Do they work?
Wild temp swings during the winter (+25F today, -20F forecast Tue/Wed, +30F
Fri) keep me changing the settings on our manual model, and sometimes I
Even with our super wiz-bang windows, we still get condensation when I
leave the humidifier set too high. If I leave it too low, we draw sparks.
What does your machine measure to determine when to turn on/off?
The inside temperature should be relatively independent of the outside
For condensation, you care about the dewpoint.
Go to engineeringtoolbox.com
Print yourself a Psychrometric chart.
For example, if your indoor conditions are 70F and 60% humidity,
your windows will have condensation if the surface temp drops
below about 57F. Sounds plausible with outside at -20F.
I run my house at 66F and 40% humidity, so the windows don't
have condensation until the inside surface hits 40-ish F.
Rarely gets lower than +20F here.
Somebody smart will chime in if I'm reading the chart wrong.
The manual AprilAire model we have presumably is measuring humidity in the
The control is just a knob, with a scale going from "OFF" to about 50%
humidity. There's a small chart with suggested settings for temperatures of
-20F (15%), 0F (25%) and +20F (35).
The thermostat's set at 68F daytime and 63F nighttime.
True, but it's the temperature of the inside panes of the windows that
determine if there's going to be condensation.
Still not clear what it's measuring. My portable unit appears to monitor
actual humidity. It's uncalibrated, but once I get it set,
the humidity in the house stays rather stable.
Yes, but what is that window temperature? A window surface with a
temperature difference of 88F may well get cold enough to hit the dew
point. Close the curtains and it likely happens. Air leaks in the
frames can make it worse.
There should be some
rules of thumb that apply. The actual math requires a bunch of assumptions
I fear you're trying to fix a problem at the wrong place.
If you have your internal temperature and humidity adjusted to the lowest
values you can tolerate. There's not much you can do to fix the
condensation problem at the humidifier...except reduce the humidity
below the level you can tolerate. You can do that by switching it off
when the outside temperature is below some limit.
I think even a perfect inside humidistat won't help you fix a dewpoint
Might be worth a look to check out air leaks. At low temperature, the
amount of moisture in the air is probably noise level.
Any of that infiltration should lower your humidity.
And when infiltration is measured in air-changes, that's a lot of air
volume carrying your added moisture outside.
I'm not seeing any glaring issues that might address the problem.
It's damn cold outside and the inside window surfaces hit the dew point.
I would be interested to hear what you find out when you measure the
window temperature when -20F outside.
Years ago, when I did more installing, we would
some times install the outdoor temp sensor. I
didn't hear back from the customers (they called
my boss, not me). The sensor might have helped.
We installed more than a few of them, so I'd dare
to guess they helped.
Christopher A. Young
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