I have a new (June 2009) goodman furnace and a typical 24v humidifier valve. The humidifier never worked and I never saw a signal on the line coming from the furnace. I've removed the furnace from the circuit and connected it to an old transformer (prev. furnace) and the water is flowing.

Now I'd like to get the fan switching working. The previous (not working) wiring was to a red wire in the harness that runs to the controller board red. The manual says "a 5inch long brown wire in the wire harnass at the low fire pressure provides 24vac humidifier control."

I see no brown wires anywhere, and have no idea with the "low fire pressure" is. It doesn't sound like a place to me.

Unfortunately the installer is so busy that I've chased him for MONTHS and can't even get the guy to call me back to help with this. I'm finally fed up enough to deal with it without him!

Goodman model? Link to manual?

If you have a two stage furnace, there is a low fire burner and a high fire burner. There is little point in humidifying at the high fire level.

Usually low fire uses a lower fan speed and thus the moisture can easily be absorbed with the fan slowed down.

The furnace isa GMV95, two stage furnace with DC motor.

Air Conditioners, (A/C), Packaged Heat Pumps and Central Air Conditioning (AC) Units for Home Healing and Cooling Systems. Residential and Commercial Heating and Cooling Equipment by Goodman Manufacturing available in 14 SEER and 13 SEER ratings for (http://www.goodmanmfg.com/Home/Products/GasFurnacesHighEfficiencyupto95AFUE/GMV95MultipositionVariablespeed95AFUE/tabid/226/Default.aspx)

As I understand it the furnace has a 24vac signal that comes on and off with the fan. This is "a 5inch long brown wire in the wire harnass at the low fire pressure provides 24vac humidifier control."

The installer hooked up the furnace but it never worked. I'm trying to trace the wiring and see it going to the controler board red connector. I'd like to find the brown wire mentioned in the manual and see whether I can use this. But I can't understand what the manual means. Is the "low fire pressure" a valve on the gasline or something? What is it, and where am I likely to find it?

Why don't you do it like everyone else?

On the link you gave me in the manual on page 13

There is a terminal labeled HUM and a bunch of secndary Neutral terminals. (white wires)

The 24 VAC for the humidifier goes on these two terminals.
On page 15 where the scematic is located, the terminals are shown as HUM and HUMN.

Because of "on blower delays and "Off blower delays", the HUM terminal is derived.

Usually heat is associated with low speed. The low fire gas valve opens for a bit before the blower turns on (blower on delay), so you don't get a burst of cold air out the registers.
There is also a "blower off delay" to extract some of the heat after the burner turns off.

So, the humidifier is activated when the low fire gas valve is open, and the fan is on low speed and in heating mode.
Low spped in many cases implies heating.

The HUM terminals should not power the humidifier, but rather act as just a control signal to turn it on. It's always advisable to not power accessories from the control transformer.

The manual explains that the HUM terminals (which I've seen clearly marked on the board) are 110V for power humidifiers. The manual also says that there is 24VAC, switched with the fan, available on that mysterious 5inch brown wire for humidifiers like mine.

If there is no hope of finding the 24VAC I guess I'll have to look at the 110V ones.

Where should I look for "at the low fire pressure"?

Hey, you have a spare transformer from the old furnace so take it and connect the HUM terminals 120 to the spare transformer and wa la, you now have 24 V to power your existing humidifier with no extra cash outlay.

Just check the transformer and humidifier ratings. Usually transformers are rated for 40 VA or 40 watts.