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How to Select the Right Size Tankless Water Heater?

Tankless water heaters are all the rave right now and for a good reason too. Tankless heaters forgo the water reservoirs on old school heaters and opt instead to heat water on demand, whenever you need it.

With this on-demand heating style, tankless water heaters significantly improve on energy efficiency compared to their tanked counterparts as they do not have to reheat water in storage continually. Furthermore, tankless water heaters are also less bulky and way more easy to install than other models.

However, like with all new developments in the home improvement, it is not uncommon for eager salespeople to oversell the many wonders on the latest product. And while “experts” may promise you the end of all your heating worries, it is essential to consider that the typical tankless water heater comes with a few drawbacks too.

Since tankless heaters only heat water when you need it, it is not uncommon to experience a slight delay before the hot water reaches your faucets. Furthermore, the number of water points the heater can serve simultaneously and the volume of water it can dispense to these points is depended mainly on the power and flow rate of the appliance.

Hence, you must select the right size of the tankless water heater to guarantee an enjoyable, uninterrupted experience. However, sizing the right heater for your home can quickly become an offputting episode, as many users are often discouraged by all the mental math involved in the process.

Worry no more, as we have created this article to fix that problem. This report will walk you step by step through the process of selecting the right size of the tankless water heater for your home.


Why You should Size a Tankless Water Heater Properly?

1. Avoid Flow Rate Drops

With tanked water heaters, the flow rate to your faucets remains constant irrespective of the size of the reservoir. Since the heater stores up sufficiently heated water in the tank, the appliance can continuously supply water at its maximum gallons per minute until the tank is empty. Hence, with these types, even when you are running low on hot water, you still get a steady supply to fuel all your tasks.

However, with tankless water heaters, this situation is flipped on its head. Since tankless water heaters do not keep hot water in storage, the rate at which they deliver water to your faucets is directly impacted by the amount of demand for hot water.

Hence, with a tankless water heater, if your home demand for hot water significantly surpasses the maximum flow rate of the appliance, the heater will only be able to provide a throttled flowed to your water points. Consequently, in such situations, expect an immediate water temperature and pressure drop at your faucets and showerheads.

This heavy reliance on the max flow rate of the device is the primary reason why the sizing is critical when selecting a tankless water heater. The right size of the tankless water heater will result in increased convenience and help save you considerable amounts of wait times.

2. Maintenance Issues

To prevent overworking your heater, you should select a heater that slightly surpasses the hot water demand at your home. A heater that is working at the max output at all times in a bid to match excessive demand is significantly more likely to malfunction and require increased maintenance costs.

3. Pricing Issues

While a tankless water heater that is too big will still get the job done efficiently, purchasing an oversized heater will cost you significantly more out of pocket. Furthermore, buying a tankless heater that is too small will result is the need to buy another heater to replace or complement the initial purchase.

How to Select the Right Size of Tankless Water Heater?

The single most important metric for adequately sizing your tankless water heater is the flow rate. Other steps you must also consider include the temperature rise, and factoring in installation and maintenance parameters.

1. Figuring Out the Right Flow Rate

Before you move on to purchasing a new tankless water heater, you must ascertain the exact flow rate, measured in gallons per minute, that you need to power your entire home. For example, if you expect to run a faucet that requires 1 GPM and a showerhead with a 2.5 GPM rating simultaneously, your heater must be able to provide at least 3.5 GPM.

Furthermore, when measuring the total flow rate for your home, you must estimate the water demand during peak periods. For example, in many homes, more hot water is needed in the early hours of the day than at any other time, since multiple showers, faucets, baths, and washbasins may need water simultaneously.

To get the perfect flow rate for your new tankless water heater, tally up the water demands of all the fixtures that will run simultaneously then add 1 or 2 GPM to provide coverage for special situations.

To find the required flow rate for a specific fixture, you can refer to its user manual or any other appropriate documentation. Alternatively, you can choose to measure the flow rate yourself using a 1-gallon container. Sit the container under the water point and record how much time, in seconds, it takes to get filled up. Then divide 60 by the total time to get the flow rate for that faucet. Repeat this process for each water point to get the overall flow rate that your water heater needs to match.

Nevertheless, if you are pressed for time and would rather skip the measuring process, you can use the following table as a quick guide to estimating the total water demand in your home.

