What Size Tankless Water Heater Do I Need?

Modern technology is a great thing. Where you once had to wait for your hot water tank to refill after running the dishwasher or taking a shower, there are now options that give you an almost endless supply of glorious heated liquid.

Best of all, tankless water heaters are more affordable and easier to install than ever before.

That means the days of having to flip a coin to decide who gets to enjoy a hot shower first thing in the morning are long over. And that’s something just about every member of your family will love!

If you’re thinking about installing a tankless hot water heater in your home, then you’ll be pleasantly surprised at all of the benefits.

Besides the obvious, homes with this type of upgrade generally see a value increase and experience lower annual energy bills.

However, making sure you have the right size unit is one of the most vital aspects of choosing the right one for your home.

If you go too small, then you won’t be able to keep up with demand during peak times. And a system that is too large only wastes energy and requires a higher upfront installation cost.

So, how do you know what size to choose? Generally, you want to think about the number of people living in your home and your desired flow rate at any given time.

You’ll also need to consider the temperature rise of the water, which is the difference between the cold water straight out of the tap and what’s required to get it up to a comfortable heat level.

Are you ready to learn about the benefits of installing a tankless hot water heater and how to choose the right size? Let’s get started.

Tankless Water Heater Basics

Before we can discuss choosing the right size tankless hot water heater for your home, you need to have a basic understanding of how they work and the comparison to traditional methods.

With a classic hot water heater, liquid feeds in from your home plumbing and into a giant reservoir.

From there, the water is heated using a gas burner or electric element. It then sits there for an undetermined amount of time before you turn on the hot water faucet. Once you do, the water is piped in and comes out of the tap.

The problem with this is two-fold. First, the water cools as it sits in the insulated tank, causing the unit to constantly have to keep up with the temperature of the liquid. Second, there’s only so much heated water available when you need it.

In contrast, a tankless design works faster and more efficiently. Just like the traditional design, the cold water comes through your plumbing and into the unit. But that’s where the two greatly differ.

With a tankless appliance, the water doesn’t leave the plumbing until you turn on the faucet. Once you do, a flow sensor tells the main control panel to start heating the water as it comes through the unit.

In some cases, this is through an electric heating element and others it utilizes a special gas burner system.

A special sensor determines whether or not the water is at the desired temperature and makes adjustments accordingly.

The end result of using a tankless heater is that the process is much more energy efficient and provides you with an almost endless supply of hot water.

Additionally, this means that you can have one person taking a shower at the same time as running a dishwasher or washing machine.

What Size Tankless Water Heater Do I Need For a Family of Four?

Choosing the right size tankless hot water heater generally comes down to a few factors. Before making a decision, you should determine:

  • The total number of people in your home or how many people are generally living in the amount of square footage that you have. Select the greater figure.
  • The flow rate in gallons per minute (GPM).
  • The heat rise necessary to warm up liquid from the typical groundwater temperature in your area of the country.

Number of People Living in the Household

It would make sense that the more people living in your home, the greater the size of the tankless hot water heater. This is an important consideration to make when purchasing a unit. However, keep in mind your future needs. If it is just you and your spouse living in the house, but you plan on having children later, you should go ahead and plan for the larger need in the future.

Flow Rate

Flow rate is measured in gallons per minute (GPM). This is the amount of hot water coming out of your tap at a given time. In order to find the right one for your needs, imagine a situation where one person is taking a shower (3 GPM) at the same time you have a high-efficiency dishwasher (1.5 GPM) running simultaneously as a standard washing machine (2.5 GPM). Add all of those numbers together to get your necessary flow rate. In this example, it would be 7 GPM.

Necessary Heat Rise

Where you live plays a role in the size of tankless hot water heater you need. For example, the groundwater in Minnesota during the winter is remarkably colder than that in summertime Texas. Thus, you might need a larger tank to meet the heat rise needs for your area.

