Which statement about heating, ventilation and air conditioning systems is true?

• A. Exhaust fans move local air and moisture out; HRVs transfer heat to incoming air.
• B. ERVs bring in fresh air with large changes in humidity.
• C. Residential AC systems usually exchange outside air with the outside.
• D. Boilers move air within the building.

Answer: (A)

Ventilation in buildings works by expelling stale air and bringing in fresh air while keeping energy use in check. Exhaust fans remove local air and the moisture it carries to the outside. An HRV (heat recovery ventilator) then takes heat from that exhausted air and uses it to warm the incoming outdoor air, so you get fresh air without a big energy loss. The idea here is that you ventilate effectively while conserving heat. The other statements aren’t generally accurate: ERVs do transfer both heat and moisture, but they don’t necessarily cause large humidity shifts; residential AC systems are usually a closed loop that doesn’t routinely exchange outside air; and boilers move and heat water, not air, so they don’t move air within the building.