What Is SFP Value?
Table of contents
SFP stands for Specific Fan Power (”Specifik Fläkteffekt” in Swedish) and describes how much electrical power the fans in a ventilation system use in relation to how much air is ventilated.
The SFP value is given in units kW/(m³/s), i.e. kilowatts per cubic metre of air per second.
The lower the SFP value, the less electricity is required to transport a given amount of air.
In connection with FTX units, the term SFPv. It refers to the specific fan power of the unit, i.e. the same principle as SFP but for a single unit instead of the whole ventilation system.
How is the SFP calculated?
The SFP is calculated by dividing the total electrical power of the fans by the greater of the supply air flow or the exhaust air flow:
SFP = total fan power (kW) / maximum air flow (m³/s)
If the fans together use 0.6 kW and the maximum air flow is 0.3 m³/s, the SFP value is:
0.6 / 0.3 = 2.0 kW/(m³/s)
A lower result means that the system transports the air more energy efficiently.
What is a good SFP value?
There is no single number that is best in all houses. A lower SFP value is better, but the comparison must be made at the same airflow and approximately the same pressure drop.
As a guideline, an FTX system with heat recovery is usually considered good if the SFP or SFPv is around or below 1.5 kW/(m³/s). If you come down towards 1.3 kW/(m³/s) or lower, it is very good.
At the same time, the value should be achieved without compromising on proper ventilation, sound level or comfort. Therefore, it is better to compare two units at the same operating point than to look only at a single catalogue value.
What affects the SFP value?
1. construction of the unit
The efficiency of the fans and the internal pressure drop in the unit affect how much energy is required to move the air.
2. channelling
Long ducts, many bends and too small dimensions increase the pressure drop in the system. This forces the fans to work harder, which increases the SFP value.
3. filters and dirt
When filters become dirty or the pressure drop in the system increases over time, fans need more power to maintain the same airflow.
4. Adjustment and operating point
A properly adjusted system provides the right airflow without unnecessarily high fan power. Therefore, two seemingly similar installations may have different SFP values in practice.
How to get the real SFP value
Data sheets normally show SFPv at a specific airflow and a specified external pressure drop. To get a more realistic value, you therefore need to specify the correct airflows and a reasonable duct pressure for your particular dwelling. For FTX units, it is often SFPv that is displayed in the calculation programme.
Below are two examples from our own data runs. They clearly show that SFPv is affected by both airflow and pressure drop - and that a smaller dwelling will not automatically have a lower value.
Example 1: Villa of about 120 m²
In the example below we use our FTX unit EvoAir A100S G3 in our calculation programme EvoCalc.
For a 120 m² house, the basic level of ventilation is 0.35 × 120 = 42 l/s. In this run, a few extra litres were added, giving 45 l/s supply air and 50 l/s exhaust air wide 100 Pa channel pressure.
The fan effect was 24 W + 26 W = 50 W. The largest air flow is 50 l/s, thus 0.05 m³/s. Then the calculation will be:
0.05 kW / 0.05 m³/s = 1.00 kW/(m³/s)
This shows that a slightly higher airflow does not automatically result in a lower SFPv value. If the unit is operating efficiently at the selected operating point, the result can still be low.
📄 Basis for the data run: villa 120 m² (120 sq ft)
Example 2: Apartment (three rooms)
In the example below we use our FTX unit EvoAir A60T G2 in our calculation programme EvoCalc.
In the apartment example, we assumed 15 l/s from bathrooms, 15 l/s from the kitchen, 5 l/s from wardrobes and 2 l/s extra for negative pressure. It provided 37 l/s exhaust air. The air was set to 10 l/s + 10 l/s + 15 l/s, thus 35 l/s supply air. The duct pressure was set to 70 Pa.
The fan effect was 20 W + 22 W = 42 W. The largest air flow is 37 l/s, thus 0.037 m³/s. Then the calculation will be:
0.042 kW / 0.037 m³/s = 1.14 kW/(m³/s)
So here the SFPv is higher than in the house example, despite lower airflow and lower duct pressure. This is a good example of why you should always look at the whole operating point - not just the size of the home.
📄 Basis for the data run: Apartment (three rooms)
A customised data run in EvoCalc
Want to test yourself with other air flows and pressure drops? You can do your own data run with our free EvoCalc tool directly on our website: Acetec energy calculation
Why is SFP important when choosing an FTX unit?
The SFP value affects the running cost of the ventilation system. A lower value means that less electricity is needed to move the same amount of air.
At the same time, SFP should not be assessed in isolation. Heat recovery, noise level, capacity reserve and how the whole system is dimensioned also influence how good the solution will be in practice.
How to keep your SFP value down over time
- Change filters as recommended
- Ensure that the system is correctly adjusted
- Avoid unnecessarily high airflows
- Make sure the ducting and dimensions are right from the start
- Have the installation carried out by a qualified installer
Frequently asked questions
Is lower SFP always better?
Basically, yes, because lower SFP means lower power output for the same air transport. But at the same time, the system needs to provide proper ventilation, low noise levels and good comfort.
Are SFP and SFPv the same thing?
Not really. SFP is used for the ventilation system as a whole, while SFPv is often used for the unit itself. The principle is the same, but the designation may differ depending on whether you are looking at the system or the unit.
Does channelling affect the SFP value?
Yes, they do. Long ducts, many bends or too small dimensions increase the pressure drop and may increase the SFP value.
Can the SFP change over time?
Yes, they do. Dirty filters, changing pressure drops or incorrect adjustment can cause fans to work harder, increasing energy consumption.
