The 10% rule states that for every 5ft added above 10ft of supply tubing, we see a 10% reduction in airflow at the outlet.
When building a fastquote, it's important to make sure you caclulate the right CFM in your system design.
Introduction
The 10% rule states that for every 5ft added above 10ft of supply tubing, we see a 10% reduction in airflow at the outlet. The longer the supply tubing is, the lower the airflow at the outlet. The 10% rule is crucial to delivering well-designed systems when working with the Unico system.
Fully Rated Outlet
If a supply tube is 10ft long, the airflow into the tubing matches what is delivered at the outlet. This is known as a "fully rated outlet" because up to 10ft, there is no loss of airflow.
10% Rule in Action: Example
If 35CFM enters 20ft of supply tubing, only 28CFM is delivered at the outlet. This outlet is delivering ~80% (28CFM) of the airflow of a fully rated outlet (35CFM).
Design Example
If a room requires 350CFM to be conditioned, and fully rated outlets are 35CFM, 10 outlets are necessary to condition the space.
When the 10% rule is applied, those 10 outlets deliver less than 35CFM, resulting in an under-conditioned room:
1. Apply The 10% Rule
| Target Outlet CFM | Supply Tubing | 10% Outlet Rule CFM |
| 35CFM | 20ft | = 28CFM |
2. Calculate the total CFM delivered by outlets into the room
| 10% Outlet Rule CFM | Number of Outlets | CFM delivered to room |
| 28CFM | 20ft | = 280CFM |
3. Check the CFM difference between what the room requires, and what is actually delivered
| Room CFM Requirement | CFM Delivered to Room | Required vs Delivered Difference |
| 350CFM | 280CFM | = 70CFM or 80% |
Solving the design challenge
To balance a design to deliver the required airflow after the 10% rule has been applied, there are three main approaches:
- Increase the plenum length to reduce the amount of supply tubing required
- Use 2.5" supply tube to allow more airflow over the same distance
- Add more outlets to the area to deliver more airflow