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How to Select the Right Tankless Water Heater Size

How to Select the Right Tankless Water Heater Size
Last Updated: 05 September 2019

In order to size up your heater, there are many things you need to consider. An electric tankless water heater is rated according to the maximum temperature rise possible at a certain flow rate. For a start, you will need to be able to determine the rate at which water flows and also the temperature rise you’ll be requiring. Even though it is tempting, do not try and undersize your tankless water heater. When selecting the perfect heater for you, think about the flow and don’t focus on the capacity.

How to Size a Tankless Water Heater?

The first step is to figure out the maximum amount of devices that you will want to run and to see how much water passes through them. This is measured in gallons per minute (GPM) and is termed the flow rate. By adding up all the flow rates of the appliances in your home, you’ll have a figure of the desired rate which you’ll need for your water heating requirements. If you’re wondering, “what size tankless water heater do I need?”, as an example, if you’re looking to run a hot water tap with a flow rate of 1 GPM as well as a shower with a 3 GPM flow rate, you should purchase a heater that can handle a GPM of at least 4. It’s advisable to purchase something that can handle a little higher flow rate in order to compensate for any additional water demand you may have.

Next, you should determine the required temperature rise by subtracting the water temperature that enters from the temperature you desire. You can assume that your liquid temperature is around 50 degrees Fahrenheit coming in so that you ensure that you don’t undersize your unit. This will depend on what climate you’ll use and the average temperature of the water coming in.

So to put this all into perspective, let’s consider an example. The average temperature coming into your home is 40 degrees Fahrenheit – If you’re looking at running two showers at once in your home, how big of a tankless water heater do I need, if you require a temperature of 105 degrees Fahrenheit? The answer is that you will need to raise the liquid temp by 65 degrees. If you’re wanting to heat 5 gallons a minute, you’ll require a heater that can produce a 65-degree rise at 5 gallons per minute.

The Benefits of a Tankless Water Heater

You may be wondering, what are the advantages and disadvantages of a tankless water heater? Tankless heaters are great in general because they ensure an endless supply of hot water. In theory, you won’t run out of water and the capacity is not an issue – it’s all about the flow.

If the flow that you’re outputting is raised over the level of the heater’s heating ability then you may have a problem, but other than that you won’t need to worry about running out, even if you’ve got an enormous bath to fill.

If you picture three showers and a washing machine running at the same time, your tankless heater may not be able to heat the large amount demanded by an appliance leading to an output of lukewarm liquid at best. However, you can solve this problem by purchasing a heater that is able to cope and put out more heat. You could also use an appliance at once, or purchase two or more of these types of heaters. So while you can heat up a huge bathtub at one time, in general, you may not be able to if you’re running other demanding appliances at the same time. You may also have to check the flow of water because if it’s too much too fast then your heater may not be able to heat, but aside from these issues, there is a definite advantage of having an endless supply of hot water coming out of the tap on demand.

Flow Rates and Desired Features

Before you make your decision on which tankless hot water heater model you want to purchase, you’ll need to think about the maximum flow rate of coming out. Many people usually overestimate their demand, so check various flow rate guidelines for the home.

A lot of homes today will only demand that everything is flowing in a single use. In general, one can expect a 3 GPM max flow rate to be sufficient here. It is best to measure all of the flow rates at the various outlets you have at home such as the shower so that you can get a better estimate of your flow rate requirements for an ideal water heater in your home. When the climate is warmer, one can use a powerful model with high flow rates of up to 8 GPM for a comfortable shower temperature, but if the climate is cooler outside then you’ll need to lower this number.

There are numerous features that one may desire for their heater system. Many heaters come with digital temperature displays that show in real time what the temperature is and allow for a lot of control. Advanced Flow Control technology automatically adjusts flow when the unit detects that flow demand exceeds the capacity of the heater. This is a great feature to use in a cold climate and in other areas where the size of electrical service doesn’t allow the installation of larger models. This particular technology also makes it more desirable to be able to handle some high flow demand and work for higher output temperatures that are needed to perform temperature critical applications, for instance, dishwashers and other appliances where it is not easy to predict the flow rate.

