What Size of Tankless Water Heater Do I Need? Your Guide to Right Sizing

what size of tankless water heater do I need

What size of tankless water heater do I need? Choosing the right size for a tankless water heater is crucial for ensuring that you have a continuous supply of hot water whenever you need it, without incurring unnecessary costs if you are switching from a tank water heater.

Unlike traditional tank water heaters that keep a ready reservoir of hot water, tankless water heaters heat water on demand, offering a continuous flow and energy savings. However, to reap these benefits, it’s necessary to select a unit that’s properly sized for your household’s hot water demand.

Selecting a tankless water heater that’s too small will lead to a shortage of hot water during peak times, while one that’s too large will overconsume energy and inflate your utility bills. Understanding the flow rate and temperature rise needed for your household will guide you in finding the right fit.

A gas-fired demand water heater can achieve a 70ºF (39ºC) water temperature rise at 5 gallons per minute flow rate, while an electric one manages this at 2 gallons per minute. Higher flow rates or cooler inlet temperatures may lower the temperature at distant faucets.

Certain performance parameters, such as the time it takes for the heater to produce hot water and the minimum flow rate to activate the heater, are essential to consider to avoid issues such as “cold water sandwiches” where unexpected blasts of cold water can occur.

Key Takeaways

  • Tankless water heaters provide on-demand hot water and energy savings.
  • Correct sizing is crucial to prevent hot water shortages and energy waste.
  • Consider flow rate, temperature rise, and appliance performance for optimal sizing.

Debunking the Myths Surrounding What Size of Tankless Water Heater Do I Need?

When you’re considering a tankless water heater, understanding the right size for your home is critical. Let’s clear up some common misconceptions you might encounter:

Myth 1: “Any size tankless hot water heater will do.”
Fact: The size of your tankless water heater should match your household’s hot water demand. Too small, and you’ll run out of hot water. Too large, and you’ll waste energy and money.

Myth 2: “Higher gallons-per-minute (GPM) means better.”
Fact: A higher GPM isn’t always better. It matters only if it aligns with the simultaneous usage needs of your household.

Myth 3: “Tankless heaters deliver instant hot water to any faucet.”
Fact: While tankless heaters provide hot water without a storage tank, there’s still a short wait as the cold water in the pipes gets replaced by hot water from the heater.

To best gauge what tankless water heater size fits your needs, consider the number of devices you’ll use at once. The ASHRAE sets national standards for sizing, debunking the notion that one-size-fits-all.

Remember, it’s not just about maximum flow rate; it’s also about the maximum temperature rise your heater can handle. Assess the coldest inflow water temperature and ensure your prospective unit can heat it to your desired temperature at your required flow rate.

  • Assessment Tips:
    • Count the number of appliances and fixtures needing hot water.
    • Calculate their combined flow rates.
    • Be realistic about simultaneous usage.

By choosing a properly sized water heater, you maximize efficiency and enjoy continuous hot water on demand.

What Size Tankless Water Heater Do I Really Need? A Step-by-Step Guide

Selecting the ideal tankless water heater hinges on understanding your home’s specific needs. Here’s how to calculate the right size for seamless, on-demand hot water.

Fixture Flow Rates

First, list each water fixture in your home and note its flow rate, which is measured in gallons per minute (GPM). Typical flow rates are around 1.5 GPM for faucets and 2.5 GPM for showerheads. Summing these rates helps determine your peak water demand.

Temperature Rise

Calculate the temperature rise by subtracting the incoming water temperature from the desired output temperature. For example, if your groundwater is at 50°F and you want a shower at 110°F, you’ll need a temperature rise of 60°F.

Fuel Type

The choice of fuel—electric, natural gas, or propane—impacts efficiency and heating capability. Gas-powered heaters often have a higher flow rate, so you’ll need to decide which fuel type suits your infrastructure and preferences.

Simultaneous Usage Considerations

You must account for possible simultaneous use. If two showers and a dishwasher might run at once, you’ll combine their flow rates. A shower at 2.5 GPM and a dishwasher at 1.5 GPM require a unit that can handle at least 4 GPM with the necessary temperature rise.

