RHVAC Help Technical Tips

Effects of changing the Outdoor Dry Bulb

We tell people that they can change the Outdoor Dry Bulb temperature if they feel that it isn’t accurate or if their area inspectors have mandated specific temperatures. (Of course, we caution that you should not change the temperature more than 5 degrees up or down and not to change it to reflect the one or two days that the heat is excessive.)

It is important to understand the affect of any temperature change.  Of course the change will affect the load of the house and Temp change 1therefore the capacity of the equipment. But did you know that it will also affect the relative humidity?

temp change 2Here is the design conditions from the ACCA Weather Data table for Harrisburg, Pennsylvania.

Note that the Relative Humidity in the Project Report is 41%.




temp change 3





Now we change the Summer Dry Bulb Temperature to 99 degrees.


Temp change 4


Note the change in the Outdoor Relative Humidity. It has dropped from 41% to 34%.

When you make a change to the outdoor dry bulb temperature you need to also change the outdoor Wet Bulb  if you want the Outdoor Relative Humidity to stay the same as the original humidity levels. This is when a Psychometric Chart comes in handy.


Elite’s Psychart is a good tool to calculate this.temp change 5

Once you have put in the outdoor dry bulb and the relative humidity for your area you get a report with the wet bulb temperature that will  maintain the relative humidity.  In this example we find that we need a Summer Wet Bulb Temperature of 78.4. Enter that under the Summer Wet Bulb Temperature field.

temp change 6



Now our outdoor relative humidity is back to 41%.




We’ve also found a couple of good places to go if you just need  to calculate the Wet Bulb Temperature from the Dry Bulb and desired Relative Humidity.




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