Hi all,

I recently rewired my home from old knob and tube wire. The one thing I am having a problem with is the attic vent. Is it possible that I somehow damaged the thermostat, because I am fairly certain I have it wired correctly, but it runs constantly whether hot or not. I set the temp to the max and it still runs... From the incoming power line, I have the white connected to the white, the incoming black to one of the two thermostat wires and the other thermostat wire connected to the black wire to the fan. I have switched the two thermostat wires and it still runs constantly... Any help would be greatly appreciated. Thanks.

The first thing I'd do is contact the manufacturer of the thermostat or, failing that, the attic fan and ask where you can download the original documentation or literature that came with that thermostat or attic fan.

If you can't see that there could have been any change to the wiring, check the literature to see if that thermostat contains an "anticipator", and if so, how to adjust it.

You see, heating thermostats for houses will contain something called a "heat anticipator" which is really nothing more than a tiny 24 VAC electric heater inside the thermostat. That's because in the case of homes, the source of heat (like cast iron radiators or warm air vents) will always be located around the perimeter of the heated area while the thermostat will be centrally located near the middle of that heated area.

So, if you set the thermostat to shut off the heat at 75 degrees F, the AVERAGE temperature in the heated area will be well above 75 degrees by the time the thermostat in the middle of that area senses that the desired temperature has been reached and shuts off the heating system.

Also, in the case of steam or hot water cast iron radiators, the very hot radiators are going to keep convecting heat into the heated area for a long time after the thermostat shuts off the heating system, thereby causing the heating system to greatly overshoot the desired temperature as well.

A heat anticipator uses the electric power that the thermostat runs on to power a variable output electric heater that's located right inside the thermostat. In 24 volt AC home heating thermostats, it's just a coil of high resistance wire that's physically located right beside the bimetallic spring holding the mercury switch. That heater fools the thermostat into thinking it's warmer in the room than it really is, causing the thermostat to shut the heat off early so as not to overshoot the desired room temperature.

It's possible that the heat anticipator setting may have been changed in rewiring the attic fan so that it's always producing maximum heat. That's going to fool the thermostat into thinking it's ALWAYS too hot in the attic, making the attic fan run continuously, even though it's only the tiny space inside the thermostat that's uncomfortably warm.

Since the attic fan runs on 120 VAC power, I expect the thermostat uses 120 VAC power as well, so don't touch the heat anticipator to see if it's hot; you might get both a burn and a shock.

That'll be 35 cents. Please contact Scripps Networks to arrange for payment.

PS: Kumar: Don't tell anyone, but I knew squat about thermostats until an hour ago. I just typed "attic fan runs continuously" into Google, and it spit out a web page that was all about heat anticipators. I just read all about them and then just parroted out that same information in here just to impress the natives. My bad.

This message has been edited. Last edited by: Nestor,

Just an FYI update in case anyone comes across a similar problem. In my case, it was a faulty device. I bought a new one, wired it up same as the old and it turned on and off according to temperature as it is supposed to.

Always good to know. Jumping a thermostat on the control [say a triple aquastat on an oil burner] will tell you if the thermostat or low voltage wiring is bad or the control itself.
PS, most if not all new thermostats no longer have heat anticipators but have switching setups. But billions of older controls are out there in everyday use.