Ideal Size of Tankless Hot Water Heater for a Family

Keeping all of these factors in mind, the average tankless hot water heater for a family of four will be somewhere around 10 to 12 GPM. This meets the example of 7 GPM mentioned above and accounts for the occasional faucet turn on or cold weather day.

Can You Oversize a Tankless Water Heater?

Now that you know about how large of a tankless hot water heater to purchase for a family of four, it is time to address whether you should go above and beyond with an oversize unit.

The truth is that there’s no set guidelines about choosing a water heater that is beyond your initial family needs. However, there are a few issues that arise when you do. In contrast, there can also be a few reasons to opt for this path, too.

The Cons of Choosing an Oversize Tankless Water Heater

  • Additional Energy Use: If keeping your energy bills low is your primary reason for switching to a tankless design, then you’ll want to stick to a smaller size unit. Larger appliances heat at their rating level, no matter how many people require hot water at the same time.
  • Higher Upfront Installation Cost: It makes sense that the larger the unit, the more expensive it is to purchase and install. This higher upfront cost might be over your project budget.

The Benefits of Choosing an Oversize Tankless Water Heater

  • Better Overall Heating in the Winter: If you live in an area that sees extreme cold weather in the winter, then you’ll want to have a larger appliance to accommodate the swing in groundwater temperatures.
  • Future Family Growth: Most people can’t tell you how many people will live in their home in ten or twelve years. Families expand, tiny babies grow to teens who like to take long showers, and in-laws move in. Installing a larger tankless system from the beginning can help grow over time.
  • Home Value and Selling: If you don’t plan to live in your home for more than a few years, it is a good idea to go ahead and choose a larger tankless unit. While you might only have two people living at your residence now, the future buyers might have more.

What Is the Downside of a Tankless Water Heater?

Now that you know how they work and the size to choose for your family’s needs, it is time to discuss the downside. When most homeowners look at the idea of having an endless supply of hot water at any given time, it can be difficult to see what any negative aspect might be.

However, like anything else in life, there are cons to installing a tankless hot water heater in your home. Here’s what you need to know.

1. Higher Upfront Costs

Tankless hot water heaters are at the cutting edge of home appliances. While the prices these days are considerably lower than when they first came out, the overall cost is still much higher than a traditional tank design.

2. Water Flow Might Not Always Keep Up

One of the biggest complaints from homeowners that purchase a tankless hot water heater is that it can’t keep up with the simultaneous needs of their family. This is especially true in situations where many different fixtures are going at once–i.e. a running shower, faucet in the kitchen, and a washing machine.

3. Need for Additional Equipment

Many homeowners are often surprised to learn that the tankless unit isn’t all they need for this type of system. Depending on the condition of the water in your area, you might also require a water softener or other item to ensure your tankless system works efficiently.

4. Rerouting of Gas Lines and Venting

Some homes aren’t equipped with the necessary natural gas lines or venting systems for a tankless hot water heater. In that case, you would need to contact a contractor to make these alterations to your residence.

5. Additional Maintenance is Required

While tankless water heaters are much more convenient, they also require more annual maintenance than a traditional design. This means tasks like flushing the system to reduce mineral buildup and more.

Wrap Up: Tankless Water Heaters

Adding a tankless unit to your home can be an excellent way to ensure everyone in the family gets the hot water when they need it most.

These appliances are much more energy efficient and can raise the overall value of your property, too.

While there are some cons, the overwhelming benefits of installing one highly outweigh the downsides. In short, if you have the budget for installing the right size for your needs, you’ll be very glad that you did.

See our water heater repair & installation service page for more info or to get a quote on installing one in your home.

So Cal Gas is currently providing rebates of $600 to $1,000 for certified Energy Star rated tankless water heaters. Check their rebates page for current availability.

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Owner - Operator at RT Olson Plumbing
Bob Olson has over 18 years experience in all aspects of residential and commercial plumbing. He's a 4th generation plumber. With over 400 five-star customer reviews, he runs one of the best-rated plumbing companies in Riverside, CA.
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