Humidifiers and heating systems, a misunderstood and often misused combination
Inspecting houses in the cold climate of Connecticut, forced air systems equipped with humidifiers are very common to find. The purpose of the 
humidifier is said to be that a forced air heating system “dries out the air” and therefore additional indoor moisture is needed.
So is this empirically true?
The answer is simply no. Elementary school science teaches us that matter can neither be created or destroyed although its form can be changed. This is the first law of thermodynamics and I would add an over simplification of a complex scientific principle. Knowing this one would have to ask, how can a heating system dry out the air inside a house? Where does the moisture go?
The indoor environment is relatively moist. Breathing, cooking, bathing all create moisture. Add to that moisture from unwanted sources such as a leaky basement or dirt crawl space without a vapor barrier and we can have a fairly moist indoor environment. In some instances too moist. So why would one want additional moisture in an already “damp” house?
In spite of the moisture generated by the occupants and other sources, the air inside the house can become dry. This can occur through several different ways. One cause is the indoor moisture is diffused out of the house, driven out by the stack effect. But what goes out must be replaced. This brings us to another scientific idiom; nature abhors a vacuum.
With the stack effect, the basement of the house will be at negative pressure. A proverbial vacuum. And we know that vacuums suck. We also know a house is anything but air tight. That is unless it’s newer and built with special attention to sealing all the little voids that are everywhere in a wood framed house. Even then, it’s not perfect. So we have a vacuum in the basement and the moist, conditioned air of the house diffusing out of the living space mostly through the attic.
Now let’s look at the outdoor air. We know in the winter the air is cold and dry. Sort of runs counter to the idea the heating system dries the air when cold air is dry and we experience the discomfort of humidity in the heat of summer. Contradictions. With a leaky house and the stack effect, cold dry air of winter is drawn into the basement through voids to displace the warm, moist air of the conditioned spaces. This air exchange happens naturally, without mechanical assistance. Of course most if not all houses have mechanical devices that further “assist” this air exchange. Bathroom vents, clothes dryers, range hoods all suck (vacuum) air from the living space and then exhaust that air we pay to heat to the exterior, exacerbating the vacuum of the stack effect.
I deliberately left one major appliance out of the list, the heating system. Depending on the type of system, the furnace or boiler is the single largest contributor to drying of the air inside your house. Not by the myth of conventional wisdom that the act of heating dries the air, but by assisting the stack effect. To add insult, the air that is pulled into your house is cold and has to be heated. Fortunately not all heating systems cause suction and drying. Closed systems such as high efficiency gas fired boilers or furnaces do not contribute to the cooling or drying. I myself saw a significant increase in indoor humidity (20%) by changing my old oil fired heating system to a new, closed loop combustion gas fired system.
One more important scientific principle regarding air and moisture, relative humidity. Relative humidity (RH) is the amount of water vapor present in air expressed as a percentage of the amount needed for saturation at the same temperature. This means that air at 25º F with 40% RH will have an RH of 8% at 70º F. Both air samples are dry although the colder sample appears not to be nearly as much.
So does the heating system really dry out the air inside the house? As I have explained, no, not by the act of warming the air. In fact the furnace can help maintain indoor RH if the unit is closed loop combustion.
As for humidifiers, my advice is to avoid using any unit connected to the heating system. These units are easy to overuse and require constant cleaning to avoid IAQ issues. Generally as a rule if the indoor RH is 25% or greater, there is no need to add humidity. Further, as discussed, RH changes with temperature. Humidifiers are typically set it and forget it gadgets. The humidity level setting is not monitored by the device, so some days it may be fine, others not so much. Too much humidity can lead to your roof sheathing in the attic looking like the frosty wood in the photo to the right. And that’s just what can be seen. Condensation is also likely occurring inside the walls.
If there is a need for humidity, an in room portable humidifier unit is in my opinion a better option. With room humidifiers the user can better monitor the device, particularly the filter and clean it as necessary. And one more scientific fact that I’m fairly certain leads to over use of humidifiers. Human beings can not sense large variations in humidity. Make certain to have a hygrometer to monitor the indoor RH.
Cold or warm, dry air is all relative.
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