Tom Szkely, P.E., LEED AP

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August 27, 1991

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All Heat is Dry Heat

Dogma, because it is dogma, is generally unexciting (unless you're on the receiving end of religious dogma la the Spanish Inquisition), and when not backed by appropriate knowledge, it approaches becoming downright moronic.


Back when I was a young lad, I worked as an Engineering Aide at Bechtel's New York Office, which used to comprise almost six office floors at 485 Lexington Avenue/750 Third Avenue. Don't ask me why, but I remember a conversation I had with the Senior Engineering Aide about the hazards of living in New York City apartments where steam heating was the system of choice, it being very dry heat. You see, hot air systems were, by definition, systems that provided "wet heat", while steam heating was, by reason of it being steam heat, "dry heat".


We went around a few times, with me asking how heat carried by water vapor could be dry, and him insisting on the flawlessness of his observation. Now that I'm older and more educated (wisdom is an entirely different matter, having little to do with age or education), I know it wasn't the heat that was dry, but the cold. The construct "relative humidity" sports the adjective because without it you're not measuring anything, you're simply describing a sensation.


There comes a point when any additional sugar mixed into a glass of iced tea will not dissolve, no matter how vigorously you stir it (unless you're The Flash). When you dissolve stuff in other stuff, you have a solution. When you've dissolved all that can be held, you have a saturated solution. Relative humidity refers how much water vapor is dissolved in air as compared to how much can be dissolved in it before it can't hold any more. That's why it's expressed as a percentage. Air can hold more water (like tea can dissolve more sugar) when warm than when cold. Thus, air at, say 30F. containing 24 grains of water per pound of air, is saturated (100% relative humidity).


When heated to a room temperature of 68F., the same 24 grains per pound comprise a relative humidity of about 23%.  And if the cold air which entered the house wasn't saturated to begin with? Then you can get indoor relative humidity levels dryer than any desert on earth. Taping your windows and doors shut won't do because you'll run into an IAQ (indoor air quality) pollution problem, which is why codes require ventilation, and why the supersealing that goes along with superinsulated structures is not necessarily a good thing.


Hot air systems generally make for spaces that are less dry because supplemental humidification is easily provided for in the design, not because of any magical difference in the quality of the heat. That is, until steam system vents and traps start to fail. Then steam heat becomes wet heat with a vengeance.


Rain, Dew, and Mirrors

When air becomes saturated, any additional moisture separates in the form of clouds or fog, and if enough "condensation nuclei" (dust particles) are about, rain (or snow, or, if you're one for whom without bad luck, there'd be no luck at all, hail). Lowering the temperature of saturated air will have the same effect, and dew is the result when saturated air in contact with a cooler surface cools to the temperature of that surface. Fog on a bathroom mirror is dew to the mirror's lower temperature. (Well, if you can't have a little pun when you write, what's the point?) My dentist, being an exceptionally clever fellow, runs his dental mirror under hot water to heat it up when my breath fogs it.


"Natural" Incandescent Light

The widely held oxymoronic view that incandescent light is more natural than fluorescent light is not only wrong in regards to its spectral energy distribution and how little a lamp's overexcited electrons have in common with the sun's fusing hydrogen nuclei, but is also off the mark in how one looks under the light. Don't get me wrong, it's certainly more flattering, but natural it ain't. You don't grade diamonds and paint by north window light because "natural" artificial light (a phrase right up there with virtual reality) does such a great job of rendering colors.


From the phrases "red hot" and "white hot" one might surmise that the color of light is related to the temperature of the body radiating the light. In this case, one would be right. A standard incandescent lamp has a color temperature of 2700K (degrees Kelvin, a scale starting at absolute zero, using Celsius-sized degrees), while quartz lamps and premium fluorescent lamps have a color temperature around 3200K. The higher the numbers (to a point), the whiter the light. Cool White fluorescent lamps come in at about 4100K, and Daylight fluorescents at 6500K. Noonday sunlight is 5250K. Candlelight (flattering as all get-out, and as natural as a forest fire) is 1800K.


There's still more to it than this. The color temperature of a lamp has less to do with its color rendering index (CRI) than its spectral power distribution; how much of its light energy is put out where on its output curve.


So, it's all right if you tell me you want to use quartz lamps because they're (as all filament lamps) point sources that fit into small fixtures as opposed to fluorescent lamps which are (as all gas discharge lamps) linear sources requiring big, clunky fixtures. But don't tell me it's because it looks more natural -- I haven't heard anyone complain lately about how strange things looked under their office's lights. If they have, it's only because someone is frozen into using the fluorescent lamps of twenty years ago, or the complainer likes caves.


Filament lamps are easier to dim, and don't flicker, but if everyone were as sensitive to 60Hz. flicker as some carry on about, nobody would work indoors. And remember, when you use a filament lamped fixture, you're using at least twice the energy that would be consumed by a fluorescent to give you the same light.



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