Air Change Per Hour Simplified

New buyers of air purifiers are often confused by the marketing trick of recommended room size. Terminology such as "air exchanges per hour" is often employed.

The terms surrounding air change rates have two separate contexts;

  • 1. The air tightness of a home.
  • 2. The airflow producing power of a room purifier or HVAC system.

    The air-tightness of a home is measured in air changes per hour (ACH); the number of times the home's air is replaced from outside in an hour.

    ACH, in this context, refers to the building's tightness and resistance to outside infiltration. Also known as the exchange rate per hour, it can also be expressed in cubic feet per minute.

    This is an energy engineering function of the whole house or specific room, requiring sophisticated pressurized measurements. A rough estimate is all that can be provided, as external factors such as wind speed, site exposure, and temperature differentials are included.

    Typically air change rates, for existing homes, average between one and two per hour. Newer construction tends to be tighter, with exchanges below 1.0 common.

    The tightest new construction can have air change rates of .35 hourly. This degree of energy efficiency usually requires a forced exchange of inside air, often a heat recovery ventilator.

    An older home without storm windows and weather sealing might go as high as 8 ACH.

    I built my home, with the goal of hyper-insulation and tight sealing, using fiberglass insulation. The fiberglass contaminated the entire living space and was removed. I found the tight design collected pollutants, especially volatile chemicals. Now my home is deliberately designed to ventilate, using a second story exhaust fan. Of course this would be difficult in a northern climate.

    Air Changes in Room Air Purifiers

    Changes hourly is used to rate the power of portable purifiers.

    For room air purifiers, the air change per hour rate is calculated by dividing the room's volume in cubic feet by the machine's airflow rate in cubic feet per minute. This result, minutes per change, is then divided into 60 (minutes per hour) to give the air exchange rate.

    For example, you have a ten by 14 foot child's bedroom with an eight foot ceiling. The volume is 10x14x8 or 1120 cubic feet. An air purifier with advertised Clean Air Delivery Rate (CADR) or airflow volume quoted in cubic feet per minute of 130 cfm is considered. Let's say the builder recommends a room size of 200 square feet. Will this air cleaner really be adequate for our 140 sq. ft. bedroom?

    So we divide the 1120 cu. ft. volume by 130 cfm, which gives 8.6 minutes per air change. 60 minutes divided by 8.6 gives us just under 7 ACH. So the "200 sq. ft." air purifier is just about right for our 140 sq. ft. room.

    Whether it is perfectly sized will depend on pollutant source levels and types in the room and the noise levels of the air purifier. Room size ratings are based on high speed operation, a stretch for many air cleaners.

    A Handy, Easy Rule for Air Changes

    Suppose we are just starting out, and want to know what range of purifier performance would provide a minimum 6 air changes per hour in our 140 sq. ft. room. Well, again using an eight foor ceiling to get 1120 cu. ft. We multiply this by 6 to get the volume which must be changed every hour: 7620 cu. ft.

    Dividing this by 60 minutes/hr gives us 112 CFM as the required power for our purifier. But look, we divided by 60 after multiplying by 6, which is the same as dividing by 10. Notice that the required airflow in cubic feet per minute (112) is the same as the room volume (1120) divided by 10.

    So the easy method is;

  • calculate the room volume, length times width times ceiling height,
  • divide room volume by 10 by shifting the decimal point one place to the left,
  • get the size machine your room really needs (for 6 changes hourly), in cubic feet per minute.

    Clean Air Delivery Rate (CADR) is usually just a little less than CFM airflow, so these may be substituted.

    Now you have a starting point. Noisy purifiers will almost always end up turned down, so I consider noise emissions alongside power. Automatic, sensor driven, quiet air purifiers can be installed in a larger space than noisy, manually-switched machines.

    Please remember to remove sources before relying on a properly powered purifier.

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