Does it ever feel that certain parts of the house are very comfortable and others are not. They could either be too hot or too cold.
This could happen even if the furnace is working well. The main culprit is the duct design.
When the furnace is actively blowing warm air into the home, they tend to pool in certain areas. Warm air rises. So the upstairs would be much warmer than the downstairs.
Even though the same type of pooling of warm and cold air happens in the summer as well, the comfort level will be better downstairs in the summer. This is because we are looking to keep ourselves cool.
If this sort of pooling occurs, then run your fan. The main job of the fan is to push the air into the room. The duct work should be designed to deliver the correct amount of warm/cool air to a specific room.
If you have a 2-story house with only a furnace, and the upstairs is warmer than needed, then most likely you have a air flow balance issue. This is corrected by adjusting the amount of air that goes into each area.
If you have a 2-story house with one furnace with AC, if you have issues of comfort, that can be achieved by automatic zoning. This is by having 2 thermostats (noe upstairs and one downstairs) and redirecting air as needed. Call your HVAC guy to achieve this.
Imagine you are filling your bathtub with warm/hot water to take a bath. The water just under the spout will be much warmer than the side of the bathtub farthest away. Before getting into the bathtub, we mix up the water or wait until the water is evenly warm everywhere.
This is exactly what we are trying to do with the fan. The fan circulates the air from all parts of the house and keeps the whole house at an even temperature.
During the fire season, you can keep your inside air clean by running the fan. This filters the air of impurities. Remember to change the filter often when the air is declared unsafe for outside activities. Check your filter more often and replace if needed.
There is another advantage to using the fan. Since the air goes through the duct is blown into the home, the air has to pass through the filters. This makes the air cleaner. So if you suffer from allergies, then running the fan is a great way to filter out the allergens in the air by running the fan.