Dehumidifiers: A Major Consumer of Residential Electricity

July 01, 2013
July/August 2013
A version of this article appears in the July/August 2013 issue of Home Energy Magazine.
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Annual electricity consumption by dehumidifiers can be 1,000 kWh or more, twice as much as an Energy Star refrigerator uses. According to a recent report in Appliance magazine, 19% of U.S. households have at least one dehumidifier. That adds up to a lot of energy used to remove moisture from the air inside homes.

However, the available data on dehumidifier operation are limited. Our company, Cadmus, conducted an engineering study, including metering dehumidifiers operating in several homes, to better understand how dehumidifiers are used and how much energy they consume.

Table 1. Current Efficiency Standards for Dehumidifiers

Average Power VS Unit Age


Figure 1. The average active power was 459 watts.

Maximum Power VS Unit Age


Figure 2. The maximum power ranged from 375 to 964 watts.

Electricity VS Water Removed


Figure 3. Electricity consumption increased with the amount of water removed from the air.

Electricity VS Unit Capacity


Figure 4. Electricity consumption showed no correlation to unit capacity.

Energy Factors


Figure 5. The relationship between measured and rated efficiency varied.

Efficiency Ratings

Dehumidifier capacity is rated by the volume of water removed from the air, usually measured in pints per day. Most residential units have rated capacities of 25–75 pints per day; some have capacities of more than 100 pints per day. Most dehumidifiers sold in recent years allow the user to adjust the relative humidity (RH) set point and the fan speed. The RH set point is often selected as a percentage of RH, but there are sometimes other options, such as low/medium/high. The efficiency of dehumidifiers is rated in terms of energy factor (EF), defined as the volume of water removed from the air per unit of energy consumed, measured in liters per kWh (L/kWh).

A federal standard regulates the minimum EF of dehumidifiers manufactured in the United States, with a scale of EF requirements based on unit capacity, and the Energy Star program provides a specification for high-efficiency dehumidifiers. In October 2012, both the federal standard and the Energy Star specification for dehumidifiers were increased. Table 1 shows the current standards.

Study Sample and Metering Period

Cadmus metered 21 dehumidifiers in 19 homes in Massachusetts, New York, Maryland, and Virginia. We began metering each unit between mid-September and early October 2011 and continued for 1 to 12 weeks. In most cases, we kept metering until the dehumidifier was turned off for the season, or for at least two months for the units that run year-round. We interviewed participants about their usage patterns and asked them to run their dehumidifiers as they normally would throughout the metering period.

The timing of this study did not allow us to measure the peak operation, which we expect would occur during the spring and summer months. Therefore the annual projections based on this study are conservative estimates of dehumidifier operation.

Metering Approach

We used power meters to measure the wattage of the dehumidifier continuously and to log power in five-minute increments. Meters placed near the units measured the ambient temperature and RH. We gave data collection sheets and graduated measuring buckets to participants who manually emptied their units, and they logged the time and volume of water each time they did so. While there is some potential for error in log data collected by users, this allowed us to track water removal throughout the metering period, and we did not find any discrepancies between operation patterns shown in log sheets and meter data.

Model Information and Configurations

Dehumidifiers in the study ranged from new to 15 years old, with an average of 4 years old. The average rated capacity was 45 pints per day, with units ranging from 25 to 70 pints per day.

All of the models purchased in the five years before the study met the Energy Star standard that was in effect at the time. The EF rating was not available for the three oldest models, but neither of the units older than five years for which we had EF ratings met the Energy Star standard.

All of the dehumidifiers were used in basements, some of which were finished and some of which were unfinished. Two participants operated two units each in the same basement. Six of the 21 units were set up to drain directly; for the remaining 15 units, the tub of collected water was emptied manually.

Participants used an average RH setting of 50%, with a range of 35–65%. Reported fan speed settings were split evenly between low and high, with a few participants changing their setting during the metering period. A few of the oldest models didn’t offer a choice of fan speed.

Usage Patterns

About half of participants leave their units on year-round, and the other half run them seasonally for a period between spring and fall. Most participants who use their dehumidifiers seasonally turn them off in October or November, and they reported that they turn them back on between April and July. The average reported operation was eight months per year.

Most of the participants without direct drains waited until the tub was full to empty their dehumidifier. Some checked their units at least once a day, while others checked and emptied their dehumidifiers only occasionally, so the tub might be full for hours or days before it was emptied. Most participants left their units on throughout their metering period. A few people turned their units on and off manually; reported reasons included saving energy and reducing noise when they were near the unit.

