverting back to atmospheric unthrottleability in fairly short order.  Actually, it takes a bit longer, but since the leaks let air in rather than letting any steam out, such systems are nearly impossible to troubleshoot and repair.  Oops.


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