Why Cold Weather Reduces Airflow

In the previous article in this series, we examined why freeze-up in through-wall HRVs is rarely caused by cold temperatures alone, and is more often the result of airflow reduction and moisture retention. This article steps back further to examine a more basic question:

Why does airflow drop in cold weather in the first place — and why does that matter more than efficiency claims when evaluating ventilation performance?

For decades, ventilation systems have been marketed, compared, and widely judged primarily by their heat recovery efficiency (thermal efficiency), allowing efficiency to become shorthand for ventilation performance itself. These percentages are easy to communicate, easy to compare, and well suited to standardized testing. As a result, efficiency has become shorthand for “good ventilation.” But this metric only remains meaningful as long as airflow remains stable.

The industry’s focus on thermal efficiency is not accidental. Heat recovery performance is measurable, marketable, and well suited to standardized testing, where results can be expressed as a single, comparable number. Over time, this made efficiency an attractive proxy for overall ventilation quality.

As a result, efficiency became shorthand for “good ventilation.” When two systems both claim heat recovery, the one with the higher efficiency rating is often assumed to be the better performer—even though that comparison says nothing about how much air is actually being delivered.

The limitation of this approach lies in what efficiency does not describe. Thermal efficiency measures how effectively heat is transferred between air streams; it implicitly assumes airflow remains stable. In real buildings, particularly in cold climates, that assumption often fails.

As operating conditions become more demanding, airflow can decline while efficiency values remain unchanged—or even improve. When this happens, efficiency and real-world ventilation performance begin to diverge.

In Canada, this concept of “efficiency” is not just a marketing term, but a formally defined performance metric used in testing and certification—one that evaluates heat recovery under the assumption that airflow is being delivered.

For LUNOS systems, cold-climate performance is considered in terms of both heat recovery and airflow stability. Heat recovery only remains meaningful if airflow can be maintained under real operating conditions, including extended winter operation.

As outdoor temperatures fall, air density increases and resistance within the ventilation path rises. Small fans, commonly used in decentralized ventilation systems, typically operate within relatively narrow pressure ranges. As resistance increases, delivered airflow drops unless the system is designed to account for these conditions.

In practical terms, every fan has a limit to how much air it can move as resistance increases. Engineers describe this relationship using a fan curve, which shows how airflow decreases as the pressure the fan must overcome rises. As colder outdoor air increases resistance in the ventilation path, the fan moves less air unless the system is designed or controlled to compensate for those winter conditions.

This is why airflow values measured under mild conditions become less predictive in winter. The colder it gets — and the longer cold conditions persist — the less representative those values are of actual delivered ventilation.

Decentralized heat recovery systems rely on balanced airflow to support regeneration timing, pressure stability, and moisture transfer. When airflow drops, these processes are disrupted simultaneously.

This is the same failure pathway discussed in C1: once airflow drops far enough, moisture purge becomes incomplete and instability compounds over time.

Once airflow falls below a critical threshold, performance does not degrade gradually — it accelerates. Moisture removal becomes incomplete, pressure balance shifts, and instability compounds over time. The threshold at which this occurs varies by system design.

This threshold behavior is explicitly considered in how LUNOS systems are configured for cold-climate operation, where maintaining stable airflow under increasingly demanding conditions becomes the primary design constraint.

In Canadian testing and certification, thermal efficiency is formally expressed as sensible recovery efficiency (SRE)—a required measure of heat recovery performance that assumes airflow is being delivered.

An overreliance on efficiency metrics can become misleading in cold climates. SRE is calculated using temperature differences between supply and exhaust air streams, and under certain operating conditions, reduced airflow can increase the apparent effectiveness of heat exchange. In cold conditions, this occurs because lower airflow extends air-to-exchanger contact time, increasing the apparent efficiency of heat transfer even while the volume of delivered ventilation declines.

This creates an efficiency illusion. Performance can appear strong on paper while the system is, in practice, moving less air. The issue is not that efficiency measurements are incorrect, but that efficiency alone does not capture whether ventilation is still occurring at meaningful rates as outdoor temperatures fall.

For this reason, cold-climate ventilation performance in Canada is not assessed on thermal efficiency alone. Airflow retention under low-temperature operation is explicitly addressed through low-temperature ventilation reduction (LTVR), as defined in CSA C439, the cold-weather performance standard referenced by the National Building Code of Canada (NBC) for residential heat recovery ventilators.

Because provincial and territorial building codes are based on the NBC (with regional amendments), this framework is carried through across Canada, establishing airflow retention under cold-weather operation as a core requirement alongside sensible heat recovery efficiency. In practice, this reflects the reality that ventilation effectiveness in Canadian winters depends not only on how efficiently heat is transferred, but on whether airflow can be sustained as temperatures drop.

This distinction aligns with how LUNOS approaches cold-climate ventilation. Rather than treating efficiency as a standalone indicator, LUNOS designs its systems with both sensible heat recovery and airflow retention in mind, reflecting the conditions under which ventilation systems are expected to operate in Canada.

Ventilation exists to achieve outcomes: pollutant removal, humidity control, odour management, and occupant comfort. When airflow drops, these outcomes deteriorate regardless of how efficient the heat exchanger may be.

Canadian buildings are challenged not by brief cold snaps, but by extended periods of low temperatures. Ventilation systems must sustain performance over days and weeks of winter operation. In this context, a system that maintains airflow will outperform a more efficient system that cannot, because the primary risk in cold climates is not wasted energy — it is lost ventilation.

In Canada, cold-climate ventilation performance is evaluated by considering both heat recovery and airflow retention under low-temperature conditions. LTVR exists precisely because airflow loss is measurable, consequential, and unavoidable in winter operation.

This approach recognizes a simple reality: sensible heat recovery (thermal efficiency) has value only after airflow is secured. Systems that maintain airflow through sustained cold conditions deliver meaningful ventilation; systems that do not may meet efficiency targets while failing to perform in practice.

This framework sets the stage for how cold-climate ventilation systems are tested, compared, and ultimately accepted in Canada—particularly when winter performance is evaluated under CSA C439.

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.