Here are the facts about my home and sytem:

Location: Indianapolis, IN
Furnace: Forced air gas (Located in garage)
Humidifer: Aprilair 500 series (Connected to hot water, electronic controller)
Home: About 1500 Sq Feet (2story + basement)

I had the humidifier installed in November of 2010. At first it was set to run only when the furnace was on and I saw humidity ~32 to ~40. I called after a couple weeks to ask how I can get it to go higher - they came out and connected it so it will humidify any time my fan runs (i.e. while heating or if I have fan set to "on"). This brought the humidity up to ~ 42-45% according to the digital control, but my in home reading was about 3-5% less. (Apparently the digital control turns off the water solenoid at 45% and back on at 42% based on what the installer showed me)

I went on a trip, left my fan on the while time (3 days) and when I came home it was only 35%. This morning (9 hours later) the fan is still running. The humidity showed 40% when I woke up, and 35% about 15 minutes later.

I've had the company who installed this out here 3 times so far and am not happy. I expected to have humdity in the mid 40's or more.

1. Am I expecting too much from this humidifier? Shouldn't I expect mid 40's constantly?

2. I am guessing that dropping from 40% to 35% in 15 minutes, when the system is running (fan on, solenoid is open) is a concern. What do you suggest I ask for from my Heating & Air company who installed this? I presume something is not working right?

Thanks in advance for your input. If this is "normal" then I guess I'll accepet it. But it doesn't seem right to me.