The home where I've lived for the past 18 years is built on sandy soil, with concrete blocks forming the basement walls. It's about 55 years old. There is a slight downward slope away from the house and I never have leakage of rain water into the basement unless I neglect to clean out the window wells. The basement has only one visable floor drain. It is next to the furnace and hot water heater and is very slow. The run-off from the humidifier will cause it to fill up and form an increasingly large puddle after only a few hours. So I have to keep the water line into the humidifier 3/4 closed. I've had this drain roto-rooted and power- rodded, but the line was impassable after about 25 feet. This floor drain would quickly be overwhelmed by significant water in the basement, say from a broken pipe or water heater. I want to avoid installing a sump pump, partly due to expense, but also because I don't want my basement floor chopped up to make basins or canals. Now here is the actual question: The floor outside of the furnace room is covered with linoleum tiles. Isn't it very likely that there's a floor drain under those tiles somewhere? And if so, might it perhaps still function? If I could find it, I could run a hose into it to test it out. So I rented a metal detector this morning, but it registered what had to be false positives over the entire surface of the linoleum. I kept adjusting the sensitivity, but to no avail. Any suggestions would be much appreciated.
I have found very few basements with more than one floor drain and doubt that you will find a second floor drain. Here are some other options:
Your experience about the windows wells is right-on. Most basement moisture problems should be solved from the outside by controlling roof and surface water runoff. So you might want to start by increasing the size or improving the configuration of the window wells.
There are some small condensate pumps that are used with furnaces etc. You maybe able to use one of those for you de- humidifier. Such pumps don't require any concrete cutting.
Check with your local building department and see if they might have some information about the design of your home and the configuration of the drain system. You may find out that the floor drain(s) runs into a dry-well and if that is the case, you can assume that after 50+ years, the dry well filled with debris and can't absorb much water.
Bring in a sewer cleaning service and have them try again to clean out the line and if that doesn't work, have them scope the line with a TV camera and find out where the line is blocked and if there might be an easy fix to the problem.
Cutting into a basement slab is not as big of a project as you imagine. Such slabs are usually thin and relatively easy to cut. You may want to have this done by a local concrete cutting company.