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Looking at endless tankless water heater specs and wondering what size you actually need? You’re not alone.
Our extensive experience installing and testing hundreds of tankless water heaters has taught us one crucial thing: getting the size right is absolutely essential for your satisfaction and comfort.
Through countless installations and real-world testing, we’ve discovered that most homeowners struggle with the same questions about tankless water heater sizing. That’s exactly why we created this straightforward, no-nonsense guide based on our practical expertise.
Drawing from our team’s background in master plumbing and extensive industry knowledge, we’ll guide you through a proven, step-by-step process to determine the perfect size for your home. We’ve refined these calculations through years of hands-on experience to make them crystal clear and easy to follow.
Using our field-tested method, you’ll be able to confidently select a tankless water heater that delivers endless hot water for your entire household. Ready to find your perfect match? Let’s dive into the details!
Through our extensive field experience, we’ve discovered that sizing a tankless water heater requires a different approach than traditional tank models. While conventional water heaters are typically sized based on household size, tankless units need to be matched to your specific fixture and appliance needs.
The key measurement here is GPM (Gallons Per Minute), also known as the flow rate. This figure tells us exactly how much hot water your system needs to deliver to meet your household’s demands. Let us walk you through our proven sizing method.
The first step is determining your home’s maximum GPM needs. In our professional installations, we calculate this by adding up the flow rates of all hot water fixtures that might run simultaneously. Here’s the basic formula we use:
Total GPM Needed = Combined GPM of All Hot Water Fixtures Used at the Same Time
Through our work, we’ve found two reliable ways to determine your fixtures’ GPM:
In our installations, we’ve found that many homeowners don’t have access to their original fixture documentation. Don’t worry – we’ve got you covered! We regularly use standardized GPM charts as a reliable alternative. These charts provide tested averages that work well for most households.
When calculating your total GPM, remember to only include fixtures that use hot water and might run simultaneously. Based on our field experience, this approach gives you the most accurate sizing requirements for your new tankless system.
| Appliance | Flow Rate (GPM) |
|---|---|
| Bathroom Faucet | 0.5 GPM |
| Kitchen Faucet | 0.5 GPM |
| Shower | 2.5 GPM |
| Dishwasher | 1.0 GPM |
| Clothes Washer | 1.5 GPM |
Pro Tip: Through years of installations, we’ve learned that it’s better to overestimate than underestimate! Consider all fixtures that might run simultaneously to ensure you’ll never run short on hot water.
After determining your flow rate needs, the next crucial factor is calculating your required temperature rise. In our experience installing systems across different climate zones, we’ve found this step is often overlooked, but it’s absolutely essential for proper sizing.
Temperature rise is the difference between your incoming groundwater temperature and your desired hot water temperature. Based on our installations and customer feedback, most households prefer their hot water around 120 degrees Fahrenheit.
To calculate your specific temperature rise:
Temperature Rise = Desired Hot Water Temperature – Incoming Groundwater Temperature
Finding your local groundwater temperature is straightforward – we recommend using a ground temperature map. You’ll find various options available, from broad regional maps to detailed local temperature charts. In our professional experience, either type works well for sizing calculations.
After installing thousands of tankless water heaters, we’ve gained deep insight into the pros and cons of both gas and electric units. Let’s break down what we’ve learned about choosing the right power source for your needs.
Electric Tankless Water Heaters: Best For:
Gas Tankless Water Heaters: Best For:
In our installations, we’ve found that your existing home infrastructure often guides this decision. Does your home already have gas lines or would you need to install them? This factor alone can significantly impact your choice.
Here’s what we’ve discovered through our years of field experience:
Gas Units:
Electric Units:
Now that we’ve covered these essential factors, let’s look at some real-world examples to help you make your decision.
When our team assessed tankless water heater requirements for a family of four living in a standard home, we focused on simultaneous hot water usage patterns. We’ll walk you through our calculation process for determining the maximum Gallons Per Minute (GPM) needed.
