Dehumidifier Metering Study

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A version of this article appears in the January/February 2011 issue of Home Energy Magazine.

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Residential stand-alone dehumidifiers can draw a significant amount of power when in operation (power draws of 300–700 watts are common). However, little is known about the total seasonal energy use of dehumidifiers. Many factors can affect this total. Dehumidifier capacity, characteristics of the ambient space, and the homeowner’s selection of set points are just some of the variables that determine how much energy a dehumidifier will ultimately consume. Wisconsin’s Focus on Energy recently commissioned a metering study to figure out just how much energy typical Wisconsin households use to dehumidify their homes.

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The Energy Center of Wisconsin monitored dehumidifier use in 40 Wisconsin homes on behalf of Focus on Energy during a four-month period in the summer and fall of 2009 (see “Focus on Energy”). The sample of homes we monitored varied in age; it consisted of 20 newer (built since 2000) Wisconsin Energy Star Homes (WESH) and 20 randomly selected homes. The randomly selected homes were located in one of three Wisconsin metropolitan areas (Milwaukee, Madison, and Eau Claire). The 20 WESH homes were located in Wisconsin’s Fox River Valley, in the east-central region of the state.

To add information about occupant use to our metered data, we randomly selected 43 WESH homes and surveyed their owners concerning their use of stand-alone dehumidifiers. We compared these survey results against 1,329 responses concerning dehumidifier use that were taken from our most recent Midwest Energy Survey (MES). (See “About the Midwest Energy Survey.”)

Metering Results

We metered several conditions in the 40 monitored homes. We used “Watts-up” watt-hour meters to collect watt-hours and wattage (minimum, maximum, and average values) in five- to ten-minute increments over four months. We used Onset Computer’s Hobo U10 data loggers to monitor relative humidity (RH) and ambient temperature in the main floor and in the basement of each home. The basement RH measurement loggers were placed near the ceiling in the same room as, but not immediately adjacent to, the dehumidifiers. We used local hourly weather station data to estimate local outdoor temperature and RH.

For our study, we divided the monitoring period into two separate seasons. The summer season included data collected during July and August. The fall season included data collected during September and October. We found that on average, the dehumidifiers we metered used more energy during the summer (6.5 kWh per day) than they used during the fall (4.8 kWh per day). The range in average dehumidifier energy use across all homes was significant; it ranged from a low value of 0 kWh per day to a high value of 17 kWh per day. During the summer, we found that the 20 newer WESH homes used an average of 1.6 kWh per day more than the non-WESH homes. We speculate that this difference in energy use is at least partially related to differences in the size of the houses; the median WESH home in our sample was 25% larger above grade and 93% larger below grade than the median non-WESH home.

We applied linear regression techniques to daily data from each site to estimate the effect that indoor basement humidity and outdoor humidity had on dehumidifier energy use. We found that high indoor basement humidity was linked to low dehumidifier energy use and that high outdoor humidity was linked to high dehumidifier energy use. We found these relationships to be effective predictors of dehumidifier energy use in general. However, the predictive ability of this model was highly variable across sites and proved to be a good fit only for dehumidifiers that were able to maintain reasonable control over basement humidity levels. For those cases where dehumidifiers did not effectively control humidity, we switched our model around and used dehumidifier energy use as a predictor of basement humidity ratios.

Daily Medians and Interquartiles for Basement Relative Humidity, by Site

Figure 1. top panel: Box plots of daily average basement RH for 40 Wisconsin homes, arranged in order of increasing daily average dehumidification energy use. Site-specific median values of daily average RH are indicated by orange diamonds. The 25th and 75th percentiles of daily average RH are indicated by blue bars. bottom panel: Average daily energy use for each site during summer months, shown in order of increasing magnitude.

Clearly a relationship exists between the amount of energy used for dehumidification and basement humidity levels. Homes that maintained lower daily humidity levels in the basement used more energy for dehumidification than homes that maintained higher daily humidity levels in the basement. During summer months, nearly half of the basements we monitored maintained a daily average RH of less than 50%. These homes used an average of 4.2 kWh per day more than the homes where the basement maintained a daily average RH exceeding 50%.

