Updated: 08.11.2026
The Reserve (also safety margin) is the surcharge on the calculated thrust of a window actuator. It covers forces that a simple calculation does not capture: seals, friction in the hardware, wind and ageing.
How much reserve is right?
- +20 %: sheltered location, interior windows
- +30 %: normal case, façade windows (recommended)
- +50 %: exposed location such as coast or tall buildings, as well as roof windows and skylights
These levels are Vent Systems guide values, not a wind load verification. Binding values are provided by the actuator manufacturer.
Example
The Window actuator calculator gives 180 N for a façade window. With 30 % reserve: 180 N × 1.3 = 234 N; the next force rating is chosen, for example a VS-KA with 250 N.
Reserve or two actuators?
From a sash width of about 1.5 m, more force alone does not help: a single actuator can twist the sash. We then recommend two actuators with Synchronous controlthat share the force.
Frequently asked questions
Can too much reserve do harm?
Not technically, but a stronger actuator costs more and is often larger. The sensible choice is the next force rating above the result including reserve.
Is the snow load included in the reserve?
No. For roof windows the snow load is calculated separately and then the reserve is added on top.
Read more
Not sure which actuator fits? Call us at 040 50694631 (Mon–Fri 8 am–5 pm) or use the actuator finder.
Related terms: Dome rooflight · Snow load · Wind load
