How to Size a Tankless Propane Water Heater: A Comprehensive Guide

Sizing a tankless propane water heater is a crucial step in ensuring your home has an efficient and reliable hot water supply. This comprehensive guide will walk you through the process of determining the right size tankless propane water heater for your needs, considering factors such as maximum temperature rise, flow rate, and the number of hot water devices you expect to use simultaneously.

Step 1: Determine the Maximum Temperature Rise

The maximum temperature rise is the difference between the coldest water temperature in your area during the winter months and the desired hot water temperature, typically 120°F. To determine the maximum temperature rise, subtract the wintertime cold-water temperature from the set temperature.

For example, if the wintertime cold-water temperature in your area is 45°F and the desired hot water temperature is 120°F, the maximum temperature rise is 75°F (120°F – 45°F). This is an important factor to consider, as it will determine the tankless propane water heater’s required heating capacity.

Step 2: Calculate the Required Flow Rate

The required flow rate is the amount of hot water you need at any given time. To calculate the required flow rate, add up the flow rates of all the hot water devices you expect to use simultaneously.

For instance, if you expect to simultaneously run a hot water faucet with a flow rate of 0.75 gallons per minute (GPM), a shower head with a flow rate of 2.5 GPM, and a washing machine with a flow rate of 2 GPM, the required flow rate is 5.25 GPM (0.75 GPM + 2.5 GPM + 2 GPM).

It’s important to consider the maximum flow rate of your tankless propane water heater, as it must be able to meet the required flow rate without compromising the desired temperature rise.

Step 3: Determine the Number of Simultaneous Hot Water Devices

The number of hot water devices you expect to use simultaneously is another crucial factor in sizing a tankless propane water heater. This will help you determine the required flow rate and ensure your chosen unit can handle the demand.

Consider the following common hot water devices and their typical flow rates:

Device	Flow Rate (GPM)
Bathroom Faucet	0.5 – 1.5
Shower Head	1.5 – 2.5
Kitchen Faucet	1.5 – 2.5
Washing Machine	2.0 – 3.0
Dishwasher	1.0 – 1.5

By adding up the flow rates of the devices you expect to use simultaneously, you can determine the required flow rate for your tankless propane water heater.

Step 4: Select the Tankless Propane Water Heater

Once you have determined the maximum temperature rise and the required flow rate, you can select the tankless propane water heater that meets your needs. Tankless propane water heaters are rated by the maximum temperature rise possible at a given flow rate.

To size a tankless propane water heater, use a manufacturer’s sizing chart, which is typically available online or in manufacturer product catalogs. The chart will list the maximum temperature rise during the coldest period of the year and the flow rates for different temperature rises.

For example, if you live in an area with a maximum temperature rise of 75°F and a required flow rate of 5.25 gallons per minute, you would need a tankless propane water heater with a flow rate of at least 5.25 gallons per minute at a temperature rise of 75°F.

When selecting a tankless propane water heater, consider the size of your household and the number of hot water devices you expect to use simultaneously. Larger households may require more than one tankless unit to meet their hot water needs.

Technical Specifications

Tankless propane water heaters are rated by the maximum temperature rise possible at a given flow rate. The technical specifications for a tankless propane water heater will include the following:

Maximum Flow Rate: The maximum flow rate is the amount of hot water the unit can produce at a given temperature rise. This is typically measured in gallons per minute (GPM).
Temperature Rise: The temperature rise is the difference between the incoming water temperature and the desired hot water temperature, typically measured in degrees Fahrenheit (°F).
Energy Factor (EF): The energy factor is a measure of the unit’s energy efficiency. A higher energy factor means the unit is more efficient, which can lead to lower operating costs.
BTU Input: The BTU input is the amount of heat the unit can produce per hour, measured in British Thermal Units (BTU).
Venting Requirements: Tankless propane water heaters require proper venting to operate safely. The technical specifications will include the venting requirements, such as the type and size of the venting system.

DIY Sizing

To size a tankless propane water heater yourself, follow these steps:

Determine the maximum temperature rise in your area during the winter months.
Calculate the required flow rate by adding up the flow rates of all the hot water devices you expect to use simultaneously.
Use a manufacturer’s sizing chart to select a tankless propane water heater that meets your needs.

When sizing a tankless propane water heater, consider the size of your household, the number of hot water devices you expect to use simultaneously, and the maximum temperature rise in your area.