Furnace humidifiers are installed in this area to add moisture to the air in our homes in the winter, because the relative humidity inside the home can become very low.


Why does the relative humidity in my house get so low in the winter?


The common misconception is that the moisture is being cooked out of the air by the furnace.But this is not the case.The term 35% ďrelativeĒ humidity means that the air contains 35% of the amount of moisture it is capable of holding relative to the temperature of that air.The higher the air temperature is, the more moisture it is capable of holding.So 70 degree air is capable of holding a lot more moisture than 20 degree air.And likewise, 70 degree air at 35% relative humidity contains much more moisture than 20 degree air at 35% humidity.


While our house might be our castle, it is in reality nothing but a big box full of holes sitting in the outdoor environment.Outside air is constantly entering our castle, just as inside air is constantly exiting it.Letís say that the outside air is 20 degrees and has 35% relative humidity.When that air enters our 70 degree home, the amount of moisture it is capable of holding increases dramatically.So the relative humidity in the house is going to plummet unless we add more moisture to it to bring it back up to 35% RH.


How big a humidifier do I need?


The required humidifier size is based on several things:


  1. The winter water temperature
  2. The outdoor temperature
  3. The temperature the thermostat in the house is set to maintain
  4. The size of the house in cubic feet
  5. The tightness of the house


Number 1, the winter water temperature is a big factor, because the colder the water going to the humidifier is, the lower the moisture output of the humidifier will be.Conventional humidifiers work by moving the warm air in the furnace through the water in the humidifier.Science dictates that as the water becomes colder, less of it will be evaporated by the warm air moving across it.Obviously we could run hot water rather than cold water to the humidifier to accelerate the evaporation process.But I absolutely donít recommend this for a number of reasons to be discussed later.


Number 2, the outdoor temperature is another very large factor, because it creates a larger difference between the indoor and outdoor temperature.And as previously mentioned, thatís a problem, because the RH of the outdoor air will amount to much less RH in the indoor air as that colder air enters the warm house.


Number 3, the thermostat setting is another big factor for the exact same reason as number 2, i.e. youíre creating a bigger temp difference between the outside and inside air when you set the thermostat to a higher temperature.


Number 4, the size of the house in cubic feet (not square feet, so weíre including ceiling height) is a big factor because there are more cubic feet of air in a larger house for the humidifier to add the required humidity to.


Number 5, the tightness of the house is a biggie.As the volume of colder air entering the house increases, the humidity in the house drops more.And as the increased volume of heated air in the house escapes, it takes the moisture that the humidifier has added with it.


There are some things that people do in a house that make the required amount of added moisture much greater.It must be kept in mind that the furnace, which the humidifier is installed on must be running before the humidifier will deliver humidity.So if you use any alternative methods of heating the house, like a fireplace with a heat exchanger setup or a woodburning stove or space heaters, youíre reducing the time that the humidifier will be operating.And if you use a fireplace with no heat exchanger system, most of your humidified air that the humidifier has generated is going up the chimney and will be replaced by colder infiltrating air that will reduce the moisture level in the home as soon as it enters.


I have a 2 story house with 1 furnace for each floor.Do I need a humidifier on each furnace?


I believe the best approach is to add one large humidifier to the first floor system.If that doesnít work, a humidifier can be added to the second floor system later.Hot air rises, and if you have the thermostats on each floor set at the same temperature, the upstairs furnace rarely comes on.So that humidifier never adds any humidity to the entire structure.So youíll have much more moisture added with one large unit on the first floor furnace.


In what cases would you recommend against using a humidifier?


Humidifiers have the ability to aggravate peopleís allergies, especially mold allergies.I always try to discourage people from installing a humidifier on a furnace for people with mold allergies.


I also donít like to see people using them in homes with older windows.Iíve seen a lot of wood rot around the windows caused by the condensation that forms on them.The condensation forms for the same reason that the humidity in the house becomes lower when thereís a large temp difference between the inside and outside air.Hereís how it works: Youíve added moisture to the air with the humidifier, thereby raising the RH of that air up to letís say 35%.The older window panes have phenomenal heat transfer.Heat moves from hot to cold.So the heat contained in the air next to the window gets sucked right out of the air, and the air temp next to the window plummets.So the RH of that very cold air can easily reach 100% at that low temp.At that point it can hold no more moisture and will give up the moisture in the form of condensation.Iíve actually seen ice build up on the panes.


