Introduction: Easy Auto Fill Humidifier

Background / Motivation:

Like many people my furnace runs much the winter,  so I need a humidifer to keep the air from getting too dry.  When the furnace runs a lot, the humidifier needs to be re-fill every day, sometimes twice a day. 

My humidfier is located in the utility room and there's a faucet nearby  - so all that was needed was a way to automatically refill the humidifer from that water source.

Basic Idea:

With just a few basic components it was surprizingly easy to create a simple auto-fill system. 

1) An electric solenoid valve & power supply - commonly used for irrigation systems and readily availabe at a Home Depot or Lowe's. 

2) A DIY float and a limit switch, this energizes the solenoid valve to refill the humidifer tank if it's low.

3) Common lamp / appliance timer (the kind you might use to turn the lights on and off while on vacation).   With this you can scheudle a few "fill check & re-fill if needed" cycles each day.

4) A few hose and electical connectors. 

You should be able to get everything locally except prehaps the limit switch, total cost should be < $50.  Say goodbye to manual refilling !!

Another Option
- Of course the best solution, allbeit at more cost and complexity, is a whole house humdifier that attaches to a forced air furnace.

Step 1: Will This Work for My Humidifier ??

The first requirement is that there be a COLD WATER source near  your humidifier.  I my case, there was an utility room faucet available. 

If you can locate the humidifier near a sink you may be able to "tee into" the water line feeding the sink. 

Secondly, check that with the humidifier's fill tank removed, you have a clear line of sight to water reservoir (see photo).  This is to accomodate the float and limit switch hardware you'll need to install. 

If space is tight, the solenoid valve can be mounted somewhere other than directly onboard the humidier itself - just route the outlet line from the solenoid valve to the humidifier water reservoir.

The instructions that follow at specific to my model humidifer, you may need to make some adjustments to suit your humidifier.

Step 2: Parts List

Stuff you can get from Lowe's or Home Depot -

1) An electric solenoid valve -  3/4 " In-Line Valve Rain Bird CP-075 or similiar.  Also called an Automatic Sprinkler Valve.

2) 24VAC power supply - Orbit Watermaster Model 57040 or similiar. used to power the solenoid valve

3) A DIY float .  I used 25 Watt  T10 light bulb and 1" PVC pipe coupling - see text¤tURL=/pl__0__s?Ntk=i_products$No=15$Ntt=25-watt

4) 1/2 " pylwood for base plate -size to fit your humidifer

5)  Lamp / Appliance Timer,  Intermatic model dt500ch or similiar

6) Hose and fittings to connect water source to solenoid valve.  Large zip-ties to attach solenoid to pylwood.

7)  Limit switch mounting hardware - 1/4 -20 screw (4 " lg), 3 hex nuts, 1 tee nut, "L" shaped brachet from scrap of aluminum or iron angle stock.  2, 4-40 screws (3/4 " lg) and hex nuts to attach limit switch to angle bracket.

Get from Radio Shack -

8) 3/16 " male and female crimp connectors Radio Shack 64-044 or similar

Probably have to get via mail order -

9) Limit Switch, Long Lever Arm Type - Omron V-10G3-1C24-K , part number 187805 or similiar

Step 3: Read Me - System Overview

Spend a minute or two reviewing the attached illustration.

The design and constrution of the float /limit switch arrangment should receive the most attention, since it detemines if the humdifier gets refilled properlly without under or worse over filling.

Step 4: Mounting Plate and Float-limit Switch

1) Remove the existing humidifier refill tank and cut a piece of plywood to fit onto the humidifier in place of the refill tank. 

The plywood should cover the same area as the original refill tank so as to not change the air flow pattern of the humidifer.

2)  Layout locations in the plywood for the  float/limit switch mechanism and the solenoid valve.  Make sure the location for the float is directly above the humidifier water tank.

3.  Cut a hole in the plywood for the float guide.  For the float guide I used a 1" PVC pipe coupling.  The coupling had a little ridge inside which can be cut away with a file or Dremel tool. 

