How To Select The Right Size Tankless Water Heater
Selecting the right size tankless water heater is crucial for ensuring you have enough hot water to meet your household's needs. Unlike traditional tank water heaters, tankless units provide hot water on demand, meaning their capacity depends on the flow rate and temperature rise they can handle simultaneously. Here's a step-by-step guide to help you choose the right size tankless water heater:
1. Determine Your Household's Peak Hot Water Demand
You'll need to calculate the total flow rate of hot water required at peak usage times, which is typically when multiple fixtures or appliances are using hot water at the same time (e.g., showering, washing dishes, and running the washing machine).
Flow rates are measured in gallons per minute (GPM) or liters per minute (LPM). Here are the average flow rates of common household fixtures:
Shower: 1.5 to 2.5 GPM
Kitchen faucet: 1.5 to 2 GPM
Bathroom faucet: 0.5 to 1 GPM
Dishwasher: 1.5 to 2 GPM
Washing machine: 2 to 2.5 GPM
Add the flow rates of all the fixtures and appliances you expect to use at the same time during peak demand.
Example:
One shower (2 GPM)
One kitchen faucet (2 GPM)
Washing machine (2.5 GPM)
Peak hot water demand = 6.5 GPM
2. Calculate the Required Temperature Rise
The temperature rise is the difference between the temperature of the incoming cold water and the desired hot water temperature.
Determine incoming water temperature: Cold water temperature varies by region and season. Here are some average incoming water temperatures based on climate:
Northern U.S. or cold regions: 40°F (4°C)
Southern U.S. or warmer regions: 55°F (13°C)
Desired hot water temperature: Most households prefer hot water to be around 120°F (49°C) to 140°F (60°C).
Calculate the temperature rise: Subtract the incoming water temperature from the desired hot water temperature.
Example:
Incoming water temperature: 50°F
Desired hot water temperature: 120°F
Temperature rise = 120°F - 50°F = 70°F
3. Choose a Tankless Water Heater Based on Flow Rate and Temperature Rise
Once you have determined your peak demand (flow rate) and temperature rise, you can select a tankless water heater that meets these requirements.
Each tankless water heater model specifies the maximum flow rate it can deliver at a given temperature rise. As the temperature rise increases, the flow rate typically decreases.
For example:
A unit may provide 6.5 GPM at a 50°F temperature rise, but only 4.5 GPM at a 70°F temperature rise.
Ensure that the tankless water heater you choose can provide the required flow rate at your calculated temperature rise.
4. Consider Other Factors
Number of bathrooms: More bathrooms may require a higher flow rate, especially if multiple showers are in use at the same time.
Climate: In colder climates where the incoming water temperature is lower, you'll need a more powerful heater to achieve the same temperature rise.
Gas vs. Electric: Gas-powered tankless water heaters typically offer higher flow rates and are better suited for larger households, while electric models are more efficient for smaller homes or point-of-use applications.
Whole-house vs. Point-of-use: For larger homes, a whole-house tankless water heater is ideal. For smaller homes or specific fixtures, a point-of-use heater may suffice.
5. Check Manufacturer Specifications
Once you've calculated your needs, compare different tankless water heater models. Manufacturers will often provide sizing guides or charts that show the flow rate at different temperature rises.
Example Sizing Scenario
For a household with the following peak hot water usage:
One shower (2 GPM)
One faucet (1.5 GPM)
Dishwasher (1.5 GPM)
Peak demand: 5 GPM
With an incoming water temperature of 50°F and a desired temperature of 120°F, the required temperature rise is 70°F.
You would need to select a tankless water heater that can provide at least 5 GPM at a 70°F temperature rise.
Conclusion
Selecting the right size tankless water heater involves calculating your household's peak hot water demand (GPM) and determining the required temperature rise. Make sure to choose a model that meets both the flow rate and temperature rise requirements. Consulting the manufacturer's specifications and considering your climate, household size, and hot water habits will ensure you pick the ideal system for consistent and efficient hot water.
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