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