The float should slide nicely within the float guide. 

Wrap the outside of the PVC coupling with duct tape to get a tight fit with the hole in the plywood and secure in place with hot melt glue.

4.  For the float I used a 25- watt T10 light bulb.  The down side of is the bulb can break.  The plus side was - it was the right size,  completely water tight, inert, wear resistant, slides with very low friction within the guide, and could be easily connected to the limit switch via a soldered-on wire loop.  Ideally, I would have used a plastic rod but I couldn't easily find that.

The only modification necessary to the 25 watt bulb was soldering a wire loop on the socket end.  There is already solder in that location, but a small pencil type solder iron does not provide enough heat due the the large thermal mass of the socket sucking away heat, so I used an old soldering gun. 

The loop needs to be sized so the limit switch lever arm will easily fit through the loop without binding.

5.  Using small 3/4 " long 4-40 screws and nuts attach the limit switch to one leg of a small piece of angle stock.  Drill a 1/4 hole in the other leg. 

In the plywood, drill a 5/16 hole near the float guide tube, see photos.  Install a 1/4-20 tee nut in the bottom side of the plywood - this creates a threaded hole in the plywood. 

A long 3 "or 4" 1/4-20 screw along with some nuts creates an "adjustable height" mounting post for the limit switch.  Check that the float and switch move freely without binding.

6.  IMPORTANT - With the water tank in the humidifier fully filled, adjust the height of the limit switch so that the switch is held in the open (or OFF) switch position, see photos. 

Step 5: Mounting the Solenoid Valve

1) First figure out the hose and fittings required to connect the solenoid valve to your cold water source. 

Make sure the water line can be routed to the humidifier without kinks. 

Note that the solenoid valve has ports marked as inlet and outlet - it's important not to get these mixed up.

2) Install a 90 elbow fitting in the outlet port of the solenoid valve and cut a hole in the plywood above the humidifier water tank. 

Drill 4 holes in the plywood to allow attachment of the solenoid valve with 2 large tip-ties, see photos.

Step 6: Electrical Connections and Test Operation


1) To the two leads coming from the solenoid valve - attach two 3/16 " female crimp connectors.  To the two leads coming from the 24VAC power supply, attach one 3/16 " female crimp connector and one male connector.

2)  Make the following connections:

a.  Connect male connector from power supply to one female connector from the solenoid valve.

b.  Connect  female connector from power supply to the common (COM) terminal of the limit switch.

c.  Connect one female connector from the solenoid valve to the N.O. terminal of the limit switch.

3) Test operation - let the humidifier run out of water, for most humidifiers this means the "Fill Light" comes on and the fan shut soff. 

Now plug the 24VAC power supply into a 110AC wall socket. 

Since at low water level, the float pulls the limit switch lever arm down closing the contacts, the solenoid valve should energize and water should begin refilling the humidifier tank. 

As the water level raises the float will lift up thus opening the limit switch contacts, de-energizing the solenoid valve, and stopping the refilling process. 

After the refilling process ends, check the water level.  

If the operation did not go as planned you may need to recheck your connections or make adjustments to the limit switch position relative to the float.

4)  A setting up the lamp /appliance timer -  Plug the timer into the 110 AC wall outlet.  You will probably need a short extension cord to connect the timer to the 24 VAC power supply. 

Set the timer to come on 2 or 3 times everyday.  The on time only needs to be about 1 minute - that's all the time it takes to refill the humidifier, but the timer probably has a minimum "On Time" of 15 minutes. 

As a precaution, I set the timer to come on when I'm likely to be home - just in case something goes wrong.  So far no problems at all.  Nonetheless, I recommend you observe a few refill cycles in person just to make sure everything is working OK. 

Technically refilling does not really need the timer, but without a timer you'll get a lot of very short refill cycles which may wear out the solenoid valve.

That's it you're done !