Humidity / AC question I'm a bit stumped

Home on a slab. No visible water leaks or problems, no sump pump.

Summer temps in mid 80’s - when running central A/C humidity goes up (mid 40’s to high 50’s)

Turn off A/C and open windows - humidity goes down to exterior levels in the mid 40’s.

A/C condensation is draining properly.

If they close windows, turn off a/c - humidity goes up, and if they run a portable dehumidifier, it fills the catch pan quickly, humidity goes up.

ANy suggestions

Sounds like they open the home at night and all the furniture and walls get saturated .
It takes a while for all the Moisture to be removed from inside the home .
I would run AC and dehumidifier for a full day and see what changes you get.

If it has under slab ductwork, look for standing water or evidence of water stains.

I had a slab on on grade recently that had almost 2" of standing water in the duct work.

It was down pouring when I inspected it.

Homes are to operated like extra large vehicles … not tiny sports cars.

Sudden starts and stops … shifting gears from A/C to open windows … etc … will result in a variety of unusual readings, reduced sustainability of the structure and shorter life spans of mechanical equipment.

Set the environmental controls where the occupants are comfortable and allow the house to attempt to maintain that climate for 24 hours and then take your humidity readings. See if that makes a difference.

One can see with a flashlight and mirrow from the register to the first offset look as Larson suggested for water marks I find standing water in the duct pretty common here. Ensure the condensate drain pan is not overflowing or leaking into the supply plenum. Last resort run a duct camera from the registers to the plenum checking for breaches in the duct.

asuming this is a downflow system being discussed

Mine with the water in them was an upflow.

The returns were in the floor and the supply was high on the walls.

I heard they do strange things up in Yankee country

This is a matter of understanding psychrometrics.

I am going to make an assumption here that you are measuring the relative humidity.

If you close the windows and turn on the air conditioner you drop the temperature within the space. When you drop the temperature within the space the relative humidity rises.

This means that the ability of the year to hold moisture is reduced. That is because the air conditioner reduces the temperature below the dewpoint and subsequently reduces the water vapor in the air.

If you repeat that same air without opening the windows, you will end up with a relative humidity of about 10% (depending on how high you raise the temperature.

There is absolutely nothing wrong with what you’re describing.

That’s how air-conditioners work!

There is nothing to suggest.

A dehumidifier is nothing but an air conditioner that discharges the hot compressed air back into the house and raises temperature.

If you’re going to take psychrometric readings with a hygrometer, I highly recommend that you brush up and hit the books and learn about psychrometrics.

Hey Buddy your Phyco readings are out the window if water is in the ducts first things first;-)

It’s odd for here too.

Bunch of cheap Town Homes built that way.

Not really Sir.
The water got there because of psychrometric properties.
One leads to another.

But if you can’t do one, it is unlikely the other won’t get do either.

I tried not to conflict with you because you can perceive (and relate to many here better than I could hope for)!

point of question: you can increase the apparent relative humidity by lowering the temperature. This does not mean that there is more moisture in the air! You must do psychrometrics to determine if it is actually proportionate when you open the window!

Relative humidity, is “relative”!

If you do psychrometrics you know exactly the "quantity"of moisture that you’re actually dealing with.

If you don’t do this, it is highly probable that you are “wrong”!.

Regardless, it is not in the scope of your inspection.

Here is a chart.

Wow, It talks to you!

Ya know me I am just ragging you I find a lot of ground water within the slab ducts here poor drainage does it every time on a slab with downflow units. I have turned the A/C unit on and had water rooster tails blowing from the registers during a heavy toad strangler.