Really hot in NJ this week, excessive heat warnings, and the like.

Daytime Temp: 96 degrees  
Daytime Humidity: 55%   
Feels Like: 105 degrees  

Nighttime Temp: 79 degrees  
Nighttime Humidity: 87%  
Feels Like: 87 degrees  

We live in a two-story colonial with a basement. We have a single unit two-zone system (with dampers to separate the 2nd floor from the 1st/basement). We keep the upstairs temperature set at around 75-76, and the downstairs temperature around 78-80.

Humidity sensor in the basement reads between 50-55%. I know above 55 is a mold threat. The unit is in an unfinished storage area.

We just bought the house this past December, it's our first summer in the house. There are decent amounts of condensation forming on some of the vents and the main unit (mostly above the inside unit and ducts to the 2nd floor. Obviously, it's working really hard to keep up with the high temps outside.

I moved some stuff out of the way and put some buckets down to catch the drips. There is a register on one of the main branches of ductwork.

Here is a video of my AC unit, the condensation, and the register.


Should I open up a register in the unfinished basement storage area where the AC/Furnace is kept to help with the condensation problem?

(And should I leave the door to the unfinished room open or closed too?)

Edit: I already have one dehumidifier running, added a second one last night.

Update: I opened the vent, increased the temp on the thermostat and ran a second dehumidifier. There was still some condensation, but nowhere near as much as the previous day.

  • 2
    Probably the best solution is to insulate those ducts - the ambient air is condensing on them. Trying to change the humidity of the ambient air seems a non-starter...
    – Solar Mike
    Jul 21, 2019 at 7:26
  • The optimum humidity zone per Honeywell is a range of 40% to 60%. Technically 55% isn't too high, but your correct they it's definitely a problem if your vents are dripping condensation! You need to either lower the humidity in your mechanical room/basement, or insulate your exposed metal duct work, especially if you intend to keep the vents closed in the basement in the summer and not put a dehumidifier down there. Try buying a dehumidifier and keep the HVAC vents closed, but open the vents if that doesn't work.
    – Dotes
    Jul 21, 2019 at 14:24

1 Answer 1


My recommendation would be to run a de-humidifier in the house to control the excess humidity. If you do not want the noise in the living space as I did not then I would put the de-humidifier in the basement. You should open 2 registers in the basement and have at least 2 floor level return grills to circulate air in the basement. Set the de-humidifier at a comfortable level and now you can control the excess humidity. This is what I did and it works great. My de-humidifier is a 70 pint model bought at the 1 of the big box stores.

  • Yes, probably get a dehumidifier. Opening vents in the basement will help lower humidity down there which is more important than making it too cold down there because of the mold risk. Think of a dehumidifier as air conditioning that doesn't make the basement too cold, so a bit better than using AC to dehumidify. A dehumidifier is more efficient than the other option of using what you already have and getting your humidity under control with AC and then running the heat (space heater or Furnace) to get it warm again.
    – Dotes
    Jul 21, 2019 at 14:13
  • 1
    @Dotes I’m going to add in that I had one dehumidifier running, and added a second one last night. Jul 21, 2019 at 14:28
  • @FrancisJohn Wait, you have dehumidifiers currently running and this is still happening? If your AC is also struggling to keep up with the heat, I'd look into seeing if you have a lot of excess air infiltration through your walls. An air blower door is one way to find out. Also, your evaporator coil or your HVAC air filter might be dirty, and make sure your compressor radiator fins outdoor aren't clogged with stuff. Two dehumidifiers and an AC unit not lowering the humidity points to your exterior walls/windows allowing outside air in. Sometimes radon fans are oversized and cause this too.
    – Dotes
    Jul 21, 2019 at 14:36
  • @Dotes the evaporator coils are probably a bit corroded/rusty. I know the filters are clean inside. I’ll have to check the outside unit. I’ve heard several opinions that although it technically complies, realistically the unit is undersized for the house. There should probably be two. Jul 21, 2019 at 15:56
  • 1
    I had excessive condensate on main supply trunk. My air handler is brand new. We changed the speed of fan from low to high and the problem stopped. Slow moving air gets much colder and the condensation increases on metal surfaces that have extreme temp variant on opposite sides. Greater volume of air moving through the coil moderated the temperature variant.
    – Kris
    Jul 21, 2019 at 15:57

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