Heat Recovery Ventilator — Temperature-Profile Diagram

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

This is a temperature-length (T-x) diagram, the classic textbook way engineers actually design counterflow heat exchangers — instead of watching parcels travel through a duct, you watch two temperature profiles cross the exchanger's length. The supply-air profile rises from outdoor temperature to its outlet value; the exhaust-air profile falls from indoor temperature to its outlet value, running in the opposite direction along the same core:

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

T_supply(x) = T_outdoor + x·ε·(T_indoor − T_outdoor)      x: 0→1 along core
T_exhaust(x) = T_indoor − (1−x)·ε·(T_indoor − T_outdoor)

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

Shaded band — the gap between the two lines is the local approach temperature. For this idealized linear model the gap stays constant at (1−ε)·ΔT along the whole core: a wider band (lower ε) means more driving temperature difference is being wasted.
Airflow rate — moves the traveling markers faster along each profile and increases mass flow ṁ, but does not change ε itself (fixed by the core's geometry).
Summer bypass — flattens both profiles to their unmodified endpoints (no heat exchanged) — free cooling instead of pointless recovery.
Frost protection — the shaded strip below 0 °C flags the region where the exhaust profile would ice the core; real units throttle supply or pre-heat to avoid it.

The lower panel plots the same recovered-heat and percentage-saved numbers as a live bar chart plus a scrolling history strip, so you can see how they respond as you drag a slider — the 2D-diagram equivalent of watching the 3D duct's parcels warm and cool in place.

Supply profile (outdoor → room) Exhaust profile (room → outdoor)