I would also never consider installing a humidifier on a furnace installed in an attic.This is just asking for trouble.At some point the unthinkable is going to happen.It will either leak or freeze or both.Itís not a matter of if.Itís just a matter of when.


Will these humidifiers keep my house at a constant RH?


Simply put, no, not in our climate.They are merely meant to add some humidity to the house.As previously mentioned, the demand for humidity is the greatest when the temperature difference between the outside and inside air is the greatest.But at the same time, the potential for condensation in the house is also the greatest at those conditions with a humidifier.So even if you have a big enough humidifier to supply the amount of moisture needed to maintain a given RH during very cold weather, you would likely have to reduce the setting of the humidistat control on the humidifier to prevent condensation from occurring.


There are some fairly new computerized humidistats available now that have an outside sensor that will automatically reduce the humidity setting as the outdoor temperature drops.But Iíve seen a lot of problems with those.


Hot water hookups to humidifiers


I just think itís a bad idea.This really scales up the humidifier parts, especially the water solenoid valve and the water panels.It can also possibly create indoor air quality issues, especially if the humidifier isnít adequately maintained on a regular basis.

Basic Humidifier Types







Bypass models are the most popular, and are quite affordable.They can typically be mounted on either the supply air side or return air side of the system.A duct is then run from the humidifier to the other side.The hot supply side air blows through the water running down through the humidifier and sends the humidified air back to the return side.


The power models contain a built in fan motor that eliminates the need for the bypass duct.These are typically mounted on the supply side of the system.They are more expensive than the bypass models, and the motors must be oiled frequently.They also tend to throw some water scale onto the cooling coil sometimes.So Iím not real fond of these types, and feel that they should only be used in installations where other types wonít fit.


The drum types are just a bad idea.They keep the same grungy water in them and merely replace the water as itís used via a float assembly similar to the ones in a toilet.They drag a drum (wheel) with a media wrapped around it through the nasty water while warm air is blowing across the water.Iíve seen some things in them that would make for a good B movie.The float assembly usually develops the same problem as its toilet float counterpart, i.e. it dribbles and wonít shut off.So if youíre lucky and the overflow drain doesnít clog up from the scum, the excess water will find its way to the drain.But in many cases the overflow drain clogs and the water runs down and rots out the ductwork and/or furnace.


Steam humidifiers are relatively new to the residential market.Theyíve been relatively slow to catch on here, possibly because they cost a lot more than the other types.They have the ability to put out a lot of humidity, but they use an electric heating element to heat the water.And that heater uses a lot of electricity, which at the present time costs a lot more than natural gas in our area.And they simply donít play well in areas where water is hard.And Greater Kansas City water is relatively hard.The installation costs are much higher, because they need a dedicated 120 volt circuit on its own circuit breaker.


There is one other type that Iím only mentioning as an ďalso ranĒ model.I consider it to possibly be the worst type.This model actually sprays a mist of water into the ductwork.You can imagine what the consequences of that are.Iíve only seen a couple of these and that was plenty for me.Iím not sure they still make them.


So the bypass and power humidifiers are pretty much the most popular types, with the vast majority of those being the bypass models.


Bad Humidifier Installs


I hate to admit it, but Iíve made a lot more money correcting bad humidifier installs than from installing new ones.Itís amazing how many possible ways there are to improperly install one.


Here are a few of the common ones I see.


