Humidistat Installation

Hi!
I've had a new Carrier furnace and humidifier installed a couple of years ago. Things are working great, but I am somewhat concerned that the humidity isn't being properly adjusted. It seems like it's really all over the spectrum. I can adjust the humidistat to 45%, and I will see 20% humidity inside. Or, I can adjust it to 5% on the humidistat, and get 20% still. I know that it's working sometimes though because the filter is wet with water.
I'm wondering, does it matter where the humidistat is mounted? They mounted it on one of the furnace intakes on the outside. It looks something similar to the picture on this page, but a little newer. ;-)
http://homerepair.about.com/od/heatingcoolingrepair/ss/humidifier_anat_9.htm
Does it need to be mounted on the first floor, next to the thermostat, in order to get an accurate reading? The one I have looks identical to this...
http://arnoldservice.com/4016_analog_humidistat_everything.jpg
Thanks in advanced for any information.
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Humidity is variable from house to house, if you have condensation on windows it is to high. Some units monitor exterior temp to set humidity, these are best.
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Have you cleaned the evaporator pad, use Lime A Way if its clogged or another acid
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Kind of.... I actually replaced the whole entire evaporator pad then scrubbed the plastic holder down.
As for condensation on the windows, there really isn't any. That's why I was wondering if the humidistat needed to be mounted somewhere else in order to obtain an accurate reading. :)
Oh, also.... I looked into different humidistat models, and it looks like I have the basic one. I saw a few that monitor outdoor temperatures, but mine does not do that.
Thanks!
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On my unit I have a damper in a vent that I close for summer, take off the cover of the humidifer when the furnace is running to be sure air is going through it, You might have a restricted water supply, there should be a rubber hose you can pull off and measure the amount that comes out then find out if it is the right amount per hour.
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Mounting it on the return air plenum if fine. That's where most of them are located. The air passing by it contains a mix of air from the various return registers throughout the house, so it should be somewhat of an average sample. The only advantage of locating it upstairs is to allow adjustment. I have an Aprilaire that includes the outside temp measuring probe and after initial setting, I never adjust it.
It's possible your humidistat is defective. Or that the humidifier itself is not operating correctly, so that it only can put a small amount of water into the air no matter what the humidistat setting. Or it could be that the humidity readings you are taking are inaccurate. Most of the simple home humidity measuring devices I've seen are not very accurate. How does the house feel? Are you getting static shocks? If you set it up high on a day when it's 15 outhside, do you get condensation on the windows?
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