Dehumidifying With Heat Pump Water Heaters: Separating Fact From Marketing

Heat pump water heaters (HPWHs) have gained significant popularity as energy-efficient alternatives to conventional electric resistance water heaters. By using the vapor-compression cycle, these appliances absorb heat from the surrounding air and transfer it to the water in the tank, achieving efficiency ratings well above 200 percent. Manufacturers and utility efficiency programs frequently highlight lower operating costs and dehumidification as key selling points. The idea is appealing: install a single appliance that provides hot water while also removing moisture from a damp basement. But does the reality match the promise? This article examines the published research and real-world testing data behind HPWH dehumidification claims. For background on how these systems transfer heat from air to water, see our article on heat pump water heaters and heat transfer technology.

How Heat Pump Water Heaters Produce Condensation

Both dehumidifiers and HPWHs rely on the refrigeration cycle. A compressor circulates refrigerant through a closed loop, creating a cold evaporator coil. When warm, humid air passes over this coil, the air temperature drops below its dew point. Water vapor in the air then condenses into liquid water on the coil surface, just as condensation forms on a cold glass of water on a humid day.

In a HPWH, this condensed water drips into a drain pan and is piped or pumped away. Because the appliance physically removes water vapor from the air, manufacturers have sometimes promoted dehumidification as a secondary benefit. The key question is whether the amount of water removed is meaningful for maintaining healthy indoor humidity levels. Understanding the vapor-compression cycle helps clarify why condensation occurs, but it does not automatically mean the HPWH can function as a dehumidifier. For more on how HPWHs deliver free cooling and moisture removal alongside hot water, see this detailed overview of HPWH benefits.

Published Research on HPWH Dehumidification

The scientific literature on HPWH dehumidification is surprisingly limited. The most comprehensive study comes from William Murphy of the University of Kentucky, who presented his findings at the 2016 International Refrigeration and Air Conditioning Conference. Murphy monitored condensate production from two HPWHs installed in his garage over a period spanning 2010 to 2015. His results showed that the HPWHs produced less than 0.2 pints of condensate per day during winter months and approximately 0.9 pints per day in summer.

To put these numbers in context, a small portable dehumidifier typically removes 35 pints per day, while a large whole-house unit can extract 200 pints or more. Even during the warmest, most humid months, the HPWH removed less than 1 pint of water daily.

Another study from 2013 by Carlos Colon at the Florida Solar Energy Center tested HPWH performance in a conditioned test building resembling a Florida garage. Researchers assumed daily hot water use equivalent to a four-person household. Even under these near-ideal conditions, the HPWH produced only 3.2 pints of condensate per day. These findings align closely with the field data collected by Jon Harrod, contributing editor at Fine Homebuilding, who documented his own results in a detailed article on this topic. You can read his full methodology and analysis at Fine Homebuilding’s coverage of dehumidifying with HPWHs.

Real World Testing Results

Jon Harrod conducted a carefully controlled experiment in his own home to measure HPWH condensate production under realistic conditions. He measured condensate collected from his HPWH over 24 days while a basement dehumidifier maintained relative humidity at around 57 percent. During this period, the HPWH produced only 4.25 total pints of condensate, an average of 0.17 pints per day.

Harrod then turned off the dehumidifier entirely and allowed the basement humidity to rise above 70 percent. Over the next five days, HPWH condensate production increased to 6.5 pints, averaging 1.3 pints per day. While this represented a nearly eightfold increase, it remained minuscule compared to the dedicated dehumidifier, which removed 36 pints per day under the same humidity conditions.

The reasons for this poor performance become clear when examining how HPWHs actually operate:

• Intermittent operation. Data from an energy monitor showed the HPWH turned on approximately 1.2 times per day, running for an average of 68 minutes per cycle. This means the compressor operated only about 5 percent of the time.
• Delayed condensation. At the start of each heating cycle, the evaporator coil takes several minutes to reach the temperature required for condensation. Water only begins dripping into the drain pan after sufficient accumulation on the coil.
• Re-evaporation losses. When the compressor shuts off, any water remaining on the coil or in the drain pan evaporates back into the room air. For a typical 68-minute cycle, much of the condensate never actually reaches the drain.

