What is ACH (or Heat is leaving my house HOW FAST?!?)

What is ACH

by Erik North on July 25, 2011


OK, I’ll admit up front that like algebra, you will never see ‘ACH’ in your daily life. Like, ever.

Maybe reading ASHRAE technical documents or attending a energy auditing training conference. But on an everyday basis? Not a scrap.

But ACH affects your home daily and can be one of your biggest home heating expenses.

ACH is an acronym that stands for Air Changes per Hour. Simply this is how much of your home’s air volume naturally changes per hour.

Sexy, no? Air, being made of oxygen and nitrogen molecules, moves. A lot. It is actually constantly on the go, getting pushed around by pressures and temps. Energy auditors need to measure this movement to determine how quickly or slowly the air is moving. In a different way, the ACH measures how air infiltrates the home, and specifically measures how much air is turned over in one hour.

The ACH for each house changes depending on the volume of air inside the home. So, your home’s ACH will differ from your neighbor’s. Also, the ACH measurement will change depending on the insulation and the air-tightness of the building. Energy auditors love the word air-tightness! Basically, the less extra air that is being turned over in your home, the less air leakage you are experiencing. So, this all boils down to the fact that you save money by only heating and cooling the air your home needs.

As you can imagine, the ACH is a mathematical formula (and I apologize in advance for this) dependent on air volume. How do you figure out the ACH of your home? Well, there are actually two ways to do it:
use the Building Air Flow Standard (BAS)
… OR …
determine the building’s Occupancy Standard.

Building Air Flow Standard

The Building Air Flow Standard can be very complicated! It’s a bit difficult to explain in a short way, but I will try. It is based on the ASHRAE (American Society of Heating, Refrigeration and Air Conditioning Engineers) ventilation standards. These standards set an airflow requirement of 15 cubic feet per minute (cfm) of air per person. The Building Air Flow Standard is also set according to the volume of the home at .35 air changes per hour.

So … the air changes per hour that you must have in your home is a bare minimum of 15 cfm or 35% of your home’s air volume must change each hour. Whew!

Occupancy Standard

The second way to find your home’s ideal ACH is to use the Occupancy Standard. Occupancy is the number of people in your home, and we all breathe! So, this standard is determined by the number of bedrooms plus one, taking into account a standard master bedroom with two occupants. Pretty neat to think that just you living in your home has such an impact on the ideal airflow calculation!

The Almighty Blower Door

Thankfully, I do not have to have these mathematical formulas in my head! So, I am not thinking and calculating while I walk about your house while giving you an energy audit. The main tool for measuring the airflow of your house – the blower door.

My blower door measures CFM50 or how much air is flowing through the house under 50 pascals of depressurization. This tests the air leakage of the building envelope (The walls, ceiling and basement of your house). From that we can calculate Minimum CFM50, the current ACH and minimum ACH.

We know how tight (or leaky) the house is currently, and how tight we can make it and still maintain plenty of fresh air.

Lots of Math to Save You Money

Yes, it is a lot of math. And I’m glad I have my blower door (and energy modeling software and spreadsheets) to help me with these calculations. The blower door assists me in measuring how much air leakage is occurring in your home and where. Once I determine that, then I can tell you exactly where you’re letting the most air, heat and money escape from your home.

Why waste money on excess lost air? Both the dollars and the air are flying right out your window (and attic and walls and chimney chase)! The math helps determine the ACH and how much it can be reduced to save you money.

{ 1 comment… read it below or add one }

Jeff Alligood December 11, 2014 at 12:59 pm

Hey there!

I really like the info on your blog and I feel like you are definitely the right guy to contact about a question I have. What do you do when the house tests in at .35ACH or lower but their bills are still high. And what is more important, the BAS or the ACH because I know you can go 30% below the BAS before you are required to bring in outside air but recommended to bring in outside air if it falls in that 30% range.

Would we air seal in a circumstance such as this? What do you recommend for houses that are already tight. And what do you recommend for HRV if need be?



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