Archive for October, 2010

How effective are central humidification systems?

October 19, 2010

I am currently working on a post discussing central versus portable humidification systems.  In the midst of it, I realized that I was discussing a lot of issues that might be too much to take in one post.  In other words, it was getting kinda long.  So, I decided to break out part of it for this post.  Well, that and the fact I am beginning to get brain fog in composing the post.

So, for this post, I am going to discuss just how effective a central humidification system might be.  Warning to those who glaze at the use of calculations, they are in here.  If you cannot dig through it, just go to the conclusions.

So, you have a central humidification system that is dumping moisture into the air.  Just how effective is that humidification system at raising the humidity level of the air flowing through the air handler?  Consider this point—to raise the humidity 1% will require about 0.00004 gallons of water for every cubic foot of air (assuming I did my psychrometrics right).  That doesn’t seem like a lot, right?  But, a typical air handler fan pushes around 1200 cubic feet of air per minute.  That means 0.046 gallons (or about 0.7 pints) of water is needed every minute just to raise the humidity in the air flowing through a typical furnace 1%.  Want to try to raise that humidity from 10 to 50%?  That would be nearly 2 gallons of water per minute.  Do you think your humidifier can do that?

Now, consider this point.  Most central humidification systems work by evaporating water into the air passing through the humidification system.  That evaporation process is not 100%.  If you want proof and have one of these systems, check for water coming out of the overfill tube when the system is operating.  Now, the water flowing through the humidification system is not pure–it contains minerals.  As the water evaporates, it leaves those minerals behind to coat the media, as shown in the photo below.  As the media becomes coating, it is less able to absorb the moisture and the surface area of the media that is available for evaporation decreases.  Water that is not evaporated into the air passes through the humidification system to the overfill tube and is wasted.  Whether the water goes into the air or down the drain, you are paying for it.


Mineral encrustation on a wetable media inside a central humidification system.

Mineral encrustations on the humidification system as shown in the photo can also channel water down through only part of the media.  That further decreases the wetted area of the media from which moisture can evaporate.  Less areas from which water can evaporate means lower efficiency.

One more issue I would like to mention.  As the temperature of the water being supplied to the humidification system drops, less water evaporates into the air.  Basically, some of the water flowing over the media is absorbing heat without evaporating into the air.  That means that less water is entering the air.  I don’t know about your water system, but mine sure seems colder in the winter than the summer.  Per my previous post, the water valve supplying water to the humidification system only allows one flow rate.  During the winter, if you water supply temperature is colder, not only is water potentially being wasted, it could also be carrying some of the heat you are supplying to the air down the drain.

In my next (or maybe the one after that or after that or . . . ) post, I am going to get more into central versus portable humidification systems.  I promise.  Stay tuned.


What is the purpose of humidifying air?

October 11, 2010

In the last post, I said my next post would be to compare central to portable humidification systems.  Well, I got well into writing that post–which will be more than one post, as I found out–when the thought came to me that maybe I should explain some of the reason I have heard for humidifying our homes.  That last post indicated one reason based on more recent research and that is to control virus. We shall see if future research supports the findings.

Another common reason I heard is to protect the wood in a home.  I am not so convinced, at least for more modern homes that use a lot of manufactured wood products rather than the real McCoy (do people still use that phrase?).  All wood expands and contracts as the wood’s moisture content changes, and the content does change with the amount of moisture in the air.  Cracks form in the wood when it is constrained from moving either by the way it is installed or by its own natural structure.


Based on a little research (and note I said a LITTLE research) I believe that most wood in homes expands less than 1/4″ and more likely the amount is around 1/8″.  In a short article published on-line (, The University of Georgia Cooperative Extension Agency published results of a little study on the amount of moisture in wood in 20 homes or offices for oak and maple.  The authors found that the average maple moisture content 7.9 to 10.3% and for oak the average was 6.3 to 8.1%.  In another article published on-line  (, Mr. Carl Hagstrom gives this rule of thumb:  for every 4% increase in wood moisture content, the wood expands 1% (for “flat gain material”).  (Mr. Hagstrom also very nicely provides a link to an on-line shrinkage calculator at  Putting these two bits of information together, wood inside homes will likely expand about 1% during a typical change in winter conditions.

Mr. Hagstrom further states that wider boards expand more than narrower boards, as you would expect based on his rule of thumb.  However, not a lot of wider boards are used in new construction.  Not that many old big trees are still around these days, and those that are usually are used for veneers.   But, craftsman builders know how to account for wood expansion–both back then and now.   Having inspected a large number of older homes, I have not seen a lot of cracks in finish wood.  I have seen plenty in structural wood, although not many I would consider bad enough to be structural issues.  At that, I have to wonder if the cracks were not caused by the wood not being adequately dried or being exposed to the more extreme variations in moisture and temperature of outdoor air.

I also do not hear many homeowners saying that they humidify because of concerns about the wood.  Instead, the issues are usually that pesky static shocks from walking across carpets in dry environments and physiological issues such as stuffy head and dry skin.  WikiHow has a list of things a homeowner can do to reduce static shock (  Apparently, some carpets are also now manufactured to reduce shock.