Water Fixture Average Flow Rate for American Standard Units
Shower 2.5 GPM
Faucet 1.5 – 2 GPM
Bathtub 4 GPM

However, it is essential to note that the GPM of each faucet measures the total water output at that point. Since no one uses purely hot water, the heater is only responsible for a portion of the overall flow rate to that faucet. Hence, your heater’s GPM can be slightly lower and does not have to be an exact match of your home total to cover your entire hot water needs.

Furthermore, you can reduce the total flow rate needs of your home by taking water conservation measures like installing low-flow fixtures or aerators.

2. Considering the Temperature Rise

It’s easy to take one look at a heater that claims to produce up to 7 GPM and immediately conclude that the unit is capable enough to match all your home needs. However, it is crucial to consider that such claims typically represent the heater operating at peak performance. One factor that can significantly the proper function of any tankless heater is temperature rise.

The temperature rise determines how much your heater has to work to get the water to the right temperature, which is typically hot water temperature.

To calculate the water rise at your location, subtract the inlet water temperature from the output temperature you require. For example, if the water temperature at your home is 50°F and you want your water heated to 110°F, your temperature rise is 60°F. Figuring out the inlet water temperature at your home is comfortable with a thermometer, or you could refer to the legend below.



To ensure that your tankless water heater can provide the flow rate estimated by the OEM, guarantee that the heater you pick can match your required temperature rise.

3. Factoring in Installation Parameters

Another critical factor to consider when sizing a tankless water heater for your home is the actual physical size of the unit. Depending on the space concerns in your home as well as your aesthetic persuasions, the perfect tankless water heater for you may vary significantly.

Some tankless heater units are small enough to fit under a sink, while others are too large to hide. Make sure to select a model that matches the available dimensions in your home before purchase.

Other Factors to Consider When Choosing a Tankless Water Heater

1. Electric or Gas

Once you have figured out the right size of the tankless water heater for your home, another almost equally important factor is the fuel type the appliance uses.

The rule of thumb with fuel type is to opt for the one that would cost you the least to set up. While the price to run the heater varies with natural gas the cheapest, then propane, then electricity, it is the installation cost that can make a significant difference and leave a big dent in your pocket.

If you are upgrading from a tanked system, opting for a tankless unit that uses the same fuel can significantly cut down the installation costs. Furthermore, you are always better off choosing a fuel type that you already have piping for in your home. Another thing to consider with the gas option is how easy it is for you to set up the venting system.

However, if you have an even spread for all installation types, select the ideal heater type for you by comparing the cost and availability of natural gas, propane, and electricity in your locality.

It is also important to note that tankless gas heaters typically heat water faster than their electric counterparts. However, generally, electric heaters cost less than gas-powered models.


2. Whole House Unit or Point of Use

When installing a new tankless water unit for your home, you can choose between opting for a single unit to power your entire house or multiple smaller unit to provide hot water at the point of use.

If you choose a whole-house unit, you must select one with enough power, flow rate, and temperature rise to power your house at peak periods. On the other hand, the point of use heater gives you more flexibility as for each heater, you only have to worry about the demand at the adjoined water outlet.

Point of use heaters are also significantly smaller in size, and they can be easily stashed away on a wall mount, under a sink, or in other obscure locations.

3. Energy Efficiency

In ideal circumstances, your heater should utilize all the energy it consumes into heating water for your daily uses. However, in the real world, this is never the case. Water heaters, like other similar appliances, have inefficiencies as they lose energy during combustion and the water heating process.

The energy efficiency of your home unit goes a long way in determining how effective the tankless water heater is at matching your home’s hot water needs. Furthermore, genuinely efficient heaters can cut your energy costs by half compared to the unoptimized unit. Hence, if you are conscious about the operating costs of your heating system, an Energy Star rated model is a no brainer.

In the U.S., the benchmark for efficient is set at an energy factor of 0.93. However, for most modern tankless heaters, energy efficiency is not typically an issue as these models often surpass the 0.95 efficiency mark.

4. Compactness

As with other home appliances and fixtures, with tankless water heaters, you also have to consider the size of the unit and how it fits into your home space and aesthetics. Ensure to select a machine that matches the available space while blending into your home aesthetics. However, while optimizing for size, you also have to keep the flow rate factor in mind.

5. Features

Tankless water heaters can come with several features that can significantly improve their user experience.

Some units feature self-diagnostic systems that continually scans the appliance checking for any malfunction and reporting it to you before any further complications. Others come with overheating sensors, and an auto-shutdown feature that alerts you of abnormal temperature rises and shuts down the heater to guarantee your safety.

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