Controlling dew point
Modern problems with high humidity, poor comfort, condensation, and mold are avoided by using a new version of an old technique.
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Why are buildings today so frequently damp, uncomfortable, and smelling a bit, shall we say “earthy”? The reasons are complicated, even if the solution is fairly simple: dew point control. A little history is important to understanding why this method, pioneered by Willis Carrier in 1902 , has become such a popular modern practice. Dew point control has solved some very thorny contemporary problems in a simple, reliable way.
A perfect storm of ignorance and good intentions
Not so long ago, HVAC designers did not have to be especially concerned with humidity. With plenty of cheap energy, the industry could afford to wallop the air with heavy-duty cooling to dry it, then fry it with reheat to keep it from freezing the occupants.
Then we got concerned about energy and started measuring (and regulating) efficiency. But we had been so accustomed to getting humidity control along with our cooling that neither regulators nor designers noticed that in pursuit of sensible cooling efficiency, we gave up latent efficiency. Measuring dehumidification efficiency and effectiveness has never been required. So we didn’t get it—especially in the low-cost, high-efficiency constant-volume cooling equipment we like to put on rooftops.
Then came the ventilation debates of the 1980s, which began by starving buildings of outdoor air and ended by flooding them. Ventilation rates tripled between 1981 and 1989. Few designers realized that the dehumidification load also had nearly tripled because of that ventilation air. So in the 1990s, we had cooling equipment optimized for sensible cooling. But it had to deal with massive dehumidification loads. Not that we knew the true dimension of that outdoor air dehumidification load, even when we bothered to calculate it.
Here’s a sad fact. Until 1997 ASHRAE’s climatic design data did not even remotely describe the peak dehumidification load. Historically, designers assumed that the peak cooling design dry bulb temperature with its average wet bulb temperature represented the peak loads for both cooling and dehumidification.
But in fact, as the new data printed in 1997 finally showed, the peak outdoor dew point comes at a time when the dry bulb temperature is moderate—not extreme.4 The dehumidification load at the peak outdoor dew point is 25% to 40% greater than the dehumidification load when the outdoor air temperature is at its peak.
So there you have it. By the turn of the century, we had cooling equipment with crippled dehumidification effectiveness just when adequate ventilation nearly tripled the dehumidification load, plus the fact that we finally realized—thanks to ASHRAE research—that our peak dehumidification load estimates for outdoor air had always been about 30% below the real-world truth.
What to do? Well, when your favorite tool is a hammer (a highly efficient cooling system) then all of your problems look like nails (must need a bigger cooling system). The usual bias toward bigger-is-better led most HVAC designers to oversize the cooling system in an effort to control humidity.
But oversizing the cooling equipment has precisely the opposite effect. When a cooling system is oversized for the sensible cooling load, it cools the space really quickly. It cools so quickly that its minor dehumidification effect happens for such a short time that the net dehumidification during thousands of off-peak hours is nearly zero.5
Dehumidification stops when cooling stops. And cooling stops often, because that big, efficient unit cools the space so quickly. Ventilation, on the other hand (with its massive dehumidification load), does not stop. So humidity from ventilation air builds up indoors and leads to problems.
This perfect storm of good intentions and ignorance helps explain why so many hotel rooms are so damp, and why so many buildings are overcooled and uncomfortable when they are adequately ventilated.
The mold problem also gets much worse when buildings are overcooled, but that’s another long and complicated story. For now, it’s enough to restate the obvious. Neither clients nor those in the legal profession are impressed with our good intentions when those nice, big, oversized cooling units lead to mold. But enough of history and problems. Let’s talk about solutions.
Dew point control
The dew point is the temperature at which the humidity in the air will begin to condense. It’s an absolute measurement of the amount of water vapor in the air, unlike either relative humidity or the wet bulb temperature. For moisture, those metrics are both, well, relative. They do not by themselves indicate the absolute amount of moisture in the air. Dew point does.
When you want to prevent humidity and moisture problems, it’s very useful to think in terms of dew point.
For example, if the outdoor dew point is above the indoor dew point, you’ll need to take water vapor out of the ventilation air. And if the outdoor air dew point is below the target indoors, you’ll have to add water vapor to the ventilation air. Easy.
For another example, in the summertime, if the cooling system chills ducts, air diffusers, or nearby walls and ceilings below the indoor dew point, you can expect condensation on those cool surfaces. In the wintertime, if the outdoor air chills the exterior walls below the indoor dew point, you can expect to have some condensation inside those chilly walls because the indoor humidity migrates outward.
Also, human thermal comfort is driven by differences between the dew point in the saturated air at the skin surface compared to the dew point in the surrounding air. A bigger difference means more drying.
That’s good in the summer when you need to release some heat, and bad in the winter when you want to conserve your body heat and keep your eyes from drying out. Either way, if you know the indoor dew point you know a lot about the potential for both comfort and discomfort in all seasons.
