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HVAC airflow - What's in a CFM?

In this episode of FAQ with AGIQ, we're going to talk about CFM - HVAC airflow and what it means to you and why you care.

Episode Transcription:
In today's FAQ with AGIQ, we're going to talk about CFM - HVAC airflow and what it means to you and why you care. Fundamentally, the airflow through your refrigeration system, through your air conditioner, or dehumidifier needs to do a set amount of work to your room. You know that - it needs to cool, and it needs a dehumidify.

In order to do that, you take a cooling coil of some kind, and you cool that airstream down so that you can apply enough work to it. In applying that work to the airstream, you've got some physical boundaries or some physics-based boundaries in how much work you can do to a given airstream. The obvious one is you don't want your airstream coming out below freezing, because then you're going to have a frozen coil and you're very rapidly not going to be able to cool anymore.

That's one of the challenges with low airflow systems, is when you're in a low airflow condition, you're trying to do a set amount of work, but you've got a relatively small amount of air to work with, which means that you need very large differences in temperature to apply that work to the airstream. That can lead to things like very low airflow conditions, freezing coils, wet ductwork…

You can end up in a situation where the air in your ductwork, the air being delivered to your space is below the space dew point. And I rather suspect that you don't have a standard operating procedure based around ensuring your ductwork isn't moldy because you're not expecting your ductwork to be wet. But it is one of the considerations of low airflow in a space. You need to be able to do enough work to that airstream in order to be able to do the work that the space needs.

In our world, we generally recommend between 300 and 400 CFM per ton cubic feet per minute, per ton of capacity. That's going to allow you to do adequate work to the airstream without vast swings in air temperature and ensure that you're not delivering air to the space that's below the dew point of the space or that's shockingly cold to your plants.

And conversely, in reheat mode, it's not going to mean that you've got a relatively small pinpoint of heat, but you've got sort of a plus or minus 10-degree Fahrenheit range on your return air. That really is the sweet spot for doing the right amount of work to the room, but not chocking plants and ensuring that you got good turnover in the space and a very stable space condition.

So that's what's in a CFM. It all matters but 300 to 400 CFM per ton is probably about the sweet spot.

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