Key Features to Consider When Buying Tankless Water Heaters

When you’re in the market for a tankless unit, there are a few critical features you’ll want to keep an eye out for to ensure you select the right model for your needs.

  • Energy Efficiency: Look for units with high energy factor (EF) ratings. The EF rating indicates the efficiency of the water heater—the higher the rating, the more efficient the heater. You can find this information on the Energy Star website, which rates devices based on their energy savings.
  • Flow Rate: The flow rate, measured in gallons per minute (GPM), is crucial because it tells you how much hot water the unit can provide. Think about your peak hot water usage to determine the right flow rate.
  • Durability: Seek models known for their longevity. Stainless steel components, corrosion resistance, and good quality construction suggest the unit will have a long lifespan.
  • Size: Ensure the physical size of the unit fits the allocated space in your home. Some compact models are designed for small spaces, available on manufacturers’ specification sheets.
  • Temperature Control: A model with a precise temperature control mechanism allows you to set your preferred water temperature, found on the product details page.
  • Warranty: Aim for a tankless water heater with a comprehensive warranty. Typically, the longer the warranty, the better the quality of the heater.

By considering these features and comparing different models on manufacturers’ websites, you’ll be well on your way to finding a tankless water heater that meets your specific needs.

Energy Efficiency and Types of Tankless Heaters

When looking for a tankless water heater, energy efficiency is paramount, as it directly impacts your utility bills and environmental footprint. Understanding the types available and their respective energy factors will help guide your choice.

Electric vs. Gas Tankless Heaters

Electric tankless heaters are typically easier to install and can be more efficient for smaller demand scenarios. However, their overall energy savings potential might be less compared to gas tankless heaters, as gas is often a cheaper energy source.

Pros of Electric Tankless Heaters:

  • Higher energy efficiency for low-use applications
  • Easier and often cheaper installation

Pros of Gas Tankless Heaters:

  • Better suited for high water demand
  • Often more cost-effective for larger households

Energy Factor (EF) Ratings

The Energy Factor (EF) rating measures a tankless heater’s efficiency; the higher the EF, the more efficient the unit is. For example, gas tankless water heaters often have EF ratings ranging from 0.80 to 0.90, suggesting that they convert 80-90% of the fuel into heat for your water.

Electric tankless heaters can exceed an EF of 0.90, indicating a very high level of efficiency. Hence, your specific usage needs and energy costs play a crucial role in determining which type is the optimal choice for your home.

Calculating Required Flow Rate

When selecting a tankless water heater, calculating the required flow rate is essential to meet your household’s hot water needs efficiently.

Average Household Usage

Begin by assessing your average household usage. This includes the water flow rates of fixtures and appliances that use hot water, such as showers, which typically use 1.5-3 gallons per minute (GPM), or kitchen faucets, which use about 1.5 GPM. Add up the flow rates of commonly used devices to get a baseline.

  • Shower: 1.5-3 GPM
  • Kitchen Faucet: 1.5 GPM
  • Dishwasher: 1-2.5 GPM
  • Washing Machine: 1.5-3 GPM

Peak Demand Calculation

Peak demand represents the maximum flow rate needed when all appliances run simultaneously. To calculate this, sum the flow rates of the highest-demand appliances that might be used at the same time.

For example, if you might run the dishwasher (2.5 GPM) while someone is showering (2.5 GPM), you’d need a water heater capable of delivering at least 5 GPM to maintain performance during peak usage.

  • Shower + Dishwasher: 2.5 GPM + 2.5 GPM = 5 GPM
  • Shower + Kitchen Faucet: 2.5 GPM + 1.5 GPM = 4 GPM

By understanding your average household usage and calculating your peak demand, you can choose a tankless water heater that suits your specific needs.

Sizing by Temperature Rise

When selecting a tankless water heater, understanding the temperature rise needed for your home is crucial. This ensures you’ll enjoy a consistent and comfortable hot water supply.

Temperature Rise Calculation

To calculate the temperature rise, subtract the incoming water temperature from your desired output temperature. For instance, if your incoming water is 50°F and you want your shower at a cozy 105°F, you will need a temperature rise of 55°F. Use this formula:

Temperature Rise = Desired Output Temperature - Incoming Water Temperature

Influence of Incoming Water Temperature

The incoming water temperature is greatly influenced by your geographical location. Colder climates can have significantly lower incoming water temperatures, which requires a larger temperature rise, hence, a more powerful tankless water heater.