  1. The humidifier isnít level.So the water doesnít run across the entire water panel evenly, i.e. it all runs down one side.So the humidity output is limited to the area of the water panel that receives the water.
  2. The humidifier (or the bypass duct) is on the dead air side of the cooling coils.So there is not enough air passing through the humidifier to pick up the moisture, because the airflow is blocked by the coil.
  3. The humidistat is mounted after (downstream in the airflow) the point where the humidity is exiting the humidifier.So as soon as the humidifier starts up and throws the humid air into the system, the humidistat senses that high humidity level and shuts back down immediately.
  4. The humidistat is mounted to the supply duct rather than the return duct.The RH of the air in the supply duct is always going to be below 5% when the furnace is running.So the humidifier never shuts off.
  5. The humidifier is wired so that it comes on when the furnace isnít running.So 100% of the water entering the humidifier just runs straight through the unit and down the drain without adding any humidity to the house when the furnace is not running.
  6. The humidistat or humidistat transformer is wired to get its power from the low speed blower motor wire.The voltage at that point in the system can be as high as 200 volts when the blower is switched to high speed.This burns out the humidifier transformer.
  7. The humidifier is getting its power from the gas control valve terminals, and the heating system is equipped with an analog thermostat.This throws off the heat anticipator adjustment in the analog thermostat.So the thermostat will either overshoot or undershoot the temperature setting part of the time.
  8. The saddle tap water valve where the water supply is coming from is mounted on the bottom of a horizontal water line.All of the sediment in the water line drops to the bottom of the line and clogs up the hole in the saddle valve.
  9. The bypass duct is run from the humidifier to a hole cut into the blower cabinet of the furnace.So all of the humidifier dust going through that hole and entering the furnace blower doesnít get filtered.
  10. The bypass humidifier is mounted to the supply duct and so is the bypass duct.Zero humidity gain there, because thereís no air moving through the humidifier.The air has to flow from the supply to the return.
  11. The humidistat is mounted on the return duct, but thereís no opening in the box the humidistat is in and thereís no hole in the return air duct there for it to sense the humidity in of the air in the duct.So it just senses the humidity of the air in the basement.
  12. A power humidifier is mounted on the return duct instead of the supply.So room temperature air is moving over the water and isnít hot enough to evaporate much water.
  13. The humidifier is mounted in a place where it canít be serviced without totally removing it.
  14. A horizontal, drum type humidifier installed on a supply trunk that serves only one area of the structure when there is room to install a bypass or power model on the duct system.So only one side of the house is humidified.


Oddball Configurations


Iíve done some unusual things with humidifier installs to try to get customers some relief from low humidity.Iíll have to post some pictures here.I once installed a monster Aprilaire model 112 power humidifier about 10í in the air on the supply plenum.They quit making these monsters a long time ago, and it left somewhat of a void, because they really put out some humidity.But they were a total pain to service.They had to be completely removed to change the water panel and oil the power motor.But this was a very large house and that was the only place available to mount it.The owner had me service it every year (with an extension ladder).I had to carry the humidifier down the ladder and back up again.He finally put the house up for sale, and I was somewhat relieved that I wouldnít have to go through that again.†† But as luck would have it, my cousin bought the house.


Then I once installed two bypass humidifiers on one furnace and ran them through the same common humidistat.There was simply no other way to get enough capacity for that house from one unit.


I had another customer with a furnace in a very tight closet with a water heater.He begged me to find a way to put a humidifier on it for several years.There was only one available place for it to go, and that was on a vertical side of the supply.But the supply duct wasnít perfectly vertical, and the humidifier must be mounted true vertical or the water wonít run down through it properly.So I took a series of measurements and fabricated a wedge shaped sheet metal box for it to mount on.The wedge shaped box mounted to the slanted supply duct and the humidifier mounted on the box, making the humidifier perfectly level.


I have another idea in the works, but Iíve yet to try it.Itís for a big house with only one furnace.I would install a bypass humidifier very high up on the large vertical return air drop coming down to the furnace.The drain hose coming out of the bottom of that humidifier would dump into the top of a second bypass humidifier mounted directly under it on the same duct.So there would be no water line to the bottom humidifier.There would only be one cold water line going to the top one.The cold water going through the top one would increase in temperature as it ran through the top humidifier.So it would be warmer when it entered the bottom one, and therefore it would evaporate more efficiently.The bottom humidifier wouldnít need a water solenoid valve.It would merely contain a water panel.So an old used humidifier could be used for the bottom unit.This would result in a very good savings in the quantity of water used and would provide a much greater humidity output, possibly greater than two separate humidifiers with cold water going to each of them.


But the most important thing to realize is that all humidifiersREQUIRE regular maintenance in our area.And if you arenít going to perform that regular maintenance, youíre better off having no humidifier at all, because you can end up with increasing inefficiency issues, water leaks, indoor air quality issues, or all of the above.††




Areas Served

Greater Kansas City including:

Johnson County, Kansas     Kansas City, Kansas       Kansas City, Missouri

Fairway, KS

Lake Quivira, KS

Leawood, KS

Lenexa, KS

Merriam, KS

Mission, KS

Mission Hills, KS

Mission Woods, KS

Olathe, KS

Overland Park, KS

Prairie Village, KS

Roeland Park, KS

Shawnee, KS

Spring Hill, KS

Stanley, KS

Stilwell, KS

Westwood, KS

Westwood Hills, KS


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Copyright 2007 Leonard Arenson Heating & A/C


Updated 12/16/15