Air Conditioning and Humidity Control

I recently received this question from one of this site’s readers:

Is it a good or bad idea to leave the ahu fan on in a commercial system?This facility is a bank and is experiencing a higher than normal moisture content. The system is a 15 ton Trane and it is a matched system.”

As we all were taught, one of the primary purposes of any air conditioner is dehumidification. So, why isn’t this system controlling the humidity in the space?

The writer says it is a “matched system” which always helps but the question that may need to be addressed is possibly the sizing of the system.  An oversized air conditioner will short cycle and will not run long enough to do proper dehumidification. The longer a cycle lasts, the more humidity the system will remove.

He asked about running a continuous fan? Most commercial applications and codes require a certain amount of air changes and, for the most part, run continuous fans during occupied periods. If the fan is moving too much air, the air does not stay on the coil long enough to dehumidify. Being a commercial system, the TXV will try  to regulate the superheat to prevent other problems which could eventually lead to compressor failures (see post on Discharge Temp as a Diagnostic).

What could be a possible factor here is whether or not the AHU is equipped with an economizer or ERV. If the unit is equipped with either of these, they might not be properly set up or not functioning properly and allowing humid air from outside into the space. The continuous fan would only compound this situation if that is the case.

Lastly, it could be an IAQ (indoor air quality) issue. If there is not proper amount of outside air allowed into the space? They could be experiencing “sick building syndrome” where the CO2 level and humidity in the space is not controlled.  This goes back to proper air changes in the space.

In my response, these were the suggestions I made for the tech to check at the site.  As we all learned — a properly sized air conditioner will be a dehumidifier. When it isn’t, then other things need to be looked at.

We all need to remember that an air conditioner is a SYSTEM and we need to look at all possible causes for a problem. I also appreciate the fact the the reader was looking for suggestions. That is the sign of a good tech — ask when you are not sure!

Please let me know if you have other possible ideas and I will pass them on to the tech who asked the question.


About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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2 Responses to Air Conditioning and Humidity Control

  1. Frank says:

    Could it have a one stage thermostat on a two stage system, not allowing the unit to run long enough.

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