For example, a home in the northern part of the country might have incoming water temperatures as low as 40°F in the winter, while in the south, the incoming water temperature could be around 70°F.

Installation Considerations

When selecting a tankless water heater, it’s important to consider the installation space and the technical requirements for safe and efficient operation.

Space Requirements

Your tankless water heater requires enough space not just for the unit itself but also for proper access and maintenance. Typically, a tankless water heater can be installed in a more compact area compared to traditional units, as there is no need for a large storage tank.

Ensure that there is sufficient clearance as recommended by the manufacturer for servicing and safety.

Ventilation and Gas Line Specifications

Proper ventilation is crucial for tankless gas water heaters to ensure safe removal of exhaust gases and to prevent back-drafting. You’ll need to consider the type of venting system required by your specific model—whether it’s power venting or direct venting.

Additionally, the gas line must be correctly sized to supply enough fuel for the heater to operate effectively. Incorrect gas line sizing can lead to inefficiency and potentially hazardous conditions.

Brand and Model Comparisons

When selecting a tankless water heater, it’s crucial to compare brands and models based on key aspects like warranty and reliability, as well as customer reviews and ratings.

Warranty and Reliability

  • Warranty Duration: Look for models offering long-term warranties, indicating a manufacturer’s confidence in their product.
  • Reliability Factors: Research the brand’s reputation for durability and the availability of replacement parts.

Customer Reviews and Ratings

  • Consumer Feedback: Online customer reviews can provide real-world insights into performance and satisfaction levels.
  • Rating Trends: Pay attention to how different models are rated across various retail platforms for a balanced view on quality and service.

Cost Analysis and Savings

When considering a tankless water heater, you’ll want to weigh the initial installation price against the long-term savings potential. Making an informed decision involves understanding both upfront costs and the future financial benefits.

Initial Installation Costs

Your tankless water heater’s price tag depends on the size and type you choose. Expect to pay $2,500 to $3,400 for a whole-house natural gas tankless water heater, including installation.

In comparison, a natural draft storage water heater installation typically costs less, but the energy savings from a tankless water heater can offset this initial difference over time.

Long-Term Savings Potential

Tankless water heaters are renowned for their energy efficiency, often leading to reduced utility bills. Over time, this can result in significant cost savings. Detailed studies confirm ongoing energy savings, as tankless models only heat water on demand, avoiding the standby energy losses associated with traditional tanks.

Remember, the actual operating cost savings will vary based on your individual usage patterns and the energy rates in your area.

Regulations and Compliance

When choosing the size of a tankless water heater, it’s crucial to consider both building codes and energy standards, which can influence the models you may legally install.

Building Codes

Building codes are set by local and state governments to ensure the safety and efficiency of construction and installations in homes. For your tankless water heater, this means that certain sizes and installation practices are required by law.

It’s important to check with your local building department to get the most accurate information. Failure to comply can result in fines or additional costs if changes are required later on. For instance, you might need specific ventilation systems or clearance space, which could affect the physical size of the heater you can install.

Energy Standards

Energy standards regulate the efficiency of water heaters. These standards are often based on factors such as the rate of hot water delivery and the heater’s energy factor (EF). An Electric tankless water heater, for example, must meet minimum energy efficiency levels set by regulations like the Energy Star program.

Moreover, the model of the tankless water heater you choose must align with these standards to ensure optimal energy savings and to qualify for certain rebates or tax credits. These standards are not only for the benefit of your utility bills but also for environmental conservation efforts.

Maintenance and Lifespan

When choosing a tankless water heater, understanding its maintenance needs and longevity can help you make an informed decision about which model best suits your home.

Routine Maintenance Needs

To keep your tankless water heater functioning efficiently, it’s important to perform regular descaling, particularly if you live in an area with hard water. This involves running a vinegar solution through the unit to remove mineral buildup.

The air filter should also be cleaned periodically to ensure proper air flow. Additionally, checking the in-line screen filter for debris can prevent damage and maintain water pressure.