For the great majority of buildings in nearly all climates, keeping the dew point at 30 to 40 F during the heating season, and below 55 F during the cooling season provides a reasonable compromise between the competing interests of energy, comfort, and building durability.
Another useful feature of controlling on dew point is that it’s easier than controlling based on relative humidity. Changes in dry bulb temperature across a room means that relative humidity (rh) varies widely throughout the building, which makes the system “hunt” to achieve control within a defined range of relative humidity.
In contrast, when the temperature/rh signal is converted to dew point and used as the control value, the system won’t be hunting up and down as sensible loads change in the space. The absolute humidity will stay much more nearly constant, so the system as a whole won’t be so twitchy.
How it’s done
To keep control of humidity, find the dehumidification loads and remove them as close to the source as possible. That way the big loads won’t mess up the stability of humidity in the rest of the building.
Figure 1: Ventilation air generates the largest dehumidification load in most buildings. Source of all images: ASHRAE Humidity Control Design Guide
The same goes for humidified buildings during the winter season. The biggest humidity deficit will be the dryness of the ventilation and makeup air. So adding humidity at that location again goes a long way toward stabilizing the humidity throughout the building.
Figures 2 and 3 show how this is accomplished. A separate unit preconditions and meters the ventilation and makeup air to the building. Then a different system supplies the heating and cooling needed to offset the loads generated inside the building in each zone.
In recent years, such ventilation air dehumidification units have become known as dedicated outdoor air systems, or DOAS units. In addition to their basic function of removing excess humidity, DOAS units often include energy recovery features and variable outdoor air volume measurement and control. These reduce annual energy consumption and avoid under- or over-ventilating the building. Sloppy ventilation is a very common problem in buildings when ventilation and makeup air is brought in through many openings rather than through one or two dedicated outdoor air systems.7
Who’s doing it and why
In 2002, the ASHRAE Design Guide for Humidity Control in Commercial and Institutional Buildings recommended dew point control in place of rh control for buildings other than museums. Also, to avoid underestimating the dehumidification load, the Design Guide also recommended that ventilation load calculations be made against the 0.4% outdoor dew point instead of the 0.4% dry bulb temperature.8 That recommendation has now been more firmly established in ASHRAE Standard 62.1—Ventilation for Acceptable Indoor Air Quality , and also in the climatic design information chapters of the 2001, 2005, and 2009 editions of the ASHRAE Handbook—Fundamentals.
In April 2003, the Public Buildings Service of the U.S. General Services Administration changed the mechanical requirements of its P-100 Facilities Standards to require dedicated outdoor air systems.9 As of that date, new designs must dry the incoming ventilation air—using dedicated units—to a 50 F dew point at all times when the outdoor air dew point is above that level, even when the building is lightly occupied. Given the ventilation air requirements of office buildings, that level of dryness in the ventilation air will keep the overall building at or below a 55 F dew point.
In 2008, the ASHRAE Guide for Buildings in Hot and Humid Climates described a 55 F indoor dew point as a prudent maximum for mechanically cooled buildings, to avoid mold and moisture problems without excessive energy costs.10
In 2009, the U.S. Environmental Protection Agency adopted the 55 F maximum indoor dew point in its new advice to building designers, contractors, and maintenance professionals titled Moisture Control in Public and Commercial Buildings.11
Finally, in late 2009, the U.S. Air Force Requirements for Mold Risk Reduction also include both dedicated outdoor air ventilation dehumidification units and a maximum indoor dew point for mechanically cooled buildings.12
All of this dew point-centric guidance comes from the ongoing concern for avoiding IAQ problems and moisture damage, while keeping energy costs associated with ventilation air to an absolute minimum. Staying focused on the indoor dew point helps both designers and building owners balance and tweak energy and comfort concerns, while avoiding the confusion generated by the traditional focus on relative humidity.
A durable approach
With this dew point focus, all of this guidance is essentially returning to an approach discovered by Willis Carrier in 1902. As a young engineer just 18 months out of Cornell University, he was asked to control humidity for the Sackett-Williams Lithographing Co. in Brooklyn, N.Y.
Carrier quickly decided the way to control indoor humidity was to control the dew point of the incoming ventilation and makeup air. That’s what he did for that project, which many believe helped accelerate more widespread adoption of mechanical refrigeration technology for air conditioning of buildings in the United States.
Interestingly, the indoor humidity control level chosen for that project was a dew point of 53 F—not much different from what ASHRAE publications, the Federal Public Buildings Service, and the EPA have returned to a century later. Circumstances and specific concerns have changed quite a bit over 100 years. But apparently, drying out the ventilation air and keeping the indoor dew point below 55 F remains a good idea.
|Harriman is director of research at Mason-Grant Consulting. He is vice chair of ASHRAE’s Technical Committee 1.12—Moisture Management in Buildings, and in 2003 he served as the Chair of ASHRAE’s Presidential Ad-Hoc Committee on Indoor Mold. Harriman was the lead author and project manager for ASHRAE’s Humidity Control Design Guide. Partly in recognition of that work, in July 2009 he was elected a Fellow of the Society.|