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    Forced-Air Systems

    I remember spending countless hours a week dealing with humidifiers, whether it was the old Skuttle drum style, the Aprilaire bypass version, or even the steam models that have actually been improved over the years - particularly Honeywell's Truesteam. The one common theme that always arose was: What do I set the humidistat for? If you loyal readers remember, a while back I wrote a short blog about ASHRAE's Psychrometric Comfort Zone. Based on this time tested theory, you can set the Relative Humidity (RH) anywhere between 30-60%! With scorched, or forced-air, systems it is desirable to keep the RH between 35-45% in order to prevent static buildup (ACCA Manual RS). But, every home is different. In order to properly sense the RH, most humidistats are located on the return duct. This may represent the RH at some point in the house, but not everywhere. So, what is the maximum RH that should be set? I say it depends on the windows - as visible condensation on the windows can cause many problems. There is a possibility of concealed condensation in walls, but if a vapor retarder is used and installed correctly, this should not happen in New England. As a Service Technician, we can only worry about the visible condensation. If concealed condensation is a concern, please contact a qualified Building Analyst!

    Equation for Surface Temp.

    Visible condensation occurs on surfaces that have a temperature below the dew point of the air. This is why we get "dew" on the grass or my car most summer mornings. Or why an evaporator actually condensates during operation in our climate. The temperature of the coil (Evaporator Saturation Temp) is lower than the dew point of the return air. In order to find out the maximum RH prior to visible condensation, you should use this equation and a Psychrometric chart. You will need to know the indoor and outdoor design temperatures, and the u-value of the device calculating for. Since the weak point in most walls is the window, you should find the lowest u-value window in the conditioned space. If you were solving for a wall or ceiling, remember that the U-Value = 1/R-value, or the inverse.

    .49 U-Value Window in MA

    For example, lets say your design temperatures were 0F (typical local outdoor) and 70F (IECC & Building Code compliant indoor), with a window that has a .49 U-Value ( typical double pane, wood frame). Using the formula, you can determine the temperature of the window at design temperatures, in this case 48F. This correlates to a dew-point of about 47F. Then, use of your psychrometric chart (I am sure "there is an app for that") you can determine the max setting for your central humidifier is about 45% RH.


    .36 U-Value Window in MA

    Control of this is very important for most new homes as this math with a Low-E window having a .36 U-Value may not yield any visible condensation until the relative humidity hits close to 60%! If technicians are using the old method of visible condensation, it may be too late for "insert issue here" (concealed condensation, mold, damage to paintings, pianos, etc) or I like to say the many possibilities of IAQ. In fact, did you know that just as much (if not more) bacteria, viruses, fungi, and mites grow above 60%RH as they do below 30%RH? (see ASHRAE Systems and Equipment Handbook, 2000) Not only does the static vanish between 35-45%, but so does the rest of the unwanted possibilities. Ever wonder why condensation builds in attic duct systems and supply registers during the winter? Check the duct insulation and do the math...