I came here to seek a bit of guidance on using my humidistat. I've only owned my home for a few months and have never had a humidistat before.

I have a GeneralAire M3 humidistat and was trying to look up how to use it properly.

From my understanding, the winter months has very low humidity and by setting the humidistat to a certain level, it will help keep some moisture in the air of the home, which helps with retaining heat. And apparently keeps your skin from drying out so bad. But also doesn't create too much humidity to where you windows sweat and things like that.

Based on the operation instructions in the manual, I should adjust the humidity setting based on the difference of the outside temperature vs the inside temperature? So if I keep my house at 72F and it's 40F...or lower, I would set it to 10% humidity?

Am I understanding this right, or am I completely wrong? Also, was I supposed to keep the humidistat off all summer or does it have benefits in the summer too?

Here is the setup I have: Humidistat Humidifier Water valve leading to humidifier

  • What is the humidistat controlling? Humidifier or dehumidifier? From your text it sounds like a humidifier, if so is the humidifier part if a forced air heating and cooling system? If I'm assuming correctly I'll write a complete answer including additional info you didn't know to ask...
    – Tyson
    Nov 12, 2016 at 18:25
  • @Tyson The humidistat is controlling a humidifier and is a part of my heating/cooling system.
    – DrZoo
    Nov 12, 2016 at 18:28
  • Those were my guesses. I'll do a detailed answer for a typical setup later this afternoon (too long for writing it on my phone). There's a few more operational details you should be aware of.
    – Tyson
    Nov 12, 2016 at 18:36
  • @Tyson I know the phone struggle all too well lol. I'll be looking forward to your answer later this afternoon.
    – DrZoo
    Nov 12, 2016 at 18:38

1 Answer 1


You asked a basic question of how to set the humidity level on the humidistat itself. The easy answer is a table that is printed right on the face of my humidistat. It's easier to use than subtracting outside temp from inside temp.

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The magic of not using an indoor temp is they just assume a comfortable indoor temp (probably around 68-72F) and build a table based on that and outdoor temps.

The humidistat is not the only control however. Most forced air systems use a "flow-thru" humidifier. The other control then is an Air Damper that controls the volume of air that is re-circulated during humidifier operation. (There are a few models in a another class this may not apply to, but virtually every humidifier I've actually seen installed is the "flow-thru" type.)

enter image description here

This pic is a typical "Flow-thru" type humidifier. When in operation warm air is fed from the duct entering on the left. The air is pushed thru a water panel, and back into the return air side of the furnace.

For the humidifier to work the air duct must be open. The knob on the left side of the second picture is fully open when the knob is on high. Low on the other hand is not "low" but essentially fully closed.

In the winter to add humidity to the air, both the humidistat must be set high enough where it's calling for humidity AND the air damper must be open.

In the summer, you want the opposite. Turn OFF the humidistat and CLOSE the air damper, so that the Air Conditioning will have maximum effect. (Air conditioning by nature DE-humidies or removes water from the air-- so you don't want to be adding water to the air just before the air is cooled.)

Three more thoughts:

1) You're probably thinking, how inconvenient to be adjusting the humidistat's setting frequently based on the outdoor temp swings. The bottom line... You don't really need to. In practice, just setting it around 30 or 40 and leave it the whole winter works out just great. The reason is that if you keep the fan (on the thermostat) on the "auto" setting, the humidifier itself will only run when the furnace comes on. If the temp is warmer outside it doesn't run much and not much humidity is added, as its colder outside it gets a chance to run more and add humidity, at the same time doors are opening and closing and letting in cold dry air. In the end it all averages out.

2) There is a disposable part in humidifiers. The water panel needs to be replaced every couple years. A worn out water panel will waste more water than it will add humidity to the house.

3) The water must be on, and the system can't be plugged up. When you switch the humidifier on the first time each year, test it. Let is run long enough that you know water is coming out the drain hose. I've seen cases where the water valve is either shut off, or orifice is plugged with calcium deposit and water won't flow. (The orifice location varies by make and model, sometimes it's part of the solenoid valve on the humidifier other times its a little plastic part with just a pin hole for water to flow thru--It's what regulates the water flow to a tiny trickle.)

  • This was amazing! I've looked all over for a while trying to find a full "how to" for the humidistat/humidifier and couldn't find anything. Hopefully this gets queried in Google and others come across it one day. Thanks!
    – DrZoo
    Nov 13, 2016 at 2:07
  • Basically keep the setting on the humidistat as high as possible without windows icing in the coldest temps of winter.
    – Tyson
    Nov 13, 2016 at 2:20
  • As far as testing it out to make sure water is coming out of the draining hose, would I turn the humidity up all the way to maximize the amount of water it uses?
    – DrZoo
    Nov 13, 2016 at 2:22
  • Yes, BUT.... Lol 😂... Turning it "up" doesn't make it use "more" water... The humidifier is either "on" or "off", turning the knob "up" doesn't make it work harder, or use more water.... You just need it to be running. (On mine there are a couple thumb screws on top that you can remove and peek at the water tray. Also on mine that "nameplate" front and center snaps out and that becomes a window you can use to check operation and the position of the air damper on the left side.)
    – Tyson
    Nov 13, 2016 at 2:30
  • I guess I was trying to say is by turning it up, the humidifier would be more likely to run lol. Maybe I'll see if my faceplate comes off. I just can't quite tell if the water is running to it or not. I do have some hard water in my area so I want to verify that it's flowing. Thanks for the great info!
    – DrZoo
    Nov 13, 2016 at 2:33

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