Relative Humidity in the Supply Air Stream

If you’ve recently used the Testo 605i hygrometer, you may have noticed that the supply relative humidity is noticeably higher than the return RH. The supply RH may even get pretty close to 100%! We’re draining water outside, so this phenomenon doesn’t make a lot of sense; the RH should be lower, if anything.

We can compare a sponge to air to explain how relative humidity works. The amount of moisture in the sponge represents the amount of moisture in the air, and the compression on the sponge represents the air temperature. A sponge that’s completely saturated would be dripping water (100% RH). A compressed sponge can hold less water before it starts dripping (compared to a less compressed sponge).

As air goes over the evaporator coil, its temperature drops. When the air’s temperature drops, it can hold less moisture than when it was warmer. So, 40% RH at a higher temperature would actually result in a much higher RH if we were to drop the air temperature. The air can warm up again and expand in the ducts, so the high relative humidity doesn’t remain that high. (The inside of a duct won’t mold like crazy because it picks up heat.)

If the supply RH doesn’t reach 100%, then the difference between 100% and the actual RH is the bypass factor. The bypass factor indicates the amount of air that isn’t making sufficient contact with the coil.