I have my own theories about the physiological effects of dry air.  I believe that the dry air dries out the mucous membranes of the nasal system.   To prevent drying, the the mucous membranes swell to increase humidification of the air going into our lungs.  Swelling of the membranes causes little fissures in the mucous membranes that cause slight bleeding, which some people see when they blow their nose during this time.  The nasal stuffiness causes some people to use decongestant sprays that can also irritate the mucous membranes and some have rebound effects that make the stuffiness worse.  Decongestants , particularly the ones combined with antihistamines, can also cause a drying effect of the membranes.  Moisturizing sprays can help relieve the drying effects; but, the effect, for me at least, is relatively short-term.

I have one other alternative to help reduce the physiological effects of dry air that most people will not find attractive.  That is, reduce the air temperature of the home.  Doing so will effectively increase the relative humidity of the air.  I confess that during the winter, I keep my home at around 65 degrees.  Even though this temperature is noticeably lower than the 75 to 85 degrees most people keep their homes at during the winter.  I have found over the years, that our bodies are amazingly adaptive.  One other lesson I have learned is that I can always put on more clothes and putting a heating pad under my butt during the coldest days can go a far way toward keeping me warm enough.

The basics of humidification systems

October 6, 2010

In a previous post on virus and humidification, I promised to discuss more about humidification systems.  In this post, I am going to discuss the basics of humidification systems.  Humidity is a measure of the moisture in the air.  Technically, the humidity that everyone is usually talking about, including the weather guys is relative humidity.  Every so often, the term absolute humidity sneaks out of the scientific and engineering circles into the public.  This is a different measurement of moisture in the air; but that is a subject for another day when I pretty much run out of other topics—and I do have a list.

Humidification systems are simply devices for putting moisture into the air.  In our area, we only want to put moisture in the air during the winter.  In the summer, we are more concerned with getting out of the air.  The reason that people want to humidify air is that the air in our homes is extremely dry during the winter.  Even though outdoor air during the winter may have what seems like a normal humidity level, when that air comes indoors and is heated, the relative humidity levels drop to levels seen in a desert—somewhere around 10%.

For homes, a homeowner has really two basic choices in humidification systems:  central and portable.  A central humidification system supplies water to the air going into or coming from the air handler.  The humidified air is then distributed to all areas of the home.

Central humidification systems typically are recognized as a plastic box mounted on a return air duct near the air handler as seen in the photo below.  However, I recently saw on system mounted on the supply air duct, which may have been the work of an installation instruction-challenged installer.

A typical central humidification system

Inside the box is a web-like media through which air flows.  The air that flows through this media usually comes from the supply air duct just downstream from the air handler (the duct coming into the side of the humidifier in the above photo).  A humidistat connected to a sensor normally in the return air duct upstream of the humidification system (the little white box above the humidifier in the above photo) operates an electronic water valve that controls the water flowing onto the media (the valve can be seen alongside the humidifier in the photo above).  The humidistat is a control system for sensing moisture in the air similar to the way a thermostat senses temperature.  The water valve is connected to the home’s water supply.

When the humidistat senses that the moisture level of the air is too low, it opens the valve to let water flow over the media.  When the humidistat is satisfied, it closes the valve.  Air flowing through the media is supposed to pick up the moisture through evaporation and carry it into the return air stream.  Water flowing across the media that is not absorbed by the air is supposed to flow into an overflow that has a drain line to a nearby drain (the black tubing below the humidifier in the photo above).  In a few cases, the humidistat is located in a living area of the home, usually next to the thermostat.  Some humidistats are incorporated into the thermostat.

Most people will be more familiar with the portable humidification system, particularly since they are sold pretty much everywhere.  These units are self-contained humidifiers that have a water reservoir and a means for converting the water into a mist.  Some units also have a small fan to power air through the unit.  Portable humidifiers have three methods for turning the water into a mist.

The first method for turning water into a mist is a nebulizer that is simply a vibrating plate onto which the water dribbles.  The vibration breaks the water into a mist that is usually carried into the air by a fan-powered air stream.  These units are pretty simple devices that use the electric current to generate the vibrations in the plate.

The second method for turning water into a mist is a rotating disk that slings water at a comb-like device, which is supposed to break the water up into a mist.  The spinning disk operates both as a kind of pump to pull water up out of the reservoir and a means of slinging the water at the comb and a fan to push the mist out of the unit.  This method tends to generate larger water droplets than the other methods, which can lead to water falling on surfaces around the unit.  But, they are also commonly the cheapest units.

The third method for turning water into a mist is the old tried and true method from many of us boomers child-hoods is steam generation.  These humidifiers have a electrical resistance element that heats up when electricity runs through it, similar to the elements on an electric stove.  Water in contact with the heated element boils and is converted into steam.  The steam literally is forced out of the unit because of the energy it absorbs as it turns to steam.  This method of humidification is also used in most commercial and higher-end home humidification systems.

So, in summary, humidifiers convert water into water vapor through three methods; but all of them ultimately work through evaporation.  Central humidification systems use direct evaporation of the moisture into the air as it passes through a wet media, similar to the way that we evaporate water from our skin.  The other methods generate a mist with the intent to get it into the room air where the water will evaporate into the surrounding air.  For these mist generators, the smaller the size of the droplets in the mist, the better the water evaporates.  That doesn’t mean that central systems are more effective humidifiers than the others.  Steam systems are actually the most effective method, but take more energy to do the humidification.

I will end this part of the discussion there.  In the next post on humidification, I will discuss the pros and cons of central and portable humidification systems.

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