Picking the right size tankless water heater is important to make sure you always have hot water and save as much energy as possible. Do you want to know “what size of tankless water heater do I need?” This complete guide will help you make the right choice for your home.
Why Sizing Matters for Tankless Water Heaters
Tankless water heaters heat water only when it’s needed, unlike traditional ones that store water in a tank. These small units use little energy and are made to give you hot water all the time. If you pick the wrong size, though, you might not have enough hot water or waste energy and money.
If you choose the right size tankless hot water heater, it will be able to handle your peak usage without stopping.
Key Factors to Determine What Size of Tankless Water Heater You Need
To answer the question, “what size of tankless water heater do I need?”, you’ll need to understand the two most important factors: flow rate and temperature rise.
1. Flow Rate (Measured in GPM)
One of the most important ways to figure out the size of a tankless water heater is to look at how many GPM it has. GPM, which stands for “gallons per minute,” tells you how much hot water your unit can send out at once.
To figure out the flow rate your house needs:
-
List all fixtures or appliances that might be used at the same time.
-
Add their GPM to get a total flow rate.
Average fixture flow rates:
-
Shower: 2.5 GPM
-
Kitchen Faucet: 1.5 GPM
-
Washing Machine: 2 GPM
-
Dishwasher: 1–2 GPM
Example: If you often run the shower (2.5 GPM) and dishwasher (1.5 GPM) simultaneously, you need a tankless water heater that can handle 4 GPM or more.
2. Temperature Rise
Temperature rise refers to the difference between incoming cold water and your desired hot water temperature.
In most U.S. regions:
-
Northern states: Incoming water is around 40°F
-
Southern states: Around 50°F
Desired temperature is typically 120°F, so if your incoming water is 50°F, your required temperature rise is 70°F.
To keep up with the temperature change, your tankless unit needs to be bigger or stronger.
3. Household Size and Usage Patterns
The number of people in your home and how often you use hot water play a big role in tankless water heater sizing.
General Guidelines:
-
1–2 people: 3–4 GPM unit
-
3–4 people: 5–7 GPM unit
-
5+ people or 3+ bathrooms: 8+ GPM unit
If multiple showers and appliances are used at once, you’ll need a larger capacity unit to avoid running out of hot water.
Examples of Tankless Water Heater Sizes
Understanding tankless water heater sizes can help you choose more confidently:
-
Small Units (3–4 GPM): Best for studio apartments or small homes
-
Medium Units (5–8 GPM): Ideal for average-sized homes with two bathrooms
-
Large Units (9+ GPM): Designed for big families and homes with 3+ bathrooms
This makes instantaneous water heater sizing easy to match to your home layout and usage.
Gas vs. Electric Tankless Water Heater Sizes
The type of unit also affects what size you need:
-
Gas Tankless Water Heaters: Offer higher GPM rates and are suitable for larger homes. However, they require proper ventilation.
-
Electric Tankless Water Heaters: More compact and efficient for smaller homes, but they may not meet high GPM demands.
Whether you’re using electric or gas, proper instantaneous water heater sizing is key to avoiding performance issues.
Steps to Calculate the Right Size
Here’s a simple method to determine what size of tankless water heater do I need:
-
Determine Total GPM Needed: Add up flow rates of fixtures you use at the same time.
-
Find Required Temperature Rise: Subtract incoming water temperature from your desired hot water temperature.
-
Match the Specs: Choose a unit that meets or exceeds your GPM and temperature rise needs.
Example:
If your total flow rate is 6 GPM and your temperature rise is 70°F, look for a tankless unit rated for 6 GPM at a 70°F rise.
Additional Tips for Choosing the Right Tankless Heater
Energy Efficiency
Look for models with a high Uniform Energy Factor (UEF). A high UEF means better energy savings and performance.
Installation Compatibility
Ensure your home’s plumbing and electrical system can support the new unit. Upgrading from a traditional system may require adjustments.
Professional Installation
You have to do more than just pick the right tool. Installation needs to be done right. Work with a licensed plumber who knows the local building codes and size standards for the best results.
Why Trust Derks Plumbing with Your Installation?
Derks Plumbing is an expert at finding the right tankless water heater for your home. Our qualified professionals do everything, from figuring out how much hot water you need to installing and maintaining it perfectly.
We give you clear answers and reliable solutions that are made just for your home when you ask, “What size tankless water heater do I need?”
Conclusion
When picking a tankless water heater, it’s not just about size; it’s also about comfort, savings, and how well it works over time. By knowing the size, flow rate, and temperature rise of a tankless hot water heater.
Need help? Get in touch with Derks Plumbing right away, and our tankless water heater services experts will help you choose and set up the best system for your home.
FAQs About Tankless Water Heater Sizing
Q1: What size of tankless water heater do I need?
A: Add up the GPM (gallons per minute) of all fixtures you use at once and factor in the temperature rise needed. That gives you the right size tankless water heater for your home.
Q2: How many GPM for a tankless water heater in a typical household?
A: Most families need 5–8 GPM, depending on how many people live there and how many fixtures run at the same time.
Q3: Are gas and electric tankless water heater sizes different?
A: Yes. Gas units handle higher flow rates better, while electric models work best for smaller homes or point-of-use needs.
Q4: Can I use a small tankless heater in an apartment?
A: Absolutely. A 3–4 GPM unit is perfect for small apartments or single-person homes.
Q5: What happens if I choose the wrong size?
A: Too small, and you’ll run out of hot water. Too big, and you’ll waste money. Proper tankless hot water heater sizing is key.
Q6: How can a plumber help with sizing?
A: A professional calculates flow rate, temperature rise, and setup. At Derks Plumbing, we make sure you get the perfect fit.
Q7: Do tankless water heaters provide hot water instantly?
A: Yes—almost. There’s a slight delay, but once running, they deliver steady hot water without a tank.