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Getting Humidification Right in Custom Make-Up Air Units

MSC gets involved every so often in projects where the type of HVAC unit required to exactly meet our client’s very specific needs is not available, or the lead time on custom equipment is as long as eighteen weeks. And oftentimes during our project meeting, someone will ask, “why don’t we just build it ourselves?”


The equipment most frequently designed and custom-built by MSC are make-up air units with modulating heat, modulating refrigeration, and modulating humidification. Along with the mechanical design of fans, cooling coils, heating coils, and humidifiers, MSC also designs and builds onboard Honeywell controls that will tell all the components what to do and when.


One of the things that make make-up air tricky is humidification. A common problem in 100% make-up air systems, also known as once-through systems, is a phenomenon in which humidity control suddenly goes haywire when the humidifier performs a drain and fill cycle. Cold water pouring on top of the boiling water effectively “kills the boil” and causes the relative humidity to drop like a rock. In essence, humidification ceases for several minutes. This produces a sawtooth effect that can put the air stream out of spec. It’s always wise to use two humidifiers in make-up air systems so that one is always on and running while the other is draining and filling. This way, there is no interruption in humidification and the air stream stays in spec.

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