Domestic Water Pipe Marking: Hot, Cold, and Reclaimed Water Labels

Domestic Water Pipe Marking: Hot, Cold, and Reclaimed Water Labels

 

Domestic water pipe marking is one of the most overlooked parts of any commercial plumbing installation. The pipes are simple — water in, water out — and it's easy to assume that everyone who works in the building already knows what's in them.

But commercial buildings are complex. A single mechanical room might have domestic cold water, domestic hot water, chilled water, condenser water, reclaimed water, and softened water all running side by side. Without labels, the only way to tell them apart is to trace each pipe back to its source.

This guide covers the correct ASME A13.1 colors, label text, and placement rules for all types of domestic water piping in commercial buildings.

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The ASME A13.1 Colors for Domestic Water

Domestic water — hot, cold, and reclaimed — falls into two categories under ASME A13.1:

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Potable Water (Hot and Cold) → Green / White

Domestic cold water and domestic hot water are classified as safe fluids under ASME A13.1 — non-hazardous, non-flammable, safe for human consumption.

The correct color for domestic water pipe markers is:

Green background — White text

This applies to:

• Domestic cold water (DCW)
• Domestic hot water supply (HWS or DHWS)
• Domestic hot water return (HWR or DHWR)
• Tempered water (mixed hot and cold)
• Water softener supply
• Reverse osmosis (RO) water supply
• Filtered water systems

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Reclaimed / Non-Potable Water → Purple / White

Reclaimed water — also called recycled water, gray water, or non-potable water — uses its own color under ASME A13.1 to signal that it is not safe for human consumption.

The correct color for reclaimed water pipe markers is:

Purple background — White text

This applies to:

• Reclaimed water supply for irrigation
• Gray water systems
• Non-potable toilet flush water
• Industrial process water that is not potable
• Stormwater capture systems used for non-potable applications

Purple/white is specifically defined in ASME A13.1 to help prevent anyone from using reclaimed water for drinking or hand washing. In buildings with both potable and reclaimed water, clear color differentiation between green/white and purple/white is essential for public health.

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Standard Label Text for Domestic Water Systems

The green/white color tells you the pipe carries a safe water-based fluid. The label text tells you exactly which one. That distinction is critical in a mechanical room where chilled water (also green/white), boiler feedwater (also green/white), and domestic water all run together.

For reclaimed and non-potable water systems, always include "NON-POTABLE" in the label text in addition to using the purple/white color. This double layer of identification is the most important safety measure for reclaimed water systems.

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Why Domestic Hot Water Needs Flow Direction Arrows

Flow arrows are required on all pipe systems under ASME A13.1 — including domestic water. On hot water systems specifically, flow arrows matter more than most people expect.

A domestic hot water recirculation system has two lines running together:

• Hot water supply — hot water flowing from the water heater toward the fixtures
• Hot water return — slightly cooled water circulating back to the heater to maintain temperature at distant fixtures

These two pipes look identical, run parallel, and both carry hot water. The flow direction arrow — and the supply vs. return label text — is the only reliable way to tell them apart without tracing the pipe back to the heater.

A plumber who accidentally adds a branch on the return line instead of the supply line will create a hot water dead-end that leads to Legionella growth risk. The arrows prevent that mistake.

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Legionella Risk and the Case for Clear Labeling

Domestic hot water systems are the most common environment for Legionella bacteria growth in commercial buildings. Legionella thrives in water that sits stagnant and cools to a temperature range of roughly 77°F–113°F.

Proper labeling supports Legionella risk management in several ways:

Identifying recirculation return lines. A Legionella management program requires monitoring temperatures at the return end of every hot water recirculation loop. Without labels, maintenance personnel can't identify the return without tracing the pipe.

Identifying dead legs. Unlabeled piping is more likely to have forgotten dead legs — sections of pipe that were capped off but left filled with water. Labels and system documentation together help identify and eliminate dead legs.

Preventing cross-connections. A cross-connection between a domestic water system and a reclaimed water system or a non-potable water system is a public health emergency. Purple/white labels on reclaimed lines and green/white labels on potable lines provide a first line of defense.

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Sizing Domestic Water Pipe Markers

Domestic water piping in commercial buildings ranges from ¾" branch lines at individual fixtures up to 4" or 6" mains in large buildings.

For most commercial domestic water branch lines (¾" to 1½"), the 8" × 1⅛" label is the right choice. Domestic water mains serving larger buildings (2" to 3") typically use the 14" × 2¼" label.

Insulated hot water pipes: measure the OD of the insulation jacket, not the bare pipe, for sizing.

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Placement Rules for Domestic Water Piping

All ASME A13.1 placement rules apply. On domestic water systems, focus especially on:

At the water service entry — The first label goes where city water enters the building. Label both cold water and any reclaimed water services at the point of entry.

