What Size Tankless Water Heater Do I Need? A Complete Guide
Choosing the right size tankless water heater is essential for ensuring a continuous supply of hot water without wasting energy or experiencing shortages. A unit that is too small won’t meet demand, while an oversized one will increase upfront costs unnecessarily.
This guide will walk you through how to calculate the right size tankless water heater for your home based on factors like flow rate (GPM), temperature rise, and household size. By the end, you’ll know exactly, what size tankless water heater I need to unit-fit your needs, whether you’re replacing a traditional tank water heater or installing a tankless model for the first time.
Why Proper Sizing Matters for a Tankless Water Heater
Choosing the right size tankless water heater is essential for maintaining comfort, efficiency, and long-term cost savings. Unlike traditional tank water heaters, which store a fixed amount of hot water, tankless units heat water on demand. This means an incorrectly sized unit can either struggle to meet demand or consume excess energy unnecessarily. Ensuring proper sizing helps optimize performance, reduce utility costs, and extend the unit’s lifespan.
A properly sized tankless water heater offers several key benefits:
- Consistent Hot Water Supply – A correctly sized unit prevents fluctuations in water temperature, ensuring you have a steady supply even when multiple fixtures are in use. This is especially important for larger households where simultaneous usage of showers, faucets, dishwashers, and washing machines is common.
- Energy Efficiency – Heating only the required amount of water, rather than maintaining a storage tank of hot water, leads to significant energy savings. A unit that’s too small may run continuously, using more energy, while an oversized heater may waste energy by producing more hot water than needed.
- Longer Lifespan – Proper sizing prevents unnecessary strain on the heating elements, reducing wear and tear. An undersized unit is forced to work harder, which can shorten its lifespan, while an oversized model may cycle inefficiently, leading to premature wear.
On the other hand, incorrect sizing can lead to several problems:
- Not enough hot water – If the heater is too small, it won’t be able to meet demand, resulting in cold showers, fluctuating temperatures, or an interrupted supply when multiple fixtures are in use.
- Increased energy costs – An oversized unit may consume unnecessary energy, leading to higher electricity or gas bills. Choosing a larger heater than needed does not improve performance—it only increases operational costs.
- Reduced unit lifespan – A constantly overworked small unit or an underused oversized heater can wear out faster, leading to more frequent repairs and an earlier replacement. Ensuring the correct size will help your heater operate efficiently and reliably for years.
Key Factors in Sizing a Tankless Water Heater
1. Understanding Flow Rate (GPM)
Flow rate, measured in gallons per minute (GPM), determines how much hot water your household needs at any given time. Since tankless water heaters heat water on demand rather than storing it, the unit must be able to supply enough hot water at maximum flow rate, to meet your peak usage. If the flow rate is too low, showers may run cold when multiple fixtures are used simultaneously. If the flow rate is too high for your needs, the unit may waste energy, leading to higher operating costs.
Each fixture and appliance in your home has a different flow rate, which is how much water it contributes to overall water usage. Understanding these rates will help you determine the correct size for your tankless water heater.
Typical Flow Rates of Common Fixtures
| Fixture/Appliance | Average Flow Rate (GPM) |
|---|---|
| Shower | 1.5 – 2.5 GPM |
| Kitchen Faucet | 1.0 – 1.5 GPM |
| Bathroom Faucet | 0.5 – 1.0 GPM |
| Dishwasher | 1.0 – 1.5 GPM |
| Washing Machine | 1.5 – 3.0 GPM |
How to Calculate Total GPM
To determine the right tankless water heater size, follow these steps:
- List all fixtures and appliances that may run at the same time in your household.
- Add their flow rates together to calculate peak demand.
- Choose a tankless water heater that meets or slightly exceeds this total GPM requirement.
Example Calculation
Let’s say your household commonly uses the following fixtures at the same time:
- Shower: 2.0 GPM
- Kitchen Faucet: 1.5 GPM
- Dishwasher: 1.0 GPM
Your total peak hot water demand would be: 2.0 + 1.5 + 1.0 = 4.5 GPM
In this case, you should choose a tankless water heater with a minimum flow rate of 4.5 GPM to ensure a consistent hot water supply for simultaneous use.
By properly calculating GPM demand, you can avoid hot water shortages and ensure that your tankless water heater efficiently meets your household’s needs.
2. Temperature Rise & Its Effect on Heater Size
Temperature rise is a crucial factor in sizing a tankless water heater. It refers to the difference between the incoming water temperature and the desired hot water temperature. Since tankless water heaters heat water on demand, they must be capable of raising the water temperature to meet household needs, especially in colder regions where groundwater temperatures are lower.
How to Calculate Temperature Rise
The formula to determine temperature rise is:
Temperature Rise = Desired Water Temp – Incoming Water Temp
Most households set their hot water temperature to 120°F, which is ideal for showers, dishwashing, and laundry. However, the incoming water temperature varies by location, affecting the heater’s performance.
