Edward Reid wrote:
On Fri, 21 Nov 2014 15:53:27 -0800 (PST), Hongyi Kang
wrote:

I set the manual humidistat to 45% since the very beginning.
And I have another small humidistat clipped right on one of the
second floor registers where the hot air comes out.

Just FYI ... the control device is a humidistat. The sensor (or gauge)
is a hygrometer.

You'd be surprised how much difference temperature makes in how much
water air can hold. I found a calculator at

http://andrew.rsmas.miami.edu/bmcnoldy/Humidity.html

and plugged in some of the numbers you'd quoted. It doesn't convert
directly from RH at one temperature to RH at another temperature, so
you have to take it in two steps.

If the temperature is 72F and the relative humidity is 45%, then the
dewpoint is about 50F.

If the dewpoint is 50F and the relative humidity is 16%, then the
temperature is 105F.

And 105F coming out of a register from a gas furnace is reasonable.

So yes, it's quite reasonable that the RH could be 16% at the register
and 45% in the middle of the room. Or IOW, 16% RH at 105F and 45% RH
at 72F are the same *absolute* humidity.

Edward

Hi,
Every house I had custom built and lived in always had humidistat
mounted side by side with thermostat. In present house I upgraded to
wireless thermostat which I can move around in the house and humidity
is monitored by humidistat still at location next to thermostat used to
be on a wall.