If you notice water around the furnace, the condensate drain may be clogged with slime. Fix the problem with chemical tablets and a larger drain hose.
Shove the tablets into the drain pan opening and push them in farther using a dowel or long screwdriver. Add new tablets monthly during air-conditioning season, or less often if you rarely use your A/C.
Pan tablets are available at most home centers and plumbing supply stores.
Screw the new fitting into the drain pan and slide on the tubing. Route it to the floor drain and secure it with tubing straps.
When you see water puddling around the furnace with the A/C running, you have a clogged condensate drain tube. Condensation from air conditioning coils contains bacteria that can form slime and clog the condensate pan drain tube. You can prevent slime and eliminate drain tube clogs in two easy steps. First, remove the drain tube and fitting from your A/C condensate pan. Toss them. Next, buy a package of slime preventing tablets (one choice is AC-Safe Air Conditioner Pan Tablets; sold at Amazon.com). Follow the package dosing directions and insert the tablets right in the drain pan (Photo 1).
Next, buy a 3/4-in. MIPT barb fitting, a small coil of 3/4-in. I.D. vinyl tubing, and several tubing straps. Then install the larger tubing (Photo 2). The pan tabs will reduce slime formation, and the larger-diameter tubing will enable faster condensate flow. That usually eliminates clogging for good.