Furnace Humidifiers - correct type and wiring

16.01.03: Is your furnace humidifier working correctly?

If you are replacing a furnace humidifier, read this blog item before deciding on the type of humidifier to use and how to wire it.

These pics are of our reservoir type Desert Spring furnace humidifier I installed a couple of years ago.
As of yesterday, it is wired correctly. Read on, for "correct".

Two Important Aspects of Furnace Humidifiers:

A. Types of furnace Humidifiers
B. Wiring of Furnace Humidifiers

A. There are basically two Types of furnace humidifiers:

1. Water reservoir - an electric motor ($20) turns a filter ($12) drum in a water reservoir and water is added only when used - Desert Spring DS-3200, Wait 1000
2. Continuous water flow - an electric water solenoid ($50) flows water (approx 12 gal/day) continuously over a stationary filter pad ($20) - Wait 5000, Desert Spring DS-PFT

New models of above types (just introduced by Desert Spring):

1. Reservoir auto-flush add-on - replaces the water in the reservoir 48 hours
2. Continuous flow pulsed-flow - uses only 20% of the std 14 gal/day water
3. Continuous flow with humidisenser - water only flows when temperature sensor (humidisenser) reads higher temperature caused by furnace running and humidistat calls for humidity

Good/bad with new models:
  a. more efficient than their predecessors and/or easier to maintain
  b. more electronics that can go wrong and require replacement

My Evaluation of Humidifier Types:

In my opinion, the original water reservoir with drum filter (Wait 1000) wired to run only when the furnace is running and humidity is required is the BEST (purchase, installation, maintenance, cost). 
With reservoir types, you need to clean each spring and replace the drum filter ($12) yearly. 
With continuous flow types, you need to replace the filter yearly.

B. Wiring of a Furnace Humidifier

There are two ways to electrically wire a furnace humidifier: 
1. wrong - connect to a 24 volt transformer connected to your 110 volt house current - runs all the time the humidistat asks for humidity
2. right - connect to a 24 volt source in the furnace - runs only when the furnace is running and the humidistat calls for humidity 

Just to protect anyone who proceeds with this blog guidance:
a. Pls read the instructions for your furnace humidifier install and do not wire to 110 volt if it indicates 24 volt. More on this later in the blog item.
b. If considering wiring to a 24 volt furnace terminal, be sure to fully understand which terminals to use or employ a furnace technician.

Note: If you are considering a new/replacement furnace humidifier, read everything before proceeding.

In summary so far:
1. The most efficient furnace humidifier is a reservoir type (with electric motor)
2. The most efficient way to connect a furnace humidifier is via the furnace 24 volt connection (to ensure it only runs when the furnace is on) 

Common problems with furnace humidifiers:
1. Humidity level - not high enough 
2. Filter pad (continuous flow) type - too much water flow 
3. Filter pad (continuous flow) type - no water flow
4. Rotating drum (reservoir)  type - drum motor running all the time 

Reason/solutions for/to above problems:
1. Humidity is not high enough - more on this later, as it can have many sources.
2. Water Solenoid is incorrectly wired or defective:
   a. the solenoid needs to be replaced, if wired to the furnace
   b. the solenoid needs to be wired to the furnace 24 volt terminal
3. No humidity required or defective solenoid:
   a. The humidistat is not calling for humidity - wired correctly - no problem
   b. The humidistat is calling for humidity - replace solenoid 
4. Incorrectly wired - move drum motor wiring to furnace 24 volt terminal

Here is a diagram with correct wiring a humidifier to the furnace HUM wire or terminal. 
The wiring is the same for drum reservoir type (electric drum motor) or continuous flow type (water solenoid).
With either type, the electric power must be 24vac instead of normal house 110vac.
If the HUM furnace terminal indicates 24vac, just connect the wires.
If the HUM furnace terminal doesn't indicate 24vac, you need a 24vac transformer.


If you already have a humidifier on your furnace, the wires to it come from the furnace or a device connected to your house 110vac. 
If the wires come from the furnace, just reconnect to the new humidifier.
If the wires do not come from the furnace, you can make the humidifier much more efficient and save electricity cost. The wires probably come from a device connected to your house 110vac. You can detach it and re-attach it inside your furnace (if your furnace HUM wire/terminal doesn't indicate 24vac). 
Call me, if this isn't easy to understand.

If you are not comfortable with wiring, you should check with your furnace technician on correct wiring for your furnace.
If you are a DIY person, check your furnace installation guide or YouTube for guidance/reassurance.

Most furnaces have a location to connect a humidifier to ensure the humidifier only runs when the furnace is on and the humidistat is calling for moisture.
It is usually identified as HUM. The newer furnaces have a 24vac HUM connection. If this connection isn't identified as 24vac, you will need to install one of these 24vac devices (see black area in pic - upper left). 

I have the Desert Spring multi-disk drum (reservoir) type and it has a common electric motor to tun the drum and a replaceable filter. 

My furnace is a Trane XV80 and was wired incorrectly to the black device connected to our house 110vac and the humidifier ran whenever the humidistat was on. Very inefficient.

To wire most efficiently, I needed to find the HUM wire for the humidifier in the junction box (see pic - just below the black device), as there wasn't a HUM terminal on the circuit board. I also had to move the 24vac black box from the 110vac to the furnace HUM wire (as the HUM wire didn't indicate 24vac).
I rewired it yesterday and the humidifier now only runs when the furnace is running.

In summary
If you are planning to replace your furnace humidifier, consider a reservoir drum type humidifiers, as the only moving part is the electric drum motor ($20 at CTC) and there is no water wasted. Cheapest to buy, install and maintain. Just clean and replace the drum filter ($12) each spring.

To minimize cost and conserve electricity and/or water, be sure to wire the humidifier to the furnace 24vac terminal/wire. 
Before buying, always check the cost of replacement parts (filter pads, electric motor, water solenoid valve), as some are very expensive and required frequently.

If at all unsure of how to wire to your furnace, pls call me or your furnace technician, as any related damage to your furnace could void your warranty and/or maintenance contract.

If you have any questions or comments, pls respond to this blog item.