High water pressure, pressure reducing valves and a 13D sprinkler system in Maryland.

Most of what gets written about residential sprinklers and water pressure is about not having enough. Our low pressure article covers that side. This one is about the houses at the bottom of the hill. In parts of Towson and Ruxton below the reservoir, in WSSC zones near a pumping station in Germantown or Laurel, and in neighborhoods downhill from the Frederick water tanks, the gauge on the riser reads well over 100 psi, and the plumber wants to put a pressure reducing valve on the house. Whether that helps or hurts the sprinklers depends entirely on where the valve goes and which kind of system is behind it.

How much pressure a 13D system can take

Residential sprinkler components are listed for a working pressure of 175 psi. That covers the CPVC pipe and fittings, the heads, the gauge and the control valve. No Maryland water utility delivers anything close to that to a house. A street pressure of 110 or 120 psi is high for plumbing and completely ordinary for the sprinkler side, which is designed to flow water at pressure, not to sit behind a faucet aerator. High static pressure is good news for a sprinkler design. The hydraulic calculation gets more margin, smaller pipe can carry the two head demand, and the system is less sensitive to a utility pressure dip on a summer evening.

The plumbing is the part that minds. The residential code adopted across Maryland requires a pressure reducing valve when the static pressure at the house exceeds 80 psi, because fixtures, supply hoses, water heaters and ice maker lines are rated for far less than the pipe in the sprinkler system. So in a high pressure zone the house needs a PRV and the sprinklers do not. The question is how to give the plumbing a valve without taking pressure away from the heads.

Stand alone system, put the valve after the tee

On a stand alone system the service splits at a tee just inside the foundation. One side feeds the sprinkler riser through the backflow preventer and the control valve. The other feeds the plumbing. The PRV belongs on the plumbing side, after the tee. The heads keep full street pressure, the faucets see 60 or 65 psi, and nobody has to recalculate anything. The sprinkler drawings for a new house in a known high pressure zone show the tee and the PRV location for exactly this reason, and the plumber is expected to follow them.

The problem arrives later. A plumber called out for a banging washer hose or a failed water heater relief valve measures 115 psi at the hose bib and installs a PRV in the most convenient spot, which is right after the meter, upstream of the tee. The sprinkler system now sits behind a valve set to 60 psi, and the design that assumed 115 is no longer valid. Nothing looks wrong. The gauge reads 60, the bell is quiet, and the house passes every plumbing inspection. It fails the next sprinkler inspection the moment the inspector compares the gauge to the design pressure on the riser tag, and it fails a resale inspection the same way. What the buyer’s inspector checks is in our buyer’s article.

Multipurpose system, the valve is in the design or it is a problem

A multipurpose system has no tee. The heads and the fixtures share one PEX network, so any PRV on the main reduces pressure at the heads too. In a high pressure zone that is handled at the design stage. The designer calculates the system at the PRV outlet pressure, the valve is sized so that it can pass the sprinkler demand without a steep pressure drop, and the drawings call out both. A PRV sized for a kitchen sink chokes at sprinkler flow, so the model and size matter as much as the setting.

On an existing multipurpose house, adding a PRV that was not in the design means the system has to be recalculated by a sprinkler designer before the plumber sets it. Sometimes it passes at the lower pressure with margin to spare. Sometimes it does not and the answer is a different valve or a conversion of part of the system. The difference between the two system types, and why the townhome tracts in Germantown, Owings Mills and Odenton tend to be multipurpose, is in our multipurpose versus stand alone guide.

Where the high zones are

• Baltimore County neighborhoods at the low end of a gravity zone, below Loch Raven or below the Towson tanks, where pressure at the lowest houses can run well above 100 psi.
• WSSC Water zones in Montgomery and Prince George’s where a house sits close to a pumping station or at the bottom of a zone, common in parts of Germantown, Laurel and Bowie. How the WSSC meter is sized for a sprinkler system is in our WSSC article.
• Frederick City and county systems downhill from the storage tanks, and Howard County lots along the lower Route 1 corridor.
• Well houses with a constant pressure pump set high, which is a homeowner setting rather than a utility, and a stored water tank with a booster pump, which delivers whatever the pump was sized for.

What a high gauge reading means

A riser gauge that reads 110 psi on a stand alone system in one of those zones is correct. It is not a fault and it does not need a valve. A gauge that reads higher than the street pressure is a different thing. Water in a dead ended pipe expands when it warms, and on a system with a check valve or a backflow preventer the pressure can creep well above static during a hot afternoon, then drop back at night. That creep is why some risers carry a small relief valve that dumps a cup of water into the drain now and then. A gauge that creeps past about 175 and stays there is a visit. Reading the gauge in general is in our gauge article.

Before the plumber installs a PRV

• Find the riser and identify the system type. A separate riser with its own control valve is stand alone.
• On a stand alone system, point the plumber at the tee and ask for the valve downstream of it. Take a photo of the finished work showing the tee, the riser branch and the PRV.
• On a multipurpose system, stop and call a sprinkler contractor first. The design pressure is on the riser tag or the approved drawings, and the recalculation is a short job compared with undoing a valve.
• Expect a thermal expansion tank on the water heater when a PRV goes in. That is plumbing code and it has nothing to do with the sprinklers, but it is the item plumbers most often skip.

Common questions

• Will high pressure wear out my heads? No. Heads are tested at pressures far beyond anything a Maryland utility supplies. The seals that weep do so from age or a factory defect, not from 110 psi.
• The house has a PRV ahead of the tee already. Is the system still legal? It is still a permitted system, but it is probably operating outside its design and it will be written up at an inspection. Moving the valve past the tee is usually a short repipe.
• Does the backflow preventer reduce pressure? A little, a few psi at flow, and the design already accounts for it. It is not a substitute for a PRV and it does not protect the fixtures.

High street pressure is a gift to a sprinkler system and a nuisance to the plumbing. Treat them separately. The valve that protects the water heater goes after the tee on a stand alone system and into the calculation on a multipurpose one, and the heads keep every pound the utility gives them.