What a home fire sprinkler system does when the power goes out

Every summer a line of storms takes out power to a slice of Loudoun or Prince William for a day or two, and every winter an ice storm does the same to Fairfax. The question comes in the next morning: is the sprinkler system still working? For most houses the answer is yes, completely, and nothing needs to be done. For a few houses the answer depends on a pump, and for every house in January the real risk is not the outage itself but the cold that follows it. Here is which house you are in.

The system itself needs no electricity

A residential sprinkler head is a mechanical device. A glass bulb or a fusible link holds a plug in place, heat from a fire breaks it, and water that was already sitting in the pipe under pressure comes out. No sensor, no circuit board, no battery. The pipe is full at all times, and the pressure comes from wherever the water comes from. On a public supply from Fairfax Water, Loudoun Water, Prince William, Arlington, or Alexandria, that pressure is the street main, and the street main does not care that your house is dark. A house on public water with a standard 13D system is fully protected through any outage, for as long as the outage lasts.

Smoke alarms, by contrast, do run on electricity, which is why they carry battery backup. The sprinkler system does not need the same because it never needed the power in the first place. The discharge article explains how a head opens and how much water follows.

The parts that do use power

Around the edges of the system there are a few electrical items, and they behave differently in an outage.

• The flow alarm. Many systems ring a water motor gong, a bell driven by the water itself, and it works without power. Systems with an electric bell or a horn strobe on a flow switch lose that sound unless it runs from a panel with a battery. The heads still open and the water still flows; you just may not hear the bell outside. The alarm article covers the flow switch.
• Monitoring. If the flow switch reports to a security or fire alarm panel, that panel has a backup battery sized for a day or so, and its cellular or internet path may or may not survive the outage. After a long outage, expect a trouble light on the panel and a call from the monitoring company. That is a communication issue, not a sprinkler issue.
• The heat. Your furnace, boiler, or heat pump stops. This is the item that actually matters, and it gets its own section below.

If your house is on a well or a tank, read this part

Houses in Great Falls, Clifton, western Loudoun, and rural Prince William that are on wells have a stored water tank and a booster pump, the design covered in the tank and pump article. The tank holds its water through an outage. The pump does not run without power. So during the outage the sprinkler system is out of service unless that pump is on a generator circuit.

NFPA 13D does not require standby power for the pump in a one or two family home, which is why a new house can pass inspection without one. But if you already own a standby generator, ask the electrician to put the sprinkler pump on the transfer switch. It is a small load, on the order of a well pump or a sump pump, and it is the single most useful thing that generator can be carrying on a stormy night. Owners with a portable generator and an inlet should know which breaker feeds the pump and include it in the list of circuits they power up.

The same applies to the rare house on public water that needed a booster pump because the street pressure was low. If a pump is anywhere in your system, the outage takes it out until the power or a generator brings it back.

The real risk is the cold, not the dark

A summer outage costs you nothing beyond the pump question above. A winter outage is different. A house that loses heat in a Northern Virginia January will drop below freezing inside within a night in the exposed places: attic pipe runs, garage ceilings, pipe in an exterior wall, the crawl space. Frozen CPVC splits, and a split pipe does exactly what it sounds like when the water thaws. The freeze protection article covers the design side. During an outage the practical rules are short.

• Keep the house above freezing by whatever means you have: generator, fireplace, gas logs.
• Open the doors on cabinets and closets that hide pipe on exterior walls so room air reaches it.
• Leave the garage door shut if there are heads in the garage ceiling.
• Do not drain the sprinkler system yourself. The valve you close may be the one that shuts off the house, and a drained system has to be refilled and checked before it protects anything. If the house is going to sit cold for days, call us and we will decide together.

When the power comes back

For a house on public water, look at the riser. The gauge should read the same pressure it always reads, and nothing else needs attention. For a house with a pump, listen for it to cycle once and confirm the tank is full. If the alarm panel shows a trouble condition, it is almost always the communication path or the battery, and the monitoring company will walk you through the reset. If you hear water running anywhere it should not be, close the main house valve and call. A pipe that froze during the outage usually announces itself right when the house warms back up.

What the outage does not change

The permit, the inspection schedule, and the insurance discount all assume the system is passive, because it is. No county in Northern Virginia asks a homeowner to generator back a 13D system, and no carrier we have seen conditions a discount on it. What they do care about is that the system holds pressure and the heads are clear, which is what the routine visit on our inspections page checks. If your house has a pump, that visit also exercises it, and if a generator feeds it, we run the pump on the generator too.