Standby Power

Meter data showed that 10 units drew no power when not operating, while the other 11 drew 0.4–1.9 watts of standby power. Units using standby power ranged from new to eight years old. One user observed that the fan operated continuously on his new unit after reaching the humidity set point, so he turned the unit off manually to reduce energy consumption.

In December 2010, DOE proposed to amend the test procedure and energy consumption equations for dehumidifiers to include the measurement of energy use in standby and off modes.

Active Power

The average power when running, excluding standby power, was 459 watts (see Figure 1). The maximum power metered for each unit ranged from 375 to 964 watts (see Figure 2). Linear regressions in these figures show a limited correlation between unit age and power. While age may affect dehumidifier power, several other factors are also expected to do so; including unit efficiency, user settings, and ambient conditions.

Water Removal

Participants who manually emptied their units logged the time and measured the water volume each time they did so. The water removed ranged from 0.2 to 12 pints per day for these units, with an average of 4.9 pints per day.

A limiting factor in water removal was the need to empty these units manually. While the rated capacity of manually emptied dehumidifiers in the study ranged from 25 to 70 pints per day, their tub volumes measured between 6 and 24 pints. Most tub volumes were less than half the rated unit capacity, meaning that the units needed to be manually emptied more than twice per day to provide the rated water removal. Depending on unit size and on ambient conditions, some units may not need to run at rated capacity to provide the desired results, but users should be aware of this potential limitation and the importance of tub volume when purchasing dehumidifiers. Other factors affecting the amount of water removed include ambient conditions and user settings.

Operating Hours

During the metering period, the average run time (not including time drawing standby power) was 8.9 hours per day, for an average duty cycle of 37%. At that rate, annual run time would be 2,160 hours, based on the average of eight months per year discussed above. We would expect actual annual operating hours to be higher, because this study did not include metering during the time of year when dehumidifier operation is at its peak. This result indicates higher dehumidifier operating hours than the 1,620 estimated in the frequently cited Little (1998) report.

As discussed above, operation of some units was limited by the need to empty the small tub frequently. Because direct-drain units were not limited by tub size, they operated longer hours, with average active run time of 11 hours per day for direct-drain units, compared to 8.1 hours per day for manually emptied units. During the peak operating season, this difference would probably be greater.

Electricity Consumption

The average metered electricity consumption was 4.2 kWh per day. As with operating hours, electricity consumption was higher for direct-drain units (5.6 kWh per day) than for manually emptied units (3.7 kWh per day). We assume that the corresponding water removal was also higher, though this study did not include measurement of water removal from direct-drain units.

We compared the average daily electricity consumption and water removal for each of the 15 units that were manually emptied (see Figure 3). This graph shows a weak correlation between water removal and electricity consumption. If the one outlying data point (a 12-year-old unit consuming 14.3 kWh per day) were removed, however, the R-squared value for a linear regression would increase to 0.47. A number of other factors affect electricity consumption, including ambient conditions, user settings, and efficiency of the unit. We found that, for this sample, electricity consumption was not related to rated unit capacity (see Figure 4). As discussed above, no manually emptied unit reached its rated water removal capacity in this period, so the capacity would not be expected to be a major factor in operation, at least for those units.

Projecting the metered daily electricity consumption across the year based on the average eight months per year of operation explained above, annual electricity consumption would be about 1,000 kWh. At the 2011 national average residential electric rate of $0.118/kWh, the average annual operating cost is $120. For comparison, most new Energy Star-rated refrigerator use less than 500 kWh per year, or 1.4 kWh per day.

According to the U.S. Energy Information Administration, the average household used 11,496 kWh in 2010. At the average consumption of 1,000 kWh, one dehumidifier would account for approximately 9% of the electricity consumption in a home.

Energy Factor

To evaluate the efficiency of the dehumidifiers, we used the measured electricity consumption and water removal to calculate the actual EF for each unit that was manually emptied. The measured EF ranged from 0.2 to 2.1 L/kWh, with an average of 0.8 L/kWh (see Figure 5). The measured EF was lower than the rated EF for all but two units. A 12-year-old dehumidifier, the oldest of the manually emptied units, had the lowest measured EF at 0.2 L/kWh, but overall in this study the correlation between unit age and measured EF was minimal. As discussed above, EF ratings were not available for three of these units.

EF ratings are based on testing at dry-bulb temperature of 80°F and wet-bulb temperature of 69.6°F, which equates to about 60% RH at sea level. Meter data showed that the actual temperature and RH were below these values for almost all units, as expected, since most units were set to less than 60% RH.

