Mechanical Systems and Ventilation
Mechanical Systems and Ventilation
This domain tests balanced ventilation, heat and energy recovery, distribution, space conditioning, and domestic hot water in a very low-load building. The exam favors design intent, commissioning, and control of moisture and pressure.
Core rules and concepts
- Use balanced mechanical ventilation with an HRV or ERV. HRV transfers sensible heat; ERV also transfers moisture, which matters in humid or very cold climates.
- Ventilation rate must satisfy the applicable standard and occupancy; a common passive-house rule is about 30 m³/h per person, but the project standard governs. Boost kitchen and bath exhaust locally without unbalancing the whole house.
- Keep ducts inside the envelope, short, insulated, and sealed. Duct leakage to outside is an energy and moisture penalty.
- Recovery efficiency is not the same as system efficiency: fan power, defrost, and distribution losses reduce delivered performance.
- Heating and cooling should be small, low-temperature systems sized from the modeled peak load; avoid oversizing.
Worked example
A house needs 120 m³/h ventilation. Outdoor air is −5°C, indoor 20°C, and the HRV has 80% sensible recovery. Supply temperature ≈ −5 + 0.8[20 − (−5)] = 15°C. Ventilation heat loss = 0.34 × 120 × 25 × (1 − 0.8) = 204 W. Without recovery it would be 1,020 W. That 816 W difference is why recovery efficiency and defrost strategy belong in the model and the commissioning checklist.
Traps
- Confusing HRV and ERV; ERV is not automatically better everywhere.
- Ignoring frost protection and its effect on ventilation.
- Running exhaust-only ventilation and calling it balanced.
- Leaving ducts in an unconditioned attic.
- Commissioning airflow by eye rather than measured balance.