Degrees of hazard: health and non-health hazards

When Maya brought her completed survey to Luis, he handed it back with a red pen. "Every line needs a hazard rating," he said. "Health hazard or non-health hazard. That rating decides the device. Get it wrong one way and the owner pays for protection they don't need. Get it wrong the other way and somebody drinks boiler chemicals." This lesson explains how backflow hazards are rated, how the rating connects to the choice of device, and how to rate the cross-connections Maya found at Kingsway Tower.

Why hazards are rated

Backflow devices are not equally protective, and they are not equally expensive to buy, install, test and maintain. An air gap is the most protective, but it breaks the pressure and needs a tank or pump to restore it. A reduced pressure principle assembly is very protective, but it discharges water through its relief port, needs a drain, and must be tested every year. A double check valve assembly is cheaper and does not discharge, but if both checks foul at the same time it gives no warning. A vacuum breaker is simple but protects only against backsiphonage. Rating the hazard lets codes and utilities match the level of protection to the level of risk, so the most protective devices go where a failure could kill and simpler devices go where a failure would mean bad taste.

Two questions decide a rating. First, what is on the other side of the connection, and what would happen to a person who drank it? Second, how likely is backflow at this connection: can it happen by backsiphonage, backpressure or both, and how often does the equipment create the conditions? A dangerous substance behind a connection that cannot backflow by backpressure may still need a lower class of device; a mild substance behind a connection with a booster pump needs a device that holds against pressure.

The two US categories

US plumbing codes, including the International Plumbing Code that Ohio's code is based on, sort cross-connections into two degrees of hazard. Many utilities and textbooks use the older pair of terms pollutant and contaminant for the same idea.

• Low hazard, also called a non-health hazard or pollutant: the substance could make the water look, smell or taste bad, but would not cause illness or death. Plain water in a sprinkler system without chemicals, or potable water in a storage tank used for non-drinking purposes, are typical examples.
• High hazard, also called a health hazard or contaminant: the substance could cause illness or death if drunk. Boiler water with treatment chemicals, sewage, medical and dental waste, pesticides, lab chemicals and industrial process fluids are typical examples.
• Some utilities and states describe an in-between group for containment, but the plumbing code itself works with these two levels, and a connection that does not clearly fit the low group is treated as high.

Utilities often publish lists of facility types that are automatically treated as high hazard for containment, such as hospitals, labs, mortuaries, car washes and buildings with auxiliary water supplies. The list is a starting point; the actual rating depends on what the survey finds inside.

From rating to device

Once a cross-connection is rated, the code points to the acceptable devices. For high hazards, an air gap or a reduced pressure principle assembly is required where backpressure can occur; for backsiphonage-only conditions an atmospheric, pressure or spill-resistant vacuum breaker may be acceptable for either hazard level, as long as its installation rules are met. For low hazards, a double check valve assembly is generally acceptable against backpressure, and the same vacuum breakers for backsiphonage only. The code's backflow table lists each device, the hazard level it suits and whether it handles backpressure, backsiphonage or both.

The phrase "or better" matters. A higher-rated device is always acceptable in place of a lower one. Many designers specify reduced pressure assemblies for all premise isolation in commercial buildings, even where a double check would satisfy the code, because the relief valve gives visible warning of a failure and because tenants change over the life of a building.

# A simple hazard-and-device lookup (US terms; check the code for the full rules)
RULES = {
    ("high", True):  "air gap or RP",
    ("high", False): "air gap, RP, AVB, PVB or SVB",
    ("low", True):   "DCVA or better",
    ("low", False):  "AVB, PVB, SVB or better",
}
connections = [("boiler plant fill", "high", True), ("fire sprinkler", "low", True),
               ("terrace hose bibb", "high", False), ("dental clinic supply", "high", True),
               ("cooling coil fill, plain water", "low", True)]
for name, hazard, backpressure in connections:
    print(f"{name:30s} {hazard:5s} -> {RULES[(hazard, backpressure)]}")

boiler plant fill              high  -> air gap or RP
fire sprinkler                 low   -> DCVA or better
terrace hose bibb              high  -> air gap, RP, AVB, PVB or SVB
dental clinic supply           high  -> air gap or RP
cooling coil fill, plain water low   -> DCVA or better

A lookup like this is a study aid, not a design tool. Real selection also considers the installation rules for each device, the utility's own requirements, whether the device must be testable, and whether it is listed for the specific equipment.

