Return to the House Plants Forum | Post a Follow-Up

electric fan, air circulation and relative humidity

Posted by kemistry 8 - Oregon (My Page) on
Tue, Nov 16, 10 at 1:25

Hello everyone,

I've been thinking of adding an electric fan to help circulate the air indoor during the Winter months (too cold to open the windows). My question is, will having a fan running caused a drop in the relative humidity level?

I don't have a humdifier to use at the moment so I'm a bit concern about this.
My plants are mainly small tropicals living together near the windows. (jasmine, gardenia, etc..)

Any inputs/thoughts are appreciated! :)

Follow-Up Postings:

RE: electric fan, air circulation and relative humidity

Having a fan on makes no difference in the humidity..It does help with prevention of mold and diseases..It can help with your containers drying out a little more rapidly...

What does help is putting your plants on top of humidity trays..They make some for window sills and you could even come up with ideas on how to make your own..

I use a big plastic saucer about 2 inches deep and fill with stones on the center on my table for my Orchids and Violets.....I also use styrofoam to raise or and set my pots on top of these, while the water settles around them..It helps my humid loving plants a lot..

Usually a lot of plants grouped together can also create a mini micro climate around them too..

Just my .02$


RE: electric fan, air circulation and relative humidity

  • Posted by tapla z5b-6a MI (My Page) on
    Tue, Nov 16, 10 at 10:22

".... will having a fan running caused a drop in the relative humidity level?


The molecules in air vibrate at a frequency based primarily on temperature. The warmer it is, the faster the molecules vibrate and the farther apart the molecules are, which is why gasses expand when they are warmed and contract when they are cooled. We can imagine this property at work if we imagine a given number of molecules of air trapped in a balloon. Stick the balloon in the fridge and it shrinks, yet the number of molecules of air remain the same, they just become closer together.

Think of air as a sponge that can only hold just so much water. When molecules are warm and excited, there are larger spaces between the air molecules that can hold more water. When the air cools, it 'squeezes' the molecules together, which literally forces the water out - just like squeezing a wet sponge.

Now to what happens in the winter: The cold air outside your house, because the molecules are vibrating slowly, is incapable of holding much water; but as that cold air finds its way into your home, it warms up, molecules become farther apart, and it suddenly becomes thirsty for water. Water evaporates quickly to fill the large, dry spaces with moisture. THEN, some of this moisture-laden air finds its way to the outside, through little openings in your home. This carries moisture OUT of the home. As this occurs, more cold air is coming in, picking up moisture ...... In a continuing process, it is: cold, dry air in, air warms and absorbs moisture, moist air moves to the outside, relative humidity falls dramatically.

Simply moving the air with a fan won't have an impact on this process unless the air movement pressurizes air against an opening and it forces more moist air outside (which would cause more cold air to come in to replace it) than would normally occur.

IF the moving air is passing over a moisture source, it will upset what is called the boundary layer of moist air that surrounds the moisture source and actually slows evaporation. Upsetting this boundary layer causes more rapid evaporation, which INCREASES humidity, so having a moisture source in front of the fan is a very good thing when trying to raise humidity. I employ this strategy in my basement grow room to help save on the cost of running humidifiers to keep humidity in the 55% range.


RE: electric fan, air circulation and relative humidity

Hi Kemistry,

I'm in NYC & I never completely close my windows; even in winter, they're usually open an inch or 1/2" minimum.

That said, I grow many succulents which can stand the cold window sills, especially when I keep the plants dry.

I don't think a pebble tray on a cold, snowy windowsill is the way to go. I'd suggest you pull your tropicals off the windowsills completely. They're unlikely to survive the combination of cold & wet, far too shocking for them.

The few tropicals I still have are on a table several feet in from the window, & if I know it's going to get very cold, I try to wait to water until it's a sunny day.

