Tom Székely, P.E., LEED AP

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January 17, 2005

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If All Heat is Dry Heat, What’s the Big Deal About Different Types of Heating Systems?


Back in 1991 (ouch!), the piece I wrote in this publication (last issued in 1997) explained, that contrary to popular belief, steam heating systems were no more likely to dehydrate people and furnishings than any other type of system.  That is, there’s no such thing as some kinds of heating systems delivering “dry heat,” while others might be more comfortable. As some of you however may have heard me say, on maybe even more than one occasion, I nonetheless dislike steam heating systems.


Now this is not because I have anything personally against water molecules becoming so antisocial that that they refuse to hang around each other, nor is it because steam heating systems make so much noise, what with clanging pipes and gasps and all. (E&E, Vol. 3,No. 1, May 1993; I’m just a font of wisdom, ain’t I?) After all, hot water heating systems sometimes exhibit waterflow noise, and hot air heating systems have been known to whoosh where they should whisper (although neither of these things should happen in systems which have been properly designed and installed).


No, it’s just because my engineering sense of economy of effort in accomplishing a goal, is offended by the amount of energy that has to be pumped into water to turn it into steam, just to take the edge off on a crisp autumn day.  I mean, it’s no biggie to pump almost 50 times as much heat into water as it gives up to keep a house warm, just to turn it into steam when it’s so cold outside that a brass monkey finds itself sans spheres, as it were, but why do all that when it’s only chilly?


In continuously cold climates the enormous amount of heat that can be transmitted via steam makes sense, and the disadvantages of its batch/digital characteristics as opposed to the continuous/analog characteristics of hot water (hydronic) heating, disappear.  Then there’s also the fact that steam heating systems require no pumps to move the steam around to where it’s needed.


Batch/digital vs. continuous/analog? Why can’t engineers learn to speak like normal human beings? Dunno. It must have something to do with the nerd gene.


Okay, steam heating systems work because of the tremendous amount of heat given up by a wee bit of steam (again. About 50 times as much as hot water gives up when heating a house) when it turns back into hot (very hot) water, or, as we engineers call it condensate.


What needs to be understood here in the making of steam, is that once the water gets to 212˚F, its temperature doesn’t change at all, and all the system’s input energy (to be exact, 48.5 times as much as it takes to change the temperature of a given amount of water by 20 degrees F.) goes into kicking around the water molecules to the point that they quit hanging around with each other, and change from water to steam.  Scientists taught us engineers to call this phenomenon change of state.


The problem, however, is, that once the steam has turned back into water, there’s not enough of it moving around fast enough to continue heating a space by giving up heat from the water. This has the effect of turning a steam heating system into an “on-off” (digital) system, with heat being delivered only so long as the boiler is firing and making steam, making it what process engineers call a batch process.


This is not to say that water gets used up and must be replaced, as one would have to do with the stock when distilling spirits. The steam heating boiler is part of a closed system, with the returned condensate being turned back into steam by the boiler, and therein lies the problem. When the thermostat tells the boiler the space has reached the thermostat’s set point, the boiler’s burner shuts off, and it’s for this reason that cast iron radiators have been the heating device of choice in steam heating systems, based upon the belief that hot cast iron would continue to radiate heat even while no steam was being condensed within.


Unfortunately, all this really can do is partially fill in the voids in system output when the boiler is off, as the cast iron will cool to near room temperature in rather short order, and it’s why a hot water heating system’s ability to fire the boiler as necessary to modulate the hot water temperature in response to outside air temperature makes for a more even heat than can be delivered by steam.


Please note that steam and hot water heating, whether the latter is delivered by baseboards or in-floor tubing, are radiant heating systems (as is infrared heating, whether gas-fired or electric), which is why one refers to their heating devices as cast iron, baseboard, or fintube radiators.


Hot air (or as engineering columnist Dan Holohan refers to it, scorched air) heating is even less efficient than steam heating because you’re warmed by being in direct contact with heated air, rather than being acted upon at a distance as is the case with radiant heat. (I don’t want to hear from any theoretical physicists about how I just screwed up.)


So what the #@!! is radiant heat anyway?


As alluded to above, you don’t heat the intervening air to heat people (and objects), and it’s why you feel warm under the infrared heaters of a Hotel’s canopy, even on a cold and windy day.  It gets even better when you heat via radiant floors because bare feet never have to walk on something cold.


Hot air is not only the worst of all systems because of its inefficiency, but also because it’s often delivered where it’s needed least, such as near the ceiling when a heating coil is put in a central air conditioning system’s ductwork.  At least residential furnace systems are designed with registers at or near the floor, although then adding a cooling coil to system delivers the cold air where it’s needed least.  In both cases the air tends to stratify, and has made for a great aftermarket in ceiling fans. While one “solution” has been to put registers at both the ceiling and the floor (closing the unneeded register during the appropriate season), the ductwork size required to move cooled air (maybe 20 degrees cooler than desired room temperature) versus that required to move heated air (at least 50 degrees warmer than desired room temperature) is but another strike against trying to heat and cool via the same system.


So with currently emerging applied technology for radiant cooling being experimented with as I write, why on earth would one want even that, i.e., “air conditioning,” to be delivered via ductwork?


Well, boys and girls, it allows you to use the system to move around room-temperature air in winter and humidify (condition) it, but that’s another story.

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