What is a chain actuator?

The chain that is rigid one way and coils the other

An ordinary chain can only pull. The chain in an actuator has links shaped with a flat shoulder on one side, so when the sprocket drives them out of the housing each link butts against the last and the whole run goes stiff. It becomes, in effect, a telescopic strut that stores itself as a coil.

Reverse the motor and the sprocket draws the chain back. As each link passes the sprocket the shoulder clears, the link is free to hinge, and it curls into a spiral inside the case. Open the sash and you see a straight bar. Close it and that bar has disappeared into a box 250mm long.

Inside the housing, part by part

There is not much in there, which is the reason these units last. A small direct-current motor, typically running at 24 volts. A worm and wheel reduction gearbox, which multiplies torque and also makes the drive very hard to back-drive, so the sash is held closed without drawing any current. A sprocket that engages the chain. A guide channel that curls the chain into its coil. Limit sensing at each end of travel, either a mechanical microswitch or, on better units, current sensing in the control electronics.

Current sensing is the arrangement worth having. Instead of stopping at a fixed physical point, the control watches the motor load and stops when the load rises, which means the sash stops when it meets its gasket rather than when it meets a switch that may have drifted.

Stroke length, and how far the sash really opens

The number quoted on a datasheet is the stroke: how far the chain extends. Common domestic strokes are in the 200mm to 400mm bracket, with longer options for smoke ventilation. The stroke is not the same as the opening you get.

Because the actuator pushes at a fixed point on the sash, the geometry converts stroke into an angle. A 300mm stroke on a wide sash gives a modest angle and a generous free area. The same stroke on a narrow sash gives a steeper angle but a smaller opening. If ventilation rate is the objective, the figure to ask for is the free area in square metres at full travel, not the stroke in millimetres.

Why the chain type suits a rooflight in particular

The constraint on a rooflight is not force, it is length. A linear actuator with a 300mm stroke needs at least 300mm of housing to retract into, and there is nowhere in a rooflight frame to hide that. A chain actuator retracts into its own coil, so the housing stays short whatever the stroke.

The second reason is the ceiling. A rooflight is looked at from directly below, all day, by people sitting under it. A chain unit tucked into the head of the frame reads as part of the frame. A rod actuator with brackets at both ends reads as machinery. On a domestic ceiling that matters more than most homeowners expect until they see it.

Concealed in the frame, or bolted to the face

There are two mounting arrangements and they are not interchangeable after the event. A concealed actuator is built into the head of the outer frame by the manufacturer, wired internally, and is the arrangement on a factory-supplied electric roof window. Nothing is visible from the room except the sash itself.

A surface-mounted actuator is a separate box screwed to the frame or to the kerb, with a bracket on the sash. It is what gets used when a manual unit is converted, when the sash is a bespoke size, or on a lantern where the opening leaf was never designed for a motor. It works perfectly well and it looks like what it is. Our page on retrofitting an electric opener covers that route.

Force, sash weight and the wind it closes against

Actuators are rated in newtons of push and pull force, and both figures matter for different reasons. Push force has to lift the sash weight resolved along the direction of travel, which on a shallow pitch is most of the weight. Pull force has to draw the sash down and compress the gasket, against whatever wind pressure is acting under the open leaf at the time.

Double glazing runs around 25kg per square metre and triple glazing considerably more, so a large triple-glazed sash on a 15 degree pitch can outgrow a small actuator quickly. This is a specification decision taken from the sash weight and pitch, not a guess, and getting it wrong shows up as a unit that opens slowly on a cold day and eventually stalls.

Voltage, current and what the electrician needs

Domestic rooflight actuators run at low voltage, almost always 24V direct current, fed from a control unit that takes a 230V supply. The actuator itself is not connected to mains. What the electrician needs to know before first fix is the location of the control unit, the location of a fused spur and isolator for it, the cable type and route to each rooflight head, and the number of units on each output.

We install rooflights. The electrical work is carried out by a qualified electrician, and we set out the requirement so the cable is in place while the ceiling is open. First fix wiring covers the sequence in full.

How a chain actuator ages, and how it ends

These are undemanding devices. There is no maintenance beyond keeping the chain track free of dust and keeping the gasket clean so the mechanism is not fighting a sticky seal. The chain runs dry by design and should not be oiled, because oil holds grit.

When one does reach the end of its life it usually announces it: slower travel, a change in the noise, or a sash that stops short of closed because the current sensing is reading a tired motor as an obstruction. On a factory-integrated unit the actuator is generally replaceable from inside without disturbing the flashing, which is worth knowing when you are weighing the twenty-year cost. We install and replace rooflights across Oxford and up to twenty five miles by road, we have fifteen years in the trade, and our workmanship carries a ten-year guarantee.