I'm on the board of a 17-story prewar cooperative in the West Village. Several years ago, the water tank was removed from the roof and the building was directly connected to the city water main. We now have brown water and low pressure on the higher floors. Someone suggested that we place a filter over the connection to the water main in the street to clear up the brown water, but wouldn't the filter need to be constantly replaced? What would be required to restore the original roof-tank system? And how do we figure out what was done (or not done) to the plumbing from the old system?
Brown water and low water pressure can be caused by several things. The first factor to consider is the building's piping. Pipes in domestic (potable) water systems are usually made of brass, copper, or galvanized steel. Over time, galvanized steel - more so than brass or copper - tends to corrode on the inside, resulting in a buildup of rust and scale. As these buildup particles flake off, they can cause a brownish discoloration in the water.
The rust and scale buildup also reduces the opening inside the pipes, decreasing the amount of water that can flow through. As a result, available water pressure often drops as well. If your building's piping is made from galvanized steel and hasn't been replaced in a long time (or ever), chances are new piping - preferably brass or copper - will need to be installed.
If, on the other hand, the building's piping is made of brass or copper, the brown water may be caused by nearby construction work disturbing the city main that supplies water to your building. Placing a filter onto the water main at or near its point of entry into the building may help remove the discoloration. Less expensive filtration systems require regular replacement of the filters, but more advanced ones have an automated filter-cleaning capability.
One drawback to installing a filter of either type, however, is that it will cause a considerable drop in water pressure across the system. This sometimes requires the installation of a supplemental pressure-booster pump to fix. In addition, a filter on the city main won't clear up the discoloration if it's caused by corroded galvanized piping inside your building.
Roof tanks. A roof-level tank is often used to supply domestic water for kitchen and bathroom faucets, showers, toilets, dishwashers, and washing machines. The tank also typically supplies water when needed to the building's closed-loop heating system. (To prevent the possibility of cross-contamination of the potable water supply, a backflow prevention device is usually installed at the domestic connection to the heating system.)
Additionally, many domestic water roof tanks supply the building's fire standpipe system. Any residential building taller than six stories or 75 feet in height is required by the New York City Building Code to have a fire standpipe system with a minimum 3,500-gallon water reserve. Fire standpipe tanks are usually installed on the roof, either standing alone or alongside the domestic water tank. At the time your building's roof tank was removed, some accommodation should have been made to ensure a standing supply of water to the fire standpipe system if one was not already in place.
In a typical roof-tank system, the city's water main enters the building and is connected to a roof tank "fill" pump in the basement. The fill pump sends water up through a supply riser to one or more wood roof tanks, usually between 5,000- and 10,000-gallon capacity. The fill pump does not operate continually; it engages only when a float gauge in a roof tank signals that water has dropped below a certain level and the tank needs to be replenished.
Wooden roof tanks operate as gravity-based systems. Water travels down from the tank to main pipes called overheads, which run horizontally either above the ceiling on the top floor of the building or across the ceiling in the cellar. Supply risers carry water from the overheads to each floor, where they branch off to feed fixtures in individual apartments.
Some buildings were designed with multiple pressure zones, with overheads located at intermediate floor levels, or in the case of a two-zone system, at both the cellar and the top-floor levels. In a two-zone setup, down-feed risers from overheads on the top floor supply water to the upper zone, i.e., the upper-floor apartments. Similarly, up-feed risers from the cellar overhead, pressurized from the city main, supply the lower zone, or lower-floor apartments.
Removing the roof tank may have very well caused a drop in available water pressure within your building, especially on the top floors. A common misconception is that the closer an apartment is to a rooftop tank, the better the water pressure in that apartment will be. In fact, the reverse is true. In a gravity system, pressure increases as water travels in a down-feed riser, and decreases as it flows in an upfeed riser. This is why the upper floors in a building tend to suffer from lower water pressure more so than the bottom floors.
Water at city main pressure can typically supply adequate pressure for the lower-floor apartments. (Most plumbing fixtures require at least 15 to 25 psi to function properly.) When your building's roof tank was removed, a water pressure booster pump was probably installed in the cellar to increase pressure to the upper floors. Without a booster pump, the pressure on the higher floors would be even less than it is now.
Reinstalling your building's roof tank may help restore the lost pressure on the upper floors. The old fill pump and supply riser to the roof tank were probably removed with the old tank, so new ones may need to be installed with the new tank. Even if the old pump and riser were abandoned in place, it's doubtful they would be usable for the new system. Furthermore, even with a new tank in place, one or two small booster pumps may be necessary to increase water pressure on the upper floors.
Surveying the system. Before undertaking such a major plumbing upgrade, the board would be well-advised to hire a professional engineer to conduct a survey of the building's domestic water system. The report should review how the current system works, how the old system was designed (including the fire standpipe system), and list the history and scope of previous repairs.
In addition, the survey should document the condition of the system components, such as pumps, piping, and valves; identify low-pressure zones in the system; and devise ways to remediate them. Replacing corroded galvanized piping and/or installing filters as possible remedies for the brown water should also be addressed. Finally, the report should include a budget and timetable for the upgrade. Based on the findings and recommendations, and the available repair funds, the board can then decide upon the best course of action.
Rand Engineering has been providing integrated engineering and architectural services to the co-op and condo community since 1987.