Heat Recovery Ventilator — Counterflow Exchanger

Weather & Setpoint

Outdoor temperature -2 °C

Indoor setpoint 21 °C

Heat Exchanger Core

Sensible effectiveness 85%

Airflow rate 150 m³/h

Live Readouts

Supply air (to room)

Exhaust air (to outside)

Heat recovered

Energy saved

❄ Frost-protection trip — the exhaust air leaving the core would drop below 0 °C, icing the channels. Real HRVs pre-heat or reduce effectiveness here; try lowering effectiveness or airflow.

Core running within a safe temperature range.

How it works

A counterflow (or crossflow) heat-recovery ventilator runs incoming fresh air and outgoing stale room air through adjacent channels that never mix, so heat — not moisture or pollutants — passes through the thin separating walls from the warm stream to the cold one. Its sensible effectiveness ε compares the actual temperature rise of the supply air to the largest rise physically possible:

ε = (T_supply,out − T_outdoor) / (T_indoor − T_outdoor)

T_supply,out = T_outdoor + ε·(T_indoor − T_outdoor)
T_exhaust,out = T_indoor − ε·(T_indoor − T_outdoor)   (balanced flow)

Q_recovered = ṁ·cₚ·(T_supply,out − T_outdoor),   ṁ = V̇·ρ_air / 3600
cₚ = 1005 J/(kg·K),  ρ_air ≈ 1.2 kg/m³

Effectiveness — set by the core's geometry (plate spacing, material, counterflow length); Passivhaus-certified units reach 75–95%.
Airflow rate — more air moves more mass but the exchanger has less time per parcel to trade heat, so ε itself is fixed by the core, not the flow.
Summer bypass — a damper routes air around the core when outdoor air is already cooler than the room needs, giving free cooling instead of pointless heat recovery.
Frost protection — if the exhaust-side outlet would fall below 0 °C, condensed moisture freezes inside the core and blocks it; real units throttle the supply fan or pre-heat incoming air to keep the exhaust outlet just above freezing.

This is the mechanism behind mechanical ventilation with heat recovery (MVHR/HRV/ERV) systems that make Passivhaus and nZEB buildings hold indoor comfort while still fully exchanging stale air for fresh, at a fraction of the heating energy a simple extractor fan would waste.

← Fresh outdoor air inStale room air out →

Supply stream (cold → warm) Exhaust stream (warm → cold)

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