Backup power and what happens in a cut

What happens in the first second

Three things stop at once and it is worth separating them.

The drive stops, so the sash freezes in position. The control system loses its supply, so the wall pad goes dead and any wired sensor stops reporting. And any programmed schedule stops running, so a timer that was going to close the unit at six in the morning will not.

What does not happen is any movement. There is no fail safe closing action on a standard rooflight actuator, and there is no fail open action either. Both exist as specialist products for smoke ventilation, which is an entirely different specification, and neither is what a domestic rooflight has. The mechanics behind that are on the actuator page.

Open, closed, or somewhere in between: three outcomes

Everything that follows depends on which of three states the unit was in.

Closed. Nothing to worry about. The sash is on its gaskets, the unit is weathertight, and it will remain so indefinitely. A cut lasting three days changes nothing.

Open a small amount for background ventilation. Usually harmless. A sash at 50 to 100mm on a pitched slope sheds normal rain over the top of the frame, and the drainage channel deals with the rest. Driving rain from the wrong direction is the exception.

Fully open. This is the state worth planning for, because a sash at full stroke on a shallow slope will take water into the room in any serious rain, and there is no sensor working to close it.

Why the cut and the rain arrive at the same time

The awkward coincidence is not a coincidence. Overhead distribution networks fail in high wind, and high wind in this country arrives with frontal rain behind it. The Forest of Dean, the Cotswold escarpment villages and the rural vale all lose supply in the same weather that puts water on the glass.

That is the specific scenario backup exists for. Not a summer afternoon cut with the unit open for ventilation, which is a non event, but a February gale at nine at night that takes the supply out and then rains for six hours onto an open sash. A rain sensor is only as good as the power feeding it, which is set out on the rain sensor page, and in a cut it has no power at all.

Battery backup, and what it is actually rated to do

A backup battery for a rooflight actuator is not an uninterruptible supply for the whole system. It is a small reserve sized for a limited number of movements, and the specification is normally expressed as a number of full cycles rather than a run time.

Typical listed products hold enough charge for something in the order of ten to twenty full open and close cycles, and they are designed to keep that reserve for the length of a realistic outage rather than for a week. Their purpose is narrow and sensible: on loss of mains, the system has enough energy left to bring the sash to a safe closed position and to allow a handful of deliberate movements afterwards.

Some systems close automatically on loss of supply when a backup pack is fitted. Others hold position and wait for a command from a battery powered control. The two behaviours are different and the difference is a specification decision, so it is worth asking which one is being supplied rather than assuming.

Where the battery lives, and getting to it later

The pack sits with the control gear rather than in the sash, and where that gear ends up is decided at first fix. Common positions are the loft void beside the unit, a cupboard on the landing, or a service void in the reveal.

The mistake is siting it somewhere that made sense on the day and is unreachable afterwards. A pack buried above a plastered reveal on a stairwell four metres up is a pack that will not be changed when its cells reach the end of their life, and cells do. Somewhere accessible without a tower is worth insisting on.

The related point is that a backup pack has an indicator, usually a small light, and an indicator nobody can see is an indicator nobody acts on. Siting it where a person passes it is part of the design.

Manual override, and what it actually gives you

Most current systems have a means of moving the sash without power, and it is worth understanding the two forms it takes.

The first is a release that disconnects the chain from the sash bracket, usually a clip or a pin at the sash end. Once released the sash swings free and can be pulled shut and latched by hand exactly like a manual unit. Reconnecting it is a matter of driving the chain back to the closed position once power returns and clipping it on.

The second is a mechanical override on the actuator itself, sometimes a crank or a knurled wheel, which winds the chain in slowly by hand. It is slow, deliberately so, and it works.

Either way the sash has to be within arm’s reach for the override to be of any use, which is the whole difficulty on the units that most need it.

Overriding a unit that is out of reach

A rooflight over a stairwell or a double height space cannot be released by hand from the floor, and that is the honest limit of manual override.

Three arrangements answer it, in ascending order of sense. A pole with a hooked end that can reach the release clip, which works to about three metres and no further. A backup pack that closes the unit automatically on loss of supply, so the question does not arise. Or, where a whole property is regularly affected, a small uninterruptible supply feeding the rooflight control circuit, which keeps the system fully alive including its sensors.

Which of those is appropriate is a conversation at survey, and it turns on the position of the unit and on how often the supply at that address actually goes.

Solar units, and why a cut passes them by

A solar operated rooflight has its own battery and its own charging module on the sash, and it is not connected to the house supply at all. A mains failure has no effect on it whatsoever, and the remote control keeps working because it runs on its own cells.

