Using PEX pipe at a water heater ensures flexible, corrosion-resistant connections, improving efficiency and longevity in hot water delivery systems.
PEX piping has revolutionized plumbing with its flexibility and durability. But when it comes to water heater connections, special rules apply. This guide covers everything you need to know about using PEX with water heaters while meeting code requirements.
PEX Pipe Basics
PEX (cross-linked polyethylene) is a flexible plastic tubing that’s become a popular alternative to copper and CPVC. It comes in three types:
| Type | Characteristics | Best For |
|---|---|---|
| PEX-A | Most flexible, best for freezing conditions | Hot water lines |
| PEX-B | Stiffer, more affordable | General plumbing |
| PEX-C | Least flexible, rarely used | Limited applications |
Why Plumbers Choose PEX
- Easier installation with fewer fittings
- Resists freezing damage better than rigid pipes
- Lower material costs than copper
- Corrosion-resistant
- Handles temperatures up to 200°F
Water Heater Connection Rules
Most building codes restrict direct PEX connections to water heaters. The International Residential Code (IRC) specifically states:
“PEX shall not be installed within the first 18 inches (457 mm) of piping connected to a water heater.”
Key Reasons for the 18-Inch Rule
- Heat exposure: The first 18 inches near the heater experience the highest temperatures
- Pressure fluctuations: Water heaters create pressure spikes during heating cycles
- Safety margin: Provides protection if the temperature and pressure relief (TPR) valve fails
Exceptions to the Rule
Some areas like Quebec don’t apply the 18-inch rule for electric water heaters. Always check your local codes.
Proper Installation Methods
When connecting PEX to a water heater, follow these steps:
1. Use Approved Transition Fittings
Connect the water heater’s metal nipples to PEX using:
- Copper stub-outs (most common)
- Stainless steel flex connectors
- Dielectric unions (when connecting to dissimilar metals)
2. Maintain Proper Clearances
Keep PEX at least:
- 18 inches from gas water heater flues
- 6 inches from electric heater elements
- 3 feet from furnace flue pipes
3. Install Shutoff Valves
Place shutoff valves within 6-16 inches of the water heater connections for easy service.
Special Considerations by Heater Type
Gas Water Heaters
Take extra precautions with gas models:
- The flue gas temperatures can exceed 300°F
- Use metal piping for the first 18 inches
- Consider direct vent models for easier piping
Electric Water Heaters
Electric units are more PEX-friendly:
- No combustion gases to worry about
- Lower surface temperatures
- Still follow the 18-inch rule unless local codes allow otherwise
Tankless Water Heaters
- Use copper for the first 18 inches of both hot and cold lines
- Install a recirculation line if needed
- Follow manufacturer’s clearance requirements
Pressure and Temperature Ratings
PEX has different ratings based on conditions:
| Condition | Pressure Rating | Temperature Limit |
|---|---|---|
| Water | 160 psi @ 73°F | 200°F |
| Chlorinated Water | 80 psi @ 140°F | 180°F |
| Recirculating Loops | 80 psi @ 140°F | 180°F |
Common Installation Mistakes
Avoid these errors when using PEX at water heaters:
1. Direct Connection Without Transition
Never connect PEX directly to the water heater’s nipples. Always use an approved transition method.
2. Ignoring Clearance Requirements
PEX too close to heat sources can melt or degrade over time.
3. Missing Shutoff Valves
Valves are crucial for maintenance and emergency shutoffs.
4. Improper Support
Support piping within 12 inches of transitions to prevent stress on connections.
Expert Tips
- Use PEX-A for best heat resistance near water heaters
- Consider manufacturer guidelines for specific products
- Insulate hot water lines to improve efficiency
- Label pipes clearly for easy identification
By following these guidelines, you can safely use PEX in water heater installations while meeting all code requirements. Always consult local building officials when in doubt about specific applications.