When the study began, one of the units had just been installed in a basement where no dehumidifier had been used in recent years, so that meter showed RH levels as high as 72% early in the study. The average measured EF for that unit was 1.15, very close to its rating of 1.2 and one of the highest EF values in the study. We looked at the EF and RH of this unit over a four-day period shortly after the dehumidifier was first turned on. The EF started out near 2, well above the rated efficiency, and decreased to 1 over a few days as the basement humidity decreased below 60%. The meter measured RH continuously, while EF was calculated once or twice a day when the user emptied the tub.

This indicates that dehumidifiers in actual operating conditions may perform at lower efficiency than they would at the test conditions used to rate dehumidifier efficiency. Additional metering could be performed in the summer to compare summer operating conditions to the test conditions and measure EF during peak operation.

Accuracy of Humidity Controls

To evaluate the accuracy of the dehumidifier controls, we graphed RH and power over time for each unit. Some units showed results tracking closely with the reported set point, while others showed potentially faulty controls. For example, one dehumidifier in a very wet basement was set at 65% RH. We found that the dehumidifier did not operate during days when the measured RH was consistently above the set point. This unit has been set up to drain directly, so operation was not limited by tub capacity. This unit does operate at times, with an average of 2.4 hours per day of run time during the metering period, but the data show a discrepancy between the RH measured during operation and the RH set point. In this study, we did not find any dehumidifiers that continued to run when the ambient RH was below the set point.

Conclusion

Our study of 21 dehumidifiers operating in Northeast and Mid-Atlantic homes showed that dehumidifiers are energy-intensive appliances that operate for several months a year in many homes. To build on this study, we plan to do additional metering in the future during the spring and summer months.

Following are our key findings to date:

  • Participants reported an average of eight months per year of dehumidifier operation.

  • The average metered run time was 8.9 hours per day. At 8 months per year, the average unit would operate 2,160 hours annually.

  • The average metered active power was 459 watts.

  • Eleven of the units drew standby power between 0.4 and 1.9 watts.

  • The average metered electricity consumption, in the fall and winter, was 4.2 kWh per day, or 1,000 kWh per year based on eight months per year of operation. This is equal to 9% of the electricity consumption in an average home. Because this study was not conducted during the peak seasons, this annual projection is a conservative estimate.

  • For the 15 manually emptied units, the average water removal was 4.9 pints per day, and the average EF was 0.8 L/kWh.

  • The humidity controls on some units did not function properly, as some units did not operate when a separate meter showed ambient RH exceeding the set point.

  • The measured EF was lower than the rated EF for all but two units. We believe that this lower operating efficiency is attributable partly to the fact that most units in this study were operating in spaces with lower temperature and RH than the standard test conditions.

  • User operation is a key factor in effectiveness and energy consumption of dehumidifiers, including frequency of emptying tubs for units that don’t drain directly.

  • While it was beyond the scope of this study to quantify the ideal water removal in each home, water removal by some dehumidifiers in the study was limited by the need to empty the tubs manually when they reached capacity. Most tub volumes were less than half the rated unit capacity, meaning that the units would need to be manually emptied more than twice per day, or set up to drain directly, to provide the rated water removal.

learn more

Mattison, Lauren, and Dave Korn. “Dehumidifiers: A Major Consumer of Residential Electricity.” ACEEE Summer Study on Energy Efficiency in Buildings. The Cadmus Group, Incorporated, 2012. Download the full report.

31st Annual Portrait of the U.S. Appliance Industry. Appliance Magazine. 2008.

Zogg, Robert and Deborah Alberino. Electricity Consumption by Small End Uses in Residential Buildings, Arthur D. Little, Inc. Report to Office of Building Equipment U.S. Department of Energy. 1998.

Recommendations

Don’t forget about the dehumidifier in your basement! They are often out of sight, but dehumidifiers can use more electricity than most other residential appliances and equipment. Follow these tips to dehumidify your home efficiently:

  • Choose an Energy Star model when buying a new dehumidifier.

  • Consider replacing older units. Federal requirements and the Energy Star program have driven significant increases in the rated efficiency of dehumidifiers sold in the United States since 2006. If possible, meter your dehumidifier to see how much energy it is using.

  • Adjust your set point to make sure your dehumidifier doesn’t run more than is necessary to reach your desired humidity level.

Lauren Mattison is an associate in Cadmus’ Energy Services Division. Cadmus is a fast-growing technical and strategic consultancy serving clients around the world in the areas of energy, environment, high-performance building, sustainability, and public health.

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