What is Proper System Airflow

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While Manual J deals with residential load calculations, Manual S deals with residential equipment selection. The equipment’s goal should be to maintain 70 degrees in the winter and 75 degrees in the summer (with a 62-degree WB). These are not Manual J goals; they are goals of the entire residential design series (Manual J, Manual S, and Manual D).

When consulting codes, remember that those are MINIMUM standards. However, some of these codes refer to Manual J and Manual S, including IECC.

The sensible heat ratio (SHR) dictates the airflow you’ll need (CFM per ton). The location and equipment manufacturer can affect the SHR, but they do not determine the CFM per ton alone. You must perform a load calculation to separate sensible BTUs from latent BTUs. Then, you divide your sensible BTUs by total BTUs to yield a ratio.

Your equipment performance data will determine blower performance. The manufacturer publishes these data sets, including heating performance data and expanded cooling performance data. (Expanded performance data can also tell you the WB, which is important in terms of meeting your design goals.) Follow that guidance for equipment selection (Manual S).

You may have heard of the 400 CFM per ton rule of thumb, but it’s about as accurate as “beer-can cold.” It’s just a rule of thumb, and your calculations will help you get a more precise CFM target for equipment selection.

It’s easier to determine capacity on a heating system, as all BTU/h are sensible. However, some cooling BTUs are latent, which complicates the capacity equation. Most manufacturers give you the total and sensible BTU/h, so you may have to do some math to determine the latent BTU/h for your SHR.

Sensible heat ratio (SHR) tells us how many BTU/h are sensible out of the total BTU/h. For example, if you have 10,000 BTU/h with an SHR of 0.75, that means that 7,500 BTU/h are sensible and 2,500 are latent. When your SHR is higher, your capacity increases. Both the system and the house have a sensible heat ratio.

When determining the CFM per ton with SHR, you can use your temperature difference (TD) in that equation. Manual S has a chart that tells you the TD for a given SHR. In that equation, you divide your sensible BTU/h by the product of 1.1 and your TD. You may get a number that’s far higher than 400 CFM per ton! Once you figure out your CFM, you can size your equipment based on airflow, capacity, and oversizing guidelines. Remove any equipment that is short on capacity from consideration.

Manual S has 3 “N” sections, which refer to standards. N1 deals with definitions and general requirements, N2 deals with sizing limits, and N3 deals with OEM verification.

N1 emphasizes the importance of using OEM performance data. Do NOT use the AHRI equipment performance data for sizing. While AHRI’s figures are good for getting a general idea, no data can compare to the manufacturer’s expanded performance data when it comes to accuracy.

One of the most important tables in Manual S is Table N2-1 in section N2-5. It tells you how to select air conditioners for single-speed, two-speed, and variable-speed equipment. The table also gives you the minimum and maximum sizing ranges for air-to-air systems as a ratio, typically with a 0.9 minimum (90%) to 1.15-1.3 (115-130%). The same table for heat pumps is in section N2-6 (Table N2-2).

When adding a dehumidifier, keep in mind that the sizing value is based on 85% of the latent cooling load. When making conversions to pints per hour, you take the latent BTU/h and divide it by 1,054.

Overall, you must remember that Manual S says to meet the sensible and latent heat gains, but don’t exceed the total gain by 15% (single speed), 20% (two speed), or 30% (variable speed).

On the heating side, you need to be able to counter the sensible losses in the heat. If a heating system cannot put out enough BTU/h to exceed those losses, you may need to add another source of supplemental heat and recalculate the BTU/h output to see if it exceeds the losses. If the BTU/h in sensible gain exceeds the losses, then the system will run satisfactorily for most of the winter.

To find the heating load CFM, you take the furnace sensible output (BTU/h) and divide it by the product of 1.1 and the temperature rise (instead of TD in cooling systems).

Ed also covers:

Design software programs
Interpolation
Default TD values for CFM approximations
Sizing for budget systems
House SHR and leaky homes
Sensible losses in the winter
Modern furnace sizing

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