Considerations for Heat Pump Water Heaters
Energy-efficient and cost-effective, but proper installation is key to avoiding moisture and mold issues
Published May 12, 2025
Why so popular?
Heat pump water heaters (HPWHs) are growing in popularity. They are an energy-efficient alternative to traditional electric water heaters, with federal and utility incentives such as rebates and tax credits making them even more appealing.
How HPWHs Work — And Why That Matters
HPWHs use ambient air to heat water through a refrigeration cycle, much like an air conditioner installed on top of a water tank. However, their operation can inadvertently create conditions conducive to mold growth.
The Hidden Risk
Because HPWHs extract heat from the surrounding air, they also cool and dehumidify that air, which can lead to condensation on nearby surfaces. When installed in enclosed or poorly ventilated spaces, such as enclosed basement areas, closets, or utility rooms, this moisture can accumulate, especially in humid environments.
The Importance of Proper Ventilation and Air Volume
All HPWH installation instructions clearly state the volume of air required for proper operation. To achieve this, the space may need to be expanded by louvered doors, transfer grills, or other approved means.
Climate Matters
It is worth noting that even if the manufacturer's recommendations are followed, if the unit's discharge is facing toward an exterior surface, it may still create conditions that allow for possible mold growth. These conditions are more prevalent in humid and mixed-humid climate zones where the dew point can easily exceed the indoor design of 75 degrees and 50% relative humidity.
Best Practices for Builders and Homeowners
The solution to prevent condensation formation is not to allow the exhaust air to cool any surfaces below the dew point. Mixing the exhaust air from the HPWH with enough warm air shortly after leaving the HPWH will raise the temperature above the dew point, preventing condensation and minimizing or eliminating the risk of moisture damage and mold.
Efficiency Without Compromise
HPWH can be a very efficient and cost-effective solution for water heating in new homes, regarding first costs and homeowners' utility bills. However, builders need to install and design HPWH thoughtfully. Homeowners should regularly inspect and clean air filters and periodically check for signs of moisture. With proper planning and maintenance, the benefits of a heat pump water heater can be enjoyed without the downside of mold-related issues.
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