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.