When a vented style furnace, heater, boiler, water heater burns gas as a fuel, it needs oxygen to burn the fuel.
The air in your building is your oxygen supply, as the burner draws air into the burner you are also burning the
moisture in the air. When you increase the use of gas appliances you will quickly lower the relative humidity in
the building. If you have a fresh air intake for the building, the air will be replaced by outside air at the humidity
level outside, winter months are usually drier in humidity levels based on the lower temperatures. The lower the
temperatures the drier the air becomes as the moisture is frozen out of the air and falls in the form of snow & ice.