Judgment calls

Some connections are hard to rate. A hose bibb is a classic example: the hose could end up in a swimming pool, a bucket of soapy water, a puddle or a pesticide sprayer. Codes resolve this by treating hose connections as a hazard that needs at least a hose connection vacuum breaker, no matter what the hose is used for today. Fire sprinkler systems are another: plain water that sits for years becomes discolored and depleted of chlorine, and some systems contain antifreeze or corrosion inhibitors, so utilities rate them differently. A dental office can be rated in the highest hazard class because of the vacuum system and chair lines, even though most of its fixtures are ordinary sinks.

When in doubt, a plumber should ask the authority having jurisdiction, which may be the building department, the utility's cross-connection inspector or both. Rating a connection higher than needed costs money; rating it lower than needed can cost lives and the plumber's license.

Ratings change

A rating is not permanent. A tenant change can turn a low-risk office floor into a high-risk lab. Adding glycol to a hydronic system raises its rating. Removing chemical treatment can lower it. Cross-connection programs require resurveys when buildings change use, and good programs resurvey on a schedule. Every time Maya's crew works on a system with a backflow preventer, Luis has her ask the same question: is the device still right for what is on the other side?

Worked examples of rating

Ratings become easier with practice. Raj gave Maya a list of real connections from past jobs and asked her to rate each one and explain why. The point was not to memorize a table but to think through what could get into the water and how bad it would be.

A residential lawn sprinkler system with no chemical injection was her first case. Water standing in the buried piping can pick up soil, fertilizer, animal waste and bacteria, so most programs treat it as a health hazard, and every program does once fertilizer or chemical injection is added. A drinking fountain fed directly from the potable supply has no hazard at all. A commercial dishwasher is often treated as a health hazard because of detergents and food soil, and a photo processor or a laboratory sink with chemical drains is a health hazard because toxic chemicals are present.

Some ratings depend on details. A fire sprinkler system with no additives, made of approved pipe and with no connection to other water sources, is usually a non-health hazard because the water in it is stale but not toxic. The same system with antifreeze, a chemical additive, or a connection to a pond or tank becomes a health hazard. A boiler with no treatment chemicals may be a non-health hazard, but a boiler with corrosion inhibitors or glycol is a health hazard. A hose bibb is rated by what the hose might reach, which is why so many programs treat any hose connection as a potential health hazard.

Who decides

The plumber, the surveyor and the designer all propose classifications, but the authority having jurisdiction decides. For devices inside the building, that is the plumbing inspector enforcing the state plumbing code. For containment at the service, it is the water utility running its backflow program. In most cases they agree, but where they differ, the stricter requirement applies, because a device that satisfies both is the only one that will pass both inspections.

When a rating is uncertain, Raj's rule was to rate it higher and document why. The cost of a better device is small compared with the cost of contaminated water, and an inspector is far more likely to accept a conservative choice than to approve one that leaves doubt. Maya wrote that rule at the top of her notes.

Ratings are also a matter of record. When Maya rated a connection, she wrote the reason next to it in one sentence, such as "boiler treated with corrosion inhibitor, toxic" or "dish machine, detergent and food soil". If the building later changed, or an inspector questioned the rating, the reason showed exactly what had been assumed. A rating without a reason is hard to defend and easy to get wrong the next time the building is surveyed, because the next surveyor cannot tell whether the original rating was a judgment or a mistake.