High-Rise and Booster Pump Building Backflow Testing

It comes down to backpressure. Phoenix's code defines it as pressure in the building's piping that rises above the supply pressure at the meter "by pump, elevation of piping, or steam and/or air pressure" (City Code 37-1). A tall building has both. Water standing in the risers presses down toward the street, and a booster pump can push building pressure above the city's. Either can try to drive water back through the service. Storage tanks add a pool of water the city no longer controls.

So the cities write buildings like these into their lists.

Mid-rise offices, condo buildings, hotels and medical office buildings all fall in here. Phoenix's list also names hospitals and medical buildings separately, so a medical tower is on Phoenix's list for that reason, whatever its height.

Here's the problem with a tower. Testing the service assembly closes its shutoff valves, and every floor above loses water while the gauges are read. For a hotel or a building with residents, that's a big ask.

The code answer is parallel assemblies. Tempe requires two sets installed side by side "if the water supply cannot be temporarily interrupted for the testing of assemblies" (City Code 33-104(b)). Scottsdale requires two or more in parallel when a user "requires a continuous water supply," and their combined cross-section must at least equal the supply line (Revised Code 49-63(c)). With a parallel pair, we close and test one while the other carries the building, then switch.

If your building has a single assembly, the test still happens. It just needs an outage window, posted notice to tenants or residents, and someone on site who can confirm the booster pumps are handled before the valves close. Adding a parallel assembly is an installation with a permit. See backflow installation for the permit path.

A booster pump sits downstream of the service assembly, and the two affect each other. Watts' manual for its large RP assemblies lists fluctuating inlet pressure as a cause of relief valve discharge. A booster pump that cycles hard can contribute to that kind of swing. If an RP on a pumped building spits water between tests, look at the pump controls along with the assembly. Our backflow assembly repair page covers the parts side.

Tall buildings rarely have just one. A typical mid-rise can carry the domestic service RP, a landscape PVB or RP, point-of-use devices in a ground-floor restaurant, and fire line assemblies. Each has its own report. Phoenix's report form records whether an assembly serves a fire system, landscaping or potable water, and whether it's primary or secondary protection. Keep an inventory so nothing goes untested. Our FAQ on how long to keep backflow test records covers the file.

Low pressure on upper floors is usually a pump, pressure regulator or piping issue, not the backflow assembly, unless the assembly is the wrong size. An undersized replacement is the one way the assembly itself can starve the upper floors, which is why Mesa and Scottsdale tie its diameter to the service connection. See water booster pumps for low pressure. And fire line and standpipe assemblies are tested by fire protection contractors: fire backflow assemblies and fire sprinkler work are outside what HQ does.

HQ tests, repairs and replaces the domestic backflow assemblies on mid-rise and high-rise buildings and files the results with the city. On parallel installations we test each leg without dropping the building. On single-assembly buildings we plan the outage window with your engineer or manager. You get an upfront price before work starts and 24/7 emergency service. Related: hotel plumbing, medical and dental plumbing, multifamily plumbing and commercial backflow testing.