Hi - I have had an attic fan installed to try to eliminate ice dams. I just had a contractor out to remove an ice dam and need to know when I should run the fam to help prevent this problem. I live in Minneapolis where it is below zero. Does it matter if I run it when it is this cold or when the snow on the roof begins melting. Can anyone help?
THANKS

I don't understand how a fan will prevent ice dams. The ice dams are forming because there is probably a leakage of heat from your living spaces into the attic, and if that isn't corrected, the attic fan will pull more heat out and the ice dams will still form.

Thoroughly check your insulation, making sure there are no gaps or thin spots, and also make certain there is positive ventilation from any soffit vents above the insulation. Find the heat leak and your problem will probably be solved.

The theory of using an attic fan to stop ice damming is sort of correct. To understand why these beasts work is fairly simple.
All agree that ice dams are caused by heat leakage in the attic and along the eves of the house because of poorly installed if at all placed insulation. Because the eave is the closest area to the heat loss the snow on the roof melts the snow faster there and runs down past the outside wall where its cold and re-freezes.
The end result a block of ice forms. This ice dam then prevents any additional water that may run down the roof because of heat loss in the attic from running off the roof. The end result is it backs up and leaks.
The reason for the fan is to keep the attic temp the same or as close to the same as the outside. In this way the entire roof including the area past the outside wall will melt at the same time and freeze at the same time. Thus help stop the ice dam from forming.

BUT this will not stop the ice from forming entirely or in the gutter which will take much longer to melt away then what is on top of the roof. This can cause water backups into the wall or soffit area causing rot and mold to develp in these areas. In some worse cases the gutter can be pulled off the house because of the weight. If you lack proper soffit vents along the entire roof edge there still will be hot spots that will melt and cause the ice to form in those areas.
Ideally natural ventilation i.e. proper sized gable vents. ridge vents along with properly sized soffit vents will be enough to stop the ice from showing its ugly head if you have some heat loss along the eave
.
But like Richard said. Ideally you should check your insulation first. It should extend to edge of the top plate (outside edge of wall area).
The money and effort spent here will help stop ice daming and also save you energy as you do not want any heat escaping the attic in the first place. Running a fan to stop this from occuring is a wast of money.