From our extensive testing, we’ve found that a typical family of four might need to run these fixtures simultaneously:
Based on our analysis of efficient fixtures’ flow rates from our comprehensive GPM chart, this adds up to a total requirement of 6.5 GPM.
Next, we calculated the necessary temperature rise for the tankless heater. Let’s use Hawaii as our example location. With a standard hot water setting of 120 degrees Fahrenheit and Hawaii’s average groundwater temperature of 77 degrees, here’s what we found:
120 degrees (desired temperature) – 77 degrees (incoming water temperature) = 43 degree rise required
Our calculations show this family needs a system capable of a 43-degree temperature RISE while maintaining 6.5 GPM flow.
For families limited to electric options, our testing has shown that a high-capacity unit like the EcoSmart Eco 27 Electric Tankless water heater meets these requirements effectively.
During our consultations, we’ve found that couples living in Florida condos have significantly different water heating requirements. Here’s what we discovered when analyzing their needs:
In our assessment of a typical Florida condo setup with one bathroom and no in-unit laundry or dishwasher, we noted some interesting findings. While their new kitchen faucet runs at 1.5 GPM, using our standard GPM calculation chart, we determined their total flow rate needs come to 5 GPM.
Our measurements showed they only need to increase water temperature from Florida’s average groundwater temperature of 68 degrees to their preferred 110 degrees. Based on these specifications, we recommend looking for a unit capable of handling a 42-degree rise at 5 GPM.
Through our testing, we’ve found that the Rheem RTEX-36 Electric Tankless Water Heater is an excellent match for these requirements. Not only does it handle the water demand perfectly, but we’ve also seen it deliver impressive energy efficiency and space-saving benefits.
In our recent evaluation of a large family home with gas line access, we encountered some unique requirements that we’d like to share.
This residence featured similar appliances to our four-person household example, with an additional half bathroom that includes a 1.5 GPM faucet. Based on our real-world observations, we calculated maximum simultaneous usage: two showers running together, plus the dishwasher, washing machine, and half bathroom faucet. Our calculations showed a total requirement of 9.5 GPM.
When measuring the local conditions in Oklahoma, we found groundwater temperatures averaging 57 degrees. With the family’s preference for 120-degree hot water, our analysis indicated they would need a system capable of a 63-degree temperature rise while maintaining 9.5 GPM flow.
Now that we’ve helped determine their requirements for an appropriate tankless water heater size, selecting the right model becomes straightforward.
In our extensive product testing, we found the Rheem RTGH-95DVLN 9.5 GPM Indoor Direct Vent Tankless Natural Gas Water Heater to be particularly suitable. Our efficiency tests showed an impressive 94 percent rating, and we confirmed it handles the projected water usage requirements effectively.
After years of installing and testing tankless water heaters, we’ve simplified the sizing process into two essential steps:
Once you’ve worked out these numbers, you’ll need to make one final decision: choosing between gas or electric power. We’ve found this choice often depends on your home’s existing infrastructure and energy costs in your area.
When shopping for your new unit, you’ll find the specifications you need right on the heater’s information plate or packaging. Most manufacturers and vendors also list these details prominently on their websites. We like to compare it to car shopping – just as you’d look at horsepower and fuel efficiency, you’ll be looking at GPM and temperature rise capabilities.
Ready to find your perfect tankless water heater? Check out our review of the best tankless water heaters today!
If you would like to calculate your own tankless water heater size, just follow the same steps as above.
Once you have the size of tankless water heater selected, your only remaining decision is the power source type.
Decide between a gas tankless water heater or electric heater, then you can start shopping! Specifications matching your calculations can be found directly on the heater’s spec plate or packaging as well as most vendors’ and manufacturers’ websites.
Think of it as shopping for a car by horsepower and mileage per gallon.
Now that you know what you’re looking for, it’s time to grab your heater. Check out our review of the best tankless water heaters today!
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