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I live in metropolitan Houston, Texas (note: high humidity), and this concerns a home I recently purchased. I have a heater (furnace) and blower unit installed in the attic, which is typical for many homes in Texas.

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I do DIY maintenance on our home, but I am not extremely familiar with A/C DIY. I noticed what could be a little mold forming on the outside of the box portion on the right. Edit: After doing some more research, I am almost certain that this section of the A/C unit is where the evaporator coil is located, as there appears to be two lines entering the top of the unit, one of which is a copper refrigerant line.

Here is a close up of the suspected mold (the drain pan, which also suggests the evaporator coil is located in this section of the A/C unit, is located on the bottom of the image):

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I would like to inspect the inside of the evaporator coil box to perform maintenance and inspect for mold, but it seems to me that it is sealed by some sort of white sealant and a bit of duct tape (tape is not an issue, but the sealant is).

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Do I just use a utility knife to cut the edges and open it up? And, if I do that and need to re-seal it, what do I use to do so?

  • You should separate your two questions into two separate posts. – Tester101 Jan 22 '17 at 19:48
  • Turns out the evaporator coil was indeed located in that section. The edges of the box were sealed with some fiberglass tape and mastic. I peeled it away by hand and cut away with a utility knife to access sheet metal screws around the box. I removed the sheet metal screws and then was able to open the top. After I finished, I inserted the screws, laid down some some fiberglass tape and held it in position as I applied mastic over the tape to seal the edges again. Found the tape and mastic in the A/C section at Home Depot. – user56530 Feb 16 '17 at 2:24
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Do not just cut into the insulation board to inspect. At the most you might end up cutting some tape to free an access panel, but the main homeowner access is for replacement of the filter and that probably does not (and should not) have tape over it that has to be cut.

My Carrier furnace and a/c evap coils are vertical so I am not familiar with a horizontal system. There will be a sheet metal access panel that slides up or to the side about 1/2 inch and then is released allowing it to be pulled out. Or there may be twist locks which free the panel. Do you know where the filter is? It could be that someone sealed the edges of the filter access panel with duct tape (or the metalized tape), but most likely the filter access panel will not be sealed with tape. Do you know how to change the filter on this unit?

I personally from your picture do not see mold. I do see rust on the bottom and this is somewhat concerning, but I don't see mold.

I am not familiar with this Honeywell switch on the top, but even in Houston (I live in Dallas) I don't think that one would need the blower fan to be on all the time. Google the override switch and see what you can find about the function.

Edit See http://yourhome.honeywell.com/en/products/ventilation/fresh-air-ventilation-system-y8150 It appears that this is for a fresh air addition to the conditioned air stream. I don't have such a system, but I have to wonder if this is will work in Houston because the humidity is so high there and the winter climate is so mild.

  • Yes, the filter is located to the left of the furnace, just out of sight in the picture (here is another picture showing it). I have indeed replaced that filter before. I am almost certain now (after doing some more research) that the evap coil is located in that compartment where the drain pipe emanates. – user56530 Jan 22 '17 at 16:50
  • Most likely the order of elements in the furnace is filter, blower motor, furnace burner/heat exchanger, finally a/c evaporator coils (where the collection pan is and condensate lines emerge. On the far left I see what looks like the electrically operated fresh air baffle which is controlled by the Honeywell switch. Personally, I would leave the switch setting so that this baffle is closed, unless you think your ducts might be leaking into the attic. – Jim Stewart Jan 22 '17 at 18:44
  • I have the same type of filter housing (mine is Spacegard) for a pleated paper filter element, Aprilaire No. 201. Except that mine is a Carrier, is vertical and in a service closet off the hall of the living floor, i.e., not in the attic. – Jim Stewart Jan 22 '17 at 18:52
  • I change the filter once a year. If you want to inspect the inside of the fan section you could remove the filter and use a mirror and flashlight. For a more extensive inspection you could remove the panel over the blower motor section, but to do that you'd have to cut the tape to remove the cover, take the panel off, inspect, then replace and retape. Unless you have good reason to do so,I don't think it is worth it. – Jim Stewart Jan 22 '17 at 18:59

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