Overview
Amply calculates the approximate or target airflow (CFM) for each room (and the overall building) if load calculations are in the project. This feature helps HVAC professionals with a starting point for sizing new, modifying existing and/or troubleshooting existing systems by translating heating and cooling loads into airflow targets using psychrometric principles.
This article explains the background behind this feature, how the calculations are performed, and how these targets should (and should not) be used in design and diagnostic workflows.
Approximate airflow CFMs are visible in the Manual J report:
What is approximate airflow CFM?
In HVAC design, CFM (cubic feet per minute) represents the volume flow rate of air delivered to a space. The amount of air required depends on:
The heating or cooling load of the room (in BTUs/hour)
The temperature difference between supply air and room air (ΔT)
Amply uses this relationship to estimate a target CFM that would be needed to deliver the BTUs/h that are required for ducted or ductless system.
How approximate CFM is calculated
We use the "psychrometric" equation for both heating and cooling sourced from Manual J8 v2.5, section 11-23, p. 108:
For Cooling:
Approximate Cooling CFM = Sensible Cooling Load / (1.1 x ACF x ΔT)
Where:
1.1 is a constant that accounts for air density and specific heat at sea level
ACF is the altitude correction factor, a number between 1.0 (sea level) and 0.0 that changes with altitude
ΔT is the temperature difference between the indoor design dr bulb temperature (75°F) and the supply air entering the room via the register or ductless head
NOTE: The assumed supply air temperature for cooling is 55°F. Actual supply air temperature will depend on the actual equipment performance and duct losses (if present).
For Heating:
Approximate Heating CFM = Heating Load / (1.1 x ACF x ΔT)
Where:
1.1, see above
ACF, see above
ΔT is the difference between the supply air temperature and the indoor room dry bulb design temperature (70°F).
NOTE: The assumed supply air temperature for heating is 100°F. Actual supply air temperature will depend on the equipment performance and duct losses (if present). For heat pumps and high-efficiency furnaces, 100°F is a common and reasonable assumption.
When and how to use approximate CFMs
Note: We report both a cooling and a heating CFM. Always pick the HIGHEST of the two values for the use cases listed below.
For example, if you have 400 CFM heating and 250 CFM cooling, use 400 CFM.
✅ Use Cases
Duct Design Guidance: Use the approximate or target CFM as a starting point when assigning airflow to each room in a Manual D duct layout.
Gut Check For Ductless Noise Concerns: The required airflow at design conditions will give you a sense of the fan speed needed. If the system needs to run at or near max fan speed to meet the load, you may run into noise issues — a common source of homeowner discomfort with ductless heads.
Troubleshooting Uneven Comfort: If a room is uncomfortable and you suspect airflow issues, compare measured supply CFM to Amply’s target. A mismatch could point to under- or over-delivery of airflow.
Installer QA/QC: Compare these targets to actual equipment airflow to ensure the system has the capacity to serve all zones.
❌ Cautions and Limitations
Not meant a substitute for Manual S: Equipment selection (Manual S) must still consider total load, capacity at design conditions, and manufacturer performance data, and fan curves.
Not a substitute for Manual D: Actual delivered air depends on fan performance, static pressure, duct size, and layout— CFM targets are starting place, NOT exact design values.