Exhaust Mode vs. Heat Regeneration

When people compare ventilation systems, airflow is usually the first number they look for. Cubic feet per minute (CFM) feels intuitive: more air must mean better ventilation. In many situations, that assumption holds. With through-wall heat recovery ventilators (HRVs), however, airflow numbers can be misleading if they are taken out of context.

Most ductless HRVs operate in more than one mode, and airflow varies depending on how the unit is running. Yet comparisons often collapse these differences into a single number, leading homeowners, builders, and even experienced professionals to compare systems that behave very differently once installed.

In practice, airflow in through-wall HRVs changes with direction, operating state, and duration. Understanding which airflow number reflects normal, continuous operation (exhaust mode vs heat regeneration) is essential for meaningful comparisons.

Through-wall HRVs can operate in several distinct ways, most notably through one-direction exhaust and alternating heat regeneration, with additional high-speed modes intended for temporary use. To understand why airflow comparisons so often go wrong, it helps to examine these operating states individually, starting with the simplest.

Exhaust mode is the simplest operating condition for a through-wall fan. Air moves in one direction only, exhausting indoor air directly to the exterior. There is no alternating cycle and no heat recovery.

Because the fan does not reverse direction or push air through a heat-storage medium, airflow measured in exhaust mode is often relatively high. From a testing perspective, this makes exhaust airflow easy to measure and easy to communicate, and it closely resembles the airflow ratings associated with traditional exhaust fans.

The issue is not that exhaust-mode airflow is incorrect. It is that it does not represent how a through-wall HRV is designed to operate on a continuous basis.

Used as a primary ventilation strategy, continuous exhaust depressurizes the space and relies on uncontrolled infiltration rather than deliberate, balanced airflow. It also provides no heat recovery. For these reasons, exhaust mode is best understood as a temporary or auxiliary operating condition, not a meaningful indicator of everyday ventilation performance

Heat regeneration mode is how through-wall HRVs are intended to operate. In this mode, airflow alternates direction on a fixed cycle. As warm indoor air is exhausted, heat is absorbed by a regenerative heat exchanger. When airflow reverses, incoming outdoor air passes through the same exchanger and picks up the stored heat.

Because air must pass through the heat exchanger and the fan repeatedly reverses direction, airflow measured in regeneration mode is lower than in exhaust-only operation. This reduction is not a limitation; it is the consequence of recovering heat while maintaining balanced ventilation.

More importantly, regeneration mode reflects how these systems operate continuously in occupied buildings. Units are paired or coordinated so that supply and exhaust occur simultaneously, maintaining near-neutral pressure—in other words, balanced ventilation—while delivering fresh air and removing stale air in equal measure.

In this context, regeneration airflow represents real-world performance rather than a best-case scenario measured under simplified conditions

Once exhaust and regeneration are understood as distinct operating states, the next challenge is how those differences are reduced to airflow numbers and compared on a spec sheet. In some cases, additional “boost” airflow values are shown, further complicating comparisons by introducing numbers that reflect temporary rather than normal operation. Even when airflow values are clearly identified by operating mode, it is not always clear which one represents normal, continuous operation.

In practice, the highest airflow value—often associated with exhaust-only operation—tends to dominate informal comparisons. Larger numbers feel intuitive, and exhaust airflow resembles the metrics people are familiar with from traditional exhaust fans. Over time, that value can become the default reference, even though it does not reflect normal operation.

The issue is not that exhaust-mode airflow is incorrect. It is that it describes a specific operating condition rather than the way a through-wall HRV is intended to function on a continuous basis. When that distinction is overlooked, comparisons can drift away from how systems actually perform once installed.

As a result, airflow values that are technically accurate can still be misleading if they are treated as interchangeable.

At this point, the distinction between airflow numbers on paper and how ventilation behaves in real buildings becomes unavoidable.

When airflow numbers are misinterpreted or compared out of context, the consequences do not remain theoretical—they show up in real buildings.

In newer, highly insulated and airtight buildings, pressure imbalance becomes immediately visible. Small imbalances are no longer diluted by incidental air leakage, and relatively modest ventilation decisions can have outsized effects on comfort, moisture movement, and long-term durability—particularly in cold climates.

Older buildings behave differently, but the underlying issue is the same. While these buildings often have more incidental leakage, ventilation systems that rely on sustained exhaust still create pressure-driven airflow through walls, floors, and assemblies that were never intended to act as air pathways. In many cases, the resulting drafts and comfort issues are attributed to “leaky construction,” when in reality they are driven by how the ventilation system is operating.

In winter, this pressure-driven airflow is typically inward, pulling cold outdoor air through joints, penetrations, and service chases where it can affect both occupant comfort and building performance. Over time, this uncontrolled infiltration can lead to cold drafts, increased heating demand, and moisture accumulation within wall and roof assemblies. These outcomes are rarely visible in airflow ratings, but they are routinely observed in both aging housing stock and newer high-performance buildings.

This is where the difference between exhaust-dominated ventilation and balanced, regenerative ventilation becomes consequential rather than theoretical. Systems that maintain near-neutral pressure while exchanging air deliberately are better suited to a wide range of building conditions, allowing ventilation to occur predictably without relying on the building envelope to compensate for imbalance.

In practice, this is where ventilation must be considered as a strategy rather than a single airflow target. In both older buildings and newer construction, effective ventilation depends on how air is introduced, exhausted, and balanced across the building over time—not just on meeting a nominal CFM value. LUNOS is designed to function as part of that broader ventilation strategy, working within existing conditions—whether that means supplementing exhaust-only systems, stabilizing buildings with minimal mechanical ventilation, or supporting balanced operation at the room or suite level—without relying on sustained pressure imbalance to move air.

In cold-climate operation, that predictability matters. Recovering heat while keeping airflow controlled and directional is not just an efficiency concern; it is a comfort, durability, and long-term building performance issue.

In practice, airflow comparisons often become further blurred by the inclusion of boost or high-speed operating modes.

Some through-wall HRVs include a boost or high-speed mode in addition to their normal operating speeds. Boost mode is a temporary override intended for short-duration events such as showers, cooking, or periods of unusually high occupancy.

In these situations, briefly increasing airflow can help remove moisture or odors more quickly. This short-term increase is intentional and situational, not a change in how the system is designed to operate day to day.

What boost mode is not designed to do is define how a ventilation system performs continuously. By design, boost operation is time-limited. Running a unit at its highest speed continuously would increase noise, energy use, and, in some cases, undermine pressure balance or thermal performance.

For this reason, boost airflow does not represent steady-state ventilation performance. It describes momentary capacity, not the airflow that governs indoor air quality under normal operating conditions.

If you’re comparing through-wall HRVs or trying to understand how airflow ratings translate into real-world performance, Lunos Canada can help clarify how these systems are designed to operate and how published specifications should be interpreted.

Reach out with questions about airflow, operating modes, or system layout—we’re happy to provide guidance based on how through-wall HRVs function in practice.