RE: electric fan, air circulation and relative humidity

Kemistry lives in Oregon, zone 8. I don't think that sitting plants next to a window will be harmful. I do it in my zone 7 home! Always have. Though I don't open any windows, lol. If I did, the heat would run all the time, making it even drier inside. My cat is all static-y as it is!

Kemistry, is your home exceptionally humid? Or do you have a great number of plants in one room? If no to both, I wonder if you really need a fan at all. I know that many people collect their plants in one location for the winter, and I can see the need for some air circulation.

RE: electric fan, air circulation and relative humidity

I keep a small fan running near my orchids to increase air circulation. The house air is very dry even with two steam humidifiers running near by. My windows are cold. Air circulating cuts back the instance of fungal problems caused by the cool-damp area.

Fans can cause drying of the leaves. Blowing dry air against leaf surface will cause rapid evaporation. This can be troublesome with thin-leafed orchids and some house plants. Leaves dry out and buds can blast. I use steam humidifiers to get more moisture higher up.

Humidity trays provide little help they are really only drip trays. If you measure humidity you will see that the moisture in the trays rarely rises. What works for me is placing the plants close together and the small micro environment provides higher humidity levels, especially if the plants are layered in rows.

I use the fans to keep the air moving. Blowing air on the humidity trays helps move the moisture but unless the air is warm, it really doesn't rise. I cram the plants close together in the window and get a better moisture content.



RE: electric fan, air circulation and relative humidity

  • Posted by tapla z5b-6a MI (My Page) on
    Tue, Nov 16, 10 at 14:09

Blowing air over a moist surface is the principle that makes evaporative humidifiers effective, and is a very effective way to raise humidity in a room. The larger the area of the moist surface and the more air moving across it, the faster it will raise humidity levels.

Case in point: I use a 16x24 humidity tray filled with gravel in my basement growing area. It sets on top of fluorescent lights suspended from chains. A 16" fan is situated very close to the tray and blows air directly on/across the tray. This set-up puts about 1/2 - 3/4 gallon of water in the air of a 20X20 room per day, and causes the RH of the growing room to rise by as much as 10-15% compared to when the tray is empty.

I figured as long as I'm going to pay for the electricity to drive the fan, I might as well let the fan do most of the work of the supplemental humidifiers that kick in when the humidity tray alone can't keep up with humidity needs.


RE: electric fan, air circulation and relative humidity

A lot of great ideas and thoughts, thanks!

I have about 30ish plants in the livingroom room and bedroom. They are grouped together to take advantage of the microclimate effect as Jane mentioned.

For now I'll go ahead and buy a fan and employ the method suggested by Al to increase the RH. When I have extra money I'll invest in a humidifier.
So again, thank you all for your contributions. : )

RE: electric fan, air circulation and relative humidity

My plants are in my living room, I can't have trays of water laying around. The plants all sit on drip trays. My ceilings are high and the area large. Hard to get humidity levels up in the room. I just concentrate on the plant area.

i prefer steam vaporizers because they warm the air and I don't have to worry about mold spores or filters. Plus, they are fairly cheap to buy...not to run.


 o Post a Follow-Up

Please Note: Only registered members are able to post messages to this forum.

    If you are a member, please log in.

    If you aren't yet a member, join now!

Return to the House Plants Forum

Information about Posting

  • You must be logged in to post a message. Once you are logged in, a posting window will appear at the bottom of the messages. If you are not a member, please register for an account.
  • Please review our Rules of Play before posting.
  • Posting is a two-step process. Once you have composed your message, you will be taken to the preview page. You will then have a chance to review your post, make changes and upload photos.
  • After posting your message, you may need to refresh the forum page in order to see it.
  • Before posting copyrighted material, please read about Copyright and Fair Use.
  • We have a strict no-advertising policy!
  • If you would like to practice posting or uploading photos, please visit our Test forum.
  • If you need assistance, please Contact Us and we will be happy to help.

Learn more about in-text links on this page here