Figure 1 illustrates the general relationship between basement RH levels and average daily dehumidification energy use. Data for the 40 sites are arranged from left to right in order of increasing average energy use per day. The bars in the top panel (representing the 25th and 75th percentiles of daily average basement RH levels) show the variability in humidity control across all 40 sites. We found that basement humidity levels were relatively well controlled at some of the sites we metered (RH levels fluctuated only 1–2%), while at other sites we metered, basement humidity levels were controlled very little, if at all (RH levels fluctuated 10–20 %).

Average Dehumidifier Duty Cycle

Figure 2. Average duty cycles of dehumidifiers in 40 Wisconsin homes during the summer and fall months of 2009.

Dehumidifier Duty Cycles

Dehumidifier duty cycles varied greatly across sites. We divided duty cycles into three modes of operation: Off, Fan-Only, and Run. Off mode consists of periods when neither the compressor nor the circulation fan is operating. Fan-Only consists of periods when the circulation fan is operating, but the compressor is not. Run mode consists of periods when both the compressor and fan are operating. Dehumidifiers in our sample drew 40–120 watts, for an average of 75 watts, when in Fan-Only mode. They drew 300–700 watts, for an average of 500 watts when in Run mode. Figure 2 shows the average summer and fall duty cycles for the 40 dehumidifiers in our sample. Note that although the average dehumidifier ran 56% of the time during summer months, we found that individual duty cycles varied greatly. Some of the units we metered were in Run mode as little as 15% of the time, while other units ran virtually nonstop.

Survey Responses on Dehumidifier Use

The most recent MES survey showed that almost 65% of Wisconsin homes have a stand-alone dehumidifier. A comparison of our survey results and the MES survey indicated that more dehumidifiers were installed in WESH homes (75%) than in non-WESH homes of a similar age (46%). The frequency and duration of reported dehumidifier use was consistent across surveys; about 60% of respondents reported using their dehumidifiers during the entire summer or more. 

In the latest MES, we found that nearly half of the dehumidifiers used in newer homes in Wisconsin were connected directly to a drain (rather than draining into a manually emptied drain pan). Dehumidifiers using drain pans are susceptible to periods of inoperation when occupants fail to empty the pans regularly. We found that the percentage of dehumidifiers connected directly to a drain was significantly higher in WESH homes (80%) than in non-WESH homes (45%). Homes in our metered sample where the dehumidifier was connected directly to a drain used slightly more energy than those where the dehumidifier was drained manually; however, the difference was not statistically significant.

A principal goal of our field study was to estimate average annualized energy use for dehumidifiers in Wisconsin. We used survey responses and the metered usage data from the 20 randomly selected homes to estimate that, on average, dehumidifiers in Wisconsin consume 477 kWh per year. This estimate is subject to the variation in use that we observed across the sites in our field study. The values used to arrive at this estimate are shown in Table 1. In generating this estimate, we assume that the average Wisconsin household uses its dehumidifier during a 90-day summer and a 60-day nonsummer period, in a manner that is consistent with use in the 20 randomly selected homes that we monitored.

Table 1. MES Responses Regarding Frequency and Estimated Levels of Dehumidifier Use

Maximize Dehumidification, Minimize Energy Use

In summary, our study suggests that households in Wisconsin use a significant amount of energy to dehumidify their homes. In addition, the degree to which dehumidifiers are used over the course of a year can vary dramatically, with some units remaining idle most of the year and other units running virtually nonstop. Households in the upper quartile of use could easily consume more than 1,000 kWh per year to dehumidify their homes. We found that all homes in the upper quartile maintained daily basement RH levels of less than 50%, and several of these homes maintained RH levels of less than 40%. Helping households to determine the dehumidifier settings that minimize energy use but maintain adequate levels of dehumidification may give Focus on Energy an opportunity to promote home energy conservation.

For more information:

The complete study described in this article, Dehumidification and Subslab Ventilation in Wisconsin Homes, is available through the Energy Center of Wisconsin at www.ecw.org/dehumidifier.

To learn more about the Energy Center of Wisconsin, go to www.ecw.org.
 

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