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My humidifier won't turn on - is my solenoid valve bad?


Is my solenoid valve bad?

Through the years, we've found that questions about solenoid valves appears to outnumber questions about all other humidifer parts combined.  This makes sense - a visual inspection shows that the solenoid valve is the "gatekeeper" that allows water to flow (or not) from your water supply to your humidifier. 

Many customers assume that if the humidifier is not running - the valve must be bad.  This is not always the case.

We have decided to demystify this simple device - and offer some tips for trouble shooting problems with the solenoid valve or other parts of the humidity control circuit.  NOTE: if you are not comfortable or knowledgeable about electricity, do not attempt to fix your own solenoid problems - consult a professsional!  Also - the scenarios described below are typical - but may not apply to your scenario.  If you aren't sure - let someone else help.

Most humidifier solenoid valves are operated on 24 Volt AC power, which is the predominant low voltage wiring standard for conventional HVAC controls.  24 Volt wiring is thin, 18 gauge wire - much smaller than 110 or 240 Volt line voltage wiring.  If you are not sure if your humidifier uses low voltage wiring - leave the diagnosing of your issues to a professional. 

All Aprilaire humidifiers (except models 110, 112, 350, 360) use the # 4040 24 Volts AC Solenoid Valve.  Most newer Honeywell, Skuttle, Herrmidifier, Autoflo, and Lobb humidifiers also use a 24 volt solenoids.

First a little info on the valve itself. The typical 24 volt solenoid valve connects on it's inlet side to a 1/4" copper water line via a compression fitting.  It connects on it's outlet side to a copper or plastic 1/4" feed tube also using a 1/4" compression fitting.  The solenoid usually has a strainer nesting inside it's inlet side - which prevents large chunks of sediment or debris from lodging in the valve.  On the outlet side, there is normally a water metering device called a fluid restrictor or orifice.  This metering device feeds just the proper amount of water through the feed tube and into the humidifier.  In the case of Honeywell humidifiers and others - the strainer and orifice are both on the inlet side of the valve.

The solenoid is normally closed and will not allow water to flow.  Only when the valve is given 24 Volts AC will it open.  The valve has 2 wires coming from it.  If there is not 24 Volts of power across these two wires - the valve should be closed and no water should flow.  Once 24 Volts is given to valve - it will "click" open and water will pass through.  If you understand these concepts - it should be easy to figure out if your valve is bad or good.  Find a known good source of 24 Volts AC (verify this with a volt meter).  Connect this 24 Volt power source directly to the solenoid.  If you hear a click and water flows through to the humidifier - you need to look elswhere in the control circuit for the issue.  If you hear no click and no water flows the valve probably needs to be replaced.

Sometimes customers say their humidifier seems to run constantly, even when the furnace is not on.   Try disconnecting power altogether from the solenoid valve.  Is the water still flowing?   If so the valve is probably bad.  If not - there is a problem elsewhere in the control circuit.

24 Volt Power Sources:

So, where does this 24 Volts come from?  It could come from a number of sources. 

There may be a stand-alone 24 Volt transformer in your mechanical room.  Often, these piggyback on a 110V or 240 Volt electrical junction box.  The visible (low voltage) side of the transformer will consist of 2 screw-down terminals for the hot and neutral sides of the low voltage circuit. 

The voltage may come from the circuit board on your furnace.  The circuit board will have a terminal strip where your thermostat connects.  This is a strip of screw down terminals, usually labelled with letters like those on your thermostat (R, W, Y, G, C, etc.).  In typical situations, there is always a constant supply of 24 Volts AC between "R" and "C" - the hot and common sides respectively of the system transformer.  There may be an "accessory terminal" closeby labelled "HUM".  This terminal will engergize any time there is a call for heat and will provide a switched source of 24 Volts (sometimes 110 Volts) that that can be used to to give power to the water valve on the humidifier.  This is pretty handy as it is an automatic "furnace proving" mechanism that eliminates the need for relays and other gadgets that help "prove" fan or furnace operation.  More on this below.