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What should be the proper gas line size for a Rinnai RU Model Series tankless water heater, such as the RU160iN?

Determining the proper gas line size for a tankless water heater, such as the Rinnai RU160iN, is a crucial step in ensuring that the unit operates efficiently and safely. The gas line size directly affects the heater's performance, and using an incorrect size can lead to issues like insufficient heating, flameout, or even damage to the appliance. In this article, we'll explore the factors that influence the gas line size selection for the Rinnai RU Series tankless water heaters, with a specific focus on the RU160iN model.

BTU Rating:
The first factor to consider when determining the gas line size for a tankless water heater is the BTU (British Thermal Unit) rating of the appliance. The Rinnai RU160iN has a BTU rating of 160,000 BTUs per hour. This rating represents the heating capacity of the unit and is a critical parameter for gas line sizing.
Gas Pressure:
Gas appliances require a specific gas pressure to operate efficiently. The Rinnai RU Series tankless water heaters, including the RU160iN, typically require a gas supply pressure of 5.0 inches of water column (WC) for natural gas and 10.5 inches WC for propane (LP) gas. It's essential to verify the gas supply pressure at the installation location, as this will impact the gas line size calculation.
Gas Pipe Sizing:
Gas line sizing is determined based on the appliance's BTU rating, gas supply pressure, and the length of the gas line. The National Fuel Gas Code (NFPA 54) provides guidelines and tables to help determine the appropriate gas pipe size. To calculate the gas pipe size, you'll need to consider the following factors:
a. Total BTU Load: Calculate the total BTU load by summing the BTU ratings of all gas appliances connected to the same gas line. For the RU160iN, consider only its 160,000 BTU rating.
b. Gas Supply Pressure: Determine the gas supply pressure and convert it to inches WC if necessary.
c. Length of Gas Line: Measure the length of the gas line from the gas meter or propane tank to the water heater's location.
d. Pressure Drop: Calculate the allowable pressure drop from the gas source to the appliance. The typical maximum allowable pressure drop is around 0.5 inches WC.
e. Gas Pipe Sizing Table: Refer to the NFPA 54 gas pipe sizing tables, which provide pipe sizes based on the factors mentioned above. Select the pipe size that can deliver the required BTUs with an acceptable pressure drop.
Local Codes and Regulations:
It's crucial to consult local building codes and regulations, as they may have specific requirements for gas line sizing and installation. These codes ensure the safety and compliance of your installation.
Professional Assistance:
While you can perform the gas line sizing calculations yourself, it is highly recommended to seek the expertise of a licensed and experienced plumber or HVAC technician. They can accurately assess your specific installation and ensure that the gas line size is appropriate for your Rinnai RU160iN tankless water heater.

In conclusion, the proper gas line size for a Rinnai RU160iN tankless water heater, or any similar RU Series model, is determined by its BTU rating, gas supply pressure, gas line length, allowable pressure drop, and local codes and regulations. Calculating the correct gas line size is essential to ensure the efficient and safe operation of the appliance. Always consult with a qualified professional to ensure that the gas line sizing meets all requirements and standards. Failure to do so can result in performance issues and safety concerns.
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