Heating, Air Conditioning, Fridge, HVAC/central air/humidity


The humidity level in my home is so high that mold is growing in places.I bought a digital humidity sensor. When I place it on a floor vent while the central air conditioner is running, it starts out at 49% and gradually rises to 99% before the unit shuts off. It seems to me that the  unit is putting excess moisture into my home instead of removing it. What would cause this?

It is normal, and actually a requirement for the air leaving the air handler to be at 90%+RH.  In order to remove moisture from the air, the air is cooled at or below dew point.   When the air is at or below dew point (saturated with moisture), the droplets of moisture are wicked out of the air via the coil. A/C units themselves DO NOT and CAN NOT add moisture to the air due to the way they work and the science behind them.  A humidifier must be installed to add moisture to the air.  Taking temp and humidity readings on discharge air is not going to give any indication of the conditioned air in the space.  

High humidity in the space can be caused by several factors.  The colder the space is kept, the harder it is to keep the RH level down.  As temp decreases, RH% increases when there is no removal of moisture.  The unit may not be running properly, not getting the air cold enough to get the moisture out of the air.  The home is not sealed very well, allowing moist outside air to gain entry to the home.  The unit is oversized and cooling the air down to quickly.  The unit has to run to remove moisture.  If the unit cools too quick, then it will not run long enough to remove moisture from the air.


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Craig HVAC Expert


I have been in the HVAC field for the past 18 years. I can help with most HVAC questions. I work on commerical buildings for the most part, and have yet to find anything I could not troubleshoot and repair, when repairable. I work on small 1 ton units to a 2500 ton chiller. Troubleshoot air flow, elect, and control problems. I attend regular classes to keep up with the latest and greatest.


I work on commerical sites, hospitals, gov't buildings. I can troubleshoot just about anything in the HVAC business.

5 years trade school, VFD training classes, Liebert factory training, some York and Trane factory training.

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