What Outdoor Data and Dew Point Graphs Can Tell You About Your Mechanical System

All the air in the storage or display environment comes from the outside, and is then moderated by heating and cooling, and in many cases by humidification in winter and dehumidification in summer. Knowing something about the design and capabilities of the mechanical system that creates the storage environment is key to achieving a good preservation environment. Comparing outdoor weather data to collected storage data can help you better understand the performance of your mechanical systems. It can help you determine which systems serve various storage spaces, identify night-time or weekend setbacks and lighting schedules, and illustrate how the moisture content of the air is being managed.

Overlay Indoor and Outdoor Dew Point Graphs

The outdoor dew points throughout the year are especially significant in determining how much “work” mechanical systems must do to create a benign environment indoors for the collections. One of the simplest comparisons is to overlay on one graph the indoor and outdoor dew points. Consider the data from Washington, DC below. The outdoor dew point data is shown in yellow, and the indoor data is shown in blue.

Dew Point Temperature Outdoors in Washington DC, and in one Storage Space, 2009


Although the indoor DP line is often different from the outdoor, a close examination reveals that major outdoor dips and rises usually have some echo of influence on indoor conditions. If your building systems have no capacity to humidify or dehumidify, then the indoor and outdoor lines will largely lie on top of each other – not the case with this example.

Look at the winter months, when the outdoor DP’s are fairly low, much too low to provide an appropriate RH when the air is heated to room temperature. The indoor DP’s are higher, moving around a bit but mainly staying in the range of 30-40°F. This indicates that the systems serving this space are humidifying the air. There are periods during the ‘transition’ months (March and April, and October and November) when the indoor and outdoor DP lines are essentially on top of each other, and near the desired range of 35-45°F. During these times, no humidification or dehumidification is apparently going on, nor is any necessary—but just in case it might be, this is a good time to check that energy is not being wasted by the systems acting like they do during the summer, sub-cooling and re-heating air to remove moisture that isn’t really there in the first place.  Such unnecessary work done on outside air can add up to very significant energy costs. 

Now look at the summer months (May through September in this location). Outdoor dew points are consistently higher than indoors, indicating that dehumidification is taking place. The blue line shows that indoor dew points average near the high 40's and are relatively well controlled within a narrow range. When the indoor summer dew point line is fairly flat, that is evidence that the climate control system is asserting control and regulating the indoor dew point. From the preservation point of view, this is good news, because in many cases properly managing the indoor summer dew point is the key to reducing the rate of chemical decay. In preservation, cooler and drier is better, and a low summer dew point allows the environment manager to have both cool temperatures and moderately low RH’s.

Determine Which System Serves the Space

Another useful analysis method involves overlaying dew point graphs to help clarify which climate control systems affect a particular room.  Although they may share the same source of outside air, two AHU’s usually don't do exactly the same amount of humidification or dehumidification. This means that their plots of dew point temperature over the course of time will not be exactly the same. Often this fact can help separate which space is served by which AHU. Consider the following graph of three spaces (yellow, blue, and green lines) in the same building served by two different systems:

Dew Point Temperature of Three Storage Spaces, 2009


You can see that the yellow line has an entirely different shape than the blue and green lines, which are almost identical. This means that the space indicated by the yellow curve has either a different AHU serving it, or else shares the same AHU but has additional equipment that modifies the air before reaching the space.

Identify Night Time Setbacks and Lighting Schedules

The graph below shows temperature from one storage location over several weeks. The days of the week are indicated at the bottom of the graph – each vertical mark indicates one 24-hour period. The saw tooth pattern you see is typical in spaces where temperature settings are lowered or air handling units are shut down during evening or unoccupied hours, then raised again, or where lights go off at night and on again in the morning.

Remember to keep the temperature range on the Y axis of the graph in mind when you see this—the swings illustrated in this graph are actually minor. Magnifying the graph makes them seem disproportionate but helps illustrate the changes.

Temperature Readings over Several Weeks in One Storage Space


Tracking Humidification and Dehumidification

Analysis of dew point graphs from storage locations can determine whether or not your mechanical system is providing adequate humidification during the winter heating season, or dehumidification during the summer. Review of your data graphs will indicate which spaces are humidified or dehumidified, when the system kicks in, and if system settings have changed over time.

The following graph shows the dew point temperature in one space over several months—including two winter heating seasons. During the first winter (December 2008 through March 2009) the dew point temperature is maintained around 48°- 49°F. Compare this data to that gathered during the same period in the following year. Clearly no humidification is in place in 2010 since the dew point temperature dropped to 35°F or less in December and even lower over the next few months.

Dew Point Temperature in One Storage Space through Two Winter Seasons


When outdoor dew points are high and the air is warm, the systems must both cool the air and wring the moisture out of it. Simply cooling the air isn’t enough; if you just cool without dehumidifying, the RH indoors will be much too high. Many systems use “sub-cooling” (cooling well below the dew point so that moisture condenses on the coils and drains away) to wring the moisture out of the air. You can see this dehumidifying effect taking place between April and October in the graph below.


This sub-cooling leaves the air at 100% RH and quite cold, too cold and humid to send into the collection spaces. The answer is to heat the air back up again (a process known as re-heating) to raise its temperature and lower the RH to acceptable levels. Just how much sub-cooling and reheating is necessary depends on the outdoor dew points and the desired indoor conditions. Unfortunately, the most desirable preservation conditions (simultaneously cool and dry) require the most energy to accomplish. When the cooling coils can’t do all the sub-cooling they need to because the chilled water supply is not cold enough or there isn’t enough chilled water available, the preservation quality of the spaces will suffer.

Our recommendation – follow the moisture!

Dew point is a measure of the absolute amount of water in the air. Dew point temperature data, established by the outdoor conditions, shows you exactly what your mechanical system is doing to manage the amount of moisture in the air in storage. Since extremely dry or excessively moist conditions are very damaging to most collection materials, your ability to control the moisture content of the air will directly affect the rate of decay your collections experience.