Average Flow GPM and Average Temp Per Fixture

How much water do you need to use at one time? This is a consideration you’ll need to pay attention to when thinking about tankless water heater sizing. What kind of fixtures do you have in your home? In order to help you decide, we’ve come up with a chart that shows the average flow GPM and temperature per fixture. We would recommend using a 2.5 GPM for showers and 1.0 for bathrooms as a point of reference when determining the total water you’re going to use at one time.

1.Fixture Type: Shower; GPM Average Flow: 3; Average Temperature (°F): 104.

2.Fixture Type: Hot tub; GPM Average Flow: 4; Average Temperature (°F): 102.

3.Fixture Type: Sink; GPM Average Flow: 1.5; Average Temperature (°F): 110.

As an example, if one was to run 2 showers at once, they would need 5 gallons per minute of hot liquid from their tankless heater. If you wanted to run a washing machine and shower at once, you’d need 4.5 gallons and so on. In either of these situations, you would want to size up the unit that meets your GPM needs or exceeds it slightly.

Other Sizing Notes

Most importantly when you’re looking for your whole house tankless water heater, it’s best to consider the flow and not the capacity. We’ve talked about how it’s all about the flow above, so when you’re looking at how to size a tankless water heater, do bear this in mind when making a purchase. If you’re wondering about what size tankless water heater do I need to replace a 50-gallon water heater, there are a number of models available offering you flow rates that vary between 1.1 and 8 GPM.

Determining the Best Power Source

The energy source that you pick, whether it be gas, propane or electricity, should be determined based on a number of factors. Where you are living is important – if you’re living in a large town where there is a copious amount of energy then perhaps a company that supplies it can offer a competitive rate of usage. If you’re living in a more rural setting, it might be difficult to get certain types of energy to your home. Rates could vary depending on where you live. Also, consider what types of energy you have available at your property. Do you have gas plugged in? Can you get enough power? Determining the optimum power source requires some consideration of these factors and a comparison of the rates of each type of energy. Make sure that you also install the cost of fixing, including any wires and pipes that you might need to purchase.

Any homeowners that are concerned about how much energy they can save should pay attention to the efficiency ratings of a unit. A rating ranges from a low of 78% and a high of 88%, indicating how much of the power source is converted to heat by your appliance. The higher the score the better and the more efficient the unit will be.


Electric water heaters are appliances that usually can be expected to consume about a third of the energy you use at home. These heaters provide hot water to meet the needs in your home without needing an enormous boiler. A lot of the newest models come with self-cleaning mechanisms that help to prevent limescale buildup and the buildup of sludge which would otherwise reduce a heater’s lifespan.

A storage tank heater is the most common of all types of an electric heater. They work by allowing water to run into a holding tank where electric elements increase the heat of the stored liquid. This is a great way of allowing the water to stay hot until it is drawn out via the plumbing system of your house. As you can probably guess from the name, these heaters don’t store anything but instead, the liquid is heated up as it passes over the heating element inside. The energy efficiency is high because when there is no demand for water to be kept hot, no extra electricity is used. One of the drawbacks, however, is that they might not be able to generate enough hot substance to meet demand. It really depends on how much hot liquid you’ll be requiring.

Natural Gas or Propane

A lot of heaters work just like an electric heater but instead, the water is heated by gas. As soon as you open the hot tap, this releases the water pressure and automatically the tankless gas water heater turns on to begin heating your water. The advantage of these types is the greater efficiency – gas can heat up water a lot faster and more efficiently. Due to the amount of heat created and the response time, you’ll find that most gas tankless hot water heater models have gas fired burners. Gas fired burners are very responsive and easy to use in your home.

Have you ever purchased a tankless water heater for your home? Do you use gas, electric or propane as your energy source? We would love for you to share your thoughts and experiences. Please leave us your comments below.

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