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ASHRAE Humidity Requirements for Systems that do not Incorporate Humidity Control

asked 2016-07-06 16:54:40 -0600

Hello Modeling Community,

I am working on a small building (<2300 m2) project that incorporates Heat Pump systems for cooling and heating. I noticed in the OpenStudio output that a significant number of hours of the year had a relative humidity <30%. I tried controlling the humidity with a humidification and dehumidification setpoint schedule but noticed no effect.

I realized that the packaged terminal heat pump does not incorporate any systems that can implicitly control humidity. I could add an air loop with a humidifier and control but began to wonder what ASHRAE Standards recommended.

I looked into ASHRAE 90.2 6.10.3, ASHRAE 90.1 and, and ASHRAE 62 5.9.1. Each of these sections discuss a relative humidity range of 30%-65% with an ideal value of 50%. However, the wording is a bit confusing regarding whether it's required. Much of the wording in these sections outline humidity control if a system is precent that controls humidity.

For example, ASHRAE 90.2 Section 6.10.3 Humidity Control states:

"If additional energy-consuming equipment is provided for adding moisture to maintain specific selected relative humidities in spaces or zones, a humidistat shall be provided. This device shall be capable of being set to prevent energy from being used to produce relative humidity within the space above 30%".

So my question is: For a system that does not implicitly incorporate humdification or dehumidification, is it required to incorporate them as an additional system into the model?

Thank you,


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answered 2016-07-07 05:19:27 -0600

Justin, As I understand it... If you are modeling to comply with ASHRAE 90.1, your Proposed and Baseline system models must each use the same design criteria. i.e., if the Proposed system has humidification capability, the Baseline must have it also. If the HVAC designer elected to ignore humidity (which is pretty common in the US Midwest), then you should not model it.

"Comfort" is a relative criteria and ASHRAE's definition is fairly US-centric. Temperature is a the main driver for comfort and humidity is a secondary (less strong) driver. I asked a colleague in India a while ago what was considered a "comfortable" office temperature in his country. He said that 30C (86F) was OK in some places. Apparently, we're all wimps in the US :)

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Excellent answer. I was leaning towards a similar interpretation but I wasn't positive. Thank you for your advice!

JustinShultz gravatar image JustinShultz  ( 2016-07-07 09:15:26 -0600 )edit

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Asked: 2016-07-06 16:54:40 -0600

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Last updated: Jul 07 '16