Why HRVs Freeze in Cold Weather and what Actually Causes It

Freeze-up is one of the most commonly cited concerns with through-wall heat recovery ventilators (HRVs), particularly in cold-climate regions such as Canada. It is also one of the most misunderstood.

The prevailing assumption is simple: cold outdoor temperatures cause HRVs to freeze. In practice, field experience and building-science research show that freeze-up is rarely triggered by temperature alone. Instead, it is almost always the result of reduced airflow, moisture accumulation, and control strategies that fail to adapt to cold-weather conditions.

Understanding this distinction matters. Systems that are designed—and evaluated—around airflow stability and moisture management behave very differently in winter than systems optimized primarily around peak efficiency. This article examines why HRVs freeze in cold weather and why airflow collapse and moisture retention—not outdoor temperature alone—are the real drivers of winter freeze-up in through-wall ventilation systems.

Outdoor temperature establishes the boundary conditions under which a ventilation system operates, but it does not, by itself, create ice. Freeze-up occurs only when moisture carried in exhaust air condenses and remains within the heat exchanger long enough to accumulate and freeze.

This principle is well established in building-science literature, including guidance published by ASHRAE, which treats frost formation as a combined moisture and airflow problem, not a simple temperature threshold.

In other words, cold air enables freezing—but airflow collapse and moisture retention drive it.

All occupied buildings generate moisture. Even in winter, indoor air contains water vapour from occupants, cooking, bathing, and basic respiration.

During the exhaust portion of a regenerative HRV cycle, warm, moisture-laden indoor air passes through the heat exchanger. As that air contacts cold surfaces, some of its moisture condenses. This is normal and unavoidable.

Under stable operating conditions, that moisture is removed during the subsequent supply cycle, as incoming outdoor air passes through the exchanger and carries residual moisture out of the system. Freeze-up risk increases when this purge process becomes incomplete.

The single most important factor in preventing freeze-up is maintaining sufficient airflow. As outdoor temperatures drop, air density increases, which raises resistance and reduces the volumetric airflow a fan can deliver at a given speed. This behaviour is fundamental fan physics and is documented extensively in fan-performance literature published by the Air Movement and Control Association and referenced in ASHRAE handbooks.

For small, through-wall ventilation systems, this reduction in airflow can be significant. When airflow drops:

• Less moisture is removed during each regeneration cycle
• Condensation persists on heat-exchange surfaces
• Ice formation becomes progressive rather than incidental

Importantly, freeze-up typically begins after airflow has already fallen below a critical threshold, not at the moment outdoor temperatures cross a particular value.

Freeze-up is rarely a single event; it is a self-reinforcing process that unfolds over time. As airflow decreases, each regeneration cycle removes slightly less moisture than the previous one. Thin layers of frost begin to form, and as ice accumulates, it further restricts airflow, accelerating the cycle.

This feedback loop—reduced airflow reducing moisture purge and allowing ice formation, which then further reduces airflow—is why systems can appear stable for extended periods before experiencing a rapid performance collapse during sustained cold weather.

Canadian research organizations, including the National Research Council Canada and the Canada Mortgage and Housing Corporation, have documented cold-climate ventilation challenges in which airflow reduction and moisture accumulation contribute to progressive performance degradation.

Control strategy is often the difference between stable winter operation and repeated freeze-ups. Many HRV control algorithms are designed to maximize heat recovery efficiency. In cold conditions, this can involve longer regeneration cycles, slower airflow, or reduced purge frequency. While these approaches may improve efficiency under certain test conditions, they also increase moisture retention within the heat exchanger.

From a cold-climate perspective, this is risky. Preventing freeze-up requires accepting some degree of inefficiency in favour of active moisture management and airflow stability. Systems that attempt to preserve peak efficiency at all costs are more vulnerable to icing.

These considerations are particularly relevant for systems that rely on alternating airflow rather than continuous exhaust. With LUNOS, the regeneration cycle is intentionally configured based on cold-climate performance considerations and operational experience, rather than set to a fixed industry convention. In practice, the cycle length is selected to balance effective heat recovery with sufficient airflow and moisture purge—recognizing that extending regeneration time beyond a certain point provides diminishing returns once airflow and moisture dynamics are taken into account.

When freeze-up does occur, many systems rely on a defrost mode. Defrost functionality is a legitimate and widely used element of cold-climate ventilation design; the distinction discussed here is not about the presence of defrost, but about relying on defrost as the primary means of maintaining winter operation.

In practice, these modes frequently reduce or suspend ventilation altogether. While this may temporarily protect the equipment, it introduces new problems:

• Ventilation rates fall below design intent
• Indoor air quality degrades
• Pressure balance within the dwelling can be disrupted

At the same time, defrost functions play an important role in breaking freeze-up cycles once airflow has already been compromised. The distinction is not whether defrost is necessary, but how often it is required and how disruptive it is to normal winter operation.

Industry guidance acknowledges these trade-offs. ASHRAE publications note that frost-control strategies which interrupt airflow may be necessary in some cases, but they are not a substitute for systems designed to operate continuously under winter conditions.

From a building-performance and indoor air quality perspective, winter operation is not an optional condition. Ventilation systems are generally expected to continue delivering airflow under cold-weather conditions, rather than suspending operation in response to freezing risk.

This distinction is important when evaluating how different manufacturers describe “defrost” functionality. In LUNOS HRVs, defrost behaviour is not intended as a primary operating mode, but as a protective measure layered onto a control strategy designed to maintain airflow and purge moisture under cold conditions, rather than as a replacement for normal winter operation. Used appropriately, defrost functions protect system integrity without becoming a substitute for stable cold-weather performance.

Freeze-up discussions frequently focus on heat-exchanger material—ceramic versus polymer versus metal. Material selection does influence durability, moisture interaction, and long-term performance limits, but it does not, on its own, determine whether a system will experience freeze-up. While material properties affect thermal behaviour, they are rarely the root cause of icing.

Both ceramic and polymer exchangers can freeze under low-airflow, high-moisture conditions. Conversely, both can operate reliably in very cold climates when airflow and control logic are appropriately managed.

Material debates persist largely because the exchanger is the most visible component. In practice, freeze-up risk is governed far more by how a system behaves under winter conditions—how airflow is maintained, how moisture is purged, and how protective functions such as defrost are integrated into normal operation.

In LUNOS HRVs, material selection is only one part of a broader cold-climate operating strategy, where control logic and system behaviour are designed to support continuous winter operation and reduce how often corrective measures, such as defrost, are required.

Understanding cold-weather ventilation performance is one thing; seeing how it performs in real buildings is another. LUNOS systems have been applied in a wide range of Canadian cold-climate projects, from large multi-unit residential buildings to compact, high-performance homes.

If you’d like to explore the building-science concepts behind cold-weather ventilation performance in more detail, the following articles provide additional technical context:

If you’re planning a project in a cold climate—LUNOS Canada can help you better understand how decentralized ventilation strategies are applied in real buildings.

Whether you’re a homeowner, builder, designer, or building professional, we’re happy to discuss how cold-climate considerations influence system selection and long-term performance.