Average Groundwater Temperatures by Region
| Region | Average Incoming Water Temperature |
|---|---|
| Northern U.S. (Cold Climates) | 40°F – 50°F |
| Central U.S. (Moderate Climates) | 50°F – 60°F |
| Southern U.S. (Warm Climates) | 60°F – 75°F |
The colder the groundwater temperature, the more powerful your tankless unit needs to be to provide an adequate temperature rise.
Example Calculation
If your incoming cold water temperature is 50°F and your desired hot water temperature is 120°F, then your temperature rise requirement is: 120°F – 50°F = 70°F
In this scenario, your tankless water heater must be capable of raising the water temperature by at least 70°F to ensure sufficient hot water supply.
Why Temperature Rise Matters
- Cold Climates Need Higher Capacity Heaters – In regions where groundwater temperatures are 40-50°F, a high-output tankless heater is required to achieve the necessary temperature rise.
- Warmer Regions Require Less Heating Power – Homes in the South or coastal areas with higher groundwater temperatures may require a lower-capacity unit, leading to greater energy efficiency.
- Performance Varies by Fuel Type – Gas tankless water heaters can handle higher temperature rise more efficiently than electric models, which may struggle in colder climates.
Understanding temperature rise helps ensure you select a unit that can consistently provide enough hot water, no matter your location.
3. Household Size & Water Usage Habits
Your household size plays a major role in determining the hot water demand and the correct size for a tankless water heater. The more people in your home, the higher the peak demand, especially if multiple fixtures or appliances are used simultaneously.
While flow rate (GPM) and temperature rise are critical factors, household size provides a general guideline for selecting the right unit.
Recommended Tankless Water Heater Size by Household Size
| Household Size | Recommended Tankless Water Heater (GPM) |
|---|---|
| 1-2 people | 4 – 6 GPM |
| 3-4 people | 6 – 8 GPM |
| 5+ people | 8 – 10+ GPM |
How Household Size Affects Sizing
- Smaller Households (1-2 people) – A unit with 4-6 GPM is generally sufficient, as fewer fixtures run at the same time.
- Medium-Sized Households (3-4 people) – With multiple bathrooms, dishwashers, and laundry usage, a 6-8 GPM unit is recommended to avoid hot water shortages.
- Larger Households (5+ people) – Homes with multiple bathrooms and high-demand appliances should opt for an 8-10+ GPM unit to ensure a steady hot water supply.
Additional Considerations
- Frequent Simultaneous Usage – If your household regularly runs showers, dishwashers, and washing machines at the same time, consider a higher-capacity unit to handle peak demand.
- Multiple Units for Larger Homes – Instead of one large tankless water heater, installing multiple smaller units (e.g., one for bathrooms and another for the kitchen/laundry) can provide better efficiency and water distribution.
- Future-Proofing – If your household is expected to grow or your hot water needs may increase, choosing a slightly larger unit can prevent the need for an upgrade later.
By factoring in household size and water usage habits, you can select a tankless water heater that consistently delivers hot water when and where you need it.
Step-by-Step Guide to Choosing the Right Tankless Water Heater Size
Selecting the right size tankless, water heater requires more than just guessing, it involves calculating your home’s peak hot water demand, understanding temperature rise, and choosing a unit that meets your needs without overconsumption or shortages. Follow these steps to ensure you get the best unit for your home.
1. Identify Peak Hot Water Demand
Start by listing all the fixtures and appliances that might run at the same time. Consider showers, faucets, dishwashers, and washing machines. The more appliances that run simultaneously, the higher the required flow rate (GPM).
2. Calculate Total Flow Rate (GPM)
Each fixture in your home has a specific flow rate, measured in gallons per minute (GPM). To determine the total hot water demand, add up the flow rates of all fixtures used at once.
Example Calculation:
- Shower: 2.0 GPM
- Kitchen Faucet: 1.5 GPM
- Dishwasher: 1.0 GPM
- Total Peak Demand: 4.5 GPM
Your tankless water heater must be able to supply at least 4.5 gallons per GPM to ensure uninterrupted hot water.
3. Determine Temperature Rise
Next, find out your incoming water temperature (this varies based on location). Subtract this from your desired hot water temperature (usually 120°F).
Formula: Temperature Rise = Desired Water Temp – Incoming Water Temp
Example: If groundwater temperature is 50°F, and you want 120°F hot water, the required temperature rise is 70°F.
4. Choose the Right Unit
Once you have determined both flow rate (GPM) and temperature rise, select a tankless water heater that meets these specifications. The unit should be able to handle your home’s peak demand without overworking or underperforming.
By following these four simple steps, you can ensure your tankless water heater is properly sized to deliver reliable, efficient, and cost-effective hot water for your household.
