Gas and Oil Home Heating Furnaces - Furnace coils

We have a Rudd central AC, but know little about the furnace. Condensation has become terrible over the last 2-3 years. I replaced the blower motor myself about 18 months ago, and did notice how filthy that end of the furnace was. I'm sure the coils are caked. I glanced at the coil end of the furnace, and it appears that it is sealed closed. What is the process for furnace coil cleaning? Also, is there a secondary drain, besides the one leaving the pan to the outside of the house? The pan is filling up, not because of being clogged, but because the drain hose is a hair higher than the pan. This hose is already as low as it will go, but if a main drain is clogged and this is only a secondary drain, it won't be that much of an issue. Please help!

a TECH.The entire system needs to be cleaned and the problem with the condensate pan should also be addressed.The condensate that lays there stagnate breeds all kinds of stuff that grows.I answer a bunch of questions on this forum,however 75% of the work I do is in commercial refrigeration.I work a lot of commercial and resturant ice machines.If customers could see the conditions the interior of the machines are they would say "No ice please".By the way does your pan have a trap installed?PDF

The coil will have to be pulled and removed to be cleaned properly if it is severely clogged. Although you may check it, by opening up the sheet metal plenum or case that it is in. If upflow furnace you want to see the underside of the coil and if downflow the topside of the coil. If it's not to bad brush with fins with a soft wire brush and remove debri. You should have a drain coming out of the coil. If not, the coil is just draining into to pan.