That independence is the strongest argument for solar in the rural parts of the county, and it is a separate argument from the usual one about avoiding a cable run. On a Forest of Dean cottage where the supply goes several times a winter, a solar unit is the one opening in the house that behaves identically whether the lights are on or not. Battery capacity, charging behaviour on north facing slopes and expected service life are on the solar rooflight page.

Power cuts in the Forest of Dean and the rural vale

This is not an abstract risk in parts of Gloucestershire. Coleford, Cinderford, Mitcheldean, Lydbrook and the smaller Forest villages sit on long rural feeders with a good deal of overhead line running through woodland, and trees and wind are what take supplies down. The vale villages north of Gloucester and out towards Newent are in a similar position.

Households in those places already know their own pattern, and it is worth saying at survey rather than being polite about it. Where the answer is that the power goes several times each winter and occasionally for most of a day, backup stops being an accessory and becomes part of the specification, particularly on any unit that cannot be reached.

Central Gloucester, Cheltenham and the larger estates are largely underground and see far less of it, which is a legitimate reason to take a different view on the same product.

Storms, and the sequence worth having agreed in advance

A named storm gives a day or two of warning, which is enough to make the whole subject manageable.

The sensible sequence is to close and secure every powered unit before the weather arrives, rather than relying on a sensor to react once it is here. That is a single press on most systems and it removes the entire problem. Where a unit is on a humidity or timer programme, that programme wants suspending for the duration, because an automatic opening at two in the morning during a gale is not what anybody intended when they set it.

The habit is easy to establish and easy to forget, which is why it belongs in the handover along with everything else rather than being left to be worked out later.

Escape openings, and a loss of supply

Where a roof window is the designated escape opening from a room in the roof, it has to open by hand. That is a fixed requirement and it does not change because the unit is powered.

In practice that means the manual release on an escape window is not an optional accessory, it is the mechanism by which the window performs its statutory function during exactly the conditions in which it is most likely to be needed. It has to be reachable from inside the room, operable without tools, and understood by whoever sleeps there. A demonstration at handover and a note kept with the Building Control paperwork are worth more than anything written in a manual in a drawer.

The dimensional requirements themselves are unchanged: 0.33 square metres clear, at least 450mm in both directions, and the bottom of the opening between 800 and 1100mm above the floor.

Smoke ventilation is a different product entirely

It comes up often enough to be worth separating. An automatic opening vent used for smoke control in a stairwell is a certified product with a certified control panel, a dedicated battery standby rated in hours, and a defined opening time under fault conditions.

That is a specification driven by fire engineering on a building where it is required, and it carries documentation, commissioning and periodic testing obligations. A domestic rooflight with a backup battery is not that, is not sold as that, and should never be described as though it were. Where a design genuinely calls for smoke ventilation, the correct product is specified as such from the outset.

When the power comes back

Most systems come back and simply sit there. The actuator does not move on restoration, which is deliberate, because a sash driving itself at the moment the lights come on would be alarming and occasionally dangerous.

What may need attention is the programming. A control unit that lost power will often have kept its pairings, since those are stored in non volatile memory, but may have lost its clock, so timed schedules run at the wrong hour until the time is reset. Rain sensors resume as soon as they are powered. Where a backup pack was used, it begins recharging, and a full recharge is typically a matter of hours rather than minutes.

The one thing worth doing after a long cut is a full open and close cycle under observation, to confirm the sash is still closing evenly onto its gaskets.

Battery service life, and replacing a pack

A backup pack is a consumable. The cells lose capacity steadily whether or not they are ever called on, and a pack that has sat on float charge for years may hold a fraction of its rated cycles by the time it is needed.

Manufacturers generally quote a service life in the region of four to five years for the packs used on domestic rooflight systems, and the sensible approach is to treat that as a planned replacement rather than waiting for a failure to reveal it. Replacement is a plug in operation where the pack was sited accessibly, and a scaffold tower where it was not, which is the argument for accessible siting made earlier.

Servicing intervals for the rest of the system, including the cells in remotes and wall controls, are set out on the maintenance page.

Specifying backup at order stage rather than adding it later

Backup is far cheaper as part of the original order than as a second visit, and on an out of reach unit the difference is not marginal, because the second visit needs the same access as the installation did.

Four questions settle it at survey. How far the unit is from anywhere a person can stand. Whether the supply at that address is reliable. Whether the unit is likely to be left open unattended, which for a night purge programme it certainly is. And whether the room below has a finish that water would damage, which for a plastered stairwell reveal or a timber stair it does.

Where the answers point to backup, it goes in the specification with the unit, the control gear and the cable route, and it appears in the fixed price after the survey rather than as an addition later. The rest of how a job is quoted is on the costs page.