What Airflow Does Your Heat Recovery Unit Need? Calculator + Sizing Guide

The question most people ask before buying a heat recovery unit isn't "which model" but "what size". And it's a good question, because an undersized unit fails to keep fresh air at a comfortable level, while an oversized one costs more to buy and wastes energy needlessly during operation.

The correct answer isn't a rough guess, it's a simple calculation based on the home's volume and the number of air changes per hour — the method most commonly used in mechanical ventilation with heat recovery (MVHR) design. The calculator below runs this calculation and immediately recommends the right model.

How the volume and air changes per hour calculation works

The method calculates the required airflow by multiplying the home's total volume (floor area × ceiling height) by a coefficient called "air changes per hour" (ACH). It is the method most commonly used by designers for mechanical ventilation with heat recovery, because it accounts for the actual size of the space rather than rough estimates based on occupants.

A coefficient of 0.5 air changes/hour is considered the minimum for very airtight homes, without risking excess humidity or CO₂ buildup. A coefficient of 0.7 is the standard recommendation for comfort in typical homes. A coefficient of 1.0 is used for dense occupancy or for people sensitive to air quality (allergies, asthma).

This method works well for anything from a studio apartment to a large house or a mixed residential-commercial space, because the result scales with the building's actual volume rather than rough estimates about rooms or occupants.

Why correct sizing matters

An undersized heat recovery unit fails to provide sufficient air exchange — humidity builds up, stale odours appear, and in severe cases mould can develop on windows or cold walls. Over time, the home loses exactly the benefit the system was installed for: fresh air without heat loss.

An oversized unit, on the other hand, costs more to purchase than necessary and may run inefficiently at constantly reduced flow, without recovering the energy-efficiency advantage it was chosen for in the first place.

The price difference between a correctly sized unit and one size up is generally far smaller than the cost of downgrading or upgrading the system after installation — which is why getting the calculation right before purchase is far more efficient than adjusting afterwards.

Residential or commercial unit — where the line is drawn

For most apartments and houses, an airflow up to 600 m³/h is sufficient and is covered by the standard residential range. Above this threshold — very large houses, homes with many occupants, or mixed residential-commercial spaces such as small guesthouses — the calculation generally points toward the commercial range, which offers units from 800 m³/h upward, with comparable recovery efficiencies but much higher airflows.

If the calculator's result above is close to this threshold, the safest decision is a direct technical conversation, not a choice based solely on the calculated figure — since other factors come into play at the boundary between the two ranges (ductwork layout, number of distribution zones, acceptable noise level).

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