Water main breaks, boil water advisories and your home fire sprinklers in Maryland: what happens when the street goes dry.

Every January the same thing happens across the Maryland suburbs. A cold snap moves the ground, a cast iron main from the 1960s splits under the street, and a few hundred houses in Silver Spring or Catonsville or Glen Burnie wake up to no water and a crew with a backhoe at the corner. WSSC Water in Montgomery and Prince George’s, Baltimore City DPW for the city and most of Baltimore County, and the county utilities in Howard and Anne Arundel all post the same notice: service interrupted, boil water advisory to follow once pressure is restored. Owners of sprinklered houses then ask us a question nobody else gets asked. What does the sprinkler system do while the street is dry, and is there anything to do about it? This guide answers that for the three events that cut the supply: a main break, a boil water advisory, and a planned shutoff for a meter swap or a plumber.

The pipe stays full. The system cannot flow.

A home sprinkler system on public water is a closed network of pipe held full of water by the street pressure, with a check valve or backflow preventer at the riser that keeps the water from running back out. When the main breaks and the utility closes the street valves, the pressure on the house side of the meter falls to nothing. The check valve holds. The pipe above it stays full, the heads stay charged, and the gauge on the riser keeps reading whatever it trapped. Our guide to reading the riser gauge explains the trapped reading.

What the system cannot do is flow. If a head opened during the outage it would discharge the water standing in the pipe, a few gallons, and then stop. That is the honest answer, and it is the same answer for the domestic plumbing. There is no fix an owner can apply from inside the house. The outage is the utility’s problem to end, and across the WSSC and Baltimore systems that is hours, not days. Cook carefully, keep the space heaters away from the drapes, and treat the house as unsprinklered until the water comes back.

Restoration is the part that actually touches the system

When the crew finishes and opens the street valves, the main refills under full pressure and a slug of water, air and loosened sediment moves through every service on the block at once. Three things can happen at a sprinkler riser in that minute.

The gauge jumps. A main that refills fast sends a pressure spike ahead of it, and a gauge that read 70 psi before the break can momentarily read well above that. On a house with a pressure reducing valve the valve absorbs it. On a house without one the spike reaches the heads and the pipe, which are built for it, but it can also trip a pressure switch if the system has one. Our guide to high water pressure and pressure reducing valves covers the houses where the spike matters.

The bell rings. On a system with a water motor gong or an electric flow switch, the surge of water moving into a riser that had drained slightly can read as flow for a few seconds, and the bell on the outside wall goes off. It stops when the pipe settles. If it keeps ringing after the first minute, that is a different problem, and our guide to the bell outside the house tells you what to check before calling.

Dirty water arrives. The advisory is about this slug, rust and sediment scoured from the main, which is why the utility asks you to run the cold taps until the water clears. Which taps matters for a sprinklered house, and it depends on the kind of system you have.

Standalone versus multipurpose during a boil water advisory

A standalone system has its own riser and its own pipe, separated from the domestic plumbing by the backflow preventer. The sediment that came in with restoration reached the check valve and stopped. The sprinkler pipe behind it did not move, so it did not take on the dirty water, and there is nothing to flush. Sprinkler water is not drinking water in any case, and the boil water advisory has no step for a standalone system. Follow the utility’s flushing instructions for the kitchen and bath taps and leave the riser alone.

A multipurpose system is the domestic plumbing. The sprinkler heads hang off the same loop that feeds the sinks, there is no separate riser and no check valve between them, and the slug of sediment that came in with restoration moved through the sprinkler branches along with everything else. Flushing the taps the way the utility asks clears the sections of pipe water moves through daily; the sprinkler drops themselves are dead ends and keep whatever settled in them. That is not a hazard; a head that opens passes the sediment harmlessly. It matters only when a head has been dripping since the last outage, which points to debris at the seat, and our guide to noises and drips covers that call. Our guide to multipurpose versus standalone systems tells you which kind your house has if you are not sure.

Meter swaps and planned shutoffs

The quieter version of the same event is the planned one. WSSC and Baltimore City both run meter replacement programs, and a technician will shut the curb stop, pull the old meter, set the new one and reopen it inside an hour. A plumber replacing a water heater or a main shutoff does the same thing from inside. Either way the sprinkler supply is off for the duration, and the same three restoration effects follow when the valve reopens, in miniature.

Two things are worth saying to the person doing the work. First, that the house has a fire sprinkler system, so they know the second riser and the backflow device are not something to cap off or bypass. Our guide to the WSSC meter on a sprinklered housecovers why the meter size matters there. Second, where the sprinkler control valve is, so nobody closes it as part of the shutoff and forgets to reopen it. A valve left shut after a plumber’s visit is one of the most common findings on a resale inspection, and our guide to the control valve shows you what open looks like. Walk to the riser after any planned shutoff and confirm the handle is where it was.

Three checks once the water is back

Read the gauge. It should settle within a few psi of what it read before the outage. A gauge that settles to zero means the check valve passed water out during the break and the pipe is now partly empty, which is a service call. Listen for the bell. Thirty seconds of ringing on restoration is the surge; ringing that continues is flow somewhere, and you go find it. Look at the ceilings on the lowest floor over the next day. A main break is also the moment an old fitting that was holding at 60 psi meets a spike, and a stain that was not there last week is how a weep shows itself.

What this costs you

Usually nothing. The utility event itself does not damage a sprinkler system that was sound before it, and the checks above are a five minute walk. When something does show up, a gauge at zero, a bell that will not quit, a drip from a head, it is a normal repair visit, priced the way our repairs page describes, and we schedule it like any other call. If the annual visit is coming up anyway, mention the outage and we will read the gauge under flow at the test valve. Our annual inspection page covers what that visit includes.