Air Gap vs High Loop [Key Differences]

Plumbing systems are designed to safely transport water and waste throughout a building. Two key components in these systems are the air gap and high loop, which serve important functions in preventing contamination from occurring. Though they sound similar, the air gap and high loop work in different ways to keep potable and non-potable water separate. Understanding the difference between these two devices is critical for both homeowners and professionals involved in plumbing work.

What is an Air Gap?

An air gap is a physical separation between the end of a water supply line and the receiving vessel, such as a sink or appliance. This separation is an “air break” that prevents backflow contamination. Air gaps are commonly installed on dishwasher supply lines, ice maker lines, commercial sinks, and other applications where a hose or direct connection could allow contaminants to backflow into the potable water system.

Here’s how an air gap works:

  • The end of the supply line stops short of the sink, dishwasher, etc., creating a gap between the supply line and receiving vessel. This gap is open to the atmosphere.
  • When water flows from the supply line, it spills over the gap and into the sink. The gap allows any backflow from the sink to spill out into the atmosphere, rather than back into the supply line.
  • The size of the air gap must meet plumbing codes, which dictate a minimum vertical distance between the end of the supply line and the flood rim of the receiving vessel. This ensures the gap can effectively prevent backflow if water attempts to flow in reverse.
  • Air gaps are inexpensive, have no moving parts, and require no maintenance. However, they can be visually unappealing and lead to splashing as water cascades from the supply line into the sink.

What is a High Loop?

A high loop is a vertical section of pipe that rises above the receiving vessel before looping down to connect to the vessel. High loops allow gravity to prevent backflow and create an “air gap” within the confines of the pipe. They are commonly installed on dishwasher and washing machine drain lines.

Here’s how a high loop works

  • The drain line from the appliance makes a vertical rise to a specified height before looping back down to connect to the sink or standpipe drain.
  • The high point of the loop must be at least 18-24 inches above the flood rim of the receiving vessel, meeting plumbing code requirements.
  • When the appliance discharges water, it flows down through the drain line and up to the high loop. Gravity causes the water to then flow back down the loop and into the drain.
  • If water attempts to flow in reverse from the drain back up into the appliance, it has to flow uphill through the loop. The weight of the vertical water in the loop prevents backflow from occurring.
  • High loops contain backflow within the piping, unlike noisy and splashy air gaps. However, high loops can collect debris and require occasional flushing.

Key Differences

While air gaps and high loops both prevent backflow contamination, there are some important differences between these devices:

  • Location – Air gaps are external to the receiving vessel. High loops are part of the drainage piping.
  • Plumbing – Air gaps require an open space and indirect connection. High loops are plumbed with standard pipes and fittings.
  • Operation – Air gaps rely on an atmospheric break to stop backflow. High loops use a vertical section of pipe to stop backflow.
  • Maintenance – Air gaps rarely need maintenance. High loops should be periodically flushed.
  • Aesthetics – Air gaps are visible and can cause splashing. High loops work discretely within the plumbing.
  • Cost – Air gaps are inexpensive to install. High loops require more materials and labor.
  • Effectiveness – Both can fully prevent backflow when properly installed. Air gaps may provide a greater physical barrier.

When to Use Each Method

Because of their different capabilities, air gaps and high loops each work better for certain applications.

Air gaps are the best choice for:

  • Dishwasher supply lines
  • Commercial sinks
  • Utility sinks
  • Ice maker supply lines
  • Applications where inspection and visibility are needed

High loops are preferable for:

  • Dishwasher drain lines
  • Washing machine drain lines
  • Applications where discreet backflow prevention is desired


What exactly is an air gap?

An air gap is a physical separation or break between the end of a supply line and the receiving plumbing fixture that prevents backflow contamination. The unobstructed gap allows water to cascade while keeping drain water from backing up into the supply line.

When should a high loop be used instead of an air gap?

High loops are preferable when an air gap would be unsightly or prone to splashing. They are commonly used on dishwasher and washing machine drain lines since they contain backflow prevention discreetly within the piping.

How high must a high loop be to prevent backflow?

Plumbing codes require high loops to rise vertically 18-24 inches above the flood rim of the receiving drain or vessel. This height uses gravity to prevent water from flowing backward up the loop.

Can air gaps dry out and allow sewer gases to enter?

No, an air gap’s physical separation prevents any gases from entering supply lines. The gap is open to the atmosphere so no drying out or sewer gas entry can occur.

Do air gaps and high loops require maintenance?

Air gaps rarely need maintenance. However, high loops should be flushed periodically to prevent buildup of debris at the top of the loop which could impair their effectiveness.


Understanding the difference between air gaps and high loops is an important aspect of plumbing system design. Both utilize gaps between supply and drainage to prevent backflow contamination through different mechanisms. When installed according to code requirements, air gaps and high loops provide highly effective protection of potable water supplies. Knowing when to use an air gap versus a high loop ensures backflow prevention is handled appropriately for each plumbing application. With this knowledge, both homeowners and plumbing professionals can make informed decisions when designing and modifying plumbing systems.

Similar Posts

Leave a Reply

Your email address will not be published. Required fields are marked *