Wiring Humidifier to furnace

[Jim Elbrecht]

Last year I installed an Aprilaire 600 humidifier on my 30 yr old oil/hotair furnace. Smacked myself for not doing it 20 yrs ago- It didn't cost much more than one of the dozen or so portable humidifiers I futzed with over those years. It did a better job of humidifying- and was no work at all.

Thanks to whoever asked about one on this group- or I probably never would have woke up.

But then I saw a great deal on a new furnace on Craigs list- and just got done installing it. I just ran it through one cycle to test for noise & leaks. The HVAC guy comes Friday to check my work & tune it up.

On the old furnace I wired a transformer to the fan so when it kicked on, the solenoid kicked on the humidifier. For the life of me I can't remember how I did that now. The schematic in the [furnace's] installation booklet don't make sense to me.

Could someone look at these and try to put into words where the transformer wires-- 2 black & a green-- go? Schematic

hook-up Thanks, Jim

[Stormin Mormon]

You're friends with a heating tech, but you're posting the question, here? What am I missing?

[Jim Elbrecht]

I *hired* a guy to come Friday. [actually he is the referral from a friend of a friend--- but I didn't confuse the original post with that bit of useless trivia]

I'm sure he'll be able to hook it up--- but it bugs me that I can't figure out those diagrams and get it done.

I'm bound to learn something in the process of hooking it up myself.

Jim

[RBM]

If you want the transformer to power up when the blower is on, connect it to the white and black wires where the diagram shows the blower, unless you're using the medium speed, then use the white and blue

[Jim Elbrecht]

Thanks-- I didn't think that was a choice *I* made. I thought the furnace chose the speed & I was looking for something to connect to before it made that choice. I'll have another read through the literature to see what it says about the two speeds.

Thanks- Jim

[trader4]

What determines if the furnace blower runs on high or med speed during heating? The diagram shows that the blower can run at med or even low if the nozzle size is small enough. I would guess that this must be set via a dipswitch somewhere on the controls. Assuming you have it set to always run at high speed, then as RBM said, the transformer goes across the black and white wires going to the motor.

If the furnace can choose itself whether to run at high or med, then you have a more difficult problem, because there is no easy way to wire the transformer so that it will have AC power in either case.

I have an Aprilaire and mine is wired so that the transformer always has 120V. There is a seperate trigger lead that is connected to the furnace controls on the 24V side to then turn it on when the furnace is running.

[RBM]

What determines if the furnace blower runs on high or med speed during heating? The diagram shows that the blower can run at med or even low if the nozzle size is small enough. I would guess that this must be set via a dipswitch somewhere on the controls. Assuming you have it set to always run at high speed, then as RBM said, the transformer goes across the black and white wires going to the motor.

If the furnace can choose itself whether to run at high or med, then you have a more difficult problem, because there is no easy way to wire the transformer so that it will have AC power in either case.

I have an Aprilaire and mine is wired so that the transformer always has 120V. There is a seperate trigger lead that is connected to the furnace controls on the 24V side to then turn it on when the furnace is running.

I've only seen an Aprilaire once, and it too was wired as you describe.

[Stormin Mormon]

I like the Can-Do attitude. Typically the humidifier water valve is wired to come on with the fan blower. And, if the humidistat is calling.

[Lefty]

You *are* using the medium speed (the blue wire). The black wire is from the air conditioning relay. The blue wire is from the fan/limit control, which is connected to the heating speed tap at (or near) the blower motor. I don't see where this furnace can choose it's own speed, based on the provided diagram. The tech who sets this furnace up will measure the temperature rise (heating) and drop across the evaporator (cooling) and physically plug in the black and blue wires into the speed tap accordingly. Anyway, the huimidifier is automatically interlocked to only run on a heating fan call, not during a cooling nor fan on (at thermostat subbase) call.

Alternately, you could use the model #50 current sensing relay for interlock.

HTH, Lefty

[trader4]

On looking at this again, I agree with your analysis. It appears the blue wire is used to power the fan for heating and the relay selects black when in AC mode. So, he would have 120V between the blue and white fan wires during heating.

Assuming of course that he has it wired up that way. That is how my Aprilaire is hooked up. It has 120V all the time and has a 24V trigger lead connected to the furnace controls.

[bud--]

Relay "relay" is connected to thermostat G-C and is energized when the stat calls for cooling or the stat is set for "fan on". The fan runs at high speed.

Control "fan & limit switch" is a plenum switch that is operated by heat. The fan (NO contacts) comes on when the switch gets hot and the fan is held on while the plenum is hot. The normally closed contacts only open if the control gets too hot. If the NC contacts open nothing operates except the fan. "Aux limit" is a redundant safety control.

(Not obvious what the low voltage connection to the "fan & limit switch" is - "if used". It is connected thermostat W-C and would be energized when the stat calls for heat. "Relay" has common symbol for a coil. "Fan & limit switch" is different - probably heat activated.)

The "blue" wire from the "fan and limit switch" terminal "F" to the "relay" terminal "4" is only on when you are heating. _It is the wire to tap_.

This is similar to what Lefty said, but you don't want to connect to the blue wire at the motor.

It is desirable to only power the dehumidifier during heating (not cooling). The wire above will do that. The other thing to avoid is connecting (directly or indirectly) to a motor wire that can feed the wrong voltage to the humidifier solenoid. The wire above also avoids that.

It is much easier to figure out what is happening with a control diagram with wiring on horizontal rungs, rather than a connection diagram provided.

[Art Todesco]

In my old house I used a special gadget where you clamp it around the neutral wire for the blower. It has 2 leads which you wire in series with the 24 volt circuit to the humidifier. Here's a link:

perfectly. As for not wanting the humidifier to turn on in the summer, the humidistat will usually take care of that .... you can just turn is as low as possible and it'll be ok. Also, you really don't want to connect the 110 volt side of a 24 volt transformer to a multi-speed blower, where speeds are determined by heating or cooling because strange voltages appear on the unused lead, i.e. blower running on medium, high lead may have 160 volts on it depending on the motor.

[bud--]

Probably what Lefty described.

That was one of my points, and it doesn't happen if connection is made where I said.