Definition
Air changes per hour (ACH) is the number of times a home's full volume of air is replaced with outdoor air in one hour. It describes both ventilation rate and airtightness: ACH50, measured with a blower-door test at 50 pascals of pressure, rates leakage, and lower ACH50 values mean a tighter house.
Also called: ACH, ACH50
Why it matters to you
Newer homes built under the Washington and Oregon energy codes are much tighter than older Clark County houses, which saves heat but means fresh air must be supplied deliberately; ASHRAE Standard 62.2 sets residential ventilation rates. Older, leakier homes get uncontrolled air exchange instead, along with drafts and crawlspace air.
Practical guidance
If you have a blower-door result from a new-home inspection or energy audit, share it with your HVAC contractor before ventilation or equipment work. Blower-door testing, ventilation sizing, and HRV or ERV balancing are done by qualified technicians or energy raters.
About this definition
Written by the Home Energy Group team — a family-owned HVAC company celebrating 25 years of comfort & care across SW Washington and the Portland metro, licensed in Washington (WA LIC# HOMEEGI003J7) and Oregon (OR CCB# 184255). Definitions inform; an on-site diagnosis decides.
