I noticed my furnace sweating a couple days ago. It's hot and humid where I live so I was only mildly concerned. It was a high of 74F today with humidity around 70% which is, relatively, nothing. My thermostat keeps the house around 72F.

Lately, I've noticed that my A/C is running 90% of the time (a lot), but chalked it up to this being the hottest month.

Side of Furnace with Sweat Drops

enter image description here

Good Size Water Trail on Floor

enter image description here

Two Small Streams Coming from the Other Side

enter image description here

Condensation Runoff Line

enter image description here

It looks slightly kinked, but has been that way since before the summer. I don't know what the blue tube is.

Just for Fun: This is How the Condensation Line Connects to the Main Sewer Line :D

enter image description here

Is this bad? What could be causing this?

I've researched online and found some informal sources that say something in the airflow is being blocked/clogged somewhere...

Edit in Reply to @Tester101's Questions in the Comments:

The System as A Whole

enter image description here

At the bottom of that "V", where the grey and off-white metal meet, is where Picture #1 above was taken.

Inside the Upper Front Panel

enter image description here

I think this section only has to do with heating, but I don't know if you guys see anything useful.

The Rear Panel

enter image description here

It looks like I can unscrew that entire rear side, but I won't do that unless necessary. There is also a smaller panel in the top of the photo. Should I open that?

  • Is the unit in a conditioned space? Is the first photo of where the unit connects to the plenum, or the seam between the furnace/air handler and the evaporator? Is there a panel that could easily be removed, where you could get a look at the coils?
    – Tester101
    Commented Aug 20, 2015 at 1:59
  • @Tester101 It's in the basement so it's usually cooler there, but probably also more humid. I tried to answer your other questions with photos edited into the question. I think I need to open that smaller panel with a screwdriver to look at the top of the coils? Commented Aug 20, 2015 at 2:52
  • Is the register above the unit open or closed?
    – Tester101
    Commented Aug 20, 2015 at 12:28
  • 70% humidity is not nothing. If your basement was tall enough, it'd be raining inside.
    – Mazura
    Commented Aug 20, 2015 at 15:59
  • @Tester101 It is closed. Would cycling more conditioned air through the space be worthwhile? If it is the freon like you say, would opening it just be putting a band-aid on a symptom? Commented Aug 20, 2015 at 16:46

1 Answer 1


There are two main contributing factors at play here. First and most importantly, the evaporator is getting too cold. The other problem is only a problem, because the first problem is a problem. And that is, that the humidity in the basement may be too high.

What's happening, is that the box that contains the evaporator is getting too cold. Once it cools below the dew point, it causes the moisture in the air around it to condense. To solve the issue, you're going to have to figure out why the evaporator is getting so cold.

Improper refrigerant level

My first guess, would be that the system is not charged to the proper level. Specifically, in this case I think it may be undercharged. Though, you may also see this if it was overcharged. However, since it sounds like it hasn't always been a problem, I'd suspect the system developed a leak.


To test this, you're going to need a set of gauges. If you don't have a set; or don't know how to use them, you're going to want to contact a local HVAC company.

Restricted air flow

If not enough air is moving through the coils, the little bit of air that is there could be getting too cold.


Check the filter, to make sure it's clean. Check the evaporator coils, to make sure they are clean. These are the two most common causes of restricted air flow. Additionally, make sure registers and returns are open and unobstructed. If your system has a multi-speed blower motor, make sure it's set to run at the manufacturer's recommended speed.

Long running system

If it's really hot outside, and your system is running almost constantly. It could just be that the evaporator doesn't have time between cycles to warm up. If this is only a problem when it's abnormally hot outside, it could be "normal" for that situation. Typical A/C systems are designed to cool between 20-30°F, anything beyond that and they'll struggle to keep up.

High humidity

If everything is operating normally, but the system just happens to be in an area where the humidity is high. It might be a good idea to insulate the system, especially in the area around the evaporator. This is common when the equipment is installed in unconditioned spaces (attics, garage, crawlspace, etc.), but if your basement has moisture problems, it might be required there too.

If the moist air cannot get to the cold metal, the moisture in the air will not condense. However, if the now insulated system gets too cold, you could have problems with the evaporator freezing. So I'd only install insulation, if I was sure the system was operating normally.

  • I'm going to take off that front register and look in tonight to see if the coils are dirty. The filter is almost brand new. I will then decide whether it is worth it to call an HVAC company since summer is almost over. Thank you! This is great, clearly-written information. Commented Aug 20, 2015 at 14:11
  • You won't be able to tell if your coils are diety by looking at the top. Air blows up through them meaning you need to look at the bottom. A good indication that there dirty is to look at your blower wheel and housing. If it's covered in dust and dirt your coil most likely will be as well. Regardless I'd clean bother evaporator and condenser coils at least twice a year, more so depending on your environment. 18yrs as a refrigeration service tech. Commented Oct 6, 2017 at 2:20
  • Two years later at a new house with a new furnace that was having some issues, I remembered to check the items under 'Restricted Airflow' first and it solved another issue for me! Thanks. Commented Dec 6, 2017 at 15:25

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.