Controlling Temperature and Dewpoint: A Guide to Preventing Mold in Your School

mechanical room

Winter is coming, so who is thinking about next summer? But summer will come soon enough. Will your school building be unoccupied for the summer? If so, how can you prevent a mold outbreak? The answer lies in temperature and dewpoint settings.

First, A Review of Terms

An important factor in setting temperature and dewpoint is relative humidity. Relative humidity is dependent on the air temperature and the amount of moisture in the air. On hot days, when air temperature is higher, the capacity for air to hold more moisture is higher than at lower air temperatures. Warmer air is more of a “sponge” than colder air.

Another important factor is dewpoint temperature. Dewpoint temperature is the temperature at which water will condense out of the air for a given relative humidity and air temperature. When you see water droplets on the outside of your cool water glass in the summer, you’re seeing high dewpoint temperature in action.

Why Colder Isn’t Always Better

What do relative humidity, dewpoint, and mold growth have to do with each other? Many of us think that if we lower the space temperature, we will dry out a building, thereby prohibiting mold growth. This is a misconception. The key is in balancing temperature with humidity and dewpoint.

Maintaining a low dewpoint (say around 55°F) and a space temperature of 75°F will result in a dry building with relative humidity near 50%. For the same dewpoint, if the space temperature drops to 72°F, the relative humidity will rise to 55%. For the same dewpoint, the lower the space temperature, the higher the relative humidity. Even dropping space temperature by a few degrees to 68°F with a 55°F dewpoint will result in 63.5% relative humidity: the right conditions for mold growth to begin.

The Sweet Spot: Setting Temperature and Dewpoint

Contrary to what many might think, maintaining a space temperature at a higher level while maintaining a dewpoint of about 55°F is the better approach. For example, a space temperature at 80°F and 55°F dewpoint will yield a space relative humidity of about 42.5%, well below peak mold growth conditions.

So how do we keep space air conditions at 55°F dewpoint and space temperature at 75°F or above to maintain a dry environment? The classic engineering answer is, “It depends.”

  • If the building HVAC system has any variable air volume systems using chilled water, this condition is easier to achieve. First, set the supply air temperature from a variable air volume air handling unit to about 56°F. This will generate air with a dewpoint of about 55°F. Ventilation rates are not a concern in an unoccupied building, so air flow in a variable air volume box zone can be allowed to modulate to zero air flow.
  • If there are no variable air volume systems such as rooftop units, air handling units, unit ventilators, or fan coil units, control dewpoint by opening most interior doors (if security considerations allow). The dewpoint can be controlled in a few central locations like the cafeteria, auditorium, and gym. With doors open, the relative humidity will equalize in the building. However, if doors are closed, cycling the constant volume systems is the best strategy. A wide dead band (turning the equipment on at 78°F-80°F and off at 75°F) will help with maintaining relative humidity near 50%. Again, no ventilation is needed during unoccupied times.
  • For buildings with chilled water systems and constant volume air systems, a very low chilled water temperature (below 45°F) may reduce humidity control in buildings. This, too, is counter intuitive. But here is why this is true. For maximum dehumidification, we want the chilled water coil face to be as cold as possible. With very cold water, a chilled water coil valve that is controlled by space temperature will start to close when nearing space setpoint temperature. The lower the chilled water flow through the coil, the less cold surface there is, and active condensation on the chilled water coil is quickly reduced. A higher chilled water temperature will require the chilled water valve to be open more and longer and provide more dehumidification over time.

 These tips are courtesy of John Hayden, a senior project development manager at Reynolds. John and his team are passionate about designing and implementing high-efficiency systems and helping clients achieve energy and operational savings. Interested in learning more about Reynolds’s energy services? Contact us today or learn more on our website.  Should mold issues ever arise, our mold mitigation and restoration team is ready to help.


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