At the water heater or boiler — Label the cold water inlet, hot water outlet, and recirculation return connections at the water heating equipment. On tankless heaters with multiple connections, label every pipe at the unit.

At every mixing valve — Tempering valves and mixing valves blend hot and cold water. Label both inlet pipes and the mixed water outlet separately.

At every zone or floor branch — When the domestic water main branches to serve different zones, floors, or wings, label each branch at the takeoff. Include the zone or floor designation in the label when useful ("DHW — 3RD FLOOR").

At every shutoff valve — Every isolation valve needs a label. This includes the main water shutoff, floor shutoffs, and equipment isolation valves.

At every wall, floor, and ceiling penetration — Both sides of each penetration.

At reclaimed water connections — Every connection to a reclaimed water system deserves special attention. Label prominently and use both the purple/white color and "NON-POTABLE" text.

Every 25–50 feet on straight runs — In long pipe chases and corridors, repeat labels at intervals.

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Reclaimed Water: Extra Attention Required

Reclaimed water systems require more careful labeling than any other domestic water pipe type. Here's why:

In buildings with dual plumbing (both potable and reclaimed water), the systems must never cross-connect. A single mislabeled valve or unlabeled branch can create a direct connection between non-potable and potable water — a public health violation with serious consequences.

In many jurisdictions, reclaimed water pipe marking is not just an ASME A13.1 recommendation — it's a legal requirement under building codes, health codes, and water authority regulations. Some authorities require specific signage at every fixture and every connection point in addition to pipe markers.

If you're working on a building with a reclaimed water system:

1. Use purple/white markers on every reclaimed water line, without exception
2. Include "NON-POTABLE" in the label text on every marker
3. Verify local code requirements for additional signage at fixtures and connections
4. Document the full reclaimed water system layout for the building owner

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Common Domestic Water Labeling Mistakes

No labels on hot water return lines. The supply gets labeled. The return gets forgotten. Both are required, and on a recirculation system, the return is just as important as the supply.

Using "WATER" alone. In a mechanical room with five different water systems, "WATER" doesn't tell anyone anything useful. Always use the full designation — DCW, DHWS, DHWR, or reclaimed water with non-potable notation.

Confusing chilled water and domestic cold water. Both are green/white. Only the text tells you which is which. "DCW" vs. "CHWS" — these labels must be right.

Missing reclaimed water labels. Skipping the purple/white color on reclaimed lines is a plumbing code violation in most jurisdictions — not just an ASME recommendation violation. Never leave reclaimed water lines unlabeled.

Labels under insulation. Hot water pipes are often insulated. The label goes on the outside of the insulation jacket, visible on the final installed surface.

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Shop Domestic Water Pipe Markers

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👉 Shop Green/White Pipe Markers (Domestic Water) → 👉 Shop Purple/White Pipe Markers (Reclaimed Water) → 👉 Need custom text for RO, softened, or specialty water systems? Order Custom Markers →

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Frequently Asked Questions

What color are domestic water pipe markers? Domestic potable water (hot and cold) uses green/white markers under ASME A13.1. Reclaimed and non-potable water uses purple/white markers.

What is the correct label for domestic hot water supply? "DOMESTIC HOT WATER SUPPLY," "DHWS," or "HOT WATER SUPPLY" are all acceptable. For systems with both supply and return lines, use "DHWS" and "DHWR" to clearly distinguish each.

What color are reclaimed water pipe markers? Purple background with white text. ASME A13.1 specifically designates purple/white for reclaimed, recycled, and non-potable water to prevent confusion with potable water systems.

Do domestic hot water return lines need labels? Yes. Hot water return lines are required to be labeled under ASME A13.1. In a hot water recirculation system, both the supply and return must be clearly identified — both for compliance and for Legionella risk management.

Can domestic cold water and chilled water use the same color? Yes — both are green/white under ASME A13.1. This is why label text is critical. "DCW" (domestic cold water) and "CHWS" (chilled water supply) must be clearly distinguished by the text on the label.

Is reclaimed water pipe marking required by law? In most jurisdictions, yes. Dual plumbing systems with both potable and reclaimed water are subject to building code, health code, and water authority regulations that require labeling and signage beyond ASME A13.1. Always verify local requirements.

What size pipe marker for a ¾" domestic cold water branch? A ¾" nominal pipe has an OD of about 1.05". That's in the ¾"–2" OD range, which requires an 8" × 1⅛" label.

What about water softener and RO water lines? Both are classified as safe/potable water systems and use green/white markers. Use specific label text — "SOFTENED WATER" or "RO WATER" — to distinguish them from raw cold water and treated hot water lines.