The optimal piping size for tankless water heaters typically ranges from 3/4 inch to 1 inch to ensure adequate flow and efficiency.
Proper pipe sizing is critical for tankless water heater performance. Undersized pipes restrict flow, while oversized pipes waste energy and increase costs. This guide covers everything you need to know about piping for tankless systems.
Why Pipe Size Matters for Tankless Heaters
Tankless water heaters require specific flow rates to activate and maintain operation. The right pipe size ensures:
- Proper water pressure throughout your home
- Efficient heater activation and operation
- Minimal pressure drops between fixtures
- Optimal energy efficiency
Minimum Flow Requirements
Most tankless units need at least 0.5 GPM to activate. Navien models like the NPE-240A2 have this minimum flow requirement. Insufficient flow can cause:
- Short cycling (frequent on/off operation)
- Temperature fluctuations
- Reduced heater lifespan
Recommended Pipe Sizes
These are the standard recommendations for tankless water heater piping:
| Application | Minimum Pipe Size | Ideal Pipe Size |
|---|---|---|
| Main supply line | 3/4″ | 1″ |
| Branch lines | 1/2″ | 3/4″ |
| Recirculation line | 1/2″ | 1/2″ |
Why 3/4″ is Standard for Main Lines
3/4″ piping provides the best balance of flow capacity and pressure maintenance. According to Rinnai specifications, this size:
- Supports flow rates up to 7-11 GPM (typical household demand)
- Minimizes pressure drop over distance
- Works with most residential water pressures
Factors Affecting Pipe Size Selection
1. Simultaneous Fixture Use
Calculate your peak demand by adding flow rates of fixtures likely to be used together:
- Shower: 2.5 GPM
- Kitchen faucet: 1.5 GPM
- Washing machine: 3.0 GPM
- Total: 7.0 GPM
2. Pipe Length and Layout
Longer pipe runs require larger diameters to maintain pressure. For runs over 50 feet, consider upsizing:
- 50-75 feet: Increase by 1/4″
- 75-100 feet: Increase by 1/2″
3. Water Pressure
Homes with lower pressure (below 50 PSI) may need larger pipes. High pressure (over 70 PSI) can use smaller pipes but requires a pressure regulator.
Installation Best Practices
Gas Line Sizing
Gas supply pipes must also be properly sized. As noted in Navien’s documentation, 1/2″ gas pipe can be used if properly sized for length and pressure drop.
Recirculation Systems
For systems with recirculation pumps, use dedicated 1/2″ return lines to maintain efficiency.
Pipe Material Considerations
Common materials for tankless systems:
- PEX: Flexible and freeze-resistant
- Copper: Durable but more expensive
- CPVC: Affordable but brittle in cold
Troubleshooting Flow Issues
If experiencing flow problems:
- Check for undersized pipes
- Look for kinks or restrictions
- Verify main shutoff valve is fully open
- Consider a low-flow showerhead if pressure is marginal
Proper pipe sizing ensures your tankless water heater delivers consistent, reliable hot water while operating at peak efficiency.