Longer cycles produced by heavy hot water demand, such as on laundry days when the HPWH ran for three-hour stretches, generated the majority of measurable condensate. This suggests that households with higher occupancy and greater hot water consumption would see somewhat more moisture removal. For guidance on choosing between different water heater types for residential use, including HPWH sizing and installation, see our article on water heater selection and installation for residential applications.

Why HPWHs Cannot Replace Dedicated Dehumidifiers

The fundamental limitation of HPWH dehumidification stems from a simple design reality: HPWHs respond to hot water demand, not indoor humidity levels. A dedicated dehumidifier operates based on a humidistat, running its compressor whenever relative humidity exceeds the set point. A HPWH runs only when the water temperature in its tank drops below the thermostat setting.

This distinction has several consequences:

1. A HPWH can only dehumidify when someone uses hot water. In periods of low occupancy or reduced hot water use, the appliance may not run for hours or even days.
2. The amount of dehumidification is proportional to hot water consumption. A household that uses 20 gallons of hot water per day sees far less moisture removal than one using 80 gallons.
3. During shoulder seasons when neither heating nor cooling systems run frequently, HPWH operation may be particularly sparse, precisely when basement humidity problems tend to peak.
4. Installing a HPWH in a location with poor air circulation further reduces its already modest condensate production.

Even with the most favorable conditions, the research consistently shows that HPWH dehumidification is best described as an incidental and intermittent benefit. It is not a substitute for proper moisture management strategies. Addressing basement moisture at the source through foundation waterproofing, exterior drainage, and subslab vapor barriers remains the primary solution. For more on integrating HPWHs into a comprehensive building envelope strategy, including thermal break considerations, see thermal breaks and HPWH installation techniques.

Practical Strategies for Basement Humidity Control

For homeowners considering a HPWH and hoping to also solve a basement moisture problem, here are the key takeaways:

• Do not rely on a HPWH as a primary dehumidification device. The data shows it removes far too little moisture to maintain healthy humidity levels below 60 percent RH.
• If you already have a HPWH, consider its condensate production a small bonus, not a moisture management solution. You will still need a dedicated dehumidifier, particularly in humid climates or during summer months.
• High-occupancy households with substantial hot water demand will see more condensate production, but the difference is unlikely to be large enough to eliminate the need for a dehumidifier.
• Address the root causes of basement moisture before relying on mechanical dehumidification of any kind. Proper grading, gutters, downspout extensions, foundation drainage, and vapor barriers reduce the moisture load that any appliance must handle.
• If you are replacing an existing water heater, consider a HPWH for its energy efficiency benefits rather than its dehumidification capabilities. The energy savings alone can make the upgrade worthwhile.

For households considering tankless options as an alternative to HPWHs, the technology operates on entirely different principles. Unlike HPWHs, tankless or instantaneous systems heat water directly without a storage tank and do not provide any dehumidification effect. Our article on instantaneous hot water systems and tankless water heaters covers the performance trade-offs between these two approaches.

Conclusion

Heat pump water heaters represent a genuine advancement in residential water heating efficiency. Their ability to absorb heat from the surrounding air and transfer it to water delivers energy savings that can significantly reduce household electricity bills. The incidental condensation that occurs during operation is a real physical effect and does remove some moisture from the air.

However, the research data is consistent and clear: HPWH dehumidification capacity falls far short of what is needed to maintain healthy indoor humidity levels. A HPWH running 5 percent of the time cannot compete with a dedicated dehumidifier that operates continuously whenever humidity rises. Homeowners should view HPWH condensation as a small side benefit, not a solution for moisture problems. The best approach combines an energy-efficient HPWH for hot water with a properly sized dehumidifier for moisture control, plus fundamental building envelope improvements to reduce moisture intrusion at the source. For more information on modern water heating technologies, see our explanation of tankless water heater technology for modern residential buildings.