Expected Heater Lifespan

Tankless water heaters generally have a longer lifespan compared to traditional storage water heaters. With proper maintenance, you can expect your tankless water heater to last about 20 years.

It’s important to note, however, that factors such as unit design, quality of installation, water quality, and maintenance frequency can affect this duration. By choosing a reliable model and adhering to maintenance guidelines, you’re more likely to enjoy the full lifespan of your unit.

Recommended Tankless Water Heaters: Expert Class

When you’re looking for the top tier of tankless water heaters, you want efficiency and reliability. Below is a curated list of expert-class tankless water heaters you can consider, each suited to a range of needs and budgets.

For Large Homes:

  • Rinnai RUC98iN Ultra Series: This natural gas heater is both powerful and efficient. It’s a great fit if you have a spacious home with multiple bathrooms. Get it here.


  • EcoSmart ECO 27: Optimal for smaller homes or moderate climates, this electric tankless water heater is cost-effective and can help save on energy bills. See user reviews.

For High Demand:

  • Stiebel Eltron Tempra 36 Plus: For homes with high hot water use, this model offers impressive flow rates without sacrificing temperature stability. Learn more.


  • Rheem 9.5 GPM Tankless Water Heater: If environmental impact is a key concern for you, this energy-efficient model from Rheem may be the perfect fit with low emissions. Available here.

Compact Size:

  • Bosch Tronic 3000: Ideal for point-of-use applications or smaller spaces, Bosch offers a reliable and space-saving design for those with limited room. Product details.

Remember to check the specifications to ensure the unit meets your home’s needs for hot water output and energy efficiency.

Common Mistakes to Avoid When Buying a Tankless Water Heater

Selecting the proper size for a tankless water heater is crucial, but many shoppers fall into common pitfalls during their purchase. Here are a few errors you’ll want to sidestep:

  • Overestimating Size Needs: Larger is not always better. A model too large for your home is not only more expensive but also less energy efficient.
  • Underestimating Hot Water Demand: Conversely, purchasing a heater that’s too small can lead to a frustrating experience. If you frequently need hot water from multiple sources simultaneously, ensure your chosen model can keep up.
  • Ignoring Flow Rates: Review each unit’s gallons per minute (GPM) capacity. It’s vital for ensuring a continuous supply of hot water when and where you need it.
  • Neglecting Temperature Rise: Tankless water heaters work by heating water as it passes through the unit. You must consider the temperature of the incoming water and the temperature you desire to ensure satisfaction with your appliance’s performance.
  • Disregarding Energy Efficiency: Look at models with high energy factor ratings. They may be pricier up front, but the long-term savings are significant.

Remember, even though customer reviews often mention delight at having endless hot water, you’ll only join their ranks by avoiding these common missteps. The right choice means you’ll enjoy the comforts of a properly selected tankless water heater without the pitfalls.


Selecting the appropriate size for your tankless water heater is fundamental to ensuring you have a continuous supply of hot water as needed, without overspending on a too-large unit or risking inefficiency with one that’s too small.

Assessing your home’s hot water needs based on the number of bathrooms, kitchen usage, and appliances is crucial.

By choosing the right-sized tankless water heater, you will enjoy the advantages of an energy-efficient and cost-effective system that aligns with your specific demands.

Modern tankless water heaters are designed for longevity and can offer significant energy savings, making them a smart investment for your home heating solution.

To confidently decide on the right heater size, consider the maximum flow rate and temperature rise you’ll need. Remember, an undersized heater will run continuously, attempting to meet demand, while an oversized one can waste energy and increase operating costs.

Armed with the insights from this guide, take action by calculating your household’s hot water requirements before switching from your tank water heaters. Ensure a future of comfortable and economical living with a tankless water heater that’s just the right fit for you.


  • Edmund Yeo

    🌱 I'm passionate about sustainable living and renewable energy, and I'm thrilled to share my journey towards a greener, brighter future with you. Join me as I explore the latest innovations, eco-friendly lifestyle tips, and inspiring stories from the forefront of the green energy revolution. Note: This website uses affiliate links which may earn a commission for purchases made at no additional cost to you.

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