Free HRV Sensible Heat Recovery Efficiency Tool
Calculate CSA C439 Sensible Recovery Efficiency (SRE), delivered supply air temperature, thermal power recovered (BTU/hr & kW), and electric defrost pre-heater requirements.
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1. Sensible Recovery Efficiency (SRE) vs Apparent Sensible Effectiveness (ASE)
When comparing Heat Recovery Ventilators (HRVs), manufacturers cite both Apparent Sensible Effectiveness (ASE) and Sensible Recovery Efficiency (SRE). Under the CSA C439 standard (and Home Ventilating Institute HVI certification):
- Apparent Sensible Effectiveness (ASE): Measures total temperature rise of the supply air stream divided by the difference between indoor and outdoor entering air. However, ASE includes heat dissipated by the electric fan motor and core cross-leakage.
- Sensible Recovery Efficiency (SRE): The true net thermal metric. It subtracts electrical fan heat, casing heat loss, and cross-leakage, reflecting pure counter-flow or cross-flow plastic/aluminum plate heat exchanger performance. Typical high-performance HRV units achieve $70\%$ to $82\%$ SRE.
2. Thermal COP & Net Energy Savings
An HRV is one of the most cost-effective continuous HVAC devices because its effective Coefficient of Performance (thermal energy recovered divided by electric fan power consumed) frequently exceeds $15$ to $30$: $$\text{COP}_{effective} = \frac{Q_{thermal} \text{ (Watts)}}{P_{fan} \text{ (Watts)}}$$
For example, at $150\text{ CFM}$ with $-5^\circ\text{C}$ outdoor air and $21^\circ\text{C}$ indoor air, a unit with $75\%$ SRE delivers fresh air at $14.5^\circ\text{C}$, recovering approximately $1.65\text{ kW}$ ($5,600\text{ BTU/hr}$) of heat while drawing only $65\text{ W}$ of fan power—a net COP of over $25$.
3. Frost Formation & Defrost Pre-Heater Requirements
As warm, humid indoor air passes through the exhaust channels of the core, it gives up its heat. If the exhaust air temperature drops below its dew point ($T_{dp}$), moisture condenses into liquid water. In cold climates (outdoor temps $< -5^\circ\text{C}$ / $23^\circ\text{F}$), the exhaust plate surfaces can fall below freezing ($0^\circ\text{C}$), causing condensate to freeze into frost that rapidly blocks the core.
While recirculating damper defrost cycles can temporarily thaw the core, they unbalance building pressure and shut down fresh air ventilation. Installing a modulated electric pre-heater on the incoming outdoor air duct sized to temper incoming air to $-3^\circ\text{C}$ to $-5^\circ\text{C}$ ensures continuous, balanced ventilation without frost accumulation.
Frequently Asked Questions
What is the difference between an HRV and an ERV?
An HRV (Heat Recovery Ventilator) only exchanges sensible (temperature) heat between exhaust and supply air. An ERV (Energy Recovery Ventilator) transfers both sensible heat and latent heat (moisture) through a permeable membrane, helping maintain indoor humidity in dry winters and reduce air conditioning load in humid summers.
Why does my HRV discharge cool air into living spaces during deep winter?
Even with 75% SRE, at -15°C outdoor temperature, delivered air will enter at around +12°C (54°F). If discharged directly into occupied rooms, this feels like a cold draft. Modern installations connect HRV supply ducts to the furnace/air handler return trunk, blending tempered air throughout the home.
How do I calculate air change per hour (ACH) requirements for ventilation?
ASHRAE 62.2 requires minimum continuous ventilation calculated by Q_tot = 0.03 * A_floor + 7.5 * (N_bedrooms + 1). For a 2,000 sq ft home with 3 bedrooms, this requires roughly 90 CFM continuous ventilation.
How often should HRV core and filters be cleaned?
Intake foam/polyester pre-filters should be vacuumed and rinsed every 2 to 3 months. The polypropylene or aluminum heat exchange core should be soaked in warm soapy water annually to remove dust and maintain rated SRE efficiency.