United States
Hong KongWater Supply and Backflow Prevention
Water must come from an approved source.
- Backflow is prevented by an air gap - twice the diameter of the supply inlet and never less than 2.5 cm (1 inch) - or by a mechanical device such as a vacuum breaker where an air gap is impractical.
On this page6 sections
One of five questions in the facilities area, and one of the most predictable questions on the paper.
Approved source
A public water system, or a private system tested and approved. Ice is food, so it is made from the same water. Bottled water comes from an approved supplier.
When the supply is interrupted - a main break, a boil-water notice - the establishment cannot operate normally. Depending on the jurisdiction that may mean closing, or operating on an approved alternative supply with the regulatory authority notified.
Cross-connections
A cross-connection is any physical link between drinking water and a potential source of contamination.
The classic is a hose attached to a tap and left lying in a mop bucket or a floor drain. Drop the pressure in the main - a burst pipe down the street, a hydrant opened - and the water flows backwards, taking the bucket with it.
Backflow is any reverse flow. Back-siphonage is the specific case caused by negative pressure in the supply line. Some questions use the second term to see whether the first still triggers the same answer. It does.
The air gap
A vertical, unobstructed space between the water outlet and the flood rim of whatever is below it.
Twice the diameter of the supply inlet, and never less than 2.5 cm (1 inch). Both halves of that sentence are examinable, and questions have offered one inch as the whole rule when the inlet was large enough to need more.
An air gap is the most reliable protection precisely because there is nothing to fail. It is physics rather than a device.
Mechanical devices
Where an air gap is impractical - a hose bibb, a dishmachine connection - a backflow prevention device is used. Vacuum breakers on hose connections are the common example. They are tested and maintained, because a device can fail in a way a gap cannot.
Where air gaps are required
- Where a sink drains into a floor drain or a grease interceptor
- On ice machine drain lines
- On drain lines from a food preparation sink
- Between a water supply outlet and any container it fills
The ice machine one is worth remembering. A drain line connected directly to a sewer line is a route from the sewer into the ice, and inspectors find it regularly.
Hot water
Supply water at the temperatures each fixture needs. Under the 2026 FDA model, a handwashing sink must be equipped to provide at least 85°F (29.4°C), while a manual warewashing sink needs a wash solution of at least 110°F (43°C). Mechanical warewashing requirements depend on the machine and method.
An establishment that cannot supply hot water cannot wash hands or dishes, which is why a water heater failure is treated as an operational problem rather than a maintenance one.
Common questions
What is an air gap?
A vertical unobstructed space between a water outlet and the flood rim below it, sized at twice the diameter of the supply inlet and never less than 2.5 cm (1 inch).
What is a cross-connection?
Any physical link between drinking water and a potential source of contamination. A hose left in a mop bucket or floor drain is the standard example, and the one exam questions use.
What is the difference between backflow and back-siphonage?
Backflow is any reverse flow into the supply. Back-siphonage is the specific case caused by negative pressure in the line. The prevention is the same for both.
Why do ice machines need an air gap on the drain?
A drain line connected directly to a sewer line creates a route from the sewer into the ice. It is a common inspection finding and ice counts as food.
What hot water temperatures are required?
Under the 2026 FDA model, a handwashing sink must be equipped to provide at least 85°F (29.4°C); a manual warewashing sink needs a wash solution of at least 110°F (43°C). Mechanical warewashing requirements depend on the machine and method. Check the locally adopted code.