How to avoid or fix a humidity problem

One of the problems that we run into during the summer months is “Our space humidity is too high”.

Or, “We would like to design our space temperature and humidity to be “68 degrees and 45% RH,” for example.

The purpose of this post is to help you quickly determine if standard unitary equipment will do the job, or if you will need a more customized solution (both of which we can help with).

In order to learn this subject, all you need is an open mind and a psychrometric chart. If you don’t have a psychrometric chart handy, don’t worry, we will show you all the info you need below.

Discussion #1

How to design for a specific space temperature and humidity requirement:

1. Choose the design space temperature and humidity requirement

2. Plot this point on a psychrometric chart

3. Draw a line straight to the left to determine the MAXIMUM discharge air temperature

4. Add reheat if necessary

5. Vapor barrier and other considerations may come into play depending on building construction and project requirements

The major point to note here is that standard air conditioning equipment is typically designed for a discharge temperature between 55-58 degF. So if the required discharge temperature is close to this, you can probably get away with unity equipment.

If, however, the required discharge temperature falls into the 40s or lower, you will need to look at a custom solution. Either way, ask your distributor or manufacturer’s rep to run an equipment selection to verify that you can achieve the necessary discharge temperature for your application.

Discussion #2

How to correct a space humidity issue:

1. Plot the current space temperature and humidity on a psychrometric chart

2. Plot the desired space temperature and humidity on a psychrometric chart

3. If you can draw a straight line to the right to achieve your desired set point, simply raise the space temperature set point

4. Vapor barrier and other considerations may come into play depending on building construction and project requirements

This is the very first step to take when diagnosing a humidity issue. It’s a common misconception that lowering the space temperature reduces the relative humidity. As you can see from the psychrometric chart, when you lower the dry bulb temperature, you actually RAISE the RELATIVE humidity.

If this doesn’t fix the problem let us know and we can help you find a solution that will work for your project. Many times an airflow or building envelope tweak can fix the issue.

Do you have a humidity problem we can help fix?

Brad Telker
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Brad Telker

Brad has been in the HVAC industry and at cfm Distributors since 2006, and is actively involved in HARDI as well as the local Kansas City chapter of ASHRAE. Brad has been a semi-professional blogger since late 2015 and is a huge fan of Apple's new 3D Touch technology.
Brad Telker
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