CSLB C-20 HVAC ContractorVentilation and Air DistributionMedium
A heat recovery ventilator (HRV) has a rated sensible recovery effectiveness (SRE) of 70%. On a winter day, outdoor air is 20°F and indoor exhaust air is 70°F. Approximately what temperature will the incoming ventilation air reach after passing through the HRV core, before any additional heating by the furnace?
- A65°F
- B55°F
- C35°F
- D45°F
Show answer & explanationAnswer & explanation
Correct answer: B. 55°F
Temperature difference between indoor and outdoor air is 70 − 20 = 50°F. At 70% sensible recovery effectiveness, the HRV recovers 0.70 × 50 = 35°F of that difference. Adding this to the outdoor air temperature: 20°F + 35°F = 55°F supply temperature leaving the HRV core.
Why the other options are wrong
- A. This would require nearly 90% effectiveness, higher than the stated 70% SRE.
- C. This equals the recovered temperature rise itself, not the final supply temperature.
- D. This undercounts the recovery effect; recalculating 0.70 × 50 gives 35, not enough to reach only 45°F.
HRV/ERV Sensible Recovery Effectiveness
SRE indicates the percentage of the sensible temperature difference between outgoing exhaust air and incoming fresh air that the heat exchanger core recovers and transfers to the incoming airstream.
- Supply Temp = Outdoor Temp + (SRE × ΔT)
- HRV recovers sensible heat only; ERV recovers sensible + latent (moisture)
- Typical residential HRV/ERV SRE ranges 60–80%
Memory trick: 'Recovery steals back some of the temperature difference.'