When you have a Dehumidifier in the system

From our instructor and friend, Tim Pierre, New Orleans, LA

We have been using dehumidifiers with increasing frequency in the new homes and retrofit projects here in the south.  A dehumidifier reduces the latent load, thus changing the size of equipment needed in the structure.

At this time, RHVAC doesn’t have an option to account for the use of a dehumidifier. But we can use some manual calculations and input them into Misc. Sup. Sen Gain (Miscellaneous Supplemental Gain) and Misc. Sup. Lat. Gain (Miscellaneous Supplemental Latent Gain) windows. These can be found in the System Data Screen window.

The first step is to determine how many pints per day the dehumidifier removes. Convert pints to pounds-per-hour.

One pint is 1 pound so you would divide the pints per day capacity by 24 – 24 hours in a day – to determine the capacity per hour. This number is then multiplied by 970 Btuh which is latent. The result is entered in the Misc. Sup. Lat. Gain as a negative number.

We determine the sensible gain created by the dehumidifier by finding the watts of the equipment and multiplying it by 3.41 btuh.  The result is entered into the Misc. Sup. Sen. Gain field.

Here is an example:

Ultra Air Dehumidifier, 98 Pints Per Day Capacity, 670 Watts @80 degree and 60% RH

Since one pint equals 16 ounces which is one pound, we then can say that in a 24 hour period the dehumidifier will remove 4.08 pounds per hour.



Multiply the lbs per hour by 970 Btu.






The result, 3957.6, is put into the Misc. Sup. Lat. Gain as a negative number.







Next multiply the watts by 3.41 to get the Btus of sensible gain.


Enter that number into the Misc. Sup. Sen. Gain.

The addition of these inputs will increase the sensible load somewhat but will reduce the latent load. The calculations seem rather simple to do and I think would provide an overall more precise outcome to the load.

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