Why do my windows have condensation on them?

I've heard this as well, both oil and propane put water into the air.

I got one of those temperature and humidity measuring things. Very handy.

It is a good idea to remove the humidifier from the furnace altogether. If you need humidity, have a shower.

Your basement is not likey a source of humidity, unless you have serious problems which you don't. It is more likely a sink of humidity from the rest of your house, and your dehumidifier takes care of that. Insulation on the outside of the foundation and under the slab would be better, but is difficult/impossible as an afterthought.

Sources of humidity on main floor.
1. Showers
2. Kitchen
3. Laundry room
4. Outside air in Summer
5. Humidity left over from summer

Ceiling fans and the drier vent take care of 1,2,3. Outside air is difficult to avoid in summer, and humidity will work its way into nooks and crannies though it will only appear serious in the basement with it cold walls. Running a dehumidifier in the basement is cheaper and more effective than heating the basement. Your problem is most likely left over humidity that is still in the house from the Summer, and literally comes out of the woodwork in the Fall to appear on windows.

Solution:
Using your humidity measuring gizmo, run your ceiling fans when you shower and close the door until the humidity in the bathroom has cleared. Run you dehumidifier in your basement whenever the humidity anywhere in the house is above 50-60%. It will be highest in the basement as it is coldest there, but that is not the source. In Fall condensation will begin to appear as outside temperatures drop and humidity leftover from summer condenses on the windows. It will be most noticeable in the morning after you shower, but should become less of a problem as drier outside air works its way in to dry the house out. Don't be afraid to let the humidity of the house drop, even to 20%, unless it causes serious problems with antique furniture and cracking wall. It should not get lower than 20% unless you have a very leaky house and run the furnace or woodstove alot. If it gets less than 20% in winter, leave the ceiling fan off when you shower. A few sticky doors in summer and loose doors in winter should be of no great concern in itself. If you allow the house to naturally breathe between 20% in late winter and 60% in late summer you should avoid moisture problems like mold and damp, and also dry out any hidden problems that do occur. A constant 40% would be ideal, but is often unreasonable.

Summary:
1. Ceiling fans at source of moisture.
2. Dehumidifier in basement in Spring, Summer and into Fall, or anytime humidity is above 60% in basement.
3. Let the house dry out in Winter and you will have less trouble in the following Summer and Fall.