Gas vs. Electric Tankless Water Heaters – Does It Affect Sizing?
Yes, the type of tankless water heater you choose, gas or electric, significantly impacts sizing and performance. Gas-powered tankless water heaters generally support higher flow rates and are better suited for larger households, while electric models have lower output and work well for smaller homes or point-of-use applications.
Gas vs. Electric Tankless Water Heater Comparison
| Factor | Gas Tankless Heater | Electric Tankless Heater |
|---|---|---|
| Max Flow Rate | 8 – 10+ GPM | 4 – 6 GPM |
| Suitable for Large Homes? | Yes | Limited |
| Installation Cost | Higher | Lower |
Choosing the Right Fuel Type for Your Home
- Gas tankless heaters are ideal for large households or homes with multiple bathrooms. They provide higher flow rates, making them suitable for homes where multiple fixtures run simultaneously. However, they require proper ventilation and may have higher installation costs.
- Electric models are better for small homes, apartments, or point-of-use applications (e.g., supplying hot water to a single sink or shower). They are cheaper to install but may struggle to support high hot water demand.
When selecting a tankless water heater, consider your household’s hot water needs, available fuel sources, and budget to choose the best option for efficiency and performance.
Common Mistakes to Avoid When Sizing a Tankless Water Heater
Choosing the wrong size tankless water heater can lead to performance issues, increased costs, and frustration. Many homeowners make sizing mistakes that result in hot water shortages, energy waste, or unnecessary expenses. Here are some of the most common errors to avoid:
- Underestimating GPM Needs – Many homeowners miscalculate how much hot water demand their household requires. If your peak usage exceeds the heater’s capacity, you may experience cold showers, reduced water pressure, or inconsistent heating when multiple fixtures are in use. Always calculate your total GPM demand to ensure your unit can handle simultaneous usage.
- Ignoring Temperature Rise – Your incoming water temperature significantly affects your heater’s performance. If you don’t account for temperature rise, your unit may struggle to heat water adequately, especially during cold winter months. Always check your regional groundwater temperature before choosing a heater.
- Oversizing the Unit – While it’s important to have enough hot water flow, choosing a tankless water heater that is too large can lead to unnecessary energy consumption and higher upfront costs. A bigger unit does not improve efficiency, it only increases operating expenses.
- Not Considering Future Needs – Your hot water needs may increase over time, whether due to a growing family, home expansion, or additional appliances. If you anticipate future changes, consider selecting a unit that meets both your current and future demands to avoid needing an upgrade later.
By avoiding these common mistakes and properly calculating your household’s flow rate and temperature rise, you can ensure your tankless water heater provides consistent hot water while maximizing efficiency and cost savings.
Conclusion
Selecting the correct tankless water heater size is crucial for maintaining a steady hot water supply, optimizing energy efficiency, and keeping utility costs in check. A properly sized unit prevents temperature fluctuations, reduces strain on the system, and ensures long-term performance. By accurately calculating flow rate (GPM) and temperature rise, you can confidently choose a properly sized water heater that meets your household’s peak demand without wasting energy or underperforming.
Experiencing temperature fluctuations, leaks, or inconsistent heating? Prestige Water Group specializes in water heater installation, repair, and maintenance across New Jersey. Whether you’re upgrading to a tankless water heater or need expert service, our licensed professionals are here to help. Call (973) 227-4740 or email info@pwgroupnj.com to schedule an appointment. Visit our website to explore our plumbing and water treatment services.
FAQs
How do I calculate the correct tankless water heater size for my home?
To determine the right size, calculate your peak demand by adding up the flow rates of all fixtures that may run at the same time. Then, determine the required temperature rise by subtracting the incoming water temperature from your desired hot water temperature, usually 120°F. Ensuring your unit meets both flow rate and maximum temperature rise, requirements will provide consistent hot water for your household.
What happens if I buy a unit that’s too small?
An undersized tankless water heater won’t be able to keep up with demand, leading to inconsistent water temperatures when multiple fixtures are in use. This can result in cold showers, interruptions in hot water flow, and overall dissatisfaction with your system’s performance. Additionally, an overworked unit may experience premature wear, leading to higher maintenance and replacement costs.
Do I need a separate tankless water heater for multiple bathrooms?
In larger homes with high hot water demand, installing multiple smaller tankless units may provide better efficiency and distribution. A single whole-house unit can work if properly sized, but it may struggle if multiple showers and appliances run simultaneously. Having dedicated units for different sections of the home can improve water availability and ensure a more consistent supply.
Is there a difference between gas and electric sizing?
Yes, gas tankless water heaters typically handle higher flow rates, making them better suited for large households with multiple bathrooms. Electric models have a lower capacity and work best for small homes or point-of-use applications, such as a single sink or shower. Choosing between gas and electricity depends on your home’s size, fuel availability, and overall hot water needs.
