I have a 10 year old Trane, heating and ac unit that has constantly had problems with the coil underneath the furnace. I have always had a service contract to have it cleaned and checked twice a year spring and fall. I have one Labrador dog. My house is very old but it is not dirty and I change the filters every month. Yet the coil is constantly getting clogged and now they want to replace the coil or the whole furnace. A couple of the service techs have mentioned that it looked like to them that when it was installed the part where the coil is is not large enough for proper air flow but the service manager says no. And how would I know who is right? They have cleaned it as much as they can and say it has to be replaced. I am going to get another estimate and opinion but how do I know?

Follow the link below for your estimate purposes.

Let's play doctor/patient.

My coil's getting dirty

By what? Dog, dust, mold? Humidifier?

How dirty are the ducts? Pop one off and look. Return & supply

What kind of filter are you using? A $2.00 one that's about 1" thick or a $50 one that's 8" thick?

Are there BIG gaps where the filter goes?

Corrosive chemicals nearby?

What are the polutants in your house?

Yep, sometimes it has to come out and put back in or replaced. The cleaning gunk is generally non-corrosive, but the gunk and a hose can work much better.

My $.02