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What Size Tankless Water Heater Do I Need?

What Size Tankless Water Heater Do I Need?

Well-maintained water heaters with tanks can last up to ten years in some cases. Yet tankless water heaters offer greater efficiency and have a much longer lifespan. If you need to replace your equipment, you might ask, “What size tankless water heater do I need?”

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It might not seem like tank size is a significant factor in tankless hot water heater replacement, but it plays a major role in the equipment’s efficiency. By understanding the importance of tankless water heater sizing, you can ensure that your home will get the hot water demand it needs for the longest time possible.

Consequences of Inadequate Water Heater Sizehot water shower from a tankless hot water heater

Water heaters come in multiple sizes to accommodate the hot water needs of different buildings and households. However, bigger doesn’t always mean better, nor does smaller relate to greater energy efficiency. 

If your water heater is too small or big for your property, you could experience these problems:

  • Hot Water Running Out: Undersized tank water heaters won’t produce enough hot water for the household. As a result, the tank could run out of hot water quicker than expected. Undersized tankless water heaters impact water flow rates.
  • High Energy Bills: When water heaters are too big, they’ll produce lots of hot water but consume too much energy. Energy bills will increase due to the wasted energy input. 
  • Shortened Water Heater Lifespan: Premature wear of water heater components produces another consequence of undersized equipment. When the system can’t meet hot water demands, it will work harder and wear parts down faster.
  • Inconsistent Temperatures: Water temperatures often fluctuate when the heater size is incorrect. 

Tankless Water Heaters vs. Regular Water Heaters

Besides asking, “What size water heater do I need?” you might also ask, “What’s the difference between tankless and traditional water heaters?” Traditional water heaters have a storage tank that collects water to heat using a heating element and electricity or gas for fuel. The tankless variety can also be gas or electric. However, instead of heating water in a tank, it runs the water directly through the heating elements to provide hot water on demand. 

Most homes have traditional water heaters with tanks that can hold 30 to 50 gallons of water. Though natural gas water heaters use less energy, they cost more upfront to install. 

Tankless hot water heaters are more energy efficient than traditional heaters. Despite having larger installation expenses, the energy savings it provides by warming water more efficiently make the initial costs worthwhile for many households. These water heaters also have a 20-year lifespan, significantly longer than the regular tank water heaters. 

Sizing a water heater is important regardless of the type of heater you install. Tankless water heaters are generally smaller than regular water heaters. Since they don’t need a tank, they’ll take up less space inside your home. 

Calculating the Right Tankless Water Heater Sizewoman enjoying a shower with hot water from a tankless water heater

Still asking, “What size tankless water heater do I need?” Here, you’ll find the primary factors to consider when calculating your property’s appropriate water heater size. 

Factors To Consider

Household Size

The number of people living in your home will impact the water heater’s size. Generally, the more people in a household, the greater the demand for hot water. It also becomes more likely that more than one tap will run at the same time, which increases the demand and strain on the water heater. 


Another consideration for calculating tankless water heater sizes includes the maximum flow rate in gallons per minute. You can find your household’s maximum flow by taking note of each fixture’s flow rate and how many will operate simultaneously. 

The average GPMs for many households include the following:

  • 3.5 GPM for up to two fixtures running simultaneously
  • 5 GPM for up to three fixtures running simultaneously
  • 7 GPM for up to four fixtures running simultaneously

Once you know the flow rate, you’ll need to calculate the temperature difference between the groundwater temperature and the temperature you want for your hot water taps. This calculation gives the temperature rise since it reflects how high your water heater has to raise the water’s temperature. 

Most residential properties have tap temperatures around 120 degrees.

Tankless Water Heater Size Calculatorplumber adjusting the temperature of a hot water heater

To calculate your ideal tankless water heater size, begin by learning the flow rate for the heaviest water usage you can expect for your household. For example, the shower and dishwasher could run at the same time. Add up the fixture flow rates to determine the maximum flow rate.

Next, input your area’s groundwater temperature into a tankless water heater size calculator. This step will help determine the necessary temperature rise. Use the maximum flow rate and temperature rise to find the right water heater size for your property.

Tankless water heater ratings go according to gas energy input or British Thermal Units. When a heater has a high BTU input, it will produce a good maximum flow. The flow should always be higher than the demand the property puts on the equipment. 

Seek Professional Advice

Tankless water heater installation isn’t something you should attempt on your own. Sometimes, buildings need updates to accommodate an upgraded water heater, like a gas line or new wiring. Sizing water heaters can also be problematic for people who are new to the concept. 

For those reasons, it’s always a good idea to seek a professional assessment of your current water heater needs. Qualified plumbers can evaluate your current system and household needs to determine the appropriate size for your new water heater. They can also handle the removal and disposal of your existing water heater before expertly installing a tankless model while adhering to all building codes and permit requirements.

Book an Assessment Today

Before getting a new water heater, you should always ask, “What size tankless water heater do I need?” Installing an appropriately sized water heater will have maximum flow rates and energy input, ensuring your property has enough hot water for everyone to use. 

At HD Air, our skilled team specializes in tankless water heater installation as well as other water heater services for Springfield, Massachusetts residents. With our same-day services and 100% satisfaction guarantee, you can be confident your new water heater will be the right size—call (413) 241-3426 for an appointment with HD Air or schedule an assessment for your water heater online.