MVHR systems aren’t designed for cooling due to low air exchange rates (0.5 air changes per hour) compared to air conditioning (4+ changes). Increasing airflow to cool would waste energy, as MVHR replaces indoor air with outdoor air instead of recirculating it. Effective cooling would require impractical temperatures (-20°C) and cause draughts, making separate AC systems more efficient.
1. The Fundamental Design Limitation of MVHR
Mechanical Ventilation with Heat Recovery (MVHR) excels at retaining heat from stale indoor air and transferring it to fresh incoming air during colder months. However, its core design focuses on energy-efficient ventilation, not active cooling. Unlike air conditioning (AC), which recirculates and chills existing air, MVHR constantly replaces indoor air with outdoor air. This process inherently limits its cooling potential because it cannot “reuse” cooled air, making it energy-inefficient for summer use.
2. Air Change Rates: Why MVHR Can’t Compete with AC
- MVHR: Provides 0.5 air changes per hour (ac/h), meaning full air replacement takes ~2 hours.
- AC: Delivers ~4 ac/h, replacing air every 15 minutes.
To match AC’s cooling capacity, an MVHR system would need to operate 8x harder, consuming far more energy and generating excessive noise. Even if this were feasible, the system would exhaust cooled air outdoors almost immediately, negating any benefit.
3. The Physics of Cooling vs. Ventilation
Cooling requires either:
- Recirculating chilled air (AC’s method), or
- Introducing cold air continuously.
MVHR does neither. Forced to use outdoor air, it would need to cool incoming air to extreme levels (-20°C) to offset its slow exchange rate. This isn’t just impractical—it’d create uncomfortable draughts and risk condensation in ducts.
4. Summer Bypass: A Partial Workaround
Some MVHR systems include a “summer bypass” mode, which stops heat recovery and draws in cooler outdoor air at night. While this can reduce indoor temperatures, it’s only effective if outdoor air is cooler than indoors—a rarity during UK heatwaves. Moreover, bypassing heat recovery sacrifices energy efficiency, undermining MVHR’s primary purpose.
5. Why MVHR and AC Work Better Separately
Pairing MVHR with a dedicated AC system avoids conflicts. While MVHR refreshes air quality, AC focuses on cooling. In fact, MVHR can enhance AC efficiency by pre-cooling incoming air during summer nights, reducing the AC’s workload. This synergy ensures both systems operate optimally without overloading either.
For effective cooling in UK homes, pair MVHR with a dedicated air conditioning system—explore VENTI’s ventilation solutions to optimise your indoor air quality year-round.