How to read the sprinkler drawings and hydraulic calculation before they go to DOB

At some point before the permit goes in, the sprinkler contractor sends a PDF with a line that says please review and approve. Almost nobody reviews it. Then the heads land in the wrong room, the rear addition gets a wet head that freezes, or the calculation quietly assumed a one inch water service the house does not have. Every one of those is a revision after DOB has the set, and the permit timeline gets longer. Here is what is in the package, sheet by sheet, and the five things an owner can check in twenty minutes without knowing anything about pipe.

What the package is

A rowhouse sprinkler system in the District is hydraulically calculated, which means the pipe sizes come from math, not from a table. The drawings and the calculation go in under the building permit, as the DOB permit steps article explains, and the DOB reviewer and later the inspector hold the contractor to them. A typical set is six parts. Some contractors bind them as one PDF, some as two.

Sheet one: the cover and the design criteria

The cover sheet names the standard and its edition, the type of system, the heads being calculated, and the water supply the design relies on. For a house it should read NFPA 13D, it should say residential sprinklers, and it should state the number of heads in the design, which is two for a 13D system, or one where a room has only one head. If the cover says NFPA 13R, you have a building, not a house, and the 13D versus 13R article explains what changed.

Sheet two: the floor plans

One plan per floor, with a symbol at every head and a line for every pipe. The symbols are standard: a filled circle for a pendent, an open circle for an upright, a half circle against a wall for a sidewall, and a note for concealed. Walk the plan room by room against the house. Every closet over the size limit, every bathroom over the size limit, the stair, the landing, the cellar. The rooms that can be skipped are a short list, and a plan that skips more than those is wrong. Look at the kitchen for the head near the range and its temperature note. Look at the rear addition for a dry sidewall or a note about routing from heated space.

Sheet three: the riser detail

A drawing of the assembly where the water service becomes the sprinkler system, in order from the street. The service entry, the meter, the backflow preventer, the control valve, the main drain, the gauge, and the waterflow switch if there is one. Each part has a size and a model. This is where to check that the backflow preventer is the type DC Water accepts, which the backflow article covers, and that the control valve is drawn as a lockable or supervised type if the alarm article made you want one.

Sheet four: the hydraulic calculation

This is the sheet people skip and it is the one that matters. It is a table that follows the water from the two most remote heads back to the street, node by node, adding up what is lost on the way. At each step it lists the flow in gallons per minute, the pipe size and length, the fittings, the friction loss, and the pressure left. Three things in it are worth understanding.

• The demand. At the bottom of the table is a flow and a pressure required at the base of the riser, or at the street, for the two heads to work as listed. That pair of numbers is what the house asks the city for.
• The supply. Somewhere on the sheet is a static pressure and a residual pressure at a flow, from a hydrant flow test or from DC Water data for the street. The water service page explains where those numbers come from and why Anacostia and Tenleytown do not get the same ones.
• The margin. Supply minus demand, in psi, at the demand flow. This is the one number to find. A healthy design has a cushion of several psi so a winter pressure dip or a slightly rough pipe does not drop the system below its listing. A margin near zero means the design is on the edge, and that is the moment to ask about a larger service or a different layout, before the permit, not after.

Two details that show whether the calculation was done honestly. The friction factor, called the C value, should be 150 for CPVC and copper and 120 for black steel, and a calculation that uses 150 on a steel riser is flattering itself. And elevation shows up as a loss of a little under half a psi for every foot of climb, so a head on the third floor of a Columbia Heights rowhouse starts about fifteen psi behind one in the cellar. If that line is missing, so is the top floor.

Sheet five: the supply curve

A graph with two lines. The supply curve slopes down from the static pressure as flow increases. The demand point sits at the flow and pressure from the table. The demand point has to sit under the supply curve, and the gap between them is the margin from the sheet before, drawn so a reviewer can see it in a second. On a multipurpose system the domestic fixtures that have to be added to the demand show up here too.

Sheet six: the schedule

A list of every head model with its identification number, K factor, temperature, and finish, plus the pipe material, the hanger type, and the valve and backflow models. This is the sheet to hold against the quote. If the bid said concealed white heads in the living spaces and the schedule says chrome pendents, the fix is one email now. The head types article explains the columns.

Five things to check before you say approve

1. Every room on the plan is a room in the house, and the rooms with no head are on the allowed list.
2. The rear addition and any unheated space show a dry head or a route from heated space.
3. The head type and finish on the schedule match the quote you signed.
4. The water service on the riser detail is the service the house actually has, or the quote says who pays to change it.
5. The cover names the standard, the edition, the designer, and a date, and the margin on the calculation is not zero.

Twenty minutes with the set is worth it. A revision after DOB review costs a week at best. A change after the rough in inspection costs an opened ceiling. The published range on the cost page assumes a set that was right the first time.

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