There should be on on-off switch on the thermostat for the fan.

Is it a good fan with a dehumidistat besides temp stat?

I didn't know there were models with a dehumidistat/temp stat. I would say that could very well be the reason it is still coming on.

I appreciate your responses.
As far as I know it only has a thermostat. I received no info on the fan when installed, though, except that the stat was set to 110degrees. The weather here has been quite dry, low humidity, so I would wonder at a humidistat coming on--not that I understand how it would function in an attic in winter. Would that be a normal occurence, then, to come on in cold temps?
Do you have any recommended brands, or are they all about the same in quality?

Think,
In winter moist humid air leaks and infiltates from the home to the attic space and once reaching the cool dry air condenses on the rafters, sheathing and sometimes insulation. Once this happens repeatily, mold and mildew occurs. The dehumidistat, activates bringing in cool dry air, thus venting the moist humid air which also eliminates ice dams that may form due to conditioned air making it past insulation or up loose access panels or venting chases.

Are you not aware of this happening, or being your old testy self?

z1700, I understand what you are saying--but wouldn't it be more likely then to come on closer to when people are active, moisture-generating activities are happening, and the heat is turned up? instead, it is coming on at 4-5 in the morning (variable, it is on right now), when the heat has been turned down for hours. And as I mentioned, the weather has been very low humidity for weeks, likewise inside the house.
Actually, I still need to know, is this a malfunction & should I switch it off? How do I tell?

To recap, dehumidistats are a very common addon for attic fans, especially in the north with colder temps, thats why they are sold as an accessory by just about every fan manufacturer.

With regards to your issue, I'm not sure without checking. If you hadn't noticed the stats that come with the fans are cheaply made and not only can be easily bent out of whack, I know mine in my garage attic new out of the box is 8 degrees off.

I would be checking the wiring, the accuracy of the stat and would even go as far as to place a data reader that measures and records temp and humidity at selected intervals which is downloaded to a computor for review. Perhaps a bit overboard, but if all other variables are checked, personally I would want an answer.

At 4-5 in the morning, is the furnace starting to come on in recovery mode due to a temp setback? Is the furnace vented through the attic? Has it been checked? All bath fans vented to exterior?

Just in case he used an aftermarket stat, some are SPDT which have three connections, in otherwords if wired wrong, is reversed in operation, instead of turning on with heat rise, turns off, thus; turns on as attic cools down.

Again, thanks for your answers.

The furnace is vented through the wall, and there is no bathroom fan. I do use a setback thermostat, which cycles on for a 6AM goal, but the power vent comes on sometimes as early as 2AM, or as late as 9AM--but never in the afternoon (whereas during the summer it was coming on in the late morning or afternoon, as expected).

The only other thing that I wonder may have affected it is the power outage after the hurricane last Sept. I can't say that I noticed the fan on in warm weather after that, but I wouldn't have really paid attention to that either, and it was with the first cold weather that I noticed it on at night.

Attaching data readers and such are a bit beyond me!
I will climb up into the attic today though and see what I can see.

I am still calling the contractor weekly. If this is normal function you'd think that he would tell me that just to stop the phone calls!

Well, to wrap this up (which was never exactly a heating or A/C issue, but definitly ventilation which affects the other)--maybe this will help another person.
I crawled up into the attic and found the brand & model of the vent. It does have a humidistat, set at 60%. The sensor was placed just below the open vent.

I called the company that made the power vent, and they say that I have several issues:

1) the power vent is way too big (powerful) for my attic, which leads to not enough inflow to satisfy the fan, which makes it noisy *and* will suck heated or cooled air from the house. The do not make a fan for an attic as small as mine (500 sq. ft).
2) the roofer at the same time installed ridge venting. They say that having both venting systems makes neither one work, and will allow weather (rain etc) to penetrate the vents. At the same time, the original gable vents are still open, and should have been/ need to be sealed, for the same reason.
3) The placement of the thermo/humidistat. It may be turning on because of dew contacting the sensor.
4) The humidistat was set too low. The said that it should be at 80%.

The vent company told me that I need to have either the ridge vent or the power vent removed!! They make both, I think that I will take their word for it -- but now my next issue will be talking to the roofing contractor. I certainly do not want to be charged to have it taken out, I want the hole in the roof wood to be repaired, and now there is the issue of having paid $375 for a fan that was not needed, was inappropriate, and that I agreed to on his recommendation.

This roofer, by the way, has great references, no problems with the BBB, good rating on Angies List, and was *not* the lowest price--he was right in the middle.

You never stop learning.

A humidistat (you mean't dehumidistat)

Imagine that (my first post)

I would just turn up the temp, move the RH sensor and let the natural ventilation with the ridge vent takeover. Up until now I don't believe you ever mentioned you had a ridge vent, which that info could have helped us earlier.

And yes the gable vents should have been sealed with adequate soffit vents, with either the ridge or power vent options.

While the air will short circuit flow, in the event of excessive moisture or heat in attic, the fan with decreased efficiency will still help. If tha fan is too big now though, what will happen if you seal up the gable vents? Do you have soffit vents? If contractor installed a ridge vent, he should know it goes hand-in-hand with proper soffit vents.

Yes, there are soffit vents, and the roofer used vented drip edge too.
All indications seem to be that it is the fan that should come out. Perhaps it would draw equally from both vents since it is oversized? So circulation would be better than from one vent + fan?

I have increased the setting to 80%, and made a little roof over the sensor with vinyl to keep dew off. This weekend I will try to seal off the gable vents. Then I hope to stay out of the attic for a good long time--it is not easy to get into.

Thanks for your comments!