I made a cheap, high capacity humidifier from a mist maker, storage crate, and 120mm fan (the fan blows into the crate, which pushes the mist out). This 4" flex pipe works fine to exhaust the mist when it is at this fully contracted state.


But humidifying one room isn't really why I made this: I wanted to be able to put it near the return air vent and humidify the entire townhouse. Unfortunately that vent is in the loft with no water access, so I've tried to connect 2 of these 8' ducts together to get up there and over to the vent. But when I do this, the vapor flow at the end is pretty weak, not nearly enough to humidify the whole townhouse. The vapor is condensing in the duct and falling back into the tank. I figure that if the fan had more static pressure it would be able to push the vapor fast enough to prevent it condensing. But I'm also trying to keep it fairly quiet, so I don't want to stick a big honking duct booster fan on there without knowing how powerful it needs to be. How do I calculate that?

  • very interesting project ...... if you are using an ultrasonic mist maker, then it does not evaporate water all that much ..... it actually "spits" out tiny droplets of water .... the droplets may evaporate after being ejected, but that depends on ambient temperature..... NOTE: any minerals in the water also get aerosolized, so you will need to use distilled water ..... this may help engineeringtoolbox.com/moisture-holding-capacity-air-d_281.html
    – jsotola
    Mar 25, 2019 at 22:39
  • It's a 5-disc ultrasonic mist maker. And indeed I've noticed the vapor evaporates at a different rate depending on the RH. At 50% it sticks around for a while! Most of the minerals get left either in the tank or the duct. Mar 26, 2019 at 0:27
  • So do you suggest heating the air that's going through the ducts? I did think about that but figured it was less practical than having a stronger fan. But it's entirely possible I'm wrong about that! What kind of heater would be suitable? Do I only need to heat the air, or the water as well? Mar 26, 2019 at 0:31
  • probably the easiest thing to do would be to get a more powerful fan and see if it works .... you could be correct about the minerals staying in the tank, but i have heard of peoples furniture getting covered with fine white dust
    – jsotola
    Mar 26, 2019 at 2:29
  • vent is in the loft with no water access ...... would you be able to run thin tubing, the same kind as refrigerator ice-maker tubing? .... it is same thickness as cat5 ethernet cable
    – jsotola
    Mar 26, 2019 at 2:32

2 Answers 2


Propeller fans aren't really good at producing any static pressure. They can move air quietly but as soon as they see any restriction air flow stops. Your basically making a remote mount fan assist humidifier.

I'm not sure if it would fit your application but if you have return air present, why not use it's draw to pull the vapour out of your box?

enter image description here

Use a short duct length connected positively to the return air register by using a boot to maximize surface area.

I know you said the vent is far away so another method would be to pipe a supply duct into the box and pipe the other side to the return.

Otherwise you will probably need a squirrel cage type of blower. Noise moving air increases with increasing static pressure, think Dyson air blade hand dryers.

I answered this in a hurry this morning. To follow up. Calculating output cfm requires specialty tools. Basically it measures feet per minute, does a little math based on technician inputs and spits out a number.

The math is as follows

cfm=fpm x pi r²

Assume 10 feet per minute

Pi =3.14

r=radius for a 6" diameter pipe r = 3"

So (3x3)3.14=28.26

1 square foot 12"x12"=144


So 5.09x10(feet per minute) =50.9cfm

  • I like this idea and would suggest increasing from a 4" duct if you're going to keep the long run otherwise you might end up with the moisture settling in the ducts. The 'intake' on your box could be larger too as you now would have a high static pressure fan for the system.
    – Dan
    Mar 26, 2019 at 17:17
  • Using the return air suction is a very interesting idea but I worry about getting too much mist condensing on the return air vent (as opposed to having the mist evaporate first so that the air going through the vent is just more humid than ambient). I'm not worried about condensate in the ducting since that's relatively easy to soak in bleach/vinegar. Mar 26, 2019 at 19:01
  • My comment and my brain got out of sync, I had the same concern about condensate in the return or your existing HVAC system, but my comment reads totally differently.
    – Dan
    Mar 26, 2019 at 19:53
  • Would a narrower duct, with a higher static pressure blower, blow the mist faster and give it less chance to condense before exiting the duct? Mar 27, 2019 at 13:28
  • You have to consider flow restriction but in general all things being equal air velocity would increase with a smaller diameter but overall cfm would decrease. Your problem seems to be in the pipe more than anything. Try using a smooth wall material. It's also helpful to have throated connections like a side take off from the link imperialgroup.ca/rectangular.cfm?c=106
    – Joe Fala
    Mar 27, 2019 at 14:13

This does not have an easy answer. The 'easy' direct answer is to use a duct calculator. To what end is where it gets complicated; I'd suggest a minimum design such that you still have velocity that you can feel on your hand (1-2mph) at the output of the vent.

The axial fan you're using can't do much against the static pressure of a 16' run of flex duct (consider that purpose-built centrifugal vent fans often have shorter max runs at 4").

Your options are to:

  • Increase the pressure the fan can generate to overcome the long, high-friction duct run
  • Reduce the pressure drop from the duct run (smooth wall duct, larger duct)
  • Play with the other variables (CFM, pump water, etc).

Personally I'd opt for smoother duct at least and probably use a DC-drive centrifugal fan designed as a quiet bathroom vent fan to pressurize the system. Delta makes some that you could probably adapt and have humidistat controls as well.

Finally, I have to caution you that HVAC is often about controlling (reducing) humidity and pushing water into your existing duct system or otherwise creating a moist environment can be a recipe for a mold disaster. I would consider smaller humidifiers in bedrooms used only in the dry season, but I probably don't live where you do.

  • Yeah I looked at duct calculators but it's tricky because I'm working with mist rather than air. And a mostly vertical run of pipe rather than horizontal. I am keeping a close eye on my hygrometers! I have learned that it takes quite a bit of airflow to distribute humidity evenly through a single room, much less the entire townhouse. Humidity has been down at 20% on cold days. I just want to keep it at least above 30% (although a previous question here got some interesting information about limiting indoor RH with very cold outdoor temps). In another month it'll be moot until next season. :) Mar 26, 2019 at 19:17

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.