Humidifiers, threat or menace?

Based on my inspections the humidifier may be the most neglected and unloved appliance in your home. Probably nine out of ten humidifiers I inspect are a mess.

Humidifiers are used in winter time to add moisture to dry air during heating season.  A humidifier mounted on a furnace uses water from a water line to add moisture to the air heated by the furnace, which is then distributed throughout the house using the hot air ducts. There are three basic types, evaporative, drum and flow through. The simplest is the evaporative type, a hole is cut in the hot air plenum of the furnace and the humidifier, consisting of a water reservoir and some metal plates, is installed in the hole.

Evaporative humidifiers
Evaporative humidifiers

Heated air absorbs the water as it passes by. They are simple, often have no controls other than a float or valve that maintains the water level. They are shut off by cutting off the water supply (which will work for any other humidifier as well).

The drum type has a hollow cylinder covered with a water absorbing foam driven by a small motor, the cylinder or drum rotates through a water reservoir with a float to control water level.

Drum type

There are automatic controls to turn the motor on and off, often there is a manual damper to close off the air passing through the humidifier, and the water shut off valve. The humidifier is mounted on either the hot air plenum or the return (cold air) plenum, there will be a short length of duct connecting the humidifier to the plenum that the humidifier is not mounted on. When the furnace fan is on air moves through the connecting duct from the supply to the return, passing through the humidifier and the rotating drum, picking up moisture as it passes through the humidifier, which then goes back through the furnace from the return side to the supply air to the house.

Flow through

A flow through humidifier is installed the same way as a drum type, on either plenum and with a duct to the other plenum but there is no drum or reservoir. A rectangular pad containing 40 mm thick metallic mesh is fed water from a valve above the pad, water is either picked up by the warm air passing through it or it flows back out through a plastic condensate line into a floor drain. Water flow is controlled by an automatic control called a humidistat.



The humidistat is an automatic control that is activated by sensing humidity, only releasing water onto the pad when it is needed.  There should also be a manual damper to shut off air flow through the humidifier.

When I inspect humidifiers, this is mostly what I see,


calcified evaportive type




Eeew! what is that? Calcified drum type
Calcified pad from flow through type
Rust on furnaces caused by overflowing humidifiers

When I find a badly maintained humidifier my recommendation in most cases is to remove or shut down the humidifier. Most homes in Edmonton don’t need one, for non Edmontonians, you probably don’t either, but don’t take my word for it 🙂 Edmonton has a humid continental climate, the outside design temperature is -32 C (the projected coldest two weeks of the year). What this means is that no one needs a humidifier in summer, and when the weather is cold inside humidity needs to be kept low to prevent condensation inside, which can lead to damaged building materials and mold.

So who needs a humidifier?

You have valuable, make that priceless, museum quality antiques made of wood, paper or textiles that will be damaged if they dry out. (yup you need a humidifier)

You have hardwood floors. (maybe) The bad news is that your floors will dry out even if you have a humidifier, if the floors are old hardwood, no need, they are already dried out, what you see is what you get, new floors will dry out too, that is what hardwood floors do, they will shrink as they dry out.

You have health problems related to dry air. (If your doctor says you need a humidifier, you need a humidifier)

You feel cold but the thermostat is set to a high temperature.  (Dry air will make you feel colder than humid air, you need to wear a sweater, you don’t need a humidifier)

You have a hygrometer, it is accurate, it reads less than 20%. (Your house may be too dry, extremely dry air can cause health problems, you may need a humidifier.)

You have an accurate hygrometer you are heating your house and the readings in your home are 40% or higher, condensation is forming on your windows. (You DO NOT need a humidifier, you need to turn it off if you have one, you may need a de-humidifier if you do not have a humidifier, or you have turned your humidifier off)

Turning off your humidifier

Turn off the water, turnoffwater
Close the damper, turnoffdamper

Turn off the humidistat turnoffhumidistat


If you must have a humidifier, the only kind I recommend are the flow through types, because they only release water when needed, when the furnace is running and the humidistat calls for more humidity. Flow through humidifier pads have to be checked regularly and replaced when they become calcified, usually once a year. The damper should be closed for the summer (the ‘off position may be labelled ‘summer’), the humidistat set to zero or off in summer, the water does not necessarily need to be turned off for a flow through, but it won’t hurt to do so.

If you are still using an evaporative or drum type, the water should be turned off for summer and the float and reservoir taken out and cleaned, and put back dry. Drums or the drum media should be replaced when calcified. You should also be measuring the relative humidity in your home in winter, never allow it to reach more than 30%. If you see condensation on windows, especially bedroom and living rooms windows, your inside humidity is too high, and you need to turn down or turn off your humidifier.

If you are installing a humidifier, the best place to mount it is on the return air duct, it can be mounted on either return or supply, except evaporative types which must always be mounted on the supply or hot air plenum. Mounting the humidifier on the return plenum will minimize (but not prevent) damage to the furnace should the water line leak or the humidifier overflow, so check your humidifier often.