Causes of High Space Humidity (In no particular order)


There are six primary causes of high space humidity.


Too much ventilation air

This one is a no brainer. Too much ventilation air (economizer minimum position) is like leaving a window open…and maybe a door…and maybe another window…anyhow, you get the idea.

The solution is not to close the outside air damper completely, the solution is to set it for the correct amount of air for the application. Hint: 10% damper position is NOT the same as 10% outside air. Read our write up on calculating ventilation air percentage the correct way.

Oversized equipment

The nice, cool evaporator coil is what removes water from the air (ie; the condensate running out of the drain line) But, it only works when it’s…well…when it’s working. Oversized equipment will drop the room temperature to set point so fast, the coil won’t be cold long enough to remove the moisture. In other words; bigger ain’t always better.

Airflow too high

Too much air flow (cfm per ton) is a big no-no for high humidity areas. Why? A couple reasons.

One-The air is not in contact with the coil long enough to allow the moisture to condense on the it.Two-Velocities approaching 600 feet per minute will pull the condensed droplets of water off the coil and back into the air stream.

Wrong coil staging

If the top circuit of a face split coil is energized first, the water will run off of its fins and onto the lower coils warm fins where it re-evaporates into the air stream. The bottom circuit must always be first stage.

Compromised vapor barrier

Humidity moves due to something called Vapor Pressure. Vapor pressure makes high humidity areas flow to low humidity areas, just like hot flows to cold for heat. And vapor travels through anything porous; sheet rock, siding, plywood, you name it. The only thing stopping the flow of moisture from high to low is….a Vapor Barrier!

Space temperature set too low

The customer says they are too humid. So, thinking back to our oversized equipment scenario, you set the thermostat another 3 degrees lower to increase the cooling run time.

Problem; when you lower the air temperature, the air becomes denser, basically shrinking. Think of it this way: you have a 16oz glass that is 50% full of water. The glass is the air in a building. When you cool the air, it contracts. Picture reducing the size of the glass from 16oz down to 10 oz, but with the same amount of water in it. The smaller glass now has a higher percentage of water even though the amount of water is the same.

Make sense?

This is why dehumidifiers always have reheat a coil after the cooling coil.

There you have it, the top six reasons for high indoor humidity….ok, really, that’s it….go read another article or something. We’re done here…..Scram!

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Patrick is Zen HVAC’s diagnostic and training guy. Patrick started in the trade the day he left technical school and never looked back. He's served in various technical and training roles in the HVAC industry but specializes in system troubleshooting and diagnostics, retro commissioning, and technical training. His moto: If I can understand it, anybody can. Patrick uses the Zen common sense approach to teach Patrick’s Likes- His Wife, kids and dog. Old pickup trucks. Hiking. The industrial Revolution. Patrick’s Dislikes- Taking work too seriously. Anything unintuitive. Emoticons :( Patrick’s Favorite famous person- Theodore Roosevelt “I am only an average man, But I work harder at it than the average man" Famous Patrick Quote- “Well, that was stupid of me”

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