Lesson 7: Water Supply, Part 2: Hot Water and Backflow Protection

Reading

Chapter 6 finishes with two jobs. Section 607 sets the rules for hot water: where it is required, how far it may travel, and how expansion is handled. Section 608 protects the drinking water from anything that could flow backward into it: which device fits which hazard, how high a vacuum breaker sits, and where only an air gap will do. Houston enforces the Uniform Plumbing Code (Lesson 1), so treat these numbers as the 2021 IPC and check the adopted code before you write a correction on a real permit.

Where hot water is required (607.1)

In residential occupancies, hot water must reach fixtures for bathing, washing, cooking, cleaning, laundry and maintenance. In nonresidential occupancies, hot water is required for the culinary, cleansing, laundry and maintenance uses, but fixtures for bathing and washing may be served with hot or tempered water. That is why many small commercial restrooms have one tempered supply to the lavatories.

Tempered water and temperature control (607.1.1, 607.1.2)

Tempered water is controlled by one of four means: a limiting device to ASSE 1070 set to not more than 110°F, a thermostatic mixing valve to ASSE 1017, or a water heater listed to ASSE 1082 or ASSE 1084. A heater thermostat alone counts as the limiting means only if the heater meets ASSE 1082 or ASSE 1085 (607.1.1).

Distance, circulation, and thermal expansion (607.2, 607.3)

The developed length of hot or tempered piping from the source to the fixtures that need it may not exceed 50 feet. A recirculation loop or heat trace counts as a source. A demand recirculation pump starts on a user signal, presence, or flow, and must keep the water entering the cold pipe to no more than 104°F (607.2.1.2); the rest of circulation design goes to the energy code.

Thermal expansion is the quiet problem. When a storage water heater is fed through a check valve, pressure reducing valve or backflow preventer, heated water has nowhere to go. An expansion control device goes on the heater cold supply, downstream of all of them (607.3). Adding a backflow assembly to an old service can create this problem overnight.

Cross connections and the two kinds of backflow (608.1, 608.7)

A cross connection is any link between potable piping and a system holding water of doubtful quality, steam, gas or chemicals, where flow can go either way. Section 608.7 prohibits them unless an approved device or method protects the supply. Backsiphonage happens when supply pressure falls below atmospheric, for example in a main break, and draws water backward. Backpressure happens when downstream pressure from a boiler, pump or elevated tank exceeds supply pressure; 4.33 psi or less (a 10-foot column of water) is called low head. Hazard is graded too: pollution only spoils taste or appearance (low hazard), while contamination is an actual health hazard such as sewage (high hazard).

Choosing a device (Table 608.1)

Ask two questions: is the hazard high, and can backpressure happen? A vacuum breaker only answers backsiphonage. A double check never protects a high hazard. An RP assembly or air gap is accepted in every case.

Air gaps and vacuum breaker heights (608.14, 608.16)

An air gap is measured straight up from the lowest end of the potable outlet to the flood level rim. Table 608.16.1 sets it by the effective opening: a lavatory (1/2 inch or less) needs 1 inch away from a wall and 1 1/2 close to one, a sink (3/4 inch) 1 1/2 and 2 1/2, an over-rim bath filler (1 inch) 2 and 3. Above 1 inch the gap is two times the diameter, or three times close to a wall. An outlet is close to a wall when one wall stands within three diameters of the spout, or within four where two walls meet. Outlets with hose threads may not use an air gap (608.16.1).

Vacuum breakers are protected by height, shown in the figure. Every hose bibb, sillcock and wall hydrant needs a breaker, except water heater and boiler drain valves and clothes washer valves protected another way (608.16.4.2).

Worked example. A mop basin in a corner is filled by a spout with a 1 1/2 inch effective opening. Over 1 inch, so the gap is a multiple of the diameter. Away from walls: 2 x 1.5 = 3 inches. Two walls count as close within 4 x 1.5 = 6 inches of the spout, so use the close figure: 3 x 1.5 = 4.5 inches. A 3 inch gap fails; 4.5 inches passes.

Specific connections (608.17)

• Sprinklers and standpipes: a double check assembly or an RP fire protection assembly. Antifreeze, additives or a nonpotable second source under continuous pressure need an RP assembly.
• Boilers: a backflow preventer with an intermediate atmospheric vent; with treatment chemicals, an air gap or RP assembly.
• Lawn irrigation: an atmospheric or pressure vacuum breaker, or an RP assembly; chemical injection needs an RP assembly.
• Beverage machines: carbonated dispensers use an ASSE 1022 device or an air gap; coffee machines may also use ASSE 1024.
• High-hazard backpressure from a nonpotable line or tank: an RP assembly.

Relief port piping ends at an approved indirect waste receptor or outdoors, sized for the maximum published discharge (608.15.2.1). No potable outlet or stop-and-waste valve may be installed below grade (608.8).

Health care, disinfection, and sizing pointers (609, 610)

Section 609 adds rules for Group I-1, I-2, ambulatory care, medical offices and laboratories, including two water services for a Group I-2, Condition 2 facility. Section 610 requires a new potable system to be flushed and disinfected by the method the health authority or water purveyor prescribes. The code's own method is 50 ppm chlorine held 24 hours, or 200 ppm held 3 hours, then flushed and repeated if a bacteriological test still shows contamination. The model code also carries an optional pipe sizing appendix, but appendices apply only where specifically adopted (101.2).

Practice questions

Glossary

Cross connection

Any physical link between a potable water pipe and another system or water of doubtful quality, where flow can go either way depending on pressure.

Backsiphonage

Backflow caused by supply pressure falling below atmospheric pressure, drawing water backward into the potable system.

Backpressure

Downstream pressure higher than the supply pressure, pushing water back into the supply. Low head backpressure is 4.33 psi or less.

Air gap

The unobstructed vertical distance through free air from the lowest end of a potable outlet to the flood level rim of the receptacle.

Flood level rim

The edge of a receptacle over which water would overflow; the reference line for air gaps and vacuum breaker heights.

Critical level

The height reference point below which backflow could occur; the bottom of the device if it is not marked.

Tempered water

Water mixed to a limited temperature, controlled at not more than 110 degrees F by a limiting device, mixing valve or listed water heater.

Developed length

The length of a pipe measured along its centerline, including fittings, rather than in a straight line.