Why Heat Pumps Corrode Faster Near the Sea

Introduction

Heat pumps are increasingly installed in coastal regions, from Northern Europe to the Mediterranean, the Gulf Coast and island environments. While energy efficiency often dominates purchasing decisions, environmental exposure is frequently overlooked.

The result is a recurring question among homeowners, installers and service technicians: why does a heat pump installed near the sea show corrosion after only a few years?

Heat pumps are outdoor equipment

Unlike many household appliances, the outdoor unit of an air-source heat pump is permanently exposed to the environment. The environmental conditions surrounding the unit often determine its long-term durability.

Why coastal locations are different

Many coastal environments experience long high-humidity periods, frequent condensation during night-time cooling, salt transport by wind and persistent surface wetness.

Even locations several kilometers inland may still experience marine influence. The combination of moisture and chloride contamination can accelerate aluminum corrosion, steel corrosion, galvanic corrosion and electrical connector degradation.

Humidity matters even without rain

Corrosion does not require direct rain exposure. When relative humidity remains high for extended periods, thin moisture films can form on metal surfaces. Condensation may occur during night-time cooling, particularly on heat exchanger fins and metallic housings.

These conditions can create long time-of-wetness periods, electrochemical corrosion, surface contamination and reduced coating performance. All five locations compared below spend between about 3,500 and 4,700 hours per year at or above 80% relative humidity.

Salt exposure is often invisible

Marine aerosols can travel several kilometers inland. Salt particles deposited on surfaces attract moisture and create conductive electrolyte films that accelerate corrosion processes.

The effects are particularly visible on outdoor coils, fasteners, mounting brackets, condenser housings and electrical terminals. The resulting corrosion is often uneven and localized, making it difficult to predict using temperature alone.

Five sites compared: chloride and humidity hours

The table combines two different quantities for five Climetry city examples. Humidity hours come from the hourly CEY10 representative year (2015-2024). Chloride is the modelled annual mean total chloride deposition at the city-centre grid cell, a screening estimate rather than a measurement. The two columns describe different things and are not interchangeable.

Three observations follow directly from the numbers:

• Humidity and chloride do not move together. San Francisco has the fewest RH80 hours of the coastal examples but almost three times Houston's chloride value; Houston has more humid hours but lower salt deposition.
• Being close to the sea does not by itself put a site in a high salinity band. Seattle and San Francisco sit 1-2 km from the water and still screen as S1 at the city-centre cell. Directly exposed shoreline positions can be higher than the city sample.
• Inland sites are not free of moisture stress. Frankfurt, 334 km from the coast and in band S0, still has more than 3,500 hours per year at or above 80 % RH.

The European and North American chloride values come from two different Climetry models with different calibration networks (EBAS/EMEP and NADP). Compare bands across continents, not small numerical differences.

What this means for material and test choices

ISO 9223 combines time of wetness, chloride and sulfur pollution for exactly this reason: neither humidity nor salt alone sets the corrosivity of a site.

What manufacturers already do

Many manufacturers offer coated heat exchanger fins, marine coatings, corrosion-resistant housings, improved drainage systems and stainless fasteners. Examples include Blue Fin coatings, Gold Fin coatings and epoxy-coated coils.

These measures acknowledge that environmental exposure can significantly influence service life. The engineering question is which locations require standard protection and which locations justify enhanced corrosion protection.

Climate exposure is more important than temperature alone

Many people assume that hot climates are always the most severe. However, a moderate coastal climate may create greater corrosion exposure than a hot dry environment.

Temperature alone rarely tells the full story. Humidity, condensation potential, marine influence, wetness duration and pollution context determine the exposure that an outdoor unit must survive.

Environmental exposure screening

Climate-based environmental screening considers humidity exposure, condensation potential, marine influence, temperature cycles and wetness duration. These indicators help answer practical questions before equipment is selected or installed.

Environmental exposure does not predict exact corrosion rates, but it helps identify locations where additional protection may be necessary.

The future of heat pump qualification

As heat pumps become increasingly common in coastal and humid regions, environmental exposure may become a more important design consideration.

Questions such as how humid the location is, how often surfaces become wet, how strong the marine influence is, and whether the environment is comparable to tropical climates may become as important as heating performance itself.

Conclusion

Heat pump corrosion is often not caused by a single defect. Instead, it results from the interaction of humidity, condensation, salt exposure and climate conditions.

Understanding environmental exposure allows manufacturers, installers and owners to make better decisions about materials, coatings, maintenance and equipment selection.

Climate does not determine whether a heat pump will fail. But it strongly influences the environment in which it must survive.

Sources

• ISO 9223:2012, Corrosion of metals and alloys - Corrosivity of atmospheres - Classification, determination and estimation (salinity bands S0-S3).
• IEC 60068-2-52, Environmental testing - Salt mist, cyclic (sodium chloride solution); ISO 9227, Corrosion tests in artificial atmospheres - Salt spray tests.
• Climetry city example pages: CEY10 2015-2024 humidity hours; North America V2 and Europe/Mediterranean v2.3 chloride screening layers (see methodology).