Could the furnace be causing the humidity problem? If no, what else could be causing the humidity problem? The installer said it is not the furnace.
Home Humidity Problems – Is 60% Humidity in the Home Too High?
Maintaining the proper humidity levels in the home is important not only for health but also for the wood products in the home. This includes doors and windows. The quick answer to your question about the furnace causing the humidity problems is no. Gas furnaces and other heating appliances tend to dehumidify (in a manner of speaking) the air and do not create any more humidification. I wonder where your supply vents are located and are they directed to blow hot air towards the windows?
Condensation is created when the temperature of air is colder than the dew point. A prime place for this to occur is windows and doors. This is part of the reason why many supply vents for forced air systems, and radiators for hot water systems, are placed near windows and doors. It keeps the windows warmer and above the dew point and also makes the air warm from your heat loss near these locations.
Home Humidity Problems – Lennox G61MP Gas Furnace
The furnace you describe, the Lennox G61MP, is a condensing furnace and having the combustion air intake in the basement actually, contributes to removing humidity from the basement. It sucks the air from the basement for use in the combustion process and the humidity in this air is condensed during the combustion process in the heat exchanger. This condensation is removed through a condensation drain near the furnace or blown out the exhaust where the combustion gasses are expelled.
If your supply vents are located near the doors and windows and you still have this problem then it may be time for new doors and windows. You have too much heat loss at these places and all the humidity is being condensed around the windows in your home.
Home Humidity Problems Maintaining Proper Home Humidity