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Correct settings for t-stat and h-stat for power roof vent

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Correct settings for t-stat and h-stat for power roof vent mapleman 08-18-2006
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Posted by mapleman on August 18, 2006, 10:41 am
Hi,

I live in a fifty year old 1100 sq. ft. (over a full basement) ranch
home in Sussex, WI. We had a power vent installed as part of a new
roof job about nine years ago. A couple of years ago the fan blades
came loose from the shaft, making an awful clatter. In the process of
shutting off the fan and figuring out the problem, the settings for the
thermostat/humidistat got all messed up. I don't know what the
original settings were (or even if that was correct).

The control unit is a Lomanco brand, and it indicates that the setting
is for the fan off temperature. The fan on temperature is fifteen
degress higher than what it shuts off at. The humidistat turns the fan
on when a certain percent relative humidity is reached.

What is the correct setting for the thermostat and humidistat for my
house and climate?


Posted by Al Moran on August 18, 2006, 7:12 pm

>Hi,
>
>I live in a fifty year old 1100 sq. ft. (over a full basement) ranch
>home in Sussex, WI. We had a power vent installed as part of a new
>roof job about nine years ago. A couple of years ago the fan blades
>came loose from the shaft, making an awful clatter. In the process of
>shutting off the fan and figuring out the problem, the settings for the
>thermostat/humidistat got all messed up. I don't know what the
>original settings were (or even if that was correct).
>
>The control unit is a Lomanco brand, and it indicates that the setting
>is for the fan off temperature. The fan on temperature is fifteen
>degress higher than what it shuts off at. The humidistat turns the fan
>on when a certain percent relative humidity is reached.
>
>What is the correct setting for the thermostat and humidistat for my
>house and climate?


So ya fucked it up huh? BWWWAAAAAHHHAAAAAA!!!!!!!

Posted by Steve Kraus on August 21, 2006, 2:07 pm
Some thoughts:

Why is there a humidity problem in the attic, at least of a sort that can
be corrected by pulling in outside air? Starting with the premise of the
attic being full of air that came in from outside and has the same %RH, the
attic warms it up. When you warm air you increase its moisture holding
capacity so the same air with the same moisture in it will now have a
*reduced* %RH. So if there is a humidity problem it can only be because of
some source such as a bathroom fan discharging into the attic. I'd correct
whatever the issue is rather than using the fan to fix it.

I suspect you don't have a humidity issue and this is simply a feature of
the fan. In that case set the humidistat to the high end so it never
switches the fan on on account of humidity. If a separate unit then
disconnect it.

As for temperature, oh I don't know. Maybe 120°? Or maybe so high that it
only comes on when the air conditioning needs every bit of help it can get.
If your AC system is good that could well mean you don't want the fan
coming on at all! Think about it. How much does that fan draw, let's say
500W (maybe a tad high but go with it). How much does the AC draw (both
inside fan and outside condensor)? Let's say 5000W. Ten times as much.
That means 10 hours of fan pays for an hour of AC. I don't know how much
heat load you get from a hot attic presuming decent insulation in the
ceiling but my gut reaction is to say I'd rather put the electricity
towards an hour of real air conditioning than 10 hrs of somewhat reduced
attic temperature.

It's different of course if you have stuff stored up there that needs
protection from overheating.

Be careful with fans that don't have ball bearings unless you are diligent
about lubrication. Fans can seize up and then you are at the mercy of the
overheat protection inside the motor. You might want to do a search for
discussions of fires caused by attic fans.

Disclaimer: Non professional opinion only. Use at own risk.

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