Rusty water at the main drain: corrosion and the five year internal pipe assessment in older DC condo sprinkler systems

A wet pipe sprinkler system in a condo building is black steel pipe full of city water that never moves. It sits there for years, and steel in standing water does what steel does. Most of the time the result is cosmetic, a brown slug at the start of the annual main drain test and some scale in the inspector's test connection. Some of the time it is a pinhole leak over a unit owner's ceiling, a head that will not flow because its orifice is packed with tubercles, or a branch line so narrowed that the hydraulic calculation on file no longer describes the system. This article is about how to tell which case a building is in, what the maintenance standard requires on a five year cycle, and what the board does with the answer.

Why steel corrodes in a sprinkler system

Two mechanisms do nearly all the damage. The first is oxygen. Every time a system is drained and refilled, for a repair, a head change, or a renovation, fresh water brings dissolved oxygen in with it, and that oxygen attacks the pipe wall at the trapped air pockets that form at every high point. A system that has been drained a dozen times for a dozen unit renovations has been through a dozen corrosion cycles. The second is microbiologically influenced corrosion, bacteria that colonize the pipe wall and eat through it from under a deposit, which is what produces the pinhole that appears in a pipe that looks fine from outside. Newer systems address the oxygen side with an air vent at the high point, which the current installation standard requires on metallic wet systems, and older systems have no such vent.The materials article explains why rowhouse systems in CPVC largely sidestep all of this.

The signs, in the order they usually appear

Discolored water at the main drain test is first, and it is on the record every year because the test is annual. Scale in the inspector's test connection and a slower than expected flow there is second, and the main drain article covers what a normal test looks like so the abnormal one stands out. Third is the weep, a damp spot at a threaded joint or a coupling, usually in a corridor or a mechanical room where the pipe is exposed. Fourth is the pinhole, which arrives as a stain on a ceiling and a unit owner on the phone, and the responsibility article covers who pays for the ceiling. The sign nobody sees until a fire is the plugged head. A head packed with corrosion product opens on heat and delivers a trickle. That is the failure the five year internal assessment exists to catch before a fire does.

What the five year assessment is

The maintenance standard that DC FEMS enforces on sprinklered buildings requires an assessment of the internal condition of the piping on a five year interval. In practice the contractor opens the system at a flushing connection or removes a section of branch line at the end of a run, and a head or two from the areas most likely to hold sediment, and looks inside. A clean pipe closes the item. Scale, tubercles, or slime triggers an obstruction investigation, which is a wider sampling of the system to find out how far the condition extends.This is a scheduled item like the five year gauge replacement and the backflow test the backflow article describes, and it belongs in the inspection contract the service agreement article covers. A board that cannot find the last one in the building's file should assume it has not been done.

What the answer leads to

Repair, replace, or repipe

A single pinhole on an exposed line is a clamp or a replaced section, and the repair or replace article covers that judgment for any system. The condo question is what happens when the pinholes keep coming. Patching a system that is corroding from the inside buys months, and every drain and refill for a patch brings in the oxygen that caused the last one. At some point the honest answer is a repipe of the worst risers and branch lines, done in phases so the building keeps protection, and that is a number for the reserve study the reserve article describes. There is no published price on this site for a partial repipe, because it depends on how much of the system is open to view. For scale, a full retrofit of a sprinklered space runs $2 to $7 per square foot per the cost page, and a repipe of exposed corridor and garage piping in an existing building sits well under the top of that band because the heads and the layout already exist.

Heads age too

The assessment is about pipe, but the same visit is the right time to pull a sample of heads, because corrosion product ends up in them and because the heads themselves have a testing schedule by age that the recalled heads article covers alongside the recall lists. A building that discovers heavy corrosion in the pipe and then leaves twenty five year old heads on the ends of it has done half the job.

What a unit owner can do

Ask the board for the date of the last internal assessment and the last main drain test report. Those two documents answer most of what this article raises. If the answer is a blank look, the inspection intervals article lists everything that should be on file and how often, and a board that is missing the five year items is usually missing the annual ones too. A buyer reading a resale package should look for the same two dates, and the buying article says where they hide.

Quick answers