ACH (Air Changes Per Hour) - Short #174

HVAC School - For Techs, By Techs

Bryan Orr

Training, Careers, Airconditioning, Self-improvement, Hvac, Business, Education, Refrigeration, Heating, Ac, Apprenticeship

4.8 • 985 Ratings

🗓️ 5 September 2023

⏱️ 10 minutes

🔗️ Recording | iTunes | RSS

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Summary

In this short podcast, Bryan talks about air changes per hour, also known as ACH, and what it means in HVAC design and indoor air quality (IAQ) discussions.

ACH tells us how frequently the entire volume of air in a room or structure is replaced; we are referring to the cubic feet of air leaving a space and then being replaced within that same space. If we have a balanced number of cubic feet per minute (CFM) of air supplied to and returned from the room in one hour, we would multiply that CFM by 60 to get the ACH, as there are 60 minutes in one hour. ACH should not be used to calculate heat loss and heat gain, even though BTUs are moved with air.

ACH is a practical guideline for HVAC design. Ventilation needs will vary based on the purpose of a room and the number of occupants in it, and ACH tends to be a more important factor for determining how we can meet ventilation needs in commercial and industrial structures than in residential structures, in which we mostly rely on Manual J calculations of sensible and latent BTU gains and losses. However, we should not confuse ACH with outdoor air ventilation requirements as described in ASHRAE Standards 62.1 and 62.2.

ACH also comes into play when it comes to infiltration and the tightness of an entire structure. When used in the context of blower door testing, the ACH will tell us if a building meets tightness standards. There is also a term called ACH50, which refers to air changes per hour at the standard pressure for blower door testing: -50 Pascals. ACH50 does not reflect ACH under natural conditions (ACH natural).

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Transcript

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0:00.0	Hello, this is the HVAC school podcast. I'm Brian. I had some people ask if AI was
0:12.0	reading my podcast nowadays and I don't know maybe I somewhere like a robot than I used to I'm not sure but anyway this is a short episode that is about air changes per hour or a ch and just talking a little bit about that and some
0:26.8	different things that that can mean because it's just one of those really
0:30.0	confusing things that people can get confused and can make mistakes, especially early on when you're trying to learn design and you're trying to learn about this sort of stuff and or equality all that.
0:39.2	So it's part of our indoor equality discussion, but definitely more just an informational piece.
0:44.4	It's not like I'm going to tell you exactly how to do anything new but just help you understand
0:48.4	some different terms and that sort of thing because this is a short episode, just short stuff.
0:52.3	So before we do that, we want to thank our great sponsors.
0:55.9	Refrigeration Technologies at refriggedec.com.
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1:35.0	The ESCCO Institute and the HVACR Learning Network.
1:40.0	Our friends Howard and Jerry Weiss, Jason Obshoot, Eugene Silverstein, Clifton Beck, all great guys who are huge
1:46.8	advocates for the industry are working hard to provide industry leading online content
1:51.1	for HVACR professionals. Visit HVACR professionals.
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1:57.0	to access reliable, up-